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  • Analytical Chemistry and Spectroscopy  (9,774)
  • 1995-1999  (2,202)
  • 1980-1984  (4,318)
  • 1975-1979  (3,254)
  • 1930-1934
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  • 1
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 2
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 3
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    Journal of Chemometrics 9 (1995), S. 229-229 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 4
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    Journal of Chemometrics 9 (1995), S. 230-231 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 5
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    Journal of Chemometrics 9 (1995), S. 232-236 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 6
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    Journal of Chemometrics 9 (1995), S. 343-362 
    ISSN: 0886-9383
    Keywords: straight line calibration ; errors in both axes ; uncertainties ; linear regression ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calibration is a fundamental step in the calculation of the unknown concentration of analyte in most analytical methods. It is known that for certain methodologies, if only the errors in the independent variable are taken into account, there may be considerable errors in the estimation of the value of the regression coefficients, the derived statistical parameters and in some cases the sought for response and concentration values. This paper reviews the calibration methods including some references to procedures for the detection of outliers and robust regression when there are errors in both axes. The advantages and limitations of the different approaches are discussed and a comparative study is made of the approaches of several techniques for which computer programmes have been developed based on the algorithms put forward by the different authors. Finally, some trends of future development in this field are envisaged.
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  • 7
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    Journal of Chemometrics 9 (1995), S. 363-372 
    ISSN: 0886-9383
    Keywords: PLS regression ; orthogonal expansion ; optimization ; Lagrange multipliers ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simple iteration algorithm that is faster and less memory-intensive than the NIPALS iteration algorithm for PLS regression is presented. The iteration algorithm is obtained by treating the orthogonal expansion or decomposition of a matrix X as an extremum problem subject to normalization and orthogonality constraint conditions and then solving the problem by use of the method of Lagrange multipliers. The main idea in this method is to find the transformation vector r. The latent variable t is expressed exactly as the linear combination of X-variables with the vector r so that the final regression coefficients can be conveniently provided. In the algorithm the recursion of the orthogonal projection is needed, which is derived by use of a matrix inverse formula. Algorithms are established from the equation for calculating the vector r that are suitable for dealing with three cases of large data sets. The first case is when the number of objects is very large, the number of variables is relatively small and the number of Y-variables is equal to or greater than the number of X-variables. The second case is when the number of objects is very large, the number of variables is relatively small and the number of X-variables is greater than the number of Y-variables. The last case is when the number of variables, either X- or Y-variables, or both, is very large and the number of objects is small.
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  • 8
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    Journal of Chemometrics 9 (1995), S. 431-432 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 9
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    Journal of Chemometrics 9 (1995), S. 451-457 
    ISSN: 0886-9383
    Keywords: non-linear regression ; optimization ; robust methods ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several robust regression methods, including a new proposal, are described and their properties discussed. Resistance to various types of outliers and non-normality is demonstrated. The techniques are applied to non-linear regression models from chemical kinetics and calibration. Optimization of the types of objective functions encountered when applying robust regression is considered.
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  • 10
    ISSN: 0886-9383
    Keywords: PLS ; kernel algorithm ; multivariate calibration ; EM algorithm ; cross-validation ; missing data ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This is Part II of a series concerning the PLS kernel algorithm for data sets with many variables and few objects. Here the issues of cross-validation and missing data are investigated. Both partial and full crossvalidation are evaluated in terms of predictive residuals and speed and are illustrated on real examples. Two related approaches to the solution of the missing data problem are presented. One is a full EM algorithm and the second a reduced EM algorithm which applies when the number of missing values is small. The two examples are multivariate calibration data sets. The first set consists of UV-visible data measured on mixtures of four metal ions. The second example consists of FT-IR measurements on mixtures consisting of four different organic substances.
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  • 11
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    Journal of Chemometrics 9 (1995), S. 439-439 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 12
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    Journal of Chemometrics 9 (1995) 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 13
    ISSN: 0886-9383
    Keywords: sequential response surface modelling ; Williamson ether synthesis ; SN2 ; optimization ; reaction kinetics ; reaction mechanism ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work the utility of a new method for determining kinetic parameters by sequential response surface modelling, previously described (Part 1), is shown by applying it to an experimental study of a reaction with known kinetics. The nucleophilic substitution reaction between ethoxide and benzyl chloride, the Williamson ether synthesis, was selected as a model reaction. This reaction is known to proceed with second-order kinetics. The method gives access to estimates of initial reaction rate which can be further used to obtain estimates of activation energy and reaction order of reactants. The results obtained are in good agreement with the estimated values of these parameters obtained with conventional kinetic experiments.
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  • 14
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    Journal of Chemometrics 9 (1995), S. 483-487 
    ISSN: 0886-9383
    Keywords: fractional factorial design ; multiresponse ; PLS ; PCA ; reduced-rank regression ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper presents an interpretation of PLS applied to orthogonal explanatory variables. In particular, it is shown that in fractional factorial multiresponse experiments PLS2 gives identical results to ordinary least squares applied to principal components of the response variables. The general relationship is that the reduced-rank regression algorithm which first projects Y onto the X-space and then truncates this matrix by principal component analysis before performing ordinary least squares estimation gives the same predictor as PLS2 and SIMPLS if all the non-zero eigenvalues of XTX are identical.
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  • 15
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    Journal of Chemometrics 9 (1995), S. 471-481 
    ISSN: 0886-9383
    Keywords: batch prediction ; continuum regression ; multivariate calibration ; sequential prediction ; simultaneous prediction ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In analytical chemistry a single fitted calibration model is used repeatedly to predict the level of the analyte of interest for the specimens comprising the prediction set. Unlike the calibration (or training) set, which is often limited in size, the prediction set can be very large.In the case of multivariate calibration a number of methods such as PLS and PCR are commonly used to construct the calibration model. The set of instrumental measurements and the reference analyte level are available for each specimen in the calibration set. For specimens in the prediction set, only the instrumental measurements are available, since the problem is to predict the analyte level for these specimens. It is not widely recognized that predictions of the analyte levels for individual specimens can be improved by utilizing seemingly unrelated information from the instrumental measurements associated with the other members of the prediction set. In the case of PCR there exists a very straightforward procedure for doing this. A description of the various sources of prediction errors is provided to explain the ability of PCR to utilize this additional information. The use of PCR in this context is illustrated with both a synthetic and a real example.
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  • 16
  • 17
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    Journal of Chemometrics 9 (1995), S. 531-531 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 18
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    Journal of Chemometrics 9 (1995), S. 509-520 
    ISSN: 0886-9383
    Keywords: canonical variates ; discriminant analysis ; partial least squares ; principal components ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new set of derived variables is proposed for exhibiting group separation in multivariate data on for preprocessing such data prior to discriminant analysis. The technique combines optimal features of canonical variate analysis and principal component analysis: the derived variables are linear combinations of the original variables that optimize the canonical variate criterion (ratio of between-group to within-group variance) but subject to the orthogonality constraints of principal components. In this formulation the canonical variates can be derived even when the within-group matrix is singular (i.e. when there are more variables than objects in the data matrix). A simple computational algorithm for extraction of these variables is proposed. The methods are illustrated on several data sets and compared with alternative techniques such as principal component analysis and partial least squares.
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  • 19
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    Journal of Chemometrics 9 (1995), S. 489-507 
    ISSN: 0886-9383
    Keywords: QSAR ; partial least squares ; robust regression ; CoMFA ; weighted regression ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A robust implementation of partial least squares (PLS) is developed in which the method of iteratively reweighted least squares is adapted for use with PLS. The result is a PLS algorithm which is robust to outliers and is easy to implement. Examples and case studies are presented, followed by two Monte Carlo studies designed to explore the behavior of the method.The paper begins with the motivation and intended applications for the procedure. A discussion is given of the method of interatively reweighted least squares (IRLS) for outlier detection. The procedure, given the name IRPLS, is then presented. Three case studies illustrate how the procedure works on various types of data and how it should be used. The first Monte Carlo study is designed to determine whether the IRPLS procedure correctly identifies multiple outliers in a wide variety of configurations. The second Monte Carlo study is designed to estimate the breakdown bound of the procedure.
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  • 20
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 21
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    Journal of Chemometrics 9 (1995), S. 309-322 
    ISSN: 0886-9383
    Keywords: simulated annealing ; messy genetic algorithms ; optimization of multimodal objective functions ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The method of simulated annealing is modified so that the concept of messy chromosomes is applied. Constituent genes of messy chromosomes are specified by their respective names (indices) and values (alleles) simultaneously. Unlike simple chromosomes (binary vectors), messy chromosomes may be either under- or overspecified with respect to the problem being solved. The messy simulated annealing algorithm is a very robust and efficient stochastic optimization method which is able to find correct minima of deceptive or highly multimodal objective functions. This is shown by way of a number of simulations.
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  • 22
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    Journal of Chemometrics 9 (1995), S. 323-326 
    ISSN: 0886-9383
    Keywords: partial least squares ; biased regression ; ordinary least squares ; minimum length least squares ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An algebraic proof is given that in partial least squares (PLS) regression the Euclidean length of the estimator is shrunk in comparison with the ordinary least squares estimator or with PLS estimators based on a larger number of dimensions.
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  • 23
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    Journal of Chemometrics 9 (1995), S. 329-330 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 24
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    Journal of Chemometrics 9 (1995) 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 25
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    Journal of Chemometrics 9 (1995), S. 389-409 
    ISSN: 0886-9383
    Keywords: multivariate image analysis ; principal component analysis ; exploratory data analysis ; projection in multivariate space ; graphical visualization ; noise ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Multivariate image analysis (MIA) is a powerful tool for many image segmentation and classification problems, but the interpretation and understanding of the original and resulting multidimensional (multivariate) data are not always easy. A strategy for MIA has been proposed which describes its usage on multivariate images for segmentation tasks. MIA starts with principal component analysis (PCA) and then continues with interactive analysis of the output from PCA. In this paper a number of extensions to MIA are proposed. The extensions are the suggestion to incorporate preprocessing of the multivariate image in MIA, the suggestion to use synthetic multivariate image models which create a clear-cut situation, and new visualization tools which improve the interactivity and understanding of the results. Extended MIA is applied on synthetic multivariate image data simulating a possible application with large noise, positron emission tomography (PET). As a result of the interactive analysis, suggestions for preprocessing emerge. The developed methodology for handling the noise is then applied on real PET image data with good results.
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  • 26
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Chemistry and Pharmacology
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  • 27
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Chemistry and Pharmacology
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  • 28
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    Journal of Chemometrics 9 (1995), S. 3-20 
    ISSN: 0886-9383
    Keywords: Deconvolution algorithms ; Instrumental analysis ; Spectrometry ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Deconvolution algorithms for measurand reconstruction are considered. Their metrological and numerical properties are briefly characterized. Six algorithms most frequently used for instrumental applications are selected for closer analysis. Their comparative study is based on the use of spectrometric-type synthetic data, calorimetric-type synthetic data and spectrometric real-world data. Conclusions concerning computational complexity and accuracy of the compared algorithms as well as their metrological applicability are drawn.
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  • 29
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    Journal of Chemometrics 9 (1995), S. 67-68 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 30
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    Journal of Chemometrics 9 (1995), S. i 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 31
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    Journal of Chemometrics 9 (1995) 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 32
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 33
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    Journal of Chemometrics 9 (1995) 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 34
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    Journal of Chemometrics 9 (1995), S. 125-135 
    ISSN: 0886-9383
    Keywords: Procrustean analysis ; FT-IR spectroscopy ; NIR spectroscopy ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Procrustean methods allow the fitting of a given matrix to another given matrix observed on the same objects. In the traditional approach orthogonal constraints are imposed upon the transformation matrix, whereas in the alternative approach Procrustean analysis may be performed without such constraints. The two methods (with and without constraints) were compared on data dealing with mid- and near-infrared spectra of oil. The aim was to reconstruct the mid-infrared spectral information using data from the near-infrared spectra. Unconstrained Procrustean analysis proved to be the more efficient for both the calibration and verification sets. Furthermore, the analysis of the transformation matrix between the two infrared ranges made it possible to indicate wavelengths and wave numbers corresponding to the same chemical groups.
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  • 35
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    Journal of Chemometrics 9 (1995), S. 139-141 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Chemistry and Pharmacology
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  • 36
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    Journal of Chemometrics 9 (1995) 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 37
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    Journal of Chemometrics 9 (1995), S. 1-2 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Chemistry and Pharmacology
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  • 38
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    Journal of Chemometrics 9 (1995), S. 137-138 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 39
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    Journal of Chemometrics 9 (1995), S. 91-123 
    ISSN: 0886-9383
    Keywords: H-principle ; PCA ; PLS regression ; latent variable models ; quadratic models ; sensitivity analysis ; outlier tests ; prediction variances ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present here an algorithmic approach to modelling data that includes principal component analysis (PCA) and partial least squares (PLS). In fact, the numerical algorithm presented can carry out PCA or PLS. The algorithm for linear analysis and extensions to non-linear analysis applies to both PCA and PLS. The algorithm allows for combination of PCA and PLS types of models and therefore extends modelling to new types of models that involve combination of regression models and ‘selection of variation’ models, which is the idea of PCA-type models. The fact that the algorithm carries out both PCA and PLS shows that PCA and PLS are based on the same theory. This theory is based on the H-principle of mathematical modelling. The algorithm allows tests for outliers, sensitivity analysis and tests of submodels. These aspects of the algorithm are treated in detail. We compute various measures of sizes, e.g. of components, of the covariance matrix, of its inverse, etc. that show how much the algorithm has selected at each step. The analysis is illustrated by data from practice.
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  • 40
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    Journal of Chemometrics 9 (1995), S. 69-89 
    ISSN: 0886-9383
    Keywords: chemometrics ; pattern recognition ; class modeling ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Different estimators of the Mahalanobis distance (such as that based on the Defrise - Gussenhoven correction) are studied and compared with respect to the bias on the distance and the characteristics (sensitivity and specificity) of the class model.Results obtained using estimators with critical values from χ2-statistics are compared with those obtained using estimators with critical values from β-statistics (training set) and Hotelling statistics (evaluation set).Tables are reported for D-statistics (useful for simulating populations of two categories with selectable theoretical sensitivity and specificity) and for critical values of the Mahalanobis distance obtained from β-statistics.For objects of the training set the estimator of the Mahalanobis distance based on the estimate of the covariance matrix produces models with the optimum sensitivity. The same model has too low a sensitivity for objects of the model category in the evaluation set, but good specificity for objects of outer categories.The estimator with the Defrise-Gussenhoven correction produces enlarged models with too high a sensitivity for objects in the training set, good sensitivity for objects of the model category in the evaluation set and low specificity for objects of outer categories.
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  • 41
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    Journal of Chemometrics 9 (1995) 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Chemistry and Pharmacology
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  • 42
    ISSN: 0886-9383
    Keywords: pattern recognition ; infrared spectra ; factor analysis ; maximum likelihood method ; entropy of information ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spectral region from 700 to 3600 cm-1 is subdivided into several wave number intervals. The peaks in each interval are summarized by means of three encoding algorithms. Using a factor model of kcommon factors, the total extractable variacnce (com) of a given set of intervals is calculated and correlated with the redundancy of information in all these intervals. The value of com is verified by analysis of the factor loadings aik (factor pattern). Finally, the information content of some chosen sets of intervals coded by the three selected feature algorithms will be correlated to the probability of information flow through a serial-parallel network. The encoding using only wave numbers was found to be the most effective.
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  • 43
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    Journal of Chemometrics 9 (1995), S. 59-66 
    ISSN: 0886-9383
    Keywords: Mathematical approximation ; Sigmoid curve ; Sensitometric curve ; Photographic material ; Non-linear regression ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper studies the application possibilities of various mathematical analytical functions to models of approximation of sigmoid sensitometric curves of photographic materials. The newly developed functions have been tested along with approximation functions taken from the literature: the former fulfil the statistical criteria of fitting the regression and experimental curve in the best way. The model functions can be used not only for objective determination of sensitometric quantities but also for determination of the general sensitivity (speed) of photographic materials.
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  • 44
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    Journal of Chemometrics 9 (1995), S. 169-178 
    ISSN: 0886-9383
    Keywords: minimum volume ellipsoid (MVE) estimators ; robust distance method ; pattern recognition ; Hotelling's T2 statistics ; near-infrared spectra ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method for pattern recognition analysis of near-infrared spectra has been developed using robust distances determined by minimum volume ellipsoid (MVE) estimators of multivariate location and scatter. Classical methods such as the Mahalanobis distance method often fail in the presence of a moderate number of outliers in a training data set, while robust distance methods can tolerate a considerably larger proportion of outliers in a training data set Outliers can be detected by their relatively large robust distances and can be excluded from a training set without a priori knowledge of the nature of the data set. In this paper the properties of a robust distance method are examined using near-infrared spectra of sulfamethoxazole and mixtures with its major degradation products, sulfanilic acid and sulfanilamide. The robust distance method successfully detected unacceptable samples (71.4%-89.3% (α = 0.05) or 78.6%-92.9% (α = 0.10)) even when they were inadvertently included in the training data set.
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  • 45
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    Journal of Chemometrics 9 (1995), S. 179-195 
    ISSN: 0886-9383
    Keywords: GRAM ; PARAFAC ; rank overlap ; second-order calibration ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: GRAM, a method for second-order calibration, has been introduced by Sanchez and Kowalski and later modified by Wilson, Sanchez and Kowalski. The methods are based on the claim that, in cases without measurement error they yield correct estimates for the concentration ratios and profiles of (rank-one) analytes present in sample and mixture. This claim has not been proven rigorously. In the present paper, rigorous proofs are given for situations where the claims are valid indeed. In addition, it is shown that PARAFAC, an alternative method for second-order calibration, can be used to obtain the same results. Next it is shown that the claims do not hold in cases with ‘rank overlap’ (partly overlapping profiles) and it is proven that a procedure by Wang et al. can still be used to assess some of the concentration ratios. A general framework is provided for a variety of second-order calibration problems and the extent to which quantitative and qualitative information can be expected is given.
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  • 46
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    Journal of Chemometrics 9 (1995), S. 223-225 
    ISSN: 0886-9383
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 47
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    Journal of Chemometrics 9 (1995), S. 197-209 
    ISSN: 0886-9383
    Keywords: proficiency test ; true value ; homogeneity robust statistic ; standard ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Profficiency tests raise two types of problems: (i) the homogeneity of a sample must be carefully checked before using it as a reference material; (ii) it is necessary to define a conventional reference value (RV) in order to rank participants. Since these topics are poorly addressed in the literature, a reference material was specially prepared in order to propose a procedure for verifying homogeneity and to establish whether the algorithms classically used to compute the conventional true value have an influence on the conclusion of the test.The homogeneity of the sample was not perfect and univariate and multivariate techniques were used to demonstrate some analytes can be suspected as heterogeneous. However, this was considered adequate for introducing the sample in a large proficiency test organized between four laboratory associations which regularly perform such tests. Altogether more than 3000 measurements were collected and eight algorithms were applied to compute the RV on 14 analytes. Thus is was possible to demonstrate that some of these algorithms are better adapted than others. It is also obvious that some analytes are better suited to proficiency testing.It can be concluded from this work that some effort towards standardization would be profitable to check homogeneity or to compute the RV, especially since the economic weight of proficiency testing is becoming even more important with the development of certification accreditation.
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  • 48
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    Journal of Chemometrics 9 (1995), S. 423-430 
    ISSN: 0886-9383
    Keywords: PLS ; neural network ; training ; interpretable ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This communication describes the combination of a feedforward neural network (NN) with one hidden neuron and partial least squares (PLS) regression. Through training of the neural network with an algorithm that is a combination of a modified simplex, PLS and certain numerical restrictions, one gains an NN solution that has several feasible properties: (i) as in PLS the solution is qualitatively interpretable; (ii) it works faster than or comparably with ordinary training algorithms for neural networks; (iii) it contains the linear solution as a limiting case. Another very important aspect of this training algorithm is the fact that outlier detection as in ordinary PLS is possible through loadings, scores and residuals. The algorithm is used on a simple non-linear problem concerning fluorescence spectra of white sugar solutions.
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  • 49
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    Journal of Chemometrics 9 (1995), S. 21-29 
    ISSN: 0886-9383
    Keywords: Near-infrared absorbance ; Partial least squares ; Principal component regression ; Root-mean-square error of prediction ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The use of principal component regression (PCR) as a multivariate calibration method has been discussed by a number of authors. In most situations principal components are included in the regression model in sequence based on the variances of the components, and the principal components with small variances are rarely used in regression. As pointed out by some authors, a low variance for a component does not necessarily imply that the corresponding component is unimportant, especially when prediction is of primary interest. In this paper we investigate a different version of PCR, correlation principal component regression (CPCR). In CPCR the importance of principal components in terms of predicting the response variable is used as a basis for the inclusion of principal components in the regression model. Two typical examples arising from calibrating near-infrared (NIR) instruments are discussed for the comparison of the two different versions of PCR along with partial least squares (PLS), a commonly used regression approach in NIR analysis. In both examples the three methods show similar optimal prediction ability, but CPCR performs better than standard PCR and PLS in terms of the number of components needed to achieve the optimal prediction ability. Similar results are also seen in other NIR examples.
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    Journal of Chemometrics 9 (1995), S. 31-58 
    ISSN: 0886-9383
    Keywords: Multivariate curve resolution ; Three-way data analysis ; Factor analysis ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new multivariate curve resolution method is presented and tested with data of various levels of complexity. Rotational and intensity ambiguities and the effect of selectivity on resolution are the focus. Analysis of simulated data provides the general guidelines concerning the conditions for uniqueness of a solution for a given problem. Multivariate curve resolution is extended to the analysis of three-way data matrices. The particular case of three-way data where only one of the orders is common between slices is studied in some detail.
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    Journal of Chemometrics 9 (1995), S. 239-262 
    ISSN: 0886-9383
    Keywords: industrial experimentation ; parameter design ; quality by design ; robust design ; Taguchi method ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aim of this paper is to present a simple structured review of the different approaches to robust process design to clarify their similarities and dissimilarities. It is primarily written for practitioners who wish to understand and compare the main ideas of each approach and to apply them to their work. Two examples are used to illustrate the different approaches and their corresponding data analysis strategies: the first one is a constructed example on a pigment kneading process and the second one is real example dealing with the validation of an HPLC method. A comparison of the different approaches is provided and some practical recommendations are formulated.
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  • 52
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    Keywords: multiway data ; standard addition method ; calibration ; matrix effects ; trilinear decomposition ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Presented here is an algorithm for analysis of second order data by the method of standard additions. The method of standard additions is applicable when matrix effects make traditional calibration unreliable. The algorithm employs a generalized eigenproblem to mathematically separate the instrument response of the analyte from the instrument response of any interfering species. A scheme for determining the eigenvectors (and hence the concentration estimate) that uniquely correspond to the analyte of interest is given. These eigenvectors can readily be distinguished from any eigenvector that corresponds to the spectrum of the interferents or both the interferents and analyte. The stability of the estimated analyte concentration is verified by Monte Carlo simulations. The algorithm is applied to the analysis of trichloroethylene in samples that have matrix effects caused by an interaction with chloroform.
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    Journal of Chemometrics 9 (1995), S. 283-308 
    ISSN: 0886-9383
    Keywords: simulated annealing ; threshold acceptance ; wavelength selection ; selectivity ; optimization ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Simulated annealing (SA) is a stochastic search method that has been applied to combinatorial search problems in chemometrics. Unlike strict iterative improvement methods, SA tolerates temporary moves to detrimental parameter configurations during an optimization. The method used by SA to decide whether or not to accept detrimental steps is a special case of a more general acceptance rule. The present work investigates the performance of various SA-type algorithms that differ only in the acceptance rule for detrimental steps when optimizing continuous or discrete problems. A method for step width modulation is introduced to overcome the poor ability of SA type algorithms to locate the exact extreme of a function. The studied search strategies are modified for the discrete problem of wavelength selection. In order to evaluate SA-type algorithms and their abilities to deal with the wavelength selection problem, two global measures of selectivity are used as criteria to determine the most suitable wavelength subset that maximizes selectivity for pure component ultraviolet-visible spectra.
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    Journal of Chemometrics 9 (1995), S. 327-328 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Journal of Chemometrics 9 (1995), S. ii 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 56
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    Journal of Chemometrics 9 (1995), S. 331-342 
    ISSN: 0886-9383
    Keywords: partial least squares (PLS) ; variable selection ; IVS-PLS ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With the aim of developing PLS models with improved predictive properties, an interactive variable selection (IVS) approach for PLS regression was introduced in Part I of this series. IVS-PLS is based on a dimension-wise selective removal of single elements in the PLS weight vector w. IVS uses cross-validation (CV) as a guiding tool. The present paper illustrates the use of IVS-PLS on both simulated data and real examples from chemistry. In the first example, spectrophotometric data were simulated according to an experimental design. The objective was to see how IVS-PLS was influenced by different levels of noise in X and Y and by the number of predictor variables (K). The results of the modelling are shown as response surfaces. In addition, four real examples were modelled by the IVS-PLS technique. The real data sets were chosen to reflect different types of data from chemistry. For each example a comparison of ‘prediction error sum of squares’ (PRESS) between IVS-PLS and classical PLS is madeFor most of the examples containing many predictor variables IVS-PLS shows an improvement in predictive properties over classical PLS. Also, improvements for IVS-PLS2 (modelling of more than one y-variable) models were found. For data sets with a moderate number of variables the influence of the IVS method becomes less pronounced.
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    Journal of Chemometrics 9 (1995), S. 373-387 
    ISSN: 0886-9383
    Keywords: robust regression ; outliers ; spectroscopy ; calibration ; MASBR (maximum sum of binary coded residuals) ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, a novel robust regression method, the maximum sum of binary coded residuals (MASBR), is proposed. Instead of the sum of squared residuals used in least squares regression as the minimization criterion, MASBR regression maximizes the sum of binary coded residuals. MASBR regression is designed for cases where the conventional robust regression methods with breakdown points less than 50% fail. To circumvent the problem of being trapped in local optima, a stepwise-varying acceptable error limit (SVAEL) algorithm is proposed. Both numerical simulation and treatment of real analytical data demonstrate the feasibility of MASBR regression in conjunction with the SVAEL algorithm.
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    Journal of Chemometrics 9 (1995), S. 411-422 
    ISSN: 0886-9383
    Keywords: gasoline ; classification ; multiple regression ; non-correlated variables ; accuracy of measurements ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A procedure for the classification of motor gasolines by their trend to carburettor icing has been proposed. For this goal a set of regression models was employed. Motor gasoline classification was carried out in terms of the isopropyl equivalent using seven physicochemical properties.
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    Journal of Chemometrics 9 (1995), S. 226-228 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Journal of Chemometrics 9 (1995), S. 211-221 
    ISSN: 0886-9383
    Keywords: diagnostics statistics ; QSAR ; MASCA ; principal component regression ; non-least squares regression ; types of multicollincarity ; flagged observations ; influential points ; high-leverage points ; outliers ; extra-carrier points ; random perturbation ; cluster correalation ; resampling ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formal application of a Hansch analysis to a series of 3-quinuclidinyl benzylates (QNBs) led to a ‘statistically significant’ QSAR equation. In contrast, the application of the MASCA model has shown that the design matrix is unsuitable for each QSAR analysis: one sample member is an outlier but not a high-leverage or influential point; another one is an influential point, a high-leverage point and an extra-carrier point. The regressors of the design matrix are multicollinear without predictive model power. The result of such flagged observation and this type of multicollinearity is a multiple cluster correlation. The QNB series is a good example for ‘sampling artifacts’ where no practically important but artificial QSARs can be found.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 580-582 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Thermal transformations of phosphorus and phosphoric acid were studied on a pyrogenic silica surface. Complete binding of phosphoric acid with the surface was observed as a result of thermal vacuum treatment. Oxidation-reduction disproportion reactions in the samples containing phosphorus acid resulted in removal of molecular phosphorus in two stages which were attributed to disproportion of phosphorus atoms in the surface monolayer (high-temperature stage) and in the bulk layers (low-temperature stage).
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 585-587 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The biologically active systems were synthesized by adsorption of vitamin B12, glucose and raffinose on an ultrafine oxide surface and studied by means of thermal analysis and temperature-programmed mass spectrometry. It is shown that adsorption of cyanocobalamine (B12) on the silica surface transformed the propionamide group to a weakly bound form. The role of the charged cobalt atom and of the propionamide and benzimide groups in the adsorption mechanism is discussed. Experimental data were obtained confirming the stable adsorption of glucose and raffinose on an ultrafine pyrogenic silica surface modified by amino groups.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 591-596 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A novel procedure to perform a membrane-inlet mass spectrometry experiment is presented, which allows semi-volatile organic compounds to be preconcentrated inside the membrane, before they are thermally released into the ion source. The method uses a standard membrane inlet with a tubular silicone membrane passing straight through the ion source of the mass spectrometer. A long slit in the ion source parallel to the membrane allows electrons from the filament to continuously bombard the membrane surface. The membrane is kept cold by a continuous flow of water or sample solution passing through the inlet. However, during a short interruption of the flow, the membrane is rapidly heated by the electron bombardment and organic compounds dissolved in the membrane are thermally released into the ion source. In this fashion a desorption peak is obtained. Using the areas of the desorption peaks, we have been able to detect semi-volatile, hydrophobic organic compounds (boiling points above 200 °C) in aqueous solution at low and sub parts-per-billion levels. In general, the detection limits were 10-100 times lower than previously published results, where membrane-inlet mass spectrometry has been used for the detection of the same compounds. Good linearity over 3 orders of magnitude was obtained and the standard deviation was 5%.
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Physics
    Notes: The accurate molecular weights for a series of 37 unknown synthetic peptides, used in research studies involving synthetic vaccines, antibacterial peptides or the de novo design of helical peptides and proteins, were determined with a magnetic sector instrument. All data were obtained with external calibration over a wide mass range during magnetic scanning. Errors between observed and theoretical monoisotopic molecular weights were typically in the 5-60 ppm range for the unknowns at sector resolutions between 2500 and 9000 (10% valley). Isotopic clusters for charge states up to 10+ were resolved through the use of high resolution. Collisionally activated dissociation (CAD) in the electrospray interface resulted in product ions that enabled either full or partial sequencing of most unknown peptides of molecular weights below 2000 Da. The complete primary sequence for one peptide was determined and the importance of high resolution was demonstrated by the differentiation of lysine from glutamine, two amino acids differing in residue mass by only 0.0364 Da. Two other peptides, with identical monoisotopic masses, but different primary sequences, were differentiated based on CAD-MS data.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 604-608 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Topics: Physics
    Notes: Many C60 derivatives form stable complexes with γ-cyclodextrin in a ratio of 1:2. These complexes can be easily analyzed by fast-atom bombardment mass spectrometry (FAB). Unlike the analysis of C60 derivatives directly by FAB, the spectra of these complexes were characterized by molecular ions with little or no fragmentation. The analysis of these complexes by FAB provides an easy and useful approach for the characterization of C60 derivatives. Most likely due to the size of the cavity, this approach is limited to C60 derivatives with molecular weights not significantly larger than that of C60 itself.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 627-627 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Notes: The influence of cyclopentane fusion on the fragmentation behaviour of 1,3-oxazines and related thiazines was studied. The differences observed in the rearrangement reactions between the hydrogen atom at the ortho position of the aromatic ring and one of the hetero atoms in the cyclohexane ring were also investigated. These rearrangements occur if the cyclohexane ring contains an electrophilic atom and the carbon atom next to it is substituted by a benzyl or arylamino group. The compounds examined were cis and trans isomers of the title compounds substituted with methyl and benzyl (in the latter case only the cis isomers) on the ring nitrogen. All the compounds exhibited a large [M — H]+ ion peak which decreased with increasing size of the substituent. This indicates an intramolecular cyclization prior to further fragmentation of the molecule. Methyl substitution seems to change the character of this intramolecular cyclization. The main routes of fragmentation resemble those of the cyclohexane-fused analogues more than those of the corresponding 5,6-fused compounds. The stereoisomers fragment so similarly that they cannot be distinguished from each other, except in the case of methyl substituted oxazines.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. i 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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    Rapid Communications in Mass Spectrometry 9 (1995) 
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 634-640 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Matrix-assisted laser desorption/ionization (MALDI) and fast-atom bombardment (FAB) mass spectrometry experiments were applied to the study of the early stages of the oligomerization reaction of dopamine with mushroom tyrosinase. Ultrafiltration was employed to remove the enzyme at various reaction times, to prevent possible attachment of the protein to the highly reactive intermediates. Two sets of five samples each, obtained at different reaction times, in one case immediately lyophilized and in the other left to react under an oxygen stream for 24 h before lyophilization, were compared. FAB showed the presence of various species and of these, that at m/z 305 increased in abundance with reaction time in the immediately lyophilized set of samples only. Accurate mass measurements and tandem mass spectrometric experiments indicated the structure of a dopamine protonated dimer for this ion. MALDI measurements showed that all samples were composed of clusters of oligomers differing in degree of oligomerization. Oligomerization increases with reaction time, resulting in the formation of species at 2643-2911 Da. These clusters in turn were formed of species with a different degree of oxidation, detected in both sets of samples.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 644-650 
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A new approach, bio-affinity characterization mass spectrometry (BACMS), aimed at providing a more rapid, sensitive and potentially more flexible alternative to techniques presently employed for the characterization of noncovalent interactions in mixtures, such as would be encountered in combinatorial chemistry, is presented. BACMS avoids some of the difficulties and potential artifacts associated with affinity chromatography since the noncovalent associations occur in solution; thus, BACMS avoids the requirement of solid support media and the development of non-interfering linker species. This paper describes the conceptual basis for the methodology and its potential use in applications which include the screening of high affinity ligands in support of new drug development. BACMS exploits new Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry technologies which, when coupled to electrospray ionization (ESI), allow the investigation of specific noncovalent complexes formed in solution. BACMS utilizes the well-known attributes of FTICR, such as the high resolution mass analysis and (MS)n (n ≥ 2) capabilities; however, it is even more directly a result of recently developed techniques involving quadrupolar excitation, such as selected-ion accumulation. These tools are demonstrated and the results illustrate the extraordinary sensitivity achievable (solution concentrations of 1 × 10-9 M without the use of separations prior to ESI). Thus, the new capabilities demonstrated here, in conjunction with ESI, will be useful for the investigation of very low relative concentration noncovalent association directly from solution, and promote a faster alternative for combinatorial mixture screening and analysis.
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  • 74
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 651-654 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The electron impact mass spectra of some diastereomeric dialkyl tartrates and isopropylidine derivatives of dimethyl tartrate show diagnostic differences in the relative abundances of some ions. The differences have been explained based on preferred conformations of these isomers. The configuration of the carbomethoxy groups has been correlated with the configuration of carbomethoxy groups in dimethyl maleate and fumarate. The interaction between the two carbomethoxy groups is relatively larger in conformations where there is a gauche interaction between them. The results could also be correlated with the solution phase conformations of these compounds reported in the literature using nuclear magnetic resonance techniques. The difference, which is small in diastereomeric dimethyl tartrates, is very reproducible and has been confirmed using six different mass spectrometers.
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  • 75
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 655-659 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A gas chromatography/negative-ion chemical ionization mass spectrometry method is presented to measure the isotopic enrichment of multiple serine and glycine isotopomers. The amino acid N-heptafluorobutyryl n-propyl ester derivatives were used. The method had good analytical linearity between 10 and 800 μg mL-1 and both precision and accuracy were 5% for plasma amino acids. Sensitivity permitted analysis of 100 pg amino acid on column. The method was applied to metabolic studies of the serine to glycine interconversion in humans. (α-15N)serine and (1,2-13C2)glycine were given as a single intravenous bolus to six healthy male subjects. Plasma concentration of glycine and serine were determined after addition of (α-15N,1,2,3-13C3) serine and (α-15N,1,2-13C2)glycine as internal standards to 500 μL of plasma. Since glycine and serine are rapidly interconverted by hepatic serine hydroxymethyl transferase, the resultant tracer spectrum requires deconvolution of the enrichment of four isotopomers of each amino acid. Deconvolution of the ion abundance ratios to yield tracer-to-tracee ratios for each isotopomer was done using Brauman's least squares approach.
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  • 76
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Fifty saponin components of Quil A, a commercially available extract from the bark of the South American tree Quillaja saponaria Molina, were partially structurally characterised. The molecular weights were determined by fast-atom bombardment mass spectrometry. The glycosyl and elemental composition of all the saponins was determined by applying our recently developed method, monomer mapping, consisting of a computer program and accurate mass measurements. Support for the presumed identity of the aglycone, i.e. quillaic acid, was found in the accurate mass determination, 1H NMR measurement and chemical reactions. The saponin composition of Quil A was shown to consist of pairs. Within the 3-O bound glycosyl moiety of a pair there was a structural difference: a pentose and rhamnose were interchanged. Structural differences between different pairs were located in the 28-O bound glycosyl moiety. A structural element, unknown to date and of which the elemental composition was deduced to be C8H12O5, was found in the 28-O bound glycosyl moiety of several saponins.
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  • 77
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 667-676 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The detection limits of a series of small peptides derivatized with 4-aminonaphthalenesulphonic acid have been determined using a magnetic sector mass spectrometer operated in the negative-ion mode. The instrument (AutoSpec-QFPD) is equipped with an electrospray ion source and both array and point detectors. Detection limits when using the array detector were on the sub-femtomole level at a mass resolution of 2000 (full width half maximum height). Cone-voltage and linked-scan collision-induced dissociation spectra were recorded using both the array and point detectors. When using the array detector, amino acid sequence information could be obtained from as little as 50 fmol of sample consumed.
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  • 78
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 677-685 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The [M — H]- ions derived from tetrapeptides generally show two different collision-induced backbone cleavages which allow the determination of the amino acid sequence of the peptide. The first of these involves the formation of the carboxylate anions of either constituent amino acids or fragment peptides. In the second, amino acids or fragment peptides are eliminated as neutrals. There are a number of residues which undergo characteristic sidechain fragmentations irrespective of their position in the tetrapeptide, e.g. Ser, Thr, Cys, Met, Phe and Tyr. However, there are also some residues which, when situated at the C-terminal end of the peptide, promote pronounced fragmentation at the C-terminal position which occurs to the exclusion of the normal backbone cleavages. We conclude that the data obtained from these negative-ion cleavages are analytically useful, and complement those provided by the cognate positive-ion technique.
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  • 79
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    Rapid Communications in Mass Spectrometry 9 (1995), S. i 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
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  • 80
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 731-734 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A new interface for coupling electrospray (ES) ionization and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been developed based on an electrostatic ion guide (EIG). In these initial studies, the EIG interface has been demonstrated to be a simple but effective means of transmitting ES ions generated outside the magnetic field to the FTICR cell. In the current system, the pressures in the EIG region and front FTICR cell are 10-5 and 10-6 Torr, respectively. Under these conditions, ions may be accumulated with low trapping voltages (0.5-0.75 V) and without a high pressure pulse of buffer gas. ES-FTICR spectra of picomole to femtomole quantities of several peptides, proteins, nucleotides and dinucleotides have been obtained. Modifications currently in progress should enhance performance with respect both to limits of detection and to mass resolution.
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  • 81
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 744-752 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Ion yields were investigated in matrix-assisted laser desorption/ionization (MALDI) as a function of amino acid composition using a variable wavelength ion source. In the case of nitrogen laser excitation (337 nm), [M+H]+ ions were abundant for short peptides containing basic or polar amino acid residues. The lack of basic residues led to diminishing ion formation at 337 nm. Increasing the chain length led to enhanced ionization even for peptides with non-polar side chains. In contrast to the liquid phase basicities the basic residues showed Arg 〉 His 〉 Lys order in their affinity to protons. Variations in the average ion yield showed decreasing trend with increasing basicity of the residues pointing to potential differences in the matrix incorporation of different guest molecules. Dye laser excitation at 280 nm resulted in extensive fragmentation and enhanced ion formation from peptides containing aromatic side chains indicating the possible role of analyte excited states in ion formation. Semi-empirical quantum chemical calculations were used to explore the sites of protonation. Heat of formation diagrams of the matrix-analyte complexes were examined as a function of their configuration. Investigation of the optimized geometries allowed the location of bifurcated and linear hydrogen bonds between the non-polar analyte (e.g., Gly-Gly) and matrix molecules. In addition to these intermolecular bonds, in the case of His-His an intramolecular hydrogen bond was also formed within the analyte molecule. Proton affinity values were calculated for every dipeptide while the site of protonation was varied. The results seemed to indicate that for every dipeptide the amino terminus was more susceptible to protonation than the peptide bond. Due to increased stabilization effects in homo-oligomers (Glyn), increasing the number of residues led to an increase in proton affinity. For basic diopeptides (e.g., His-His), the side chains had the highest proton affinity, underlining their role in MALDI of proteins.
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  • 82
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 753-760 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: N-Phenyllinoleamide (NPLA), the anilide of linoleic acid, has been regarded as a marker of the case oils associated with toxic oil syndrome, but the mechanisms of toxic injury remain enigmatic. Experimental data have related an increased systemic toxic effect of heated linoleic anilides to chemical structural modifications that might also be possible by in vivo metabolism; however, little is known about their metabolism. Taking into account that NPLA is a derivative of linoleic acid, a fatty acid that can be metabolized by lipoxygenase activity to a vast array of derivatives possessing biological activity, the objective has been to elucidate the oxidative metabolism of NPLA by mouse peritoneal macrophages, a cellular model with high lipoxygenase activity. Cells were incubated with 0.1 mM NPLA spiked with N-phenyl[1-14C]linoleamide. The metabolites were separated by high-performance liquid chromatography and individually collected prior to GC/MS analysis. Identification of trihydroxy-, monohydroxy- and epoxy-derivatives of NPLA, suggests that this xenobiotic can be metabolized via the same oxidative processes as for linoleic acid. Furthermore, identification of the non-amidated monohydroxylated and trihydroxylated derivatives of linoleic acid arising from NPLA suggests an amidase-like activity with release of aniline and the free fatty acid. These results provide information about possible biological structures arising from NPLA, and open the way to evaluate the biological significance of these metabolites in the inflammatory reactions associated with toxic oil syndrome.
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  • 83
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 775-777 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The quantitative investigation of the four RNA nucleosides (adenosine, guanosine, cytidine and uridine) using fast-atom bombardment mass spectra is reported. Nucleoside concentrations are varied in the 0.8-4% range. The nucleoside hydrogen-bonded complexes are shown to form primarily on the liquid matrix surface.
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  • 84
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 783-787 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: This paper gives the first demonstration that the kinetic method can be used to determine molecular pair thermochemical values. Fragmentation of proton-bound trimers is used to determine the gas-phase basicities (GB) of molecular dimers (the so-called molecular-pair gas-phase basicities). The set of equations necessary for data evaluation, i.e. extension of the so-called kinetic method, is discussed. Proton-bound amino acid trimers have been studied as examples. The GB of glycine dimers (G2) is 190.1 kcal/mol at 970 K, 4.0kcal/mol higher than that of glycine (G). The GB of alanine (A) and valine (V) dimers have also been determined; these are 192.4 (A2) and 194.4 (V2) kcal/mol. The GB of mixed dimers is very close to the average of the respective homo dimers, 191.4 (GA); 192.0 (GV) and 193.2 (AV) kcal/mol. Experimental errors in the relative GB values of various dimers are estimated to be less than 0.3 kcal/mol.
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  • 85
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 851-853 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The methane chemical ionization mass spectra of N-carbonyldihdrazones of 2,4-diacetylresorcinol were recorded. The protonated molecular ion was found to be of variable abundance and was often accompanied by [M + 29]+ and [M + 41]+ ions. Electron impact mass spectra displayed weak molecular ions and were characterized by the scission of a N—N bond and subsequent loss of XCONH and XCONHNH2.
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  • 86
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 870-870 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
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  • 87
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Frozen solutions of a cryoprotector, glycerol, in water were studied by low temperature fast-atom bombardment mass spectrometry in the temperature range from - 196° to 0°C. Strong dependence of the mass spectral pattern on the water-glycerol ratio and temperature of the sample was observed. The evolution of mass spectra of the frozen 2000:1 water-glycerol mixture with increase of the sample temperature is reproducible: in the lower portion of the temperature range (-196° to -120°C) the spectra contain a set of cluster ions, (H2O)nH+, (n = 1-15), characteristic of pure water; then the spectra show a superposition of peaks due to water clusters and glycerol itself, the latter with characteristic changes with temperature rise: degraded ‘peak at every mass’ pattern in the -120° to -80°C range followed by a build-up of glycerol clusters, GnH+, (n = 1-3) in the -80°C to -55°C range. At approximately -55°C to -50°C a sublimation of water component occurs, detected by [H2O]+· ion, and the remainder of the spectral pattern coincides with that of pure glycerol up to ambient temperature. Mixed water-glycerol clusters were never observed. An explanation of such spectral behavior is proposed based on ideas about the morphology of the frozen solutions, characterized by water microcrystallites separated by eutectic channels containing cryoprotector. The independent sputtering of the two substances from different domains of the frozen sample surface is discussed.
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  • 88
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The separation of trace impurities in cimetidine drug substance by high pressure liquid chromatography (HPLC) showed a series of minor components. These were analysed using an on-line electrospray source connected to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Working typically with a resolution of 10 000-20 000 accurate mass measurement of the components confirmed their empirical formulae to generally better than 5 ppm accuracy. This work shows that the combination of conventional analytical HPLC using 4.6 mm columns with a low flow rate electrospray source interfaced to an FT-ICR mass spectrometer enables structural information to be obtained on low levels of low mass components.
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  • 89
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 1038-1043 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: We report the electrospray (ESI) mass spectra of four fluoroquinolones, enoxacin, norfloxacin, ofloxacin and ciprofloxacin, a group of highly potent antibiotics that have not been previously characterized by this ionization technique. At low sampling cone voltage settings, 50 and 100 V, the electrospray mass spectra contained primarily molecular adduct ions. At a sampling cone voltage setting of 150 V, collisionally activated dissociation mass spectra containing structurally significant product ions were produced and accurately mass measured at a magnetic sector resolution of 3000. Gradient separation and characterization of these compounds was demonstrated by analysis using packed capillary column liquid chromatography with UV and ESI-MS detection. UV detection limits in the 1 to 2 ng range ((signal-to-noise ratio S/N) = 10:1) were estimated and interpretable mass spectra with S/N ratios in excess of 100:1 were observed for 1.5 ng of analyte during electrospray mass spectrometric analysis.
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  • 90
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 1051-1055 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Electroblotting proteins separated by gel electrophoresis provides a suitable support for further manipulations and analysis of small amounts of relatively pure samples. On-membrane digestion, peptide mapping by mass spectrometry, and database searching offer sensitive and fast tools to identify the analyte. By providing sequence information, tandem mass spectrometry can go a step further, confirming the database identification, solving problems connected with post-translational modifications and sequence variations, or supplying the stretches of internal sequence necessary to synthesize an oligonucleotide probe for gene isolation. The viability of this approach was successfully evaluated using different tandem mass spectrometric techniques: metastable decomposition in a matrix-assisted laser desorption/ionization (MALDI) time-of-flight instrument with a curved-field reflectron; low energy collision-induced dissociation in a MALDI quadrupole ion trap mass spectrometer; and high energy collision-induced dissociation in a high-performance four-sector mass spectrometer with massive cluster-impact ionization.
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  • 91
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 1067-1076 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A suite of software programs called the BioToolBox is described. The primary purpose of the BioToolBox is to facilitate the characterisation of proteins and peptides using data acquired by IonSpraTM mass spectrometry and tandem mass spectrometry. As an example, the characterization of a 50 KDa protein (transcription termination factor-rho) is described. The programs which comprise the BioToolBox produce complementary information which is exchanged between the software modules.
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  • 92
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 1115-1119 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The potential energy surface of n-hexane radical cations is probed theoretically. The structures with low internal energies are characterized by extensive lengthening of one of the three innermost C—C bonds. This C—C bond stretch is induced by localization of the positive charge. For an elongated terminal C—C bond we calculated an energy which is 0.4 eV higher than that for a stretched central C—C bond. The n-hexane geometries with several C—C bonds stretched do not represent stationary points on the potential energy surface. Comparison with previous dissociative-neutralization experiments of low-internal-energy n-hexane ions shows that the fragmentation pathways reflect the two lowest energy structures.
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  • 93
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The behaviour of three fluorinated acyclic nucleoside analogs has been examined under different ionization conditions, i.e. by electron ionization, fast-atom bombardment and positive-ion chemical ionization using methane, isobutane and ammonia as reactant gases. The protonated molecules have been studied by collisional spectroscopy. In general, protonation takes place on the functional groups exhibiting a proton affinity higher than that of the reactant species but some discrepancies are present. They have been explained by the presence of quasi S—H—F structures implicating proton bridging between sulphur and fluorine atoms.
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  • 94
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Electrospray ionization mass spectrometry is an accurate and sensitive analytical method to characterize the purity of oligodeoxynucleotides being tested for pharmacological studies. We report the preparation procedure (‘desalting’) of natural and modified oligodeoxynucleotides (ODNs) and their analysis by negative-ion electrospray mass spectrometry. We evaluate the sensitivity and the accuracy of the method for two antisense ODN sequences. Mass analysis of the 25-mer phosphorothioate can be performed to within 0.001% accuracy (standard error of 0.05 Da) for a sample concentration of 12 pmol/μL. In addition, the adduct ion and the failure sequence can be identified to characterize the antisense ODN.
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  • 95
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Several studies have investigated stereochemical effects observed in the electron ionization and low-energy collision-induced dissociation (CID) mass spectra of molecular ions from synthetic and natural alkaloids. More recently, it has been shown that low-collision-energy CID spectra of protonated epimeric alkaloids can yield additional information. For instance, three epimeric tetracyclic alkaloids studied in the present work are characterized by different cis/trans ring junction geometries and by different configurations of the ethyl group linked to the C-ring. Low-energy collisional spectra of the epimeric MH+ ions show large differences depending both on the ring junction geometry and the stereochemistry of the side chain. Interpretation of certain specific processes invokes ion-dipole formation, allowing internal electron transfer. Consecutive decompositions have also been scrutinized.
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  • 96
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    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The aim of the present work is to study the oxidation reactions between an aromatic hydrocarbon, toluene and oxygen. Experiments have been performed in a low-pressure plasma tubular reactor supplied by an RF 13.56 MHz generator. On-line mass spectrometry and off-line gas chromatography were used for a qualitative and quantitative analysis of the effluents. The reaction can lead to the formation of products in the solid (deposited on the wall of the reactor), liquid or gas state. Hydrocarbons (benzene, ethylbenzene, diphenylethane), as well as oxygenated species resulting from competition between the oxidation of the aromatic ring (phenol, cresols) and the oxidation of the alkyl side chain (benzyl alcohol, benzaldehyde), can be obtained. The power and the amount of added oxygen were studied by comparing the mass spectrometry and gas chromatography results. Reaction mechanisms are discussed.
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  • 97
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The decomposition of carbon tetrafluoride (CF4) in two types of plasma reactor has been investigated by using mass spectrometry. The optimization of the plasma process requires the establishment of an adequate chemical balance between major components and also traces in the reactor effluent, in order to minimize toxic emissions by control of the proper reactor parameters. On-line mass spectrometric characterization of the effluent has been used to optimize the CF4 decomposition yield. A GC/MS protocol has been developed to quantify both major and minor components of the effluent in order to evaluate its toxicity.
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  • 98
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 67-70 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Translational energy spectra have been obtained for single-electron capture by 8 keV Ar4+ ions in He, Ne and Ar. The spectra for He and Ne targets display eight well-resolved peaks, the majority of which correspond to capture involving a rearrangement of the electron core configuration. By contrast, the five channels observed in the spectrum for the Ar target are all associated with core-conserved electron-capture processes. One pronounced peak in the spectra for He and Ne cannot be explained on the basis of capture from the ground 3P or metastable 1D and 1S states of the Ar4+ ions, and appears to originate from capture by Ar4+ in the hitherto unidentified metastable 5S0-state at ∼ 7.5 eV above the ground state. All observed channels are spin-conserved.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 57-60 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The mass spectrometric behaviour of 2-fluoro, 2-chloro, 2-bromo and 2-iodo ethanols has been studied in detail with the aid of metastable ion studies and deuterium labelling experiments. Clear differences are observed and rationalized in terms both of carbon-halogen bond strengths and electronegativity.
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    Rapid Communications in Mass Spectrometry 9 (1995), S. 64-66 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A steady production of positive hydrated oxygen molecular ions has been achieved by the introduction of H2O vapour into the ion source of a mass spectrometer under chemical ionization conditions using O2 reagent gas. Collision-induced dissociation, mass-analyzed ion kinetic energy (CID MIKE) spectra (R. G. Cooks, J. H. Beynon, R. M. Caprioli and G. R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973) of the m/z=50 ions, which correspond to the H2O·O2+· ions, show a prominent loss of a water molecule, but no significant hydrogen loss. By contrast, the CID MIKE spectra of the H2O2+· ions obtained under identical conditions indicate an important successive loss of hydrogen and oxygen atoms, giving rise to an intense H2O+· peak. Conversely, with H2O·O2+·, a relatively loosely-bound adduct of the two stable moieties is indicated, a ‘cluster’ that fragments into its constituents with a preferential charge retention on the species of lower ionization energy.
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