Advantages of very thick coatings
Separation efficiency correlated with film thickness
Preparation of columns with very thick coatings
Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
It has become common practice with capillary columns to call film thicknesses of 0.1 - 0.3 μm regular or standard, whereas 0.3 - 1.0 μm is considered to denote thick films. Consequently, films with thickness beyond 1μm (we have explored the range up to 8.0 μm) may be termed very thick. Increasing film thickness, at least to about 5 μm, brings important additionl advantages primarily in terms of loading capacity. Columns with very thick coatings may replace the traditional SCOT columns thanks to at least equivalent loading capacity combined with higher separation efficiency, and better stability characteristics. Additional advantages are increased retention, essential for the analysis of very volatile substances, and increased elution temperature, providing a basis for direct water injection.We observe that TZ (the separation number) drops to roughly 50% when the film thickness is increased by a factor of 300 from 0.025 to 8.0 μm, provided TZ is measured with a standard flow of hydrogen. The reduction is only 30% when the thick-film colums are run with individual optimization with nitrogen as a carrier gas. In contrast to standard columns, very thickly coated columns show slightly increased separation efficiency with nitrogen at drastically reduced flow rate. The HETP increases very slightly in the indicated range of flim thickness. This contradicts widespread theoretical expectations, which probably account for the fact that very thick coatings have so far been neglected. Since the preparation of very thick coatings involves some deviations from the standard procedure, complete and detailed instructions are given.
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