ISSN:
1089-7623
Source:
AIP Digital Archive
Topics:
Physics
,
Electrical Engineering, Measurement and Control Technology
Notes:
In Fourier transform mass spectrometry (FTMS), static electric and magnetic fields are used to store gaseous ions in an analyzer cell. Typically, the ions can be stored for several minutes if the pressure in the analyzer cell is below 10−8 Torr. While they are stored, the ions move in circular cyclotron orbits having a radial frequency given by w=qB/m, where q/m is the charge-to-mass ratio of an ion, and B is the magnetic field strength. With most FTMS instruments, a mass spectrum of the stored ions is obtained by using a rapid radio frequency (rf) sweep, or "chirp,'' to accelerate the ions. The chirp excitation signal produces an oscillatory electric field perpendicular to the magnetic field, and as the frequency of the oscillatory electric field approaches the cyclotron frequency of an ion, the ion is accelerated to a larger cyclotron orbit. In this article the theory of operation, design, and performance of an alternative excitation method called "impulse excitation'' is described. Impulse excitation is different from chirp excitation because the ions are accelerated by a very short, high-voltage pulse rather than an oscillatory, radio frequency signal. One of the advantages of impulse excitation is that all the ions in the analyzer cell are accelerated simultaneously in less than a microsecond. This gives more stable FTMS signals and more reliable isotope ratios. An impulse excitation amplifier is inexpensive to build and far less complex than the hardware needed for chirp excitation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1140390
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