Mass spectrometer, detectors double-focusing

Figure 4.4 Schematic (a) and photograph of ion detector with extended dynamic range (b) in the double-focusing sector field mass spectrometer Element. (Reproduced by permission of Thermo Fisher Scientific, Bremen.)...

B = magnetic sector E = electric sector. Six types of magnetic mass spectrometers are displayed. Instruments 4, 5 and 6, with double focusing, are most frequently used. Instrument 4 is dispersive, while 5 and 6 are scanning types. The photographic plate in 4 can be replaced by an array detector for limited mass ranges. Reproduced from Finnigan MAT documentation, with permission. 

The mass spectrometer we now use for zinc analysis, in the laboratory of Maynard Michel of Lawrence Berkeley Laboratory, is a thermal ionization mass spectrometer, a single direction focusing instrument with a 12" radius magnetic sector, double filament, rhenium ionizing source and electron multiplier detector. In addition, have done some preliminary work for Fe and Cu analysis with an automated TI/MS which speeds analysis considerably with excellent precision. We hope to be able to develop methods to use this automated Instrument for zinc analysis as well. 

There are many types of mass spectrometer, from high-resolution double-focusing instruments, which can distinguish molecular and fragment masses to six decimal places, to bench-top machines with a quadrupole mass detector which can resolve masses up to about m/z = 500, but only in whole-number differences. Routinely you are most likely to encounter data from bench-top instruments and therefore only this typie of spectrum will be considered. 

A small fraction of the reactant ions collide with Cm molecules, and product ions scattered into the forward hemisphere in the laboratory frame are collected by the octapole. Product and unscattered reactant ions are mass selected by a double focusing electric and magnetic sector mass spectrometer, and detected by an on-axis Daly detector [16]. In the following we give only relative cross sections, due to uncertainties in the absolute pressure of Cm, the product collection efficiency, and the alkali-metal ion beam intensities in the scattering cell. 

---