Field ionization mass spectra

The product ions appearing in an FI mass spectrum are formed within picoseconds, following formation of the molecular ion. This is provided the decomposition is unimolecular. Referring to eqn. (9), the limits t and t2 to the observation window are zero and of the order of 1 or 10 ps, respectively. The limit t2 depends upon the relative masses m/M of the reactant M+ and fragment ions m+ and therefore is different for different peaks in the mass spectrum. As m/M increases towards unity, so t2 increases, (i.e. At increases as the mass of the neutral eliminated decreases). 

An FI mass spectrum typically contains peaks due to ions formed in chemi- and physisorbed layers on the emitter. The expressions given in Sect. 2 would be unlikely to be applicable to these surface processes. The numbers of these ions detected will depend upon, among many other factors, the kinetics of desorption from the surface. 

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Beckey, H.D. Field-Ionization Mass Spectra of Organic Molecules. Normal C, to C9 Paraffins. Z. Naturforsch. 1962,17A, 1103-1111. 

Beckey, H.D. Wagner, G. Analytical Use of an Ion Field Mass Spectrometer. Fresenius Z Anal. Chem. 1963,197,58-80. Beckey, H.D. Schulze, P. Field Ionization Mass Spectra of Organic Molecules. III. n-Paraffins Up to C16 and Branched Paraffins. Z Naturforsch. 1965, 20A, 1329-1335. 

Rollgen, F.W. Heinen, H.J. Levsen, K. Doubly-Charged Fragment Ions in the Field Ionization Mass Spectra of Alkyl-benzenes. Org. Mass Spectrom. 1976, 77, 780-782. 

The maximum residence times in many electron-impact instruments is generally considered to be about 10 sec. (Chupka, 1959) but field-ionization mass spectra are usually obtained after residence times of about 10 sec. and these contain no rearranged ions (Beckey al, 1969). To explain these observations it was suggested that field ionization residence times were too short to allow the slower rearrangement processes to occur and only reactions with low entropies of activation were observed. 

The HPLC was a Du Pont 8800 quaternary solvent system with a Hewlett-Packard (HP) 1040A photodiode array detector (5-8). Flexible disks were used for data storage, and postrun data evaluation was performed by the detector s computer (HP 85). Samples were injected with a loop injector 10 p,L was used for analytical scale, and 200 jlL was used for preparative scale. Spectra were run on a spectrophotometer (Perkin-Elmer Lambda 3) for static absorbance and on a spectrofluo-rometer (Perkin-Elmer MPF-66) for fluorescence. Field-ionization mass spectra were obtained at a resolution of 45,000 (corresponding to a 0.01-daltons error for a mass of 450). 

I thank C. E. Rechsteiner for the field-ionization mass spectra of the new PAHs and P. Shah and L. B. Rogers (Department of Chemistry, University of Georgia, Athens, GA) for the NMR data on the octadecyl bonded-phase (Vydac) packing. I also thank Chevron Research Company for permission to publish this work. 

Figure 16.17. Comparison of the electron-impact and field-ionization mass-spectra of d-ribose. The molecular ion is found at mje = 150.

Figure 12 Field-ionization mass spectra of dibenzothiophenes (DBTs) in a diesel fuel analyzed by GC-TOF-MS in FI mode. Error in accurate mass measurement for DBTs is less than 3 mmu. Note that there are no significant fragment ions present.

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