Chemical ionization source, Finnigan

The chemical ionization mass spectrum of cimetidine and the major fragmentation ions are presented in Figure 8 and Table 4. The spectrum was obtained using a Finnigan lYbdel 3200 quadrupole mass spectrometer fitted with a chemical ionization source. The sample, applied to the probe from an acetone solution, was introduced via the direct inlet system. Methane was used as the reactant gas. 

The Finnigan chemical ionization source was modified by the addition of two cartridge heaters. Eluate entered the source through a heated 1/2" probe and excess solvent was removed by a mechanical vacuum pump connected directly opposite the eluate entrance. The interface consisted of 1 meter deactivated silica capillary tubing (ID, 60 OD, 0.008"), led from the outlet of microbore column and threaded through the probe. The probe design has been previously described (26). For all analyses, the probe was operated at 240 C and the source at 250 C. The analyzer pressure was 10 b torr. 

The standard MAT 90 ion source is used for optimized FD/FI mode by means of the newly designed FD/FI probe. Conversion from electron impact (El), chemical ionization (Cl) or fast ion bombardment (FAB) to FD/FI operation does not require the exchange of the ion source. The FD/FI probe accommodates both the field emitter and the extraction electrodes, mounted at the probe tip. Both are introduced as a unit into the ion source through the ionization volume exchange lock without breaking vacuum. The fast and simple changeover illustrates the versatility of the Finnigan MAT 90 with no compromise on the performance. 

Figure 6 presents the average values with standard error obtained for A9-THC, 11-0H-A9-THC and cannabinol from plasma of male volunteers receiving A9-THC by intravenous infusion. The measurements covered a 24 hour period. A9-THC values obtained with the LKB-9000 El source were in close agreement with the data obtained on the Finnigan 3300 chemical ionization (Cl) source. 

The second problem is that ion-ion or ion-neutral reactions can occur. Reactions (e.g., proton transfer) result in high abundance of protonated molecular ions in the mass spectrum. Thus QIT can be disadvantageous for determining chemical composition in manual spectral interpretation, because the presence of the (M + l)" ion tends to confuse the interpretation. Library spectral matching, however, is not affected if the spectral matching algorithm reflects the unique features of the QIT spectrum. An external ionization source with ion injection into the QIT is an alternative solution, because only ions are present in the trap (i.e., neutral analyte molecules that could participate in ion-molecule reactions are not present). The Thermo Finnigan PolarisQ GC/MS is an example of such an instrument. 

The chemical ionization spectrum, shown In Figure 11, was obtained on Finnigan 4000 mass spectrometer using methane gas as a reagent with Ion electron energy of 100 eV, Ion source pressure of 0.3 Torr, Ion source temperature of 150 C and emission current of 300 pA. The spectrum Is dominated by a quasimolecular Ion (H + 1). Two peaks appearing at m/e 353 and m/e 365 are attributable to the transfer of carbocatlons from the carrier gas. The mass spectra assignment of the... 

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