NICI spectrum

Figure 1. FTMS NICI spectrum of a mixture of fluorene [(M+14) at m/z 180], fluoranthene [M- at m/z 202)], and benzo(a)pyrene [(M at m/z 252)]. Methane was used as the reagent gas at a static pressure of 7 x 10 6 torr.

Figure 2. FTMS NICI spectrum of a mixture of fluorene and 9-methyl fluorene, using argon as the reagent gas. Observed anions at 180.058 and 180.094 correspond to C H 0 and C H O , respectively. (Reproduced with permission from ref. 18. Copyright 198 7 Valley.)...

When the electron beam time was increased to longer times (up to 500 msec), additional anions were observed. This is illustrated in Figure 3, which is the NICI spectrum obtained from a reaction of... 

Figure 3. FTMS NICI spectrum of 9,9-d2 fluorene using 1802 as a reagent gas (1 x 10"5 torr) and a 200 msec electron beam.

FIGURE 7.20 (a) (CH4/CH3I) NICI spectrum of 5,5-dimethyl-l,3-cyclohexanedione. The following conditions were used reagent gas methane at 0.4 Torr reagent gas methyl iodide at 10 Torr electron energy 500 eV repeller at —9 V mass range 50-500 p, ion source temperature at 115°C. A solids probe was used as the inlet. (Masucci and Caldwell, unpublished data.)... 

TABLE 7.10 Effect of Ion Source Temperature on NICI Spectrum of p-Toluic Acid... 

The acidity/hydrogen-bonding NICI technique gives primarily proton abstraction anions [M—H] (Reaction 7.38) and/or cluster adduct anions [M- -C] (Reaction 7.39). Exactly what anion is observed in the NICI spectrum depends on the... 

The MCI spectrum of psoralen (Fig. 9.25) indicates that resonance capture is occurring so that most of the ion current is carried hy the molecular ion at m/z 186. The associated GC trace indicates that ionisation is ca 10 times more efficient for the same amount of psoralen in comparison with El. In addition, since most of the ion current is channelled into the molecular ion, an analytical method which selectively monitored the molecular ion of psoralen would be ca 40 times more sensitive if NICI conditions were used rather than El. The technique only works for compounds which are electron capturing. The small ion at m/z 218 indicates that the psoralen has also formed an adduct with traces of oxygen present in the instrument, i.e. true chemical ionisation. 

Structural information about toxaphene components can be obtained by gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS), positive ion chemical ionization mass spectrometry (GC/PICI-MS) or negative ion chemical ionization MS(GC/NICI-MS). Mass separation is performed by low resolution quadrupole or high resolution magnetic field instruments, by ion trap systems (GC/IT-MS), or by tandem mass spectrometry (MS/MS) offering a broad spectrum of possibihties. 

The importance of these NICI (negative ion chemical ionization) spectra of modified polypeptides led Howe and co-workers [222] to study the spectra of the free compounds. Thus, with OH as reagent ion with a mixture of three tripeptides, in addition to fragment ions, [M-H] molecular ions are produced in sufficient abundance (Fig. 77) to be analyzed by collisions (MIKE/CAD spectrum). 

Figure 77. OH /NICI mass spectrum of tripeptide mixture [222].

Curtis and Boyd utilized this feature to the full by optimizing NICI conditions for achieving hard ionization, viz., DEA, by which chloride ions are predominantly produced from chlorinated compounds and detected by SIM at m/z 35 and 37. Operated in this way, NICIMS is effectively turned into a chlorine-specific GC detector with selectivity and sensitivity comparable to those of the ELCD. By using this technique, Milley et al. successfully identified dichlor-otetradecanoic acid in a GPC-enriched sample from lobster digestive gland lipids (but with the chlorine position remaining undetermined). The identification of this compound was supported by the mass spectrum obtained with soft NICI optimized for molecular anions, which resembled that of a synthesized 9,10-dichlorotetradecanoic acid. 

Figure 7.20 illustrates an example of the methane/methyl iodide NlCl spectrum of 5,5-dimethyl-l,3-cyclohexanedione. In this example, methane is the [Ri] reagent gas that produces thermal electrons upon electron ionization. Methyl iodide is the [R2] reagent gas used to generate the reactive iodide anion ([ ] ) at m/z 127. This [I] anion reacts with 5,5-dimethyl-l,3-cyclohexanedione ([M] = 140 x) to produce the cluster [M+I] peak at m/z 267 (Reaction 7.14). Note there is no fragmentation for this NICI technique. 

Example The NICI mass spectrum of tetraiodoethene, I2C=CI2, has been obtained using isobutane reagent gas (Fig. 7.12). The negative molecular ion, NT, at m/z 531.6 has a relative intensity of just 0.15%, while the product of nucleophilic addition, [M+I]", tn/z 658.5, yields the base peak [77], Losses of T and I2 from M"" are also observed. The series of peaks at m/z 126.9, 253.8, and 380.7 corresponds to traces of iodine present as impurity of tetraiodoethene. The iodine is also ionized by both electron capture (EC, next paragraph) and iodide addition. The spectrum nicely exemplifies the superimposition of mass spectra of two components of a mixture. It is not always simple to tell the corresponding peaks apart accurate mass measurements or tandem mass spectrometry may be required. It is worth noting the mass defect introduced by the iodine and the C isotopic peak of merely 2% due to only two carbon atoms present. 

Fig. 7.12. NICI mass spectrum of tetraiodoethene (isobutane reagent gas, electron emission 300 pA at 200 eV, and ion source temperature of 200°C). Adapted from Ref. [77] with permission. Wiley-VCH, Weinheim, 2007.

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