PICI spectra

Isobutane is an especially versatile reagent gas, because i) it provides low-fragmentation PICI spectra of all but the most unpolar analytes, ii) gives almost exclusively one well-defined adduct ([Mh-C4H9], [M+57]) if any (Fig. 7.7), and iii) can also be employed for electron capture (Chap. 7.4). 

An interesting artifact that appears in methane PICI spectra involves the following set of reactions (84) ... 

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

The positively charged ions can either associate with the analyte or can transfer a proton to the analyte. The most commonly observed adduct ions are illustrated by the PICI spectrum of psoralen (Fig. 9.23). In this case, the ions arise via addition... 

The methane PICI spectrum of TATP obtained with a linear quadrupole mass analyzer has been reported to contain ions of m/z 103,117,133, and 223, corresponding to the pseudomolecular ion [TATP -i- H]" [35]. A full-scan methane PICI spectrum of TATP has also been reported to contain ions of m/z 43(100%), 59,74,75,91, and 223(<5%), as shown in Figure 16.4B [30].The LOD for methane PICI analysis of TATP with linear qudaru-pole and ion trap mass analyzers were reported in the same work as 1 and 2 ng, respectively. Quantitation was based on extracted ions m/z 43, 59, 75, and 91 in both cases. When the m/z 91 ion was isolated in an ion trap mass analyzer and fragmented under CID, ions of m/z 43 and 74 were observed [30]. The third-order tandem mass spectrometry (MS ) experiment involving isolation of m/z 91 with subsequent CID and isolation of the m/z 74 product ion leads to the formation of a m/z 43 ion upon further CID. These results provide a scheme for analysis of TATP by selected reaction monitoring (SRM). 

Although the ammonia PICI spectrum of TATP is dominated by the m/z 240 [TATP + NH4] ion, attempts to perform CID studies of the ion trap-isolated ion failed to produce a significant yield of product ions. Calculated interaction energies between TATP and the ammonium cation were reported to be less than the peroxide bond dissociation energy [30].The ammonium ion that would presumably be formed by ejection from... 

Reactions 7.8 and 7.9). Figure 7.13b is the methane PICI spectrum of the same compound in which the [M- -H]+ cation (m/z 430) is the base peak (Reaction 7.12). Note there is less fragmentation in PICI as compared to El. Figure 7.13c is an example of a phenomenon called self-PICI, which can show up in spectra if the pressure of the reagent gas is low and the pressure of the sample molecules... 

TABLE 7.6 Effect of Ion Source Pressure on Methane PICI Spectrum for Di-n-Butyl Phthalate"... 

FIGURE 7.18 (a) PICI spectrum of V-nitroso-di-n-propylamine (peak 4 in Figure 7.17a) with ammonia as the reagent gas. (b) PICI spectrum of V-nitroso-di-n-propy-lamine (peak 4 in Figure 7.17b) with isobutane as the reagent gas. (c) PICI spectrum of V-nitroso-di-n-propylamine (peak 4 in Figure 7.17c) with methane as the reagent gas. (Masucci and Caldwell, unpublished data.)... 

Example El vs. methane reagent gas PICI spectrum of methionine. The comparison shows greatly reduced fragmentation in the PICI spectrum (Fig. 7.4). Only small intact molecules such as NH3, HCOOH, and MeSH are eliminated from the [Mh-H]" ion, m/z 150, which yields the base peak. In addition, the PICI spectrum perfectly reveals the isotopic pattern that indicates the presence of sulfur. The horizontal line (Fig. 7.4b) means that this range was not acquired in the PICI spectrum as to keep the spectrum free from reactant ion signals. The spectra were, however, plotted on the same m/z scales to simplify the comparison. 

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. 

Other Species Observed by El and PICI GC-MS In addition to TATP, DADP, the cyclic tetra-mer tetraacetone tetraperoxide (TrATrP), and the oli-goperoxides H[OOC(CH3)2] OOH (n = 2,3) have been observed by GC-EI-MS and gas chromatography-mass spectrometry using chemical ionization (GC-CI-MS) [33]. The El spectrum of DADP contains ions at m/z 43 (base peak), 58, 59, and 101, but does not exhibit a molecular ion (Figure 16.5A). The first El mass spectrum of DADP was reported by Bertrand et al. [43]. The... 

In Figure 7.13 examples of El, methane PICI and an interesting cation/molecule reaction called self-PICI are shown. Figure 7.13a is the 70 eV El spectrum of an aromatic compound in which the [M]+ cation (m/z 429) is not observed. For this molecule, the amount of internal energy retained by the [M]+ cation, after electron ionization, was so great that the [M]" " cation totally fragmented... 

Fig. 7.6. 70-eV El (a) vs. isobutane-PICI spectmm of glycerol (b). Even in El mode, an [M+H] ion by autoprotonation is observed rather than M. Besides [M+H], the PIC3 spectrum shows only few fragment ions and a weak [2M+H] cluster ion signal. 

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