Methane Reagent Gas PICI Spectra

Occasionally, hydride abstraction may occur instead of protonation. Electrophilic addition fairly often gives rise to [Mh-C2H5] and [M-nCaHs] adduct ions. Thus, [Mh-29] and [Mh-41] peaks are sometimes observed in addition to the expected - usually clearly dominating - [Mh-1] peak. 

Note Hydride abstraction is only recognized with some difficulty. To identify a [M-H] peak occurring instead of a [Mh-H] peak it is useful to examine the mass differences between the signal in question and the products of electrophilic addition. In such a case, [Mh-29] and [Mh-41] peaks are observed as seemingly [Mh-31] and [Mh-43] peaks, respectively. An apparent loss of 16 u might indicate an [Mh-H-H20] ion instead of an [Mh-H-CHJ ion. 

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. 

Note Resulting from the large excess of reagent gas, its spectrum often is of higher intensity than that of the analyte. Therefore, Cl spectra are usually acquired starting above the m/z range occupied by reactant ions, e.g., above m/z 50 for methane or above m/z 70 for isobutane. Background subtraction can alternatively be applied to remove these peaks, but this approach suffers from variations in the reactant ion abundances upon admission of the analyte. 

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GC/MS analysis of PFBOA-derivatives can be performed by either El or chemical ionization. Chemical ionization is performed in positive ion mode (PICI) using methane as a reagent gas. In this analysis, abundant protonated [M + H]+ ion of acetaldehyde-derivatives at m/z 240, diacetyl-mono derivatives at m/z 282 and of internal standard o-chlorobenzaldehyde derivatives at m/z 336, form, and an abundant formation of [M + H—18]+ ion of acetoin-derivatives at m/z 282, is observed. Mass spectra of acetaldehyde, diacetyl monooxime, acetoin and o-chlorobenzaldehyde PFB-derivatives recorded in the PICI analysis of a standard solution are reported in Figure 1.19. 

Negative ion chemical ionization (NICI) may also be used for the analysis of TATP, although El or PICI are more commonly used. The major ions observed in NICI spectra of TAIP are fragment ions at m/z 57, 56, 59, and 41. The choice of reagent gas (ammonia or methane) produces some effect on the spectra, with ammonia giving a greater abundance of m/z 106 and 107 ions (Figure 16.4C,E) [30].The LOD determined by ammonia NICI were 1 ng for both linear quadrupole and ion trap mass analyzers, with quantitation of extracted ions m/z 57,56, and 59. The LOD by methane NICI was 0.5 ng for both mass analyzers, with quantitation of extracted ions m/z 57,56,59 and 41. 

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... 

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