Isotopolog ion

The position of the most intensive peak of an isotopic pattern is termed most abundant mass [6], and the corresponding ion should be named most abundant isotopolog (ion). For example, the most abundant mass in case of C120 is 1441 u corresponding to M+1 (Fig. 3.2). The most abundant mass is relevant in the case of large ions (Chap. 3.4.3). 

We started our lesson on isotopic pattams with the unspoken assunption of nominal mass resolution, which makes it much easier to understand how isotopic patterns are formed. The procedure of summing up all isotopic abundances contributing to the same nominal mass is correct as long as very high resolution is not employed. In that case, following such a simplified protocol is acceptable, because generally isobaric isotopolog ions are very similar in mass [24]. 

If isotopolog ions are resolved or if certain isotopologs are free from interference, and provided sufficient mass accuracy is available, the distance between isotopic peaks can yield a new type of analytical information. As are the isotopic masses themselves, the differences between isotopic masses are characteristic for certain elements. Boron for exanple, has a small mass difference of just 0.9964 u... 

If isotopolog ions are resolved and sufficient mass accuracy is available, the distance between isotopic peaks can yield a new type of analytical information the differences between isotopic masses are characteristic for certain elements. 

The upper limit on the molecular mass of analytes that are amenable to trace quantitative analysis is currently considered to be 2000u (Da). This is the result of several factors, some of which are intrinsic to the problem, e.g., the relative abundances of higher isotopologs that increase significantly with increasing mass, and result in dilution of the total ion current derived from the analyte over too many m/z values. 

Even if such matrix interferences do not contribute signals directly to the particular miz value(s) monitored for the analyte, they can affect the analyte signals indirectly by competing for the ionization power of the mass spectrometer ion source this ionization suppression effect is particularly important for electrospray ionization, as discussed in Sections 5.3.6a and 9.6. Note, however, that an isotopolog SIS is doubly advantageous in this regard as it most closely resembles the analyte in the extraction and... 

Figure 6.25 Scan function for MS spectrum of the molecular ions of tetrachlorodibenzodioxin (TCDD, or dioxin ) m/z 320 and 320 correspond to the Cl4 and Cl3 Cli isotopologs, respectively. MFI refers to resonance activation using multi-frequency irradiation. The pre-scan for AGC is not shown. Reproduced from March, J. Mass Spectrom. 32, 351 (1997), with permission of John Wiley Sons, Ltd.

If the particular analog happens to be a structural isomer (Table 2.1), chromatographic resolution is necessary to distinguish the analytical signals for analyte and SIS (identical molecular masses) unless MS/MS is used and the isomers yield different product ions. Since the molecular mass of a homologous structure differs from that of the analyte by at least 14 Da, there is seldom any problem of cross-contributions arising from overlap of the isotopolog distributions (Section 8.5.2c). 

LOD, defined here as the lowest quantity of analyte injected on-column to be detected with respect to peak identification criteria of S/B > 3, plus verification of the isotope abundance ratio for at least two isotopologs of the molecular ion and retention time consistency in both GC dimensions, was 0.5 pg for 2,3,7,8-TeCDD (Figure 11.24). This represents an improvement by a factor of 5-10 compared with conventional GC-TOFMS but is an order of magnitude higher than for GC-HRMS instruments (typically 0.04pg for 2,3,7,8-TeCDD). 

Sulfur produces a series of intense peaks in both ionization modes. Peaks are more prolific in the negative mode, where signals are observed for the entire Sf to Si5 series. In contrast, in the positive mode, fewer peaks are observed from sulfur. Higher laser fiuences are required to observe peaks for very low mass ions such as mJz 32 and 64. Unfortunately, higher laser fluence results in the deterioration of peak resolution, shape, and intensity of the peaks for more abundant ions. However, with optimum adjustment of delayed extraction time and voltage, resolution of all isotopolog peaks can be achieved. Sulfur exists in nature as a mixture of four isotopes [94.93%], [0.76%],... 

Example Effects of a secondary kinetic isotope on the a-cleavage of tertiary amine molecular ions occurring after deuterium labeling both adjacent to and remote from the cleaved bond (Chap. 6.2.6). This reduces the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes [80]. The reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated by field ionization kinetic measurements, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isotopologs [68,78]. 

Molecules and ions of identical elemental conposition but differing in isotopic composition are termed isotopic homologs or simply isotopologs. For example, H3C-CH3 and H3C- CH3 are isotopologs. Molecules and ions of identical isotopic composition but differing in position of the isotopes are termed isotopomers. For example, H2C=CD2 and HDC=CHD are isotopomers. 

Example According to Eq. 4.7 the Qo " ion, m/z 720, of the preceding example will travel through a field-free flight path of 2.0 m in 27.665 ps, while it takes slightly longer for its isotopolog m/z 721. The proportionality to the... 

Cyclohexanone has been examined by deuterium-labeling to reveal the mechanism of propyl loss, [M-43], m/z 55, from its molecular ion, M = 98 [35,36]. The corresponding signal is the base peak of the spectrum (Fig. 6.11). Clearly, one deuterium is incorporated in the fragment that is shifted to m/z 56 in case of the [2,2,6,6-D4]isotopolog, which is consistent with the above three-step mechanism. 

Fig. 8.9. FI spectrum of toluene. The molecular ion and its isotopolog are the only observed entities. (The QD spectrum of field-ionized toluene is shown in Chap. 9, the El spectrum is discussed in Chap. 6.4.3).

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