Isobaric ion

The alkane series is present in mass spectra of any compound containing an alkyl group. In case of isobaric series (e.g., alkanes and ketones) one should pay attention to the intensities of the isotopic peaks. Thus, for the isobaric ions of m/z 43 (CH3CO and C3H7) the abundance of the isotopic peak (m/z 44 ion) will be 2.2% and 3.3%, respectively. The situation is very simple with A + 2 elements. In this case there are two homologous ion series due to A and A + 2 ions. It is worth emphasizing that... 

Example the molecular ions of nitrogen, N2, carbon monoxide, CO, and ethene, C2H4, have the same nominal mass of 28 u, i.e., they are so-called iso-baric ions. The isotopic masses of the most abundant isotopes of hydrogen, carbon, nitrogen and oxygen are 1.007825 u, 12.000000 u, 14.003070 u and 15.994915 u, respectively. Using these values, the calculated ionic masses are 28.00559 u for Nz"" , 27.99437 u for CQ-", and 28.03075 u for CjH/. This means they differ by some millimass units" (mmu) from each other, and none of these isobaric ions has precisely 28.00000 u (Chap. 3.3.4 and Chap. 6.9.6). 

Isobaric ions are ions having the same nominal mass. However, their exact mass is different. Isomers are (almost) perfect isobars. 

Fig. 3.14. Exact masses of isobaric ions of nominal mass 310 u. The Am scale is plotted in mmu relative to the saturated hydrocarbon, C22H46, at 310.3599 u. Adapted from Ref. [13] with permission. University Science Books, 1993.

Given infinite mass accuracy we should be able to identify the empirical formula of any ion from its exact mass. The emphasis is on infinite mass accuracy, however, and the meaning of infinite is perfectly illustrated by an example from McLafferty s book (Fig. 3.14). [13] Even though the compositions are restricted to CxHyNo 30o-4, there is a considerable number of isobaric ions of nominal mass... 

High and in particular ultrahigh-resolution in combination with a soft ionization method such as ESI, MALDI, or FD presents another way to achieve the separation of the molecular species contained in a mixture. Given a sufficient level of resolution, isobaric ions are displayed separately in the range of their common nominal mass value (Chap. 3.3.2, 3.4). 

Advances in high resolution mass analyzers (TOF, FT-ICR, orbitrap) have greatly improved the detection and identification of metabolites based on accurate mass measurements. In single MS mode accurate mass determination is mainly used to differentiate between isobaric ions. Combined with LC-MS, it allows the detection of predicted metabolites by performing extracted ion current profiles... 

Figure 16.19—ICP - MS recording plasma torch ionisation for elemental analysis. A spectrum of a sample containing metals in the mass range of 42 to 68 Da is shown. The sample contains a high concentration of calcium that constitutes a major peak at 40 Da (not shown on the figure). The resolution in this recording is not sufficient to separate isobaric ions, thus peaks in the spectrum are the sum of different species with the same nominal mass.

Accurate mass measurements play a major role in distinguishing nominally isobaric ions and structure assignment based on observed fragmentation mechanisms... 

ISOBARIC IONS AT NOMINAL MASS 124 RESOLUTION 24,000 FWHH... 

Figure 5. Expansion of the region around m/z 123 of Figure 4 showing three well-resolved isobaric ions.

Isobaric Ions Ions of the same nominal mass but having... 

In arsenic assays, one possible way to eliminate interference resulting from the isobaric ions referred to above is application of correction equations. The contribution of argon chloride to the value of the signal measured for the As ion can be calculated because chlorine occurs as two isotopes, Cl and Cl. The latter forms the " Ar CE ion in plasma (signal at w/z 77). The intensity of the signal makes it possible to assess the necessary correction of ion signal intensity at w/z 75 by applying an appropriate mathematical equation [24, 62]. The approach is only possible if the ratio between the As and Cl content in the sample does not exceed... 

Mass accuracy is also closely bound up with mass resolution as failure to achieve sufficient resolution of the ion of interest, away from interfering isobaric ions, will seriously impinge on the attainable mass accuracy. An... 

Indeed, it is possible to obtain MIKE/CAD spectra of isobaric ions [154a,b]. For example, after elimination of 43 amu fragment, cyclohexanone leads within the... 

The MIKE/CAD spectra of each ion are sufficiently different so that we may admit the separation of these ions, as shown by the non-existence of ions arising from the loss of 4,5 or 6 hydrogen atoms, as well as that of the m/z 39 ion in the MIKE/CAD spectrum of the isobaric ion CaHjO]". Inversely, the very low intensity of the m/z 41 peak in the MIKE/CAD spectrum of the C4H7] ion is proof of the good separation of these isobaric ions Fig. 29). 

In order to increase the possibility of collisional activation, the current trend is to develop this mode (on universally accessible conventional geometry instruments), utilizing the detection of decomposition produced in the first FFR by the B/E linked scan method. Nevertheless, although the resolution of fragment ions is better in this mode, that of precursor ion resolution remains mediocre and does not lead to as good a separation of isobaric ions as in the case of MIKE mode. ... 

Figure 29. MIKE/CAD spectra of C3H3O and QH, isobaric ions [m/z 55) produced from cyclohexanone under El conditions [154a].

The presence of isomeric (or isobaric) ions could eventually represent a limitation of the linked scan method when working on a unique transition. This is why coupling with GC is interesting. 

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