Isobaric Overlap

Isobaric overlaps appear when isotopes of different elements have the same nominal mass. Many of them can be overcome by choosing an alternative less interfered isotope of the element of interest, although a sacrifice in sensitivity may result. 

If the spectra consisted only of elemental ions, every element except In would have at least one isotope that was free of spectral overlaps from other elemental ions (Table 3.2). In most cases, the isobaric overlaps are easily predictable. The contribution of the interfered ion to the signal measured at the mass of the analyte elemental ion can be subtracted by measuring the interfered ion signal at a second mass. The known isotopic distribution of the interfered element can be used to determine the contribution of the interfered ion at the analyte ion mass. Alternatively, a solution containing only the interfered element can be measured in order to determine the interfered signal at the analyte mass and at another isotope of the interfered element. 

Molecular ions present a more complex problem in ICP-MS. With a combination of molecular ion interferences and isobaric interferences, all of the isotopes of the analyte ion of interest may suffer from a spectral overlap. The molecular ion signals can also be strongly dependent on the sample composition and experimental parameters. It is often more difficult to identify and correct for molecular ion spectral overlaps than for isobaric overlaps. Because the resolution of the commercial quadrupole mass spectrometers is 0.5 dalton or less, isotopic patterns, rather than exact mass, must be used in an attempt to identify the interfering molecular ion. 

Mathematical correction procedures can be used to remove the contribution of a spectral overlap from a measured signal. However, if the signal due to the spectral overlap is much larger than the analyte signal, the signal-to-noise ratio of the corrected signal may be poor. Furthermore, it may not be easy to predict and account for quantitatively all of the potential sources of spectral overlap, particularly those due to polyatomic ions. For isobaric overlaps (Table 3.2), for which the relative isotopic abundances are predictable, mathematical corrections are straightforward. Instrument software often has built-in correction equations for this case. 

ICP-MS can provide semiquantitative analysis for about 70 elements by using element response functions built into the instrument software and calibration of only a few elements [205,206]. Most elements are more than 90% ionized in the ICP (with the exception of elements with ionization potentials greater than about 8 eV). Ion transmission efficiency is a smooth function of mass. The natural isotopic abundances of the elements are well known. Therefore, it is possible to predict the relative sensitivities of the elements and any isobaric overlaps. 

In addition, quantitative and qualitative elemental analysis of inorganic compounds with high accuracy and high sensitivity can be effected by mass spectrometry. For elemental analysis, atomization of the analysed sample that corresponds to the transformation of solid matter in atomic vapour and ionization of these atoms occur in the source. These atoms are then sorted and counted with the help of mass spectrometry. The complete decomposition of the sample in the ionization source into its constituent atoms is necessary because incomplete decomposition results in complex mass spectra in which isobaric overlap might cause unsuspected spectral interferences. Furthermore, the distribution of any element in different species leads to a decrease in sensitivity for this element. 

Most elements in the periodic table have one, two, or even three isotopes that are free from isobaric overlap, Att exception is indium, which has two stable isotopes. In and Mn .The former overlaps with Cd and the latter with Sn. More often, an isobaric interference occurs wiih the most abundant and thus the most sensitive isotope. I )r example, the very large peak for " Ar (see Figure I I-I5b) overlaps the peak for the most abundant calcium isotope Ca (97%), making it necessary to use the second-most abundant isotope Ca (2.17o). As another example, ilie most abundant nickel isotope, suffers from... 

Use of isotopes that are free of isobaric overlap. The isobaric elemental interferences can be eliminated by choosing as the analyte a less abundant isotope of the target element. For example, " Ar+ interferes in the analysis of calcium because of its overlap with " Ca" ". This interference can be eliminated when measurements are done with the second-most abundant isotope, " Ca. ... 

ICP-MS (the signals of and Ar" show isobaric overlap), hence the corresponding isotope ratios can be used to study isotope fractionation during different processes and the isotopic composition in different sulfur species can be determined. Sulfur isotope ratio measurement is complicated as a result of interference from O2, which can be overcome either by using a higher mass resolution or by using dry plasma conditions. 

The final classification of spectral interferences is called isobaric overlaps, prodnced mainly by different isotopes of other elements in the sample creating spectral interferences at the same mass as the analyte. For example, vanadinm has two isotopes at 50 and 51 amu. However, mass 50 is the only practical isotope to use in the presence of a chloride matrix because of the large contribution from the interference... 

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