OVERCOMING SPECTRAL OVERLAPS: QUADRUPOLE ICP-MS WITH REACTION CELL AND SECTOR BASED ICP-MS

摘要

Spectral overlaps have been recognized as one of the main limitations of ICP-MS since its initial development. Every element except iodine has at least one isotope that does not occur at the same nominal mass as an isotope of another element. However, molecular ions are the main source of spectral overlaps in ICP-MS. Two different approaches to overcome spectral overlaps in ICP-MS based on selective chemical reactions using a pressurized reaction cell or mass spectral resolution will be discussed here. Ions from the plasma gas (such as Ar~(+), Ar_(2~(+))), plasma gas and entrained air (such as ArN~(+)), plasma gas and aqueous solvent (ArO~(+), ArH~(+), ArOH~(+), O_(2~(+))) and plasma gas and organic species (ArC~(+)) can result in spectral overlaps with ~(40)Ca~(+), ~(80)Se~(+), ~(54)Fe~(+), ~(56)Fe~(+), ~(41)K~(+), ~(57)Fe~(+), ~(32)S~(+), and ~(52)Cr~(+), respectively. Acids used to digest and preserve samples can also result in intense signals from molecular ions that can overlap with major isotopes of some elements. ~(15)N ~(16)O~(+) from HNO_(3), has the same nominal mass as ~(31)P~(+); SO~(+) from H_(2)SO_(4) overlaps with ~(48)Ti~(+); SO_(2~(+)) from H_(2)SO_(4) overlaps with ~(64)Zn~(+), ~(65)Cu~(+) and ~(66)Zn~(+); ArCl~(+) from HCl or HClO_(4) overlaps with ~(75)As~(+); ClO~(+) from HCl or HClO_(4) overlaps with ~(51)V~(+); ClO_(2~(+)) from HCl or HClO_(4) overlaps with ~(69)Ga~(+). High concentrations of dissolved solids or major elements in materials sampled by laser ablation can also result in significant molecular ion signals. For example, high concentrations of B (such as from lithium metaborate fusion lead to ArB~(+) that is at the same nominal mass as the only major isotope of Al. High concentrations of Na produce ArNa~(+) that has the same nominal mass as ~(63)Cu~(+). Molecular oxides formed from elements in the sample are also common. Elements with low second ionization potentials, such as Ba, can produce significant signals from doubly charged ions (Ba~(2+) has the same mass/charge ratio as ~(69)Ga~(+). Finally, there are isobaric overlaps when an isotope of one element occurs at the same nominal mass an isotope of an analyte element of interest. For example, the major isotope of calcium, ~(40)Ca~(+), occurs at the same nominal mass as ~(40)Ar~(+). The major isotope of nickel, ~(58)Ni~(+), has the same nominal mass as ~(58)Fe~(+). Extensive lists are available to assess potential spectral overlaps as well as a convenient computer database and software with a very complete list of overlaps that simulates mass spectra of user selected mass resolution. Most commercial ICP-MS instruments have a database of spectral overlaps included in the software. ICP-MS instruments based on a quadrupole mass spectrometer typically provide mass resolution no better than of 0.6 mass units. Therefore, several strategies were previously developed to avoid or correct for spectral overlaps. Mathematical corrections could be used if the overlap ion signal/analyte ion signal is not too large. For example, consider the overlap of ~(204)Hg~(+) on ~(204)Pb~(+). Mercury could be measured at mass 202, multiplied by the ratio of the isotopic abundance of ~(204)Hg/~(202)Hg and then subtracted from the signal measured at mass 204 to obtain the signal due only to ~(204)Pb~(+). Isobaric corrections are typically provided in quadrupole ICP-MS software. Mathematical corrections can also be made for molecular overlaps, using either correction equations or spectral fitting. Correction equations for molecular ion overlaps require experimental measurement of the ratio of signal related to the overlap ion signal and the signal at the analyte mass of interest due to the overlap. As the ratio of spectral overlap ion signal to analyte ion signal increases, the precision and uncertainty of the analyte concentration degrades. Most molecular ions observed in ICP-MS are probably produced in the plasma and representatively sampled although they may also be