Figure 1

Recent advances in collision–reaction cell technology have all but eliminated interferences from quantitative inductively coupled plasma mass spectrometry (ICP–MS) for most matrix types. However, in general, this technology requires specific information about the matrix and target analytes to set up the correct collision–reaction chemistry to eliminate the interferences. Additionally, the conditions required to eliminate one interference in one matrix frequently are not effective for all analytes in all matrices. For this reason, multiple sets of collision–reaction conditions typically are used. This results in significant deviations from predicted relative response factors for elements acquired under varying conditions. Making useful semiquantitative measurements requires that all elements be acquired under the same conditions. As a result, it has not been possible to use collision–reaction cell technology to reduce interferences in semiquantitative analysis in the same way as in full quantification. However, polyatomic interferences can be eliminated using carefully controlled kinetic energy discrimination (KED) in helium collision mode (Agilent 7500ce Octopole Reaction System), which can allow good semiquantification in most matrices for most analyte isotopes. Because helium is a nonreactive gas, no new interferences are formed in the cell, and plasma-formed polyatomic interferences are eliminated generically as a function of their larger ionic radii (Figure 1).

Figure 2

When only a single set of collision–reaction cell conditions is used, accurate relative response factors are maintained. Because helium collision mode eliminates only polyatomic interferences, interference-free isotopes of elements suffering from isobaric interferences still must be used. For the most part, the only significantly affected isotope is ⁴⁰Ca. In this case, ⁴⁴Ca can be used. KED eliminates the transmission of the larger polyatomic ions to the quadrupole by placing an energy barrier between the octopole and quadrupole (Figure 2).

Figure 3

Because the larger polyatomic ions undergo more energy-depleting collisions with He in the cell than do the smaller atomic ions, they lack sufficient energy to cross the barrier. Figure 3 depicts the effects of KED on ion energy. Only the high-energy atomic ions exceed the stopping potential and are transmitted.