ICP-MS Measurement

Interferences

This next section presents the most common interferences encountered in ICP-MS using the quadrupole mass filter (hereafter referred to as low-resolution ICP-MS). You can also find the major interferences for the popular isotopes in our interactive periodic table.

Isobaric Interferences

Isobaric interference is a result of equal mass isotopes of different elements present in the sample solution. Low-resolution instruments cannot distinguish between the isotopes. There are many examples in the intermediate mass regions where the second and third row transitions and the rare earths appear. Fortunately, there are no elemental singly charged isotopes that overlap with monoisotopic elements (⁹Be, ²³Na, ²⁷Al, ⁴⁵Sc, ⁵⁵Mn, ⁷⁵As, ⁸⁹Y, ¹⁰³Rh, ¹²⁷I, ¹³³Cs, ¹⁴¹Pr, ¹⁵⁹Tb, ¹⁶⁵Ho, ¹⁶⁹Tm, ¹⁹⁷Au, and ²³²Th). However, for the monoisotopic elements, be aware of the other interferences to be discussed later. For elements having more than one isotope, the quickest fix may be ('may' is used because other interferences could be encountered) to use another isotope of that element. If the interference is from an isotope with roughly the same or lower peak intensity, it is possible to perform a correction by measuring the intensity of another isotope of the interfering element and subtracting the appropriate correction factor from the intensity of the interfered isotope. If you are working with an unknown sample composition, a semi-quantitative analysis is suggested with low-resolution instruments using a quick scan of the sample and the rather sophisticated semi-quantitative programs available on current instrumentation.

Polyatomic (Molecular) Interferences

Molecular interferences are due to the recombination of sample and matrix ions with Ar and other matrix components such as O, N, H, C, Cl, S, F, etc. The light elements (Li, Be, B) are not affected due to their small masses.

Starting with ³⁹K, this type of interference becomes a significant issue. For example, ³⁹K is interfered with by ³⁸ArH and ²³Na¹⁶O. Some polyatomic interferences can be avoided by eliminating certain matrix elements such as the classic ⁴⁰Ar³⁵Cl interference upon the monoisotopic element ⁷⁵As, where the use of HCl in the sample preparation is to be avoided. The isotopes ⁵⁶Fe, ³⁹K, and ⁴⁴Ca or ⁴⁰Ca are all interfered with by combinations of the Ar, O, and N isotopes.

As we go to the heavier elements the major polyatomic interferences come from isotopes that are 16 atomic mass units lower than the analyte isotope through molecular oxide (MO) interference. The lanthanide element isotopes are especially prone to molecular oxide formation.

The use of cool plasma techniques, reaction / collision cells, desolvation, and chromatographic separations -- to name a few approaches -- have resulted in reduction and, in some cases, complete elimination of many polyatomic interferences. The severity of the MO interference can be reduced through reduction of the sample argon gas flow rate. Mass corrections may be an option in cases where the use of an alternate isotope is not an option. Polyatomic interferences are particularly troublesome in the determination of first row periodic table elements (K thru Se) due to the vast number of combinations of Ar with matrix components.

Doubly Charged Ion Interferences

Doubly charged ion interference is due to doubly charged element isotopes with twice the mass of the analyte isotope. For example, interference from ²⁰⁶Pb⁺⁺ (m/e = 103) upon ¹⁰³Rh is likely at high Pb concentration levels. Reduction in the sample Ar will minimize this interference. Fortunately, this type of interference is not as prominent in Ar plasmas, but care should be exercised in matrices containing high levels of mid to heavy mass element isotopes. The alkaline and rare earth elements form doubly charged ions to a extent that is greater, relative to the other elements.

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