Division 2.4.8 Specific requirements for inductively coupled mass spectrometry (ICP-MS)(11))

The determination of trace elements of average atomic mass, such as chromium, copper and nickel may be subject to strong interference from other isobaric and polyatomic ions. This can be circumvented only when a resolution power of at least 7000-8000 is available. Difficulties associated with the MS techniques include instrumental drift, matrix effects and molecular ion interference (m/z < 80). Multiple internal standardisation covering the same mass range as the elements to be determined is required to correct instrumental drift and matrix effects.

Complete decomposition of organic matter in samples prior to ICP-MS measurements is required. As in the AAS, after digestion in sealed vessels, volatile elements e.g. iodine are to be transferred to a stable oxidation state. Most severe interference results from molecular ion combinations of argon (plasma gas), hydrogen, carbon, nitrogen and oxygen (dissolution acids, plasma gas impurities and entrained atmospheric gases) and the sample matrix. Complete digestion, background measurements, appropriate choice of analytical masses sometimes associated with a lower abundance (poorer detection limit) and of dissolution acids, e.g. nitric acid, are required to avoid interferences.

For the elements to be determined, interferences are to be excluded by the appropriate choice of specific analytical masses including confirmation of isotope ratios. Instrument response considering Fano-factors shall be checked for each measurement by the use of internal standards.