Suppression of the incoherent molecular formation effect by oxygen gas in arsenic analysis by ICP-MS

The Incoherent Molecular Formation (IMF) effect, which involves the molecular formation of arsenic (As) atoms depends, on its oxidation state in the inductively coupled plasma. This leads to determination errors, when As is measured by inductively coupled plasma mass spectrometry (ICP-MS). The sensitivity of As(V) is 4-7% higher than that of As(III) at m/z 75, since the final ionization rates of As(V) and As(III) are different from each other due to the IMF effect. A precise measurement of As by ICP-MS is generally very difficult. Recently, the collision/reaction cell is widely used for ICP-MS to eliminate spectral interferences caused by polyatomic molecules. Especially, oxygen gas (O2) is one of the most useful reaction cell gases for As analysis, because As(+) is oxygenated into AsO(+), which is free from the such interferences as ArCl(+). In addition, the use of O2 as a reaction gas is extremely effective for reducing the IMF effect and eliminating the sensitivity difference between As(III) and As(V).