Interference-free determination of ultra-trace concentrations of arsenic and selenium using methyl fluoride as a reaction gas in ICP–MS/MS

Interference-free conditions, allowing straightforward As and Se determination, can be obtained by using CH 3 F (a mixture of 10 % CH 3 F and 90 % He) as a reaction gas in tandem ICP–mass spectrometry (ICP–MS/MS). Both target elements react via CH 3 F addition and subsequent HF elimination, rendering AsCH 2 + and SeCH 2 + the respective favored reaction product ions. Instrumental limits of detection were 0.2 ng L −1 for As and below 10 ng L −1 for Se, using either 77 Se, 78 Se, or 80 Se. Neither addition of carbon to the solutions, nor admixing of additional He into the octopole reaction cell resulted in a further improvement of the LoDs, despite the increase in analyte signal intensity. By using synthetic matrices, containing elements giving rise to ions interfering at either the original mass-to-charge ratios or those of the reaction products, absence of spectral overlap could be demonstrated. This conclusion was corroborated by successful As and Se determination in a collection of reference materials from plant, animal, or environmental origin, displaying a considerable range of As and Se contents. These accurate results were obtained via external calibration using Te as an internal standard. The high efficiency reaction between As and CH 3 F and the possibility to use the major isotope of Se provides enhanced detection power versus other techniques, such as sector-field ICP–mass spectrometry, while the possibility to monitor at least three Se isotopes interference-free also enables isotopic analysis. Graphical abstract ᅟ