Blank correction in IDMS: possible pitfalls

Abstract

The blank correction in isotope dilution mass spectrometry (IDMS) is investigated using the double IDMS exact matching technique. The effects of different amounts of blanks in the sample, reference and spike solutions are simulated for the determination of the mass fractions of Pb and Cu. The simulations are compared with and confirmed by the respective interlaboratory CCQM key comparisons. The most accurate results are obtained when the blend bx of the sample and spike and blend bz of the reference and spike are treated in the same way (digestion, matrix separation, choice of the same solvent, amounts of sample, reference and spike, etc.) starting from sample preparation. When applying this procedure, it is not necessary to subtract the blank subsequently. A similar “blank-matching” approach has previously been reported by Pagliano, Mester, and Meija (Anal. Chem., 2015, 87: 10724–10727). In the present study, it is shown that in the case where exact matching is applied, the subsequent blank subtraction is not only superfluous but also yields a systematically wrong result with a bias of up to ten times the uncertainty depending on the blank concentration. If the procedural blank is “small”, a subsequent subtraction yields a result similar to that of the procedure presented in this work. This small effect has obviously hidden a systematic bias in the past. By varying the mass fraction of the blank w_bl in the range 0.0001 µg g⁻¹ < w_bl < 0.0025 µg g⁻¹ for Pb, a subsequent blank subtraction yields a significantly biased result of w_x from −0.57% to −14%, exceeding the associated standard uncertainty u(w_x) at w_bl ≈ 0.0004 µg g⁻¹.