Blank correction in IDMS – possible pitfalls

Abstract

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 lead and copper. The simulations are compared to and confirmed by respective interlaboratory CCQM key comparisons. The most correct results are obtained when the blend bx of sample and spike and blend bz of 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 been reported by Pagliano, Mester, and Meija previously (Anal. Chem. 2015; 87: 10724-27). In the present study it is shown that the subsequent blank subtraction is not only superfluous but will yield a systematically wrong result with a bias up to ten times of the uncertainty depending on the blank concentration. If the procedural blank is "small", a subsequent subtraction will yield a similar result as the procedure presented in this work. This small effect obviously has hidden a systematic bias in the past. Varying the mass fraction of the blank w_bl in a range 0.0001 µg/g < w_bl < 0.0025 µg/g in case of 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.