Comparison of methods for the determination of total selenium in plasma by magnetic sector inductively coupled plasma mass spectrometry

Abstract

Three sample preparation methods (dilution, microwave digestion and ethanol addition) were evaluated for the determination of Se in human plasma using magnetic sector inductively coupled plasma mass spectrometry (ICP-MS). A number of instrumental parameters were also considered, namely the choice of internal standard (Sc, Rh, In) and pre-defined spectral resolution ( m/Δ m ∼300 (low resolution, LR) and ∼7500 (high resolution, HR)). The isotopes 77 Se and 82 Se were selected for analysis avoiding the major and unresolvable Ar dimer interferences associated with other Se isotopes. Ethanol addition was found to be the most suitable and reliable sample preparation method. The optimum ethanol concentration for a 1+9 dilution of the plasma sample was 0.5% (v/v). Indium or Sc were selected over Rh as internal standards as Seronorm™ Level 1 target values were achieved at lower concentrations of ethanol modifier. Representative ICP-MS detection limits for this method using 77 Se (HR) and 82 Se (HR), were 0.1 and 0.2 μg l −1. Accurate analysis of Se in Seronorm™ Level 1 (MI0181) was found using either 77 Se or 82 Se and HR (86±5 and 83±5 μg l −1 respectively, n=5, In internal standard, target value=83±6 μg l −1). Although offering improved precision (e.g. 82 Se LR: 0–3% versus HR: 1–6%), accurate results were only obtained with LR for the Seronorm™ Level 1 sample when using 82 Se (86±1 μg l −1). The major 40 Ar 37 Cl interference precluded accurate Se analysis using 77 Se and LR for all samples considered. Reliable Se concentrations were only found for a “real” pooled sample when using 82 Se (HR), in part a result of non-negligible and unresolved 81 Br 1 H interference when using LR. Consistently elevated Se concentrations were found under all conditions when Seronorm™ Level 2 (NO0371) was analysed. These results were confirmed by independent GF-AAS analysis. Magnetic sector ICP-MS with 82 Se (HR), in combination with the ethanol addition sample preparation method, was used for the analysis of Se in human plasma samples as part of a small pilot study. Average measured selenium concentrations were 102±18 μg l −1 ( n=18).