Double calibration (external calibration and stable isotope dilution) for determining selenium in plant tissue by hydride generation inductively coupled plasma mass spectrometry

Both external calibration (EC) and stable isotope dilution (SID) calibration are reliable methods for determining selenium by hydride generation inductively coupled plasma mass spectrometry (HG-ICP-MS), but they are rarely used simultaneously. Because these methods are independent of each other, they could be used simultaneously to test each other and to evaluate data quality directly. A double calibration approach using EC and SID simultaneously was developed to evaluate data quality directly for analysis of Se in plant tissue by HG-ICP-MS. Natural Se was used for EC. 77Se-enriched Se (0.004 microgram; 77Se = 94.38%) was spiked to 0.5 g sample for SID (78Se or 82Se isotope used as reference isotope). Sample preparation and measurement was not doubled, because one spiked sample served for both EC and SID. All samples were digested with acid and heat, selenate was reduced to selenite, and selenite was measured by HG-ICP-MS. The Se determined by SID corresponded to that determined by EC with a concentration ratio of 1.02 +/- 0.08 (n = 207 alfalfa samples). The measured Se in 8 reference materials also agreed well with certified values. The study showed that the double calibration approach was a direct way to evaluate data quality. This approach should be applicable to other multiisotope elements