Determination of isotope enrichments of magnesium in microwave-digested biological samples by thermal ionization mass spectrometry using a direct loading technique.

The isotope ratios of magnesium were determined in isotopically normal and 26 Mg-enriched samples of human blood, blood plasma, urine and faeces and bovine muscle. The measurements were made with a magnetic sector, thermal ionization mass spectrometer (TIMS) equipped with a multiple ion collector system for simultaneous detection of the ion currents. The samples were decomposed using microwave digestion with HNO3 and HCI. Without further chemical treatment, the mineralized samples were deposited together with silica gel and boric acid on rhenium filaments, which served as thermal ionization source filaments. This method, called the direct loading technique (DLT), results in stable ion signals of the magnesium isotopes with isotope ratios indistinguishable from those of natural Mg standards within experimental error. Fractionation-corrected 26 Mg/24 Mg ratios of natural Mg standards were determined with a relative external precision of 0.02%. The magnesium recoveries for all of the analysed matrices were > or = 97%; 26 Mg was added to calibrated sample solutions to produce isotopic enrichments within a range typically appearing in samples of human tracer studies. Linear regression analysis of measured versus expected per 1000 (/1000) enrichments yields y = 0.998x + 0.79. The DLT described here is a simpler and quicker method than other methods reported hitherto. It has the advantage of avoiding magnesium separation and purification steps prior to TIMS analysis for all of the analysed biological samples and thus reduces contamination and guarantees optimum magnesium recovery. The reported method improves the applicability of stable isotopes of magnesium in human tracer studies.