Accurate measurement of stable isotopes of magnesium in biological materials with inductively coupled plasma mass spectrometry

Abstract

A method is described for the accurate isotopic determination of magnesium (²⁴Mg, ²⁵Mg, ²⁶Mg) in biological materials, which is based on inductively coupled plasma mass spectrometry (ICP-MS). The analytical performance of the method was examined with respect to the requirements of stable isotope tracer studies. When applied to the measurement of base-line isotope ratios (MR_a/b) in rat tissue the following results were obtained (data are given in terms of MR_25/24 and MR_26/24± 1RSD, n= 4 or 5): standard solution of Mg (0.05 µg ml^–1), 0.1394 ± 0.6, 0.1697 ± 0.8; bone, 0.1401 ± 0.5, 0.1706 ± 0.8; brain, 0.1404 ± 0.2, 0.1720 ± 0.3; kidney, 0.1392 ± 0.5, 0.1702 ± 0.6; liver, 0.1388 ± 0.4, 0.1696 ± 0.5; muscle, 0.1396 ± 0.2, 0.1716 ± 0.4; plasma, 0.1385 ± 0.3, 0.1691 ± 0.3; red cells, 0.1383 ± 0.1, 0.1694 ± 0.2; and urine, 0.1402 ± 0.1, 0.1721 ± 0.3. The measurement precision for replicate analyses of each matrix was in the range 0.1–1.0% and the mean value of the isotope ratio for different matrices agreed with the corresponding ratio for the standard solution to within 1.5%. When measured over a 10-h period, the isotope ratios appeared to be independent of the observed drifts in the ion beam intensities. The instrument blank contribution to the ion beam intensities was about 0.1% of the values obtained for a solution containing 0.05 µg ml^–1 of natural Mg. The absolute detection limit for Mg (based on the experimental standard deviation of blanks run over 10 h) was <2 ng for all three stable isotopes. The ICP-MS phase of the analysis was capable of making 50–150 separate measurements of both ratios in a 10-h period, depending on the desired level of measurement precision within the range 0.1–1.0%. A complete analytical scheme for the accurate determination of the three stable isotopes of Mg, which is based on precipitation with ammonium phosphate, is described. The accuracy of the method was tested using the standard reference materials Bovine Liver (NBS 1577a, 600 ± 15 µg g^–1 of Mg) and Animal Bone (IAEA H-5, 3550 ± 90 µg g^–1). The proposed method provided the following data (µg g^–1): SRM 1577a, 617 ± 4 and IAEA H-5, 3585 ± 16. For other biological matrices of interest, the accuracy of the method was compared with atomic absorption spectrometry. The complete analytical procedure, up to the point of mass spectrometric measurement, can be performed on about 20 samples per working day.