Measurement of stable isotopes of bromine in biological fluids with inductively coupled plasma mass spectrometry

Abstract

A method is reported for the accurate measurement of the two stable isotopes of bromine in biological fluids of interest in human metabolic studies. The method is based on inductively coupled plasma mass spectrometry (ICP-MS). It is shown that the background ion beam intensities at m/z= 79 and 81 are typically in the range 70–335 and 600–7200 ions s^–1, respectively, when de-ionised water is aspirated into the plasma. The corresponding range for 1.0 µg ml^–1 of natural Br is 9700–18 500 ions s^–1 at m/z= 79. The detection limit (3[graphic omitted]) for Br is in the range 2–5 ng ml^–1. A method is given for automatic correction of the argon contribution at m/z= 81. Data are presented which show that the isotope ratio ⁸¹Br/⁷⁹Br can be measured routinely with a precision (relative standard deviation) of 1% or better. The measured ratio is independent of the Br concentration in the range 3–20 µg ml^–1. Linear regression equations are obtained for stable isotope calibration graphs over the range 0.997–5.322 (MIR_81/79). However, the slopes of these plots deviate considerably from the expected value of one. Two chemical separation schemes are described, Scheme I, based on cation exchange and Scheme II, based on distillation from acidified solutions. The former is applicable to plasma (and possibly saliva) samples whereas the latter is successful for urine. The presence of large amounts of sulphate produces significant enhancement of the ion intensity at m/z= 81 (due to ³²S¹⁶O₃ ¹H⁺). Distillation permits the required separation of Br from sulphate, whereas precipitation with Ba(NO₃)₂ does not appear to be satisfactory. Application of the method of standard additions and stable isotope dilution analysis to samples of urine from several subjects indicates that this method permits quantitative analysis of bromine to be carried out with a precision (and accuracy) of about 2%.