Accurate measurement of stable isotopes of lithium by inductively coupled plasma mass spectrometry

A method based on inductively coupled plasma mass spectrometry (ICP-MS), for accurate isotopic analysis of lithium in human blood and urine is reported. The method permits the precise and accurate measurement of isotope ratios for 6Li/7Li to within ±1%(RSD) for the lithium concentration range typical of urine and blood from patients on lithium therapy. The measured isotope ratio is independent of typical drifts in the ion beam intensities. The instrument response to increasing isotope ratio is highly linear for at least an order of magnitude above the natural ratio. Complete isotope ratio data acquisition for a measurement precision of 1%(RSD) can be carried out in less than 7 min per sample.Applying the method of in vitro isotope dilution with 6Li as the spike, the over-all accuracy of the method is better than 3% for any single sample, with a mean deviation in 12 measurements of better than 1%.