Isotope ratio measurements using gas chromatography inductively coupled plasma mass spectrometry for the assessment of organolead sources

Abstract

The measurement of isotope ratios (within individual species), for a range of organolead compounds, was carried out by coupling a capillary gas chromatograph (HP 6890) to an inductively coupled plasma mass spectrometer (HP 4500). Optimum conditions for the separation and detection of the derivatised lead organometallic compounds were studied focusing on the precise and accurate measurement of isotope ratios on each chromatographic peak. Three lead isotopes were monitored, 206, 207 and 208 using 66 ms integration time per isotope. Under optimum conditions adequate chromatographic peak profiles could be measured. Based on peak area ratios, isotope ratios were determined for five butylated lead compounds (trimethyllead, dimethyllead, triethyllead, diethyllead and inorganic lead) and mixed methyl-ethyl tetraalkyllead compounds from leaded gasoline. Mass bias was corrected using both ethylated and butylated NIST 981 isotopic standard (Common Lead) and a certified enriched 204Pb solution. No isotope fractionation effects were observed during derivatisation of the different organometallic compounds. Applications of this methodology include the differentiation between Pb sources of the different standards used, the measurement of Pb isotope ratios in the organolead compounds present in airborne particulate matter of Oviedo to ascertain their origin and the comparison with isotope ratios in the BCR CRM 605 urban dust reference material. Significant differences between lead isotope ratios in the different species of organolead compounds and in total lead (after acid digestion of the samples) were observed. These differences can be used to trace back sources of lead contamination, by resorting to the speciation data, in cases where the isotope ratios of total lead in the samples are inconclusive in identifying Pb contamination sources.