Isotope ratio measurements
The relative isotopic abundances of a variety of elements vary in nature due to kinetic and equilibrium fractionation processes. This variability has been widely exploited in many scientific fields, including geochemistry, climatology, hydrology, ecology and forensics. We have recently been studying two elements which exhibit natural isotopic variability: sulphur and lead.
Sulphur
Relative differences in sulphur isotopic abundances mainly result from the reduction of sulphate ions by anaerobic bacteria. An accurate and precise method for the determination of δ34S measurements by multi-collector inductively coupled plasma mass spectrometry has recently been developed. Resolution of the measured δ34S values was better than 1 ‰ after consideration of all uncertainty components.
The technique was evaluated for practical applications by measurement of δ34S for a range of mineral waters by pneumatic nebulisation sample introduction, and the analysis of genuine and counterfeit pharmaceuticals using both laser ablation sample introduction and liquid chromatography. For the former two cases, polyatomic interferences were resolved by operating the MC-ICP-MS in medium resolution, whilst for the chromatographic analyses polyatomic interferences were minimised by the use of a membrane desolvator, allowing the instrument to be operated at a resolution of 400. Using this method δ34S values were obtained for a range of bottled waters on sale in the UK, ranging from -12 to +22 ‰ relative to the VCDT scale. Significant differences were also observed in δ34S values for genuine and counterfeit samples of Viagra tablets which ranged from -10 to +8 ‰ relative to genuine Viagra.
MC-ICP-MS δ34S values for a suite of mineral waters.
Lead
Lead is a global pollutant due to a variety of historical and contemporary anthropogenic emissions. As differences in lead isotope ratios are small, extremely high precision measurements are necessary to differentiate between samples and sources. One such example of our recent work in this field is the use of the 207Pb: 206Pb ratio as a cross check on 210Pb dating for the top 40 cm of a salt marsh core from Chezzetcook , Canada . This approach identifies known changes in the composition of atmospheric lead fallout, such as the onset of industrialisation, the post-war increase of car use and the introduction of unleaded petrol. The 207Pb: 206Pb profile obtained by MC-ICP-MS shows a slow increase in the ratio of 207Pb: 206Pb from ca. 1850 to the early 1900s, marking increased use of coal in the early industrial period. Levelling off of the 207Pb: 206Pb ratios was not observed with depth, indicating that preindustrial sediments were not sampled. A second increase, starting in the 1930s (22 cm depth), is the result of expansion of car use and the increased consumption of leaded petrol. The introduction of unleaded petrol has resulted in a fall in the 207Pb: 206Pb ratio since 1976 (top 7 cm of the sequence).
In conjunction with other dating techniques for the lower part of the core, a high-resolution record of sea-level change spanning the past 1000 years has been derived. Between AD 1000 and AD 1800, relative sea level rose at a mean rate of 17 cm per century. Between AD 1900 and AD 1920, sea-level rise accelerated to the modern mean rate of 3.2 mm/yr. This acceleration corresponds in time with global temperature rise and may therefore be associated with recent global warming.
Variation in the 207Pb: 206Pb isotope ratio in a salt marsh core
Publications
Clough, R., Evans, P., Catterick, T. and Evans, E.H. (2006). Delta 34S Measurements of sulfur by multicollector inductively coupled plasma mass spectrometry. Analytical Chemistry 78, 6126-6132.
Gehrels, W.R., Kirby, J.R., Prokoph, A., Newnham, R.M., Achterberg, E.P., Evans, H., Black, S. and Scott, D.B. (2005). Onset of recent rapid sea-level rise in the western Atlantic Ocean. Quaternary Science Reviews 24, 2083-2100.