A new separation procedure for Cu prior to stable isotope analysis by MC-ICP-MS Fiona Larner, * a Mark Rehk€ amper, ac Barry J. Coles, ac Katharina Kreissig, a Dominik J. Weiss, ac Barry Sampson, b Catherine Unsworth c and Stanislav Strekopytov c Received 22nd February 2011, Accepted 14th April 2011 DOI: 10.1039/c1ja10067j A novel ion exchange chromatography was developed for the separation of Cu from biological samples prior to stable isotope analyses. In contrast to previous methods, the new technique makes use of the different distribution coefficients of Cu(I) and Cu(II) to anion exchange resin and this helps to significantly improve the purity of the Cu separates obtained from biological samples, whilst maintaining crucial quantitative yields. Careful method validation confirmed that the procedure yields sufficiently pure Cu fractions after a single pass through the anion exchange columns, with a recovery of 100  2%. Subsequent isotopic analyses of the Cu by multi-collector inductively coupled plasma mass spectrometry, using admixed Ni for mass bias correction, produced accurate Cu stable isotope data with a reproducibility of 0.04& for pure standard solutions and of 0.15& for samples of biological origin. 1. Introduction The measurement of stable 65 Cu/ 63 Cu isotope ratios in geological, biological and anthropogenic samples is of wide interest for research in Earth, 1–6 environmental 7–11 and life sciences. 12–16 Such Cu isotope studies utilize the technique of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to obtain Cu isotope data with a precision of about 0.1 to 0.2& (2s). These measurements require that Cu is separated from the sample matrix and an admixed element (generally either Ni or Zn) is utilized for correction of the instrumental mass bias by external normalization. A double spike procedure 17–19 cannot be used for mass bias correction of Cu, as this element has only two stable isotopes ( 63 Cu and 65 Cu). Previous work has shown that the separation of Cu from complex sample matrices for isotopic analysis is best accom- plished by ion exchange techniques. Carefully optimized methods are required, however, to achieve both the essential quantitative yields and a high purity for the Cu separate. The first requirement follows from the observation of significant Cu isotope fractionation during elution from ion exchange resins 14,20 whilst the second is important to prevent spectral and non- spectral interferences that would be detrimental for the collection of accurate and precise isotopic data. At present, the most commonly used method for the separa- tion of Cu uses macroporous anion-exchange resin as Cu 2+ is moderately retained on such resin at higher HCl concentrations. For the separation of Cu from diverse natural samples, the samples are commonly dissolved in 7 M HCl (containing small amounts of H 2 O 2 to ensure that Cu is present as Cu 2+ ) for loading onto the resin columns. The same acid is then used to first remove major matrix elements and then purified Cu 2+ from the resin column. 21–24 This chromatographic separation is not perfect, however, and variable overlap between the elution of Cu and matrix elements is commonly observed, depending on sample size and type. 21 In order to obtain Cu fractions, which are pure enough for accurate stable isotope analyses, it is therefore often necessary to adopt slight modifications to the elution procedure for different sample matrices and/or to carry out a second column pass. 21,23 Application of these methods for the separation of Cu from complex biological samples appears to be particularly problem- atic. 21 This was confirmed by our own work, which showed that the application of these methods leads to a significant overlap of the Cu elution with matrix elements. As a result, Cu isotope analyses of biological samples are readily compromised by significant interferences at mass 63, due to relatively high amounts of Na producing 23 Na 40 Ar + in the plasma. 12,21,25,26 Based on our own experiences, it is often necessary to purify biological samples by at least two stages of ion exchange chromatography to obtain sufficient Cu for isotopic analysis. Such multi-stage procedures, however, are time consuming, suffer from higher blank and are more readily compromised by loss of Cu during elution. In this study, we have developed a novel ion exchange proce- dure for the separation of Cu from biological samples prior to stable isotope analyses. This procedure utilizes differences in the affinity of Cu + and Cu 2+ to anion exchange resins, to enable a simple yet efficient separation of Cu from biological samples. a Department of Earth Science & Engineering, Imperial College London, London, SW7 2AZ, UK. E-mail: fiona.larner04@imperial.ac.uk b Department of Clinical Biochemistry, Imperial College Hospitals NHS Trust, Charing Cross Hospital, London, W6 8RF, UK c Department of Mineralogy, Natural History Museum, London, SW7 5BD, UK This journal is ª The Royal Society of Chemistry 2011 J. Anal. At. Spectrom., 2011, 26, 1627–1632 | 1627 Dynamic Article Links C < JAAS Cite this: J. Anal. At. Spectrom., 2011, 26, 1627 www.rsc.org/jaas PAPER