Abstract
The need for inter-laboratory comparability is crucial to facilitate the globalisation of scientific networks and the development of international databases to support scientific and criminal investigations. This article considers what lessons can be learned from a series of inter-laboratory comparison exercises organised by the Forensic Isotope Ratio Mass Spectrometry (FIRMS) network in terms of reference materials (RMs), the management of data quality, and technical limitations. The results showed that within-laboratory precision (repeatability) was generally good but between-laboratory accuracy (reproducibility) called for improvements. This review considers how stable isotope laboratories can establish a system of quality control (QC) and quality assurance (QA), emphasising issues of repeatability and reproducibility. For results to be comparable between laboratories, measurements must be traceable to the international δ-scales and, because isotope ratio measurements are reported relative to standards, a key aspect is the correct selection, calibration, and use of international and in-house RMs. The authors identify four principles which promote good laboratory practice. The principle of identical treatment by which samples and RMs are processed in an identical manner and which incorporates three further principles; the principle of identical correction (by which necessary corrections are identified and evenly applied), the principle of identical scaling (by which data are shifted and stretched to the international δ-scales), and the principle of error detection by which QC and QA results are monitored and acted upon. To achieve both good repeatability and good reproducibility it is essential to obtain RMs with internationally agreed δ-values. These RMs will act as the basis for QC and can be used to calibrate further in-house QC RMs tailored to the activities of specific laboratories. In-house QA standards must also be developed to ensure that QC-based calibrations and corrections lead to accurate results for samples. The δ-values assigned to RMs must be recorded and reported with all data. Reference materials must be used to determine what corrections are necessary for measured data. Each analytical sequence of samples must include both QC and QA materials which are subject to identical treatment during measurement and data processing. Results for these materials must be plotted, monitored, and acted upon. Periodically international RMs should be analysed as an in-house proficiency test to demonstrate results are accurate.
Notes
A critical review of the use of these terms is presented by Thompson [1]
The form of this equation follows SI guidelines [2]. Historically, the value was multiplied by 1000 to give values in permil or by 106 to give values in per meg (ppm)
Also known as high temperature conversion, high temperature pyrolysis, and carbon reduction
Analogous problems have been reported when analysing reduced forms of nitrogen by TC/EA [11]
The difference between the VSMOW and VPDB scales is not additive. The conversion determined by Coplen [14] is widely accepted but authors should always state when this formula has been applied (δ 18OVSMOW = 1.03091 × δ 18OVPDB + 30.9)
At the time of ILC#3, the IAEA assigned the values δ 13C = −10.43 ‰ and δ 15N = +0.4 ‰
The δ-value and uncertainty assigned to many of the international RMs has been re-assessed and reassigned over time and it is important to check these periodically. The most reliable source of information is the CIAAW website: http://www.ciaaw.org
Because isotopic differences can be measured most precisely when these differences are small, the δ-value of the working gas should be within the range of isotopic compositions to be measured
With the exception of hydrogen, RMs spanning a range of approximately 50 ‰ will encompass most of the natural variation observed
This is not the same as linearity correction, which compensates for changes in δ-value in response to variations in sample size
This may not be true for elements in extreme oxidation states, specifically nitrates
LIMS for isotopic measurements can be obtained from http://isotopes.usgs.gov/research/topics/lims.html
Peak integration settings will have a profound effect on the data and must be applied uniformly
The δ-values measured for very small peaks can vary unpredictably, because of integration issues, but acceptable measurements of peak area are still possible
The introduction of a second anchor point for the VPDB scale in 2006 [15] resulted in the reassignment of many of the δ 13C values for international RMs so it is important to report the values assigned together with any analytical data
Higher temperatures and/or chemical additives, for example AgCl, have been proposed as remedies
Drying over phosphorus pentoxide, at elevated temperature and under vacuum for several days, is usually recommended [13]
Although it is possible to perform measurements on SO+ this is generally considered less sensitive and less reliable because of problems with blanks and interference
Oxygen isotope corrections for δ 34S are theoretically possible when both SO+ and SO2 + are measured from the same sample [46, 47]. These general formulae apply [36]:
$$ {\delta^{18}}\mathrm{O}=24.02\times {\delta^{66}}-23.024\times {\delta^{50 }}\,\mathrm{and}\,{\delta^{34}}\mathrm{S}=1.0908\times {\delta^{66}}-0.0908 \times {\delta^{18}}\mathrm{O} $$The latter formula also gives the conventional way of calculating δ 34S from δ 66 values under the assumption that samples and standards are prepared with the same oxygen isotope composition, so that δ 18Ο is zero and can be ignored
If the copper is omitted higher temperatures can be used to speed equilibration
References
Thompson M (2012) Precision in chemical analysis: a critical survey of uses and abuses. Anal Meth 4:1598–1611. doi:10.1039/c2ay25083g
Coplen TB (2011) Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results. Rapid Commun Mass Spectrom 25(17):2538–2560. doi:10.1002/rcm.5129
Brand WA (2011) New reporting guidelines for stable isotopes – an announcement to isotope users. Isotopes Environ Health Stud 47(4):535–536. doi:10.1080/10256016.2011.645702
Coplen TB, Qi H (2012) USGS42 and USGS43: Human-hair stable hydrogen and oxygen isotopic reference materials and analytical methods for forensic science and implications for published measurement results. Forensic Sci Int 214(1–3):135–141. doi:10.1016/j.forsciint.2011.07.035
Schimmelmann A, Albertino A, Sauer PE, Qi H, Molinie R, Mesnard F (2009) Nicotine, acetanilide and urea multi-level 2H, 13C and 15N abundance reference materials for continuous-flow isotope ratio mass spectrometry. Rapid Commun Mass Spectrom 23(22):3513–3521. doi:10.1002/rcm.4277
Hagopian WM, Jahren AH (2012) Elimination of nitrogen interference during online oxygen isotope analysis of nitrogen-doped organics using the “NiCat” nickel reduction system. Rapid Commun Mass Spectrom 26(16):1776–1782. doi:10.1002/rcm.6285
Qi H, Coplen TB, Wassenaar LI (2011) Improved online δ18O measurements of nitrogen- and sulfur-bearing organic materials and a proposed analytical protocol. Rapid Commun Mass Spectrom 25(14):2049–2058. doi:10.1002/rcm.5088
Coplen TA, Qi H (2009) Quality assurance and quality control in light stable isotope laboratories: A case study of Rio Grande, Texas, water samples. Isotopes Environ Health Stud 45(2):126–134. doi:10.1080/10256010902871952
Carter JF, Hill JC, Doyle S, Lock C (2009) Results of four inter-laboratory comparisons provided by the Forensic Isotope Ratio Mass Spectrometry (FIRMS) network. Sci Justice 49(2):127–137. doi:10.1016/j.scijus.2008.12.002
Carter JF, Barwick VJ (eds) (2012) Good practice guide for isotope ratio mass spectrometry. LGC/FIRMS, ISBN 978-0-948926-31-0
Kornexl BE, Gehre M, Höfling R, Werner RA (1999) On-line δ 18O measurement of organic and inorganic substances. Rapid Commun Mass Spectrom 13:1685–1693. doi:10.1002/(SICI)1097-0231(19990830)13:16<1685::AID-RCM6991>3.0.CO;2-9
David GE, Coxon A, Frewa RD, Hayman AR (2010) Isotope fractionation during precipitation of methamphetamine HCl and discrimination of seized forensic samples. Forensic Sci Int 200(1–3):123–129. doi:j.forsciint.2010.03.043
Qi H, Coplen TB (2011) Investigation of preparation techniques for δ2H analysis of keratin materials and a proposed analytical protocol. Rapid Commun Mass Spectrom 25(15):2209–2222. doi:10.1002/rcm.5095
Coplen TB, Kendall C, Hopple JA (1983) Comparison of stable isotope reference samples. Nature 302(5905):236–238. doi:10.1038/302236a0
Coplen TB, Brand WA, Gehre M, Gröning M, Meijer HAJ, Toman B, Verkouteren RM (2006) New guidelines for δ13 C measurements. Anal Chem 78(7):2439–2441. doi:10.1021/ac052027c
Coplen TA (1994) Reporting of stable hydrogen, carbon and oxygen isotopic abundances (Technical Report). Pure Appl Chem 66(2):273–275
Werner RA, Brand WA (2001) Referencing strategies and techniques in stable isotope ratio analysis. Rapid Commun Mass Spectrom 15:501–519. doi:10.1002/rcm.258
Skrzypek G, Sadler R (2011) A strategy for selection of reference materials in stable oxygen isotope analyses of solid materials. Rapid Commun Mass Spectrom 25(11):1625–1630. doi:10.1002/rcm.5032
Skrzypek G, Sadler R, Paul D (2011) Error propagation in normalization of stable isotope data: a Monte Carlo analysis. Rapid Commun Mass Spectrom 24(18):2697–2705. doi:10.1002/rcm.4684
Porter TJ, Middlestead P (2012) On estimating the precision of stable isotope ratios in processed tree-rings. Dendrochronologia 30(3):239–242. doi:10.1016/j.dendro.2012.02.001
Coplen TB, Qi H (2010) Applying the silver-tube introduction method for thermal conversion elemental analyses and a new δ 2H value for NBS 22 oil. Rapid Commun Mass Spectrom 24(15):2269–2276. doi:10.1002/rcm.4638
Qi H, Gröning M, Coplen TB, Buck B, Mroczkowski SJ, Brand WA, Geilmann H, Gehre M (2010) Novel silver-tubing method for quantitative introduction of water into high-temperature conversion systems for stable hydrogen and oxygen isotopic measurements. Rapid Commun Mass Spectrom 24(13):1821–1827. doi:10.1002/rcm.4559
Brand WA, Coplen TB, Aerts-Bijma AT, Böhlke JK, Gehre M, Geilmann H, Gröning M, Jansen HG, Meijer HAJ, Mroczkowski J, Qi H, Soerge K, Stuart-Williams H, Weise SM, Werner RA (2009) Comprehensive inter-laboratory calibration of reference materials for δ18O versus VSMOW using various on-line high-temperature conversion techniques. Rapid Commun Mass Spectrom 2009(7):999–1019. doi:10.1002/rcm.3958
Wassenaar LI, Hobson KA (2003) Comparative equilibration and online technique for determination of non-exchangable hydrogen of keratins for use in animal migration studies. Isotopes Environ Health Stud 39(3):211–217. doi:10.1080=1025601031000096781
Landwehr JM, Meier-Augenstein W, Kemp HF (2011) A counter-intuitive approach to calculating non-exchangeable 2H isotopic composition of hair: treating the molar exchange fraction fE as a process-related rather than compound-specific variable. Rapid Commun Mass Spectrom 25(2):301–306. doi:10.1002/rcm.4854
Brand WA (2009) Maintaining high precision of isotope ratio analysis over extended periods of time. Isotopes Environ Health Stud 45(2):135–149. doi:10.1080/10256010902869097
Fry B (2007) Coupled N, C and S stable isotope measurements using a dual-column gas chromatography system. Rapid Commun Mass Spectrom 21(5):750–756. doi:10.1002/rcm.2892
Craig H (1957) Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide. Geochim Cosmochim Acta 12:133–149. doi:doi.org/10.1016/0016-7037(57)90024-8
Brand WA, Assonov SS, Coplen TA (2010) Correction for the 17O interference in δ13C measurements when analyzing CO2 with stable isotope mass spectrometry (IUPAC Technical Report). Pure Appl Chem 82(8):1719–1733
Santrock J, Studley SA, Hayes JM (1985) Isotopic analyses based on the mass spectrum of carbon dioxide. Anal Chem 57:1444–1448. doi:10.1021/ac00284a060
Kaiser J, Röckmann T (2008) Correction of mass spectrometric isotope ratio measurements for isobaric isotopologues of O2, CO, CO2, N2O and SO2. Rapid Commun Mass Spectrom 22(24):3997–4008. doi:10.1002/rcm.3821
Sessions AL, Burgoyne TW, Hayes JM (2001) Correction of H3 + contributions in hydrogen isotope ratio monitoring mass spectrometry. Anal Chem 73:192–199. doi:10.1021/ac000488m
Fry B, Brand W, Mersch FJ, Tholke K, Garritt R (1992) Automated analysis system for coupled d13C and d15N measurements. Anal Chem 64(3):288–291. doi:10.1021/ac00027a009
Polissar PJ, Fulton JM, Junium CK, Turich CC, Freeman KH (2009) Measurements of δ 13C and δ 15N isotopic composition of nanomolar quantities of C and N. Anal Chem 81(2):755–763. doi:10.1021/ac801370c
Ogawa NO, Nagata T, Kitazato H, Ohkouchi N (2010) Ultra-sensitive elemental analyzer/isotope ratio mass spectrometer for stable nitrogen and carbon isotope analyses. In: Ohkouchi N, Tayasu I (eds) Earth, life and isotopes. Kyoto University press, Kyoto, pp 339–354
Fry B (2006) Stable isotope ecology. Springer, New York. doi:10.1007/0-387-33745-8
Gröning M (2011) Improved water δ2H and δ18O calibration and calculation of measurement uncertainty using a simple software tool. Rapid Commun Mass Spectrom 25(19):2711–2720. doi:10.1002/rcm.5074
Paul D, Skrzypek G, Fórizs I (2007) Normalization of measured stable isotopic compositions to isotope reference scales – a review. Rapid Commun Mass Spectrom 21(18):3006–3014. doi:10.1002/rcm.3185
Thompson M, Wood R (1993) The international harmonized protocol for the proficiency testing of (chemical) analytical laboratories (Technical Report). Pure Appl Chem 65(9):2123–2144
Santrock J, Hayes JM (1987) Adaptation of the Unterzaucher procedure for determination of oxygen-18 in organic substances. Anal Chem 59(1):119–127. doi:10.1021/ac00128a025
Hunsinger GB, Stern LA (2012) Improved accuracy in high-temperature conversion elemental analyzer δ18O measurements of nitrogen-rich organics. Rapid Commun Mass Spectrom 26(5):554–562. doi:10.1002/rcm.6132
Gehre M, Geilmann H, Richter J, Werner RA, Brand WA (2004) Continuous flow 2H/1H and 18O/16O analysis of water samples with dual inlet precision. Rapid Commun Mass Spectrom 18(22):2650–2660. doi:10.1002/rcm.1672
Stricker CA, Rye RO, Johnson R, Rye RO, Johnson CA, Bern C (2006) An automated cryo-focusing approach for sulfur isotope analysis of organic and other low-level sulfur materials. In: The 5th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, Belfast, Ireland
Hansen T, Burmeister A, Sommer U (2009) Simultaneous δ 15N, δ 13C and δ 34S measurements of low biomass samples using a technically advanced high sensitivity elemental analyzer connected to an isotope ratio mass spectrometer. Rapid Commun Mass Spectrom 23:3387–3393. doi:10.1002/rcm.4267
Dugan G (1977) Automatic carbon, hydrogen, nitrogen, sulfur analyser chemistry of sulfur reactions. Anal Lett 10:639–657. doi:10.1080/00032717708059229
Fry B, Silva SR, Kendall C, Anderson RK (2002) Oxygen isotope corrections for online d34 S analysis. Rapid Commun Mass Spectrom 16:854–858. doi:10.1002/rcm.651
Holt BD, Engelkemeir AG (1970) Thermal decomposition of barium sulfate to sulfur dioxide for mass spectrometric analysis. Anal Chem 42(12):1451–1453. doi:10.1021/ac60294a032
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Published in the topical collection Isotope Ratio Measurements: New Developments and Applications with guest editors Klaus G. Heumann and Torsten C. Schmidt.
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Carter, J.F., Fry, B. Ensuring the reliability of stable isotope ratio data—beyond the principle of identical treatment. Anal Bioanal Chem 405, 2799–2814 (2013). https://doi.org/10.1007/s00216-012-6551-0
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DOI: https://doi.org/10.1007/s00216-012-6551-0