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Journal of the Korean Society for Environmental Analysis 2014;17(3):173-181.
Comparison of Mass Bias Correction Methods for Lead and Cadmium Stable Isotope Analysis by Multi-Collector ICP-MS
다중 검출 유도결합 플라즈마 질량분석기를 이용한 납 및 카드뮴안정동위원소비 분석의 질량 편향 보정방법 비교
김지영1, 김명진1, 유은진1, 이강현1, 이경석2, 이원석1, 최종우1, 권오상1
1국립환경과학원
2한국표준과학연구원
Abstract
To obtain precise and accurate isotope ratios with a multi-collector ICP-MS, three methods such as interelementalinternal calibration, sample-standard bracketing (SSB), and log-log linear regression correctionmodel were applied to isotope ratio measurement of lead (Pb) and cadmium (Cd) for the correction of instrumentalmass discrimination. Certified reference materials including NIST SRM 981 (Pb), NIST SRM 3108(Cd), and BAM I012 (Cd) were used as reference materials for the isotope ratios of lead and cadmium to comparethe corrected results obtained from the respective correction models. The accuracies of lead isotope ratiomeasurements were in the range of -0.002 (207/206Pb) to -0.035 (208/204Pb) % when the inter-elementalinternal calibration applied. The same range of -0.002 (207/206Pb) to -0.035 (208/204Pb) % was also found withthe SSB method. The log-log linear regression correction model, on the other hand, showed a differentrange of -2.160 (207/206Pb) to 0.219 (208/204Pb) %. The precisions of lead isotope ratios with these three correctionmethods ranged from 0.027 (206/204Pb) to 0.127 (208/204Pb), 0.025 (206/204Pb) to 0.124 (208/204Pb), and1.265 (207/204Pb) to 1.367 (207/206Pb), respectively. Therefore, both of the inter-elemental internal calibrationand the SSB method were selected as good methods for mass bias correction of lead isotope ratios. Theε114/110Cd values obtained from the application of the three different correction models were -14.5±0.7 (Agas an internal standard), -13.0±2.1 (SSB), and -15.2±1.8 (log-log linear regression correction model),respectively. For ε114/110Cd values, the inter-elemental internal calibration showed the best precisionamong the three correction methods. Isobaric interferences with cadmium were also corrected when correctioncoefficients were applied to each isotope of cadmium. The difference of correction coefficientsbetween the isotope ratios with natural abundances and the empirical ratios in the isobaric interferencesreached up to 11%. The heavier isobaric interferences (106, 108, 110Pd vs. 105Pd) were underestimated in theapplication of the natural ratios, while the lighter isobaric interferences (112,114,116Sn vs. 118Sn and113In vs. 115In) were overestimated. In conclusion, the inter-elemental internal calibration and the SSB method canproperly correct the isotope ratios of interest in the isotope ratio measurement using the MC-ICP-MS. Further research on the isotope ratio analysis of environmental samples should follow to improve precisionand accuracy.
Key Words: Mass bias correction, Multi-collector ICP-MS, Stable isotopic ratio
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