Achieving traceable chemical measurements: inter-laboratory evaluation of a simplified technique for isotope dilution mass spectrometry (IDMS). Part 1: Methodology for high accuracy analysis of trace metals

A high accuracy measurement procedure developed and validated at the Laboratory of the Government Chemist (LGC) has been transferred to a number of expert United Kingdom laboratories and their experience in applying the technique has been evaluated by inter-laboratory comparisons. It is an approximate matching calibration procedure for analysis of trace metals using isotope dilution mass spectrometry (IDMS). Use of a calibration blend and a sample blend having closely matched isotope amount ratios reduces systematic errors. Four comparisons were carried out, each evaluating an increasingly difficult aspect of the methodology. The final study required determination of trace levels of cadmium in an artificial food matrix acid digest containing tin, using both external calibration and approximate matching IDMS. The results of the trials clearly showed the superior accuracy of the IDMS approximate matching procedure; a result within 1% of the target reference value could be achieved. These trials and a training workshop enabled LGC to transfer its methodology to other expert laboratories, helping to improve the United Kingdom delivery of traceable reference measurements.