Fast and accurate isotope dilution mass spectrometry method for the assignment of trihalomethanes in water in an uncommon accuracy-based proficiency testing programme in Singapore

Abstract

ABSTRACT In the light of the instability of trihalomethanes (THMs) in water, a fast and high accuracy method based on headspace gas chromatography-isotope dilution mass spectrometry (HS-GC-IDMS) technique was developed. The optimised method was validated using fortified water samples. Recoveries between 96.1% and 108.8% with an intermediate precision between 3.9% and 5.7% were achieved. The method was applied in the value assignment of THMs in water in an accuracy-based proficiency testing (PT) programme organised by the Health Sciences Authority (HSA) Singapore. The assigned values obtained for chloroform, bromodichloromethane, dibromochloromethane and bromoform were 73.6 ± 9.3 µg/L, 49.1 ± 5.2 µg/L, 47.7 ± 4.4 µg/L and 70.8 ± 6.8 µg/L, respectively. The PT programme was especially valuable and uncommon as it allowed local water testing laboratories to have access to traceable reference values for the benchmarking of their technical competencies. The procedure undertaken by the Laboratory to prepare a batch of PT sample comprising the unstable analytes, conduct homogeneity and stability testing, as well as value assignment, are presented. The between-samples relative standard deviations (RSDs) obtained in a feasibility batch of THMs concentrate in methanol were improved by four to six times compared to those from the first feasibility batch of THMs in water. Based on z-scores, evaluated using the assigned reference value determined by HS-GC-IDMS, 85% of the participating laboratories in the PT programme achieved satisfactory results. The provision of metrological assigned values heightened the testing laboratories’ awareness on the importance of accuracy-based PT programmes for evaluation of true method biases, which is particularly valuable for analytes with regulated limits.