Objective To evaluate the uncertainty in the determination of trihalomethanes (THMs) including chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and tribromomethane (TBM) in drinking water using headspace capillary gas chromatography (HS-GC) to assess the quality of measurements.

Methods An automated headspace sampler was used for sample injection, and an electron capture detector (ECD) was employed to measure THMs levels in water. A measurement model was established to quantify the uncertainty of components, ultimately assessing the uncertainty of HS-GC in determining THMs in drinking water.

Results According to the sources of uncertainty, relative standard uncertainties (RSUs) from standard curve fitting of TCM, BDCM, DBCM and TBM were 0.067, 0.051, 0.051, and 0.056, respectively; RSUs from repeated sample measurements of TCM, BDCM, DBCM and TBM were 0.012, 0.012, 0.009, and 0.010, respectively; RSUs from sampling, reference standards and standard curve preparation of THMs were all 9.6×10^-4, 0.015, and 0.006, respectively. The contents of TCM, BDCM, DBCM and TBM determined by HS-GC were (1.44±0.20), (2.50±0.27), (2.61±0.28) and (2.69±0.32)μg/L, respectively.

Conclusion The primary source of uncertainty in the determination of THMs in drinking water HS-GC is the residual error from standard curve fitting. Standardized operations should be strictly followed, and the number of calibration curve measurements should be increased to reduce uncertainty in curve fitting.