Addition of thiourea and hydrochloric acid: Accurate nanogram level analysis of mercury in humic-rich natural waters by inductively coupled plasma mass spectrometry

An analytical method was developed for the direct determination of total mercury in natural waters at low ng L-1 level by inductively coupled plasma mass spectrometry (ICP-MS). The presented method overcomes previously observed problems relating to poor spike recoveries by adding 0.12% thiourea in addition to 3% HCl to all samples and standards. The sample preparation is fast and easy to perform by the developed method since it requires only the addition of HCl and thiourea to the water samples. A very low instrument detection limit (0.4 ng L-1) was obtained without time-consuming preconcentration procedures. The accuracy and precision of the developed method were found excellent by the analysis of a certified groundwater reference material (ERMCA615). The determined Hg concentration of 38.6 +/- 0.5 ng L-1 was within the 95% confidence interval of the certified concentration of 37 +/- 4 ng L-1. The analysis of natural water samples showed that total mercury levels ranged from concentrations lower than the method detection limit (2.0 ng L-1) to 10.9 ng L-1. Excellent recoveries of 96-108% for inorganic mercury (iHg) and 102-110% for methylmercury (MeHg) were obtained for spiked humic-rich natural water samples. To our knowledge, the developed method is the first ICP-MS method for the analysis of humic-rich natural water samples at ng L-1 concentrations without the need for hyphenated techniques or preconcentration procedures.