Simultaneous determination of 10 N-nitrosamines in drinking water by tandem solid phase extraction-gas chromatography tandem mass spectrometry

Background N-nitrosamines, a group of by-products of drinking water disinfection, have strong cytotoxicity to mammals. N-nitrosamines in drinking water are at the ng·L⁻¹ level, and its accurate qualitative and quantitative analysis is difficult, so it is necessary to develop a sensitive and accurate method to determine N-nitrosamines in drinking water.

Objective To establish a solid phase extraction-gas chromatography tandem mass spectrometry (GC-MS/MS) method for simultaneous determination of 10 kinds of N-nitrosamines in drinking water. To apply the established method to determine the levels of 10 kinds of N-nitrosamines in drinking water in Nanjing, and to understand the pollution status.

Methods Coconut charcoal solid phase extraction (SPE) cartridge and HLB Pro SPE cartridge were compared for the extraction efficiency of 10 N-nitrosamines in drinking water. A coconut charcoal SPE cartridge and a HLB Pro SPE cartridge were concatenated using a SPE connector, and then formed two combinations: coconut charcoal (top)-HLB Pro (bottom) and HLB Pro (top)-coconut charcoal (bottom), to extract the spiked samples, and combined with direct and independent elution ways to obtain the best extraction efficiency. From November to December 2021, 9 raw water, 10 finished water, and 7 tap water samples were collected from 9 municipal water supply units in Nanjing with 1 L brown glass sampling bottles. An 1.0 L drinking water sample was added with the isotope internal standard to prepare a test sample containing an isotope internal standard concentration of 25 ng·L⁻¹. The automatic SPE instrument loaded all the 1.0 L drinking water samples to the tandem SPE cartridge of the HLB Pro (top)-coconut charcoal (bottom) at the rate of 15 mL·min⁻¹. After extraction, the HLB Pro SPE cartridge and coconut charcoal SPE cartridge were transferred to the solid phase extraction vacuum device and eluted with 10 mL of dichloromethane respectively, then the dichloromethane eluents were combined, and concentrated to about 1.0 mL by nitrogen blowing after a small amount of the upper aqueous phase was removed. The concentrated solution was detected by GC-MS/MS and quantified by isotope internal standard method.

Results The comparison of sample spike recovery experiments showed that coconut charcoal solid phase extraction (SPE) cartridge and HLB Pro SPE cartridge presented highcomplementarity for the extraction efficiency of 10 N-nitrosamines in drinking water. Using HLB Pro (top)-coconut charcoal (bottom), independent elution, and combined with eluents, the optimal extraction efficiency was obtained. Under these conditions, by GC MS/MS, the 10 N-nitrosamines showed a good linear relationship within the range of 2–50 ng·L⁻¹, the correlation coefficients were all greater than 0.9996, the method detection limit was 0.149–0.211 ng·L⁻¹, and the limit of quantification was 0.596–0.844 ng·L⁻¹. At the spiked concentrations of 5.0, 15, and 30 ng·L⁻¹, the average recoveries of the 10 kinds of N-nitrosamines were 88.0%–104.8%, and the relative standard deviations were 1.22%–4.87%. When applying the method to determine the concentrations of the 10 N-nitrosamines in Nanjing drinking water, the results showed that the 10 N-nitrosamines were positive in different degrees in raw water, finish water, and terminal water, the detection rates were 0%–100%, and the concentrations were ND–27.6 ng·L⁻¹.

Conclusion This tandem solid phase extraction-gas chromatography tandem mass spectrometry method can achieve simultaneous determination of a variety of N-nitrosamines in drinking water with high sensitivity and high throughput.