Determination of four odorous substances in water by purge and trap-gas chromatography-tandem mass spectrometry

Background With the events reporting on odors in drinking water, odorous substances in water have become a hot topic in water quality analysis. Due to the low concentration of the odor threshold and the complexity of the odor components in water, it is difficult to make accurate qualitative and quantitative analysis. So it is necessary to develop a highly sensitive and accurate qualitative and quantitative analysis method.

Objective To establish a method for simultaneous determination of four odorous substances, including dimethyl disulfide, dimethyl trisulfide, 2-methylisoborneol, and geosmin in water by purge and trap-gas chromatography-tandem mass spectrometry.

Methods A certain amount of water sample was stored in the sample vial of a purge and trapinstrument. Through nitrogen purging, the odorous substances in water were purged out and enriched in the trap. Subsequently, the odorous substances were rapidly released at high temperatures after heating the trap, and then carried by carrier gas into gas chromatograph. After temperature programming, the substances were separated by an Agilent DB-624 capillary chromatographic column (30 m×0.25 mm, 1.4 μm) and determined by tandem mass spectrometry in multiple reaction monitoring modes, with internal standard method for quantification. The current project optimized purge time, sodium chloride concentration in water sample, desorption temperature, desorption time, and split ratio during the experimental process. Under the optimized experimental conditions, the standard curve, detection limit, and quantification limit were validated. Recovery tests with spiking concentrations of 5.0, 10.0, 30.0, 80.0 ng·L⁻¹ and precision tests were conducted on water samples. Finally, the established method was applied to detect odorous substances in source water, finished water, and pipeline water in Deqing County of Huzhou City.

Result After the optimization, the purge time was 20 min, the desorption temperature was 280 ℃, the desorption time was 2 min, the split ratio was 10∶1, and no sodium chloride was added during the purge process. Under the optimized experimental conditions, the calibration curves for the four odorous substances showed an excellent linearity in the range of 1 to 100 ng·L⁻¹ (R>0.999), with 0.3 ng·L⁻¹ limit of detection and 1.0 ng·L⁻¹ limit of quantitation. The average recoveries were from 85.5% to 102.4% and relative standard deviations (RSD) from 1.6% to 5.2%. After applying this method to detect local source water, finished water, and pipeline water, it was found that the positive rates of 2-methylisoborneo, and geosmin were relatively high, while the positive rates of dimethyl disulfide and dimethyl trisulfide were relatively low. Only one sample of source water tested positive for dimethyl disulfide, and all samples were negative for dimethyl trisulfide.

Conclusion Combined with the superiority of purge and trap and tandem mass spectrometry, the method has the advantages of easy to perform, strong anti-interference ability, good accuracy and precision, which meet the limit requirements of the four odorous substances in the expanded indices and reference indices of Hygienic standards for drinking water (GB 5749-2022). It also provides technical support for water quality assessment and analysis of odorous substances.