Rapid and sensitive on-line monitoring 6 different kinds of volatile organic compounds in aqueous samples by spray inlet proton transfer reaction mass spectrometry (SI-PTR-MS)

- A novel SI-PTR-MS was presented for on-line monitoring volatile organic compounds (VOCs) in aqueous samples.
- The response time of the SI-PTR-MS was much shorter than other methods.
- Six different kinds of VOCs were simultaneously detected with fast response and low limits of detection (LODs).

Rapid and sensitive monitoring of volatile organic compounds (VOCs) in aqueous samples is very important to human health and environmental protection. In this study, an on-line spray inlet proton transfer reaction mass spectrometry (SI-PTR-MS) method was developed for the rapid and sensitive monitoring of 6 different kinds of VOCs, namely acetonitrile, acetaldehyde, ethanol, acetone, aether, and methylbenzene, in aqueous samples. The response time, limit of detection (LOD), and repeatability of the SI-PTR-MS system were evaluated. The response of the SI-PTR-MS was quite rapid with response times of 31-88 s. The LODs for all these VOCs were below 10 μg/L. The LOD of methylbenzene was 0.9 μg/L, much lower than the maximum contaminant level (MCL) in drinking water. The repeatability of this method was evaluated with 5 replicate determinations. The relative standard deviations (RSDs) were in the range of 0.8-3.1%, indicating good repeatability. To evaluate the matrix effects, the SI-PTR-MS system was employed for on-line monitoring of these 6 VOCs in different aqueous matrices, including lake water, tap water, and waste water. The relative recoveries were in the range of 94.6-106.0% for the lake water, 96.3-105.6% for the tap water, and 95.6-102.9% for the waste water. The results indicate that the SI-PTR-MS method has important application values in the rapid and sensitive monitoring of VOCs in these aqueous samples. In addition, the effect of salt concentration on the extracting efficiency was evaluated. The results showed that the LOD of the SI-PTR-MS could be further decreased by changing the salt concentration.

As shown in the above figure, the aqueous sample was sprayed as small droplets. Because these small droplets have much larger contact area with the air than the aqueous sample, the volatile organic compounds (VOCs) in the sample evaporated quickly to the air around the droplets. And then, the air containing VOCs was delivered to the PTR-MS. If the proton affinity (PA) values of these VOCs were higher than that of H2O (PA = 691.0 kJ/mol), they can undergo proton transfer reaction with H3O+ as shown in the figure. Lastly, the protonated VOCs was detected by the mass spectrometry.277