| dc.description.abstract | Hazardous air pollutants (HAPs) are extensively emitted into the ambient air due to industrial development, posing significant risks to human health. The most commonly used analytical method for toxic volatile organic compounds (VOCs) as a class of HAPs is the standard method of U.S. EPA TO-15. Taiwan′s Environmental Protection Administration (EPA) has adopted a similar standard method, i.e., NIEA A715.16B, based on the revision of TO-15. Both methods are offline types which involve sampling VOCs using treated stainless canisters that are brought back from the field to the laboratory for analysis. These offline techniques do not provide real-time data to reveal the instantaneous variation of analytes in ambient air.
In this study, we build upon previous research and utilize online thermal desorption gas chromatography/mass spectrometry (TD-GC-MS) technology. This method enables direct and continuous analysis at fixed locations, allowing direct near real-time analysis of numerous VOCs in the air. The online TD-GC-MS method greatly improves the drawbacks of offline methods in terms of sample preservation, data representativeness, and the.
From the past experience, when MS are used for online analysis, the ion source could degrade rapidly by as much as 80% in sensitivity in 4 days. Maintenance usually takes several hours, causing interruptions in continuous data collection. Observations have revealed that the surface of the ion source becomes contaminated with presumably oxides and carbon residues after a period of usage. Moisture and oxygen in the air are likely the cause for contamination, leading to rapid degradation.
To reduce the rate of contamination of the ion source, this study conducted tests on the optimization of MS parameters. By adjusting the emission current of the ion source, which reduces the intensity of ionization, the contamination rate can be effectively reduced. When reducing the emission current from 50 µA to 15 µA, the effective runtime of the ion source can significantly extend to 15 days or more, thereby greatly reducing the frequency of ion source maintenance.
By using the optimized parameters, the quality assurance/control (QA/QC) results are as follows: the relative standard deviation (RSD) of the calibration curves ranging from 0.226% to 7.204%, linearity (R2) ranging from 0.993 to 1.000, recoveries between 75.95% to 94.11%, the precision (RSD) results between 1.20% to 11.16%, and MDL for all species between below 0.1 ppb. All the QA/QC results complied with the requirements set by the NIEA A715.16B method.
With the optimized parameters, field monitoring was conducted for six months. The instrument′s stability and sensitivity trend were closely monitored using internal standards. Throughout the monitoring period, the instrument′s data completeness exceeded 80%. The average runtime of the ion source was 14 days, with the most extended single continuous runtime reaching up to 21 days. This study significantly prolonged the instrument′s runtime, reduced maintenance frequency, and minimized data loss caused by maintenance. These results have a positive impact on the online continuous monitoring method. | en_US |