| dc.description.abstract | Atmospheric volatile organic compounds (VOCs) are complex in composition and variable in abundance. To accurately assess their presence is qualitatively and quantitatively challenging. The reliable calibration of VOC concentrations often depends on the availability of robust standard gas mixtures. They are either analyzed by inserting in between sample aliquots in a continuous measurement process called external standardization, or blended into each sample aliquot called internal standardization. For both types of standardization, the gas mixtures must show high chemical stability overtime. For polar or reactive compounds, dynamic permeation method is preferred to static high pressure cylinders as the stable standard source. However, both types of standards are costly and the long-term stability cannot be ensured. As a result, this study attempted to develop a low-cost and simple external calibration method with self-made permeation tubes and a delivering system. Internal standardization employing a group of airborne compounds persistently existing in the atmosphere – chlorofluorocarbons (CFCs) – was also adopted for validating permeation tubes.
In this study, we set up a heart-cut gas chromatographic system that can measure VOCs and CFCs simultaneously, by flame ionization detection (FID) and electron capture detection (ECD), respectively. The homogeneity of CCl2FCClF2 (CFC-113) in this study was used as an “intrinsic” internal standard to facilitate the validation of our self-made permeation tubes used for external standardization.
The structure of this investigation was divided into three stages. At the first stage, we used atmospheric CFC-113 to validate the stability of the self-made permeation tubes. Both of them were smaller than 4% for the relative standard deviation (RSD), and the relative variations were less than 11%. At the second stage, both CFC-113 (internal standard) and a commercial VOC standard mixture (external standard) were used to assess the stability of the self-made permeation tubes. The results showed that the RSD for CFC-113, the VOC standard mixture, and permeation tubes was less than 1.5%, and the relative variations were less than 5%. Their comparable stability thus validated the self-made permeation method as an external standard source. At the last stage, selected VOCs in outdoor air were targeted continuously, for which the permeation method was the only source of external standardization for the two purposes of concentration calibration and system’s drift check. The RSD for permeation tubes and CFC-113 was less than 2.5%, and the relative variations were less than 6%. In contrast, the variability of VOCs in outdoor air was far more dramatic than that of CFC-113 and permeation tubes. For example, pentane in outdoor air varied between 0.1 and 4 ppbv, with an RSD of 82.0% and relative variation of 704%.
Our results suggested that the self-made permeation tubes can be easily made and adopted as a stable calibration source. The light weight, simplicity, and low cost make this method highly desirable in field and lab applications. Future studies will include more polar and reactive VOCs to further broaden its applicability.
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