Activated carbon as a medium for sampling and analysis of volatile organic compounds in thermal desorption-gas chromatography-mass spectrometry

Abstract

In this study, the possible use of activated carbon (AC) as sorbent media for sampling and thermal desorption (TD) analysis of volatile organic compounds (VOCs: benzene, toluene, m-xylene, and styrene (BTXS)) was investigated using TD-gas chromatography (GC)/mass spectrometry (MS). In all experiments for assessment of AC, BTXS standard gas was prepared and analyzed via a series of calibration experiments. The desorption conditions of AC were determined such as 300 °C for 5 min at 50 mL min-1 by computing the recovery of each VOC in reference to common three-bed (Carbopack (C+B+X)) tube sampler. Under such conditions, quality assurance (QA) data for BTXS were obtained with good linearity (R2>0.99) and reproducibility (RSE of 2.38 ±0.21%). The thermogravimetric analysis (TGA) analysis showed that AC with minimal weight loss was stable up to 50 adsorption/desorption cycles. Also, the calibration response factor (RF) value during the repeated measurements was reliable within mean 0.5±0.09% range. The storability of AC, when tested over 1, 3, 7, and 14 days, was stable up to 3 days period relative to CBX tube (1 week). This result corresponds with the subject of recommendation ASTM d3686 for tube storage period (within 5 days). Finally, concurrent analysis of BTXS in lab air by AC and CBX sampler showed great compatibility (e.g., errors below 5%). As such, the results of this study support that the AC can be used as suitable media for sampling VOCs in air. For analysis of volatile organic compounds in the atmosphere, gaseous standards and liquid standards can be selectively used. For this end, the recovery percentage (RP) of RF value for liquid phase VOC standard in AC was assessed in comparison with CBX tube in TD-GC-MS system. Three groups of liquid standards were prepared: (1) aromatic compounds (BTXS, L1), (2) four others ((methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), butyl acetate (BuAc), and isobutyl alcohol (i-BuAl), L2), and (3) mixture 8 VOC (L3). In case of aromatic compounds, no significant difference between AC and CBX was observed in liquid phase. Compared with RF value of gas phase standard in CBX tube method, the experimental bias between two phases was very low in BTX below 5% while S was not (11.3%). Although the RDs of BTX was observed below 10% in AC tube method, relatively higher than those of CBX tube. For other 4 compounds which have –OH and C=O, stable RP (e.g. above 95%) of 3 compounds was observed in AC method except for MEK. However, RP of RF value for all compounds are stable over 90% compared with CBX in mixture of 8 target compounds. This is suggested that recovery could be controlled by boiling point, vapor pressure, and structure characteristics of some compounds. As such, quantitative analysis of VOC using AC tube through TD-GC-MS was carried out to confirm the possibility of expanding potential and target components as an adsorbent medium at low cost. However, it is necessary to further investigate the bias that arises from the difference phase of other components when it comes to using AC tube.