The analysis and monitoring of atmospheric volatile organic compounds via thermal desorption gas chromatography mass spectrometry
Wong, Gwendeline Kee Shien
Date of Issue2014
School of Physical and Mathematical Sciences
An analytical method has been established for the quantitative determination of 48 gaseous volatile organic compounds (VOCs) that were detected in the outdoor environment in Western Singapore by Thermal Desorption Gas Chromatography Mass Spectrometry (TD-GCMS). Tenax/Carbopack X multi-sorbent tubes were evaluated for active sampling performance and the method was validated using VOC standard solutions. The procedure exhibited repeatability with relative standard deviation (%RSD) values ≤ 10%, linearity with R2 values ≥ 0.99 for concentrations from 0.02 to 500 ng, VOC standards breakthrough ≤ 5% , tube desorption efficiencies ~100% and the majority of recoveries were between 61% to 120%. 30 mL/min flow rate coupled with sampling volumes of 1 L and 5 L gave the best results for sampling breakthrough and reproducibility during air sampling. Most of the target analytes exhibited acceptable breakthrough ≤ 5%, reproducibility ≤ 20% and method detection limits below 0.5 ppbv. The analyte exceptions were pyridine that remained undetected during sampling experiments and dichloromethane that failed the breakthrough requirement. 517 air samples were collected between February 2012 and January 2013 at 30 mL/min for 5 L samples. 60% of the daily trend profiles for hydrocarbons were linked to anthropogenic activities whereas 44% of the carbonyl compounds’ intra-day trends were potentially related to biogenic sources. The annual statistical analysis indicated that the VOCs with high maximum concentrations were toluene, 2-methylpentane, hexane, ethyl acetate and styrene. The highest overall maximum concentration is from toluene, at 100 μg m-3, which is comparable to Kolkata, India. Monthly box plots revealed that 8 VOCs had their largest monthly averages in September 2012. A major proportion of the target analytes also showed spikes in the monthly means between August 2012 and October 2012, likely attributable to the September 2012 transboundary haze (originating from forest fires in Sumatra, Indonesia). Strong Spearman coefficients of ρ ≥ 0.8 and ρ ≤ -0.8 were found between 26 pairs of hydrocarbons and 2 pairs of OVOCs respectively. 3 pairs of hydrocarbons had coefficients of determinations R2 ≥ 0.8. Positive matrix factorization (PMF) analysis confirmed 7 source profiles for the modeled hydrocarbons. Health risk evaluation of non-cancer effects was implemented for 16 compounds, while cancer effects were studied for 5 carcinogenic compounds. Benzene had the highest average Hazard Ratio (HR) and Lifetime Cancer Risk (LCR). 44% of benzene HRs were above the potential level of concern. 37% of benzene LCRs were beyond the definite risk of 10-4 and the maximum LCR obtained was as high as 6.41 x 10-4. Single-walled (SWCNT) and multi-walled (MWCNT) carbon nanotubes and their carboxylated derivatives (i.e. COOH-SWCNT and COOH-MWCNT) were evaluated for their potential as sorbent materials for trapping and analyzing VOCs using TD-GCMS. The first and subsequent conditioning durations and temperature were optimized for all types of carbon nanotubes (CNT) sorbent tubes to remove organic contaminants prior to usage. The primary artifacts in the CNT blanks were benzene, toluene and hexane. Thermogravimetric analysis (TGA) confirmed that all CNTs were stable when heated at 380 ◦C for several hours during conditioning the different CNTs. Desorption recoveries of 48 VOCs dissolved in methanol and loaded onto the CNTs demonstrated that there were minimal solvent interferences on the adsorption and desorption recoveries of VOCs with different functional groups. Hydrocarbons and aromatic compounds in MWCNTs had high peak area ratios > 0.7. 25 to 31 VOCs of varying polarities had peak area ratios > 0.7 when desorbed from SWCNTs, when the solution injection approach was employed for loading. VOCs with electron donor accepter (EDA) functionalities such as carbonyls, alkenes and alcohols demonstrated poor recoveries on all CNTs, suggesting that they may partake in reactions with residual metal impurities which act as catalysts during high temperature desorption. Inductively coupled plasma-mass spectrometry (ICPMS) experiments led to the detection of high amounts of nickel and molybdenum impurities in all CNTs. The reactions of methanol or formaldehyde with EDA VOCs were deemed to be unlikely due to the absence of an acid medium. Raman spectroscopy offered evidence of defects largely on MWCNTs, indicating that defective sites, together with a large number of methanol molecules could be the reason for high dichloromethane (DCM) breakthrough when using MWCNTs. Both polar molecules (DCM and methanol) can compete for adsorption on defects due to their enhanced affinity for these sites. Exposure to the "normal" analytical laboratory environment revealed that organic contamination of CNT materials likely occurs during transfer of the material in open air and during long-term storage. Desorption profiles from active sampling of atmospheric VOCs showed good agreement with those obtained from the injection of solution standards. SWCNT was established to possess the highest potential as a sorbent material for VOCs analysis using TD-GCMS. The outcomes gave useful insights, expanding the scope of future studies on CNTs.