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Fused-silica-lined (FSL) canisters and SUMMA polished (SUMMA) canisters were compared for the recoveries and the stabilities of 58 volatile organic compounds (VOCs) at low ppbv (volume/volume) levels under various humidified conditions using a three-stage preconcentration method followed by GC-MS analysis. The target VOCs included non-polar VOCs (e.g. halogenated hydrocarbons and aromatic hydrocarbons) and polar VOCs (e.g. alcohols, ketones, esters, ethers, nitriles and thiols). The three-stage preconcentration method was initially optimized for simultaneous analysis of non-polar and polar VOCs because determination of canister stability is dependent on the accuracy of analytical measurements. The method showed good linearity over the concentration range from 1 to 25 ppbv for all target analytes, and the correlation coefficients were higher than 0.9974. The method detection limits ranged from 0.023 to 0.39 ppbv. The test mixtures loaded in both type of canisters (n = 3) had concentrations of 1.7-2.5 ppbv per compound at ambient pressure under various humidified conditions (%RH = 1.6, 8.0, 27, 39, 53 and >99% with excess water present). All canister samples were initially analyzed on day 0 (after 6-12 h). The effect of competitive adsorption of water vapor and polar VOCs on active sites of interior surface was remarkably observed for SUMMA canisters. Polar VOCs had a greater requirement for water vapor to be present. The RH percentages that ensured good recovery on day 0 were RH > 8% for non-polar VOCs and RH > 27% for polar VOCs (except alcohols under the condition of RH > 99%). All thiols were not recovered from SUMMA canisters under all conditions. FSL canisters showed good recoveries of more than 86% for all VOCs under all conditions on day 0 (except alcohols under the condition of RH > 99%). The recoveries of alcohols in both canisters under the condition of RH > 99% displayed relatively low recoveries in the range 25-76% because of the partitioning effect into condensed water. The canister samples under the conditions of RH 8.0, 27, 53 and > 99% were analyzed for the stability test on days 3, 7, 14 and 28 after loading. All non-polar VOCs were reasonably stable in the FSL canisters under all examined conditions over 28 days. However, several polar VOCs that have relatively lower vapor pressure, e.g. MIBK, butyl acetate and alcohols except ethanol, showed unstable characteristics under relatively dry conditions (RH 8 and 27%) during 28 days. RH > 53% was needed to ensure good stabilities of all analytes except thiols with the recoveries of > 80% over 28 days for both canisters. Although the FSL canister showed good recoveries of more than 86% for thiols on day 0, drastic degradations were observed after day 3 and they were not detected after day 14.

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A multi-residue method to determine 85 pesticides, including organochlorine pesticides, carbamates, organophosphorus pesticides, and pyrethroids, in vegetables, fruit, and green tea, has been developed. The method is based on stir bar sorptive extraction (SBSE) coupled to thermal desorption (TD) and retention time locked (RTL) GC-MS operating in the scan mode. Samples are extracted with methanol and diluted with water prior to SBSE. Dilution of the methanol extract before SBSE was optimized to obtain high sensitivity and to minimize adsorption onto the glass wall of the extraction vessel as well as to minimize sample matrix effects (particularly for the pesticides with high log K(o,w) values). The optimized method consists of a dual SBSE extraction performed simultaneously on respectively a twofold and a fivefold diluted methanol extract. After extraction, the two stir bars are placed in a single glass thermal desorption liner and are simultaneously desorbed. The method showed good linearity (r2 > 0.9900) and high sensitivity (limit of detection: < 5 microg kg(-1)) for most of the target pesticides. The method was applied to the determination of pesticides at low microg kg(-1) in tomato, cucumber, green soybeans, spinach, grapes, and green tea.