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A Study of Needle Trap Technology Followed by Gas Chromatography for the Determination of Organophosphorus in Groundwater

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Tutor: ZhangLanYing
School: Jilin University
Course: Environmental Engineering
Keywords: Groundwater,Organophosphorus,Needle trap
CLC: X832
Type: Master's thesis
Year:  2013
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Organophosphorus (OPPs) pesticides are located among the most widely employedpesticides in China, causing residues of these compounds to be presented in ground water.Since the concentrations of the OPPs are in trace amounts, pre-concentration techniquesare required to determine these compounds in the groundwater samples. The conventionalliquid¨Cliquid extraction (LLE) often needs large amounts of toxic solvent andtime-consuming procedures. There is, therefore, an increasing demand for developingsafe, simple, rapid and sensitive analytical methods for monitoring these compounds.This study develops a method based on needle trap (NT) and gas chromatography todetermine trace concentrations of four organophosphorus pesticides, dichlorvos, phorate,methyl-parathion, malathion, in aqueous samples. And this paper consists of three parts infollows:(1) This paper proposes six sampling methods,£¨a£©Ultrasonic assisted sequentialpurge and trap-NT sampling technique, the purging needle in the water,£¨b£© Sequentialpurge and trap-NT sampling technique, the purging needle in the water, without ultrasonic,£¨c£©Ultrasonic assisted sequential purge and trap-NT sampling technique, the purgingneedle in the headspace,£¨d£©Ultrasonic assisted simultaneous purge and trap-NTsampling technique, the purging needle in the water,£¨e£©Ultrasonic assisted staticheadspace-NT sampling technique,£¨f£©Ultrasonic assisted static headspace-gas-tightsyringe sampling technique. Method £¨a£© displayed good recovery in the range of87.9~101.4%, good repeatability (RSD2.16~7.06%, n=6), low limits of detection atS/N=3(1.9~21.9¦Ìg/L) at the following sampling conditions, concentration of OPPs1mg/L, sample volume5mL, salt addition20%£¨w/v£©, N2purging time5min, samplingrate12.5mL/min. Method £¨a£© showed an effective extraction compared to the other fivesampling technique, hence, method £¨a£©was chosen as the sampling method in thefollowing experiments.(2) Breakthrough volumes of NT, filled with TENAX/Carbopack/Carboxen,Tenax-TA, Porapak-Q, Porapak-P, were investigated. To test the breakthrough volume, afixed concentration of OPPs1mg/L, fixed sample volume5mL, fixed salt addition20%£¨w/v£©, fixed N2purging time5min and fixed sampling rate12.5mL/min were used. Itis observed that the breakthrough volume for OPPs were in the range of525~600mL,400~562.5mL,475~600mL,437.5~587.5mL when using the four kinds of NTrespectively, indicating that NT filled with TENAX/Carbopack/Carboxen and Porapak-Q has the stronger adsorption ability. Breakthrough experiments were completed using NTfilled with TENAX/Carbopack/Carboxen and Porapak-Q, it was found that breakthroughvolume was proportional to sampling temperature and the concentration of the analyte,inversely proportional to the sampling rate.Important parameters influencing the sampling process were optimized and theoptimum parameters were as follows, sampling time25min, vapor-liquid ratio7:1,sampling rate11.11mL/min, salt addition25%£¨w/v£©, extraction temperature30¡æ.(3) Carry over is the parameter that reflects desorption efficiency of needle trap.Under the optimum extraction conditions, needle trap filled with TENAX/Carbopack/Carboxen, Tenax-TA, Porapak-Q, Porapak-P, were used to extract OPPs from aqueoussamples, then each NT was desorbed and analyzed two times consecutively by means ofGC¨CECD. The amount of analyte determined after the second desorption was comparedto the sum amount of analyte determined after the first and the second desorption. Thelower carry over is, the stronger desorption ability is. The order of the NT desorptionability is as follows, Tenax-TA£¾TENAX/Carbopack/Carboxen£¾Porapak-P£¾Porapak-Q, and their carry over are0.53~0.96%,0.42~1.06%,0.83~1.09%,0.87~1.13%,respectively. Tenax-TA NT and TENAX/Carbopack/Carboxen NT provided the bestresults regarding carry over. The influence of desorption temperature on desorption abilitywas investigated, and carry over was inversely proportional to desorption ability. Carryover was below0.83%for TENAX/Carbopack/Carboxen needles at300¡æ.Desorption temperature was optimized under the optimum extraction conditions, andthe optimum desorption temperature260¡æfor TENAX/Carbopack/Carboxen needles.To sum up, TENAX/Carbopack/Carboxen needle showed best results and wassuitable for the determination of OPPs in aqueous samples.(4) The developed NT-GC method was successfully applied to real samples analysisof the four OPPs in aqueous. Limits of detection ranged from1.6¦Ìg/L, forTENAX/Carbopack/Carboxen needles, to15.7¦Ìg/L, and from2.9¦Ìg/L, for Porapak-Pneedles, to19.1¦Ìg/L. Excellent recoveries and RSDs of the OPPs were obtained fromwater samples (recoveries:81~112%, RSD2.7%~6.7%, n=6) for TENAX/Carbopack/Carboxen needles and (recoveries:83%~108%, RSD2.3%~5.5%, n=6) for Porapak-Pneedles. The newly proposed NT-GC method demonstrates the potential as an alternativeeffective and simple technology for the determination of OPPs in groundwater.
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