SummaryThe design and features of an on-line purge and cold-trap preconcentration device for rapid analysis of volatile organic compounds in aqueous samples are discussed. Excessive water is removed from the purge gas by a condenser or a water permeable membrane in order to avoid blocking of the capillary cold-trap. Synthetic mixtures covering concentrations ranging from tenths to tens of ppb's and different chemical classes are used to study the effect of various process factors on the efficiency and selectivity of water removal as well as on the purging recovery. The importance of the concentration of the solutes, the flow rateinconjunction with thevolumeof the purge gas, and the temperatureof thecondenser, thecold-trapand the sample is emphasized. Theoretical models describing the purge process and the blocking of the cold-trap agree fairly well with the highlyreproducibleexperimental results (o = 2-4'10). Both the condenserand the Nafion membrane successfullyremove water, although some compounds, dependent on volatility and polarity, are partly or completely lost. It is shown that non-polar volatile organic compounds are efficiently enriched so that recoveries between 80-100% and a detection limit of 1 ppt can be obtained. The applicability of the system is illustrated on some examples.
In order to meet the requirements of analyzing very low concentrations of pesticides in water (typically at 0.1 pg/l or less), samples have to be concentrated prior to GC-analysis. Sample pre-concentration by off-line methods based on solid phase extraction (SPE) or liquid-liquid extraction are very time consuming and cumbersome. Moreover, the quantitative performance of the analytical method as a whole in terms of accuracy and reliability is seriously hindered by elaborate, manually performed sample pre-treatment steps. This paper describes an automated method based on solid phase extraction and capillary gas chromatography. The technique was applied for the analysis of 31 polar organophosphorus and organonitrogen pesticides. A commercially available HPLC/GC instrument i s modified, using the LC-part for solid phase extraction. The sample, of which only a few ml's is required to obtain sufficiently low detection limits, is delivered by a robotic large volume autosampler. After solid phase extraction and elution, the eluate is transferred into the GC via a so called "loop type interface". In this paper the instrumentation and analytical methodology is described, as well as the main validation results. The quantitative performance (ie. recovery and repeatability) of the most polar solutes like metamitron and dimethoate appears to be better than obtained with off-line SPE as a result of the more beneficial ratio between the amount of sorbent and the sample volume. As the loop-type interface causes losses of the most volatile compounds, a co-solvent is added. This co-solvent provides sufficient trapping capacity in the capillary pre-columns to allow quantitative analysis of even the most volatile pesticides. Moreover a better separation of early eluting compounds is also established.
SummaryExpressIons for the minimum detectabie amount 0 0 and the minimum analyte concentration Co as functlons of the chromatographic parameters are derlved for both mass and concentratlon sensltlve detectors. The effects of pressure drop, column inner diameter, and film thickness are given.The minimum analyte concentration for mass flow sensitlve detectors, Cam, can be reduced conslderably by selecting the carrier gas velocity weil above lts optimum value (related to Hml n ), however, at the cost of long columns and long analysis times. For 0 0 the improvements can be neglected, and 80 the analysis can best be performed at Uopt.When the flow rate In the detector, F d, is equal to the column flow rate F c, the maximum permissible detector volume of concentration sensitlve detectors is proportional to d c 2 up to d c 3 , and so narrow bore columns requlre detectors of extremely sm all volume. Make-up gas has to be added when the actual volume is too large, th us worsening the detectability. Another approaCh, vacuum operation of the detector cell, appears to be very attractive. On the other hand, when wide bore columns are used in comblnation with smaU volume concentration sensltive detectors, very smaU values of Qoc and Coc are obtalnable when the abundant carrier gas can be removed betare entering the detector cell.Digital nolse filtering can furtherreducethe obtalnable 0 0 and Co values, especially for broad peaks and thus for wide bore columns.
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