Summar y Matrix effect is one of the major problems in quantitative analysis of organic compounds in environmental sample. The aim of this study was to determine the matrix effect in the analysis of 42 pesticides in river water by liquid chromatography-tandem mass spectrometry (LC/MS/MS) with two kinds of approaches. One approach was a conventional way to compare pesticide responses between the pesticide-spiked solution of river water sample and the pesticide standard solution. The other was

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... ng a chromatographic profile of a pesticides constantly added to a postcolumn eluate of river sample (PCI method). The matrix-effects were varied among the pesticides. Almost no ion suppression was observed for the ions of metalaxyl, cafenstrole and isoxathion. On the other hand, the ionization efficiencies of some other pesticides added by PCI were varied with the retention times of river water components. Ion suppression in the analysis of the pesticides was examined with river water samples collected at 7 of sampling sites. Because the matrix components were varied among the sites, it is difficult to prepare a "matrix-matched standard" which eliminates the matrix-effects. A method for diluting sample solution was examined to reduce the ion suppression. The suppression between 30 and 50% could be reduced to below 10% by 5-fold dilution of water sample, and for the suppression above 50%, the sample dilution was not effective to reduce the suppression. A sample clean-up method with solid phase extraction (SPE) was examined in two different pHs to reduce the matrix-effects. The ion suppression in water sample of non-adjusted pH (pH around 9) was smaller than that in the sample adjusted pH3. Then, in the case of measuring non-acidic pesticides, better results can be expected with smaller matrices from water sample of non-adjusted pH.