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170 ug/L). 3 of the 8 samples (37%) had unacceptable RPD values and interestingly in all three <br /> cases repotted EPA 8020 values were significantly greater than the 8240 value. The small <br /> number of samples in this class limits any definitive conclusions,however, it appears that <br /> overestimation of the MTBE concentration by EPA method 8020 may occur when gasoline <br /> concentrations are high and MTBE concentrations are low in LUF r ground water samples. As <br /> discussed above, co-eluting gasoline components present in the LUFT ground water sample may <br /> lead to overestimates of MTBE concentrations by EPA 8020 analysis. <br /> Finally, 42 samples contained both high concentrations of gasoline (B=and/or B compounds <br /> at concentrations greater or equal to 150 ug/L) compounds and high concentrations of MTBE <br /> (greater than 240 ug/L). Unacceptable RPD values increased to 43% (18 out of 42 samples). <br /> The increased level of non-agreement between values reported by EPA 8020 and EPA 8240 may <br /> reflect the combined effects of small errors in dilution and calibration and in some cases the <br /> interference of co-eluting compounds. <br /> CONCLUSIONS- <br /> • As expected, false positives reported by EPA 8020 only occurred in samples that contained <br /> elevated concentrations of gasoline components. False negative samples did NOT occur. <br /> • The overall occurrence of false positives by EPA 8020 was low (4% of the samples, 2 out of 50 <br /> samples, that contained elevated BTEX and/or B concentrations). <br /> • The 2 false positive values detected by EPA 8020 were greater than 100 ug/L (240 and 5300 <br /> ug/L). <br /> • Amoug this data set, EPA 8020 may Dead to produce a significantly lower analytical value than <br /> EPA 8240 in approximately 20% of the samples that do not contain elevated concentrations of <br /> gasoline compounds. <br /> • Overestimation of the MTBE concentration by EPA method 8020 may occur when gasoline <br /> concentrations are high and MTBE concentrations are low (20 to 170 ug/L) in LUFT ground <br /> water samples. <br /> RFCOMMENDATIONS_ <br /> • Based on the data contained in this report, it is recommended that EPA method 8020A (or <br /> 8021B) continue to be used to detect BTEX and MTBE compounds in LUFT ground water <br /> samples. <br /> • Due to the low level occurrence of false positives, it is recommended that at least one sample <br /> per site which was positive for MTBE by EPA method 8020A (or 8021B) be analyzed by EPA <br /> method 8240B (or 8260A) to verity the correct identification of MTBE in the =ground <br /> water sample CLe. split samples from a minimum of one well from each release). We <br /> recommend that confirmation analysis be performed for a sample containing the highest MTBE <br /> concentration from the site based on method MMA values. Subsequent comparison of 8020A <br /> and 8240B data should be performed in six months to evaluate whether continued verification <br /> by EPA methods 8240B (or 8260A) is requi-ed_ <br /> • We suggest that analytical laboratories perform an "interference check" analysis of a 3- <br /> methylpentane and 2-methylpentane standards in the presence and absence of MTBE. This test <br /> will evaluate whether these compounds co-elum with MTBE under routine conditions used in <br /> individual laboratories for EPA 8020A analysis. <br /> 3 <br />