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Discussions with laboratory analysts and remediation engine..-rs indicate that it is widely believed that the greatest number of false positives will occur when low concentrations (<100 ug/L.) MTBE are reported by EPA 8020_ This is supported by recent studies by Mobile Oil and Shell Development Company which predict that false positive detections of MTBE will tend to occur when <20-50 ug/L concentrations of MTBE are reported by EPA method 8020_ The data analyzed in this report strongly contradict these expectations. The reported EPA 8020 values for the 2 false positives in this data set are 240 and 5300 ug/L. Therefore, presently, the reported EPA 8020 concentration (above 20 ug/L) have no predictive value for indicating the probability of false positive mis-identifications. New samples were obtained from the well previously reporting a false positive EPA 8020 value of 5300 ug/L. Both GC/PID and GC/MS analysis performed at Lawrence Livermore National Laboratory (L.L.NL) confirmed ND levels of MTBE. Our analysis also confirmed the presence of 3-methylpenmm a gasoline component which can co-elute with MTBE.leading to false detection or overestimation of MTBE concentrations by EPA 8020. 3-methylpemmne was also independently detected in these samples by Shell Development Company research laboratories. Therefore, this example shows that gasoline components present in LL'FT ground water have been mis-identified as MTBE by EPA 8020 analysis. Other laboratories have reported that the presence of 2-methylpentane in LUFT ground water samples can result in false detection of MTBE. Therefore;, we suggest that analytical laboratories perform an "interference check-'T analysis of a 3-methylpentane and 2-methyipentane standards in the presence and absence of MTBE to evaluate whether this compound co-elutes with MTBE under routine conditions used in their laboratory for EPA 8020 analysis. It is our opinion that as analytical laboratories optimize conditions to detect and quantity MTBE by EPA 8020 in the presence of 3- and 2-methylpentane,both the number and concentration of reported false positives will decrease. In addition, current State of California regulations require use of a single coltunn for analysis of ground water samples containing gasoline. The Association of California Testing Laboratories (acd ABS) has recommended the use of a second confirmation column for detection of MTBE in the presence of gasoline to possibly reduce the number of false positives detected by EPA 8020 (a&NEWS vol. 7, issue 3. July 1996). Because the quality of EPA 8020 data may improve due to optimizations of GOPID separation conditions and/or the use of a second column, we recommend than a comparison of EPA 8020 and 8240 data should be performed in the future to evaluate whether trends from this data set continue. Agreement of analytical values reported for EPA 8020 and EPA 8240: For our analysis, RPD values Beater than 40% were unacceotable, demonstrating a significant diaerence between the values obtained by these two analytical methods. We reviewed three classes of MTBE data samples with ND or low concentrations of gasoline compounds, samples with high concentrations of gasoline and Iow concentrations of MTBE, and samples with both high concentrations of gasoline and MTBE. In genual, we found agreement between the MTBE concentrations reported by both methods was fair for the 48 samples that did not contain elevated concentrations of gasoline compounds: 27% had RPD values greater than 40% (13 of 48 samples containing :tD to 73 ug/L BTFX and/or 7 ug/L B compounds). Interestingly, in the majority of these cases (9 out of 13), the MTBE concentration reported by EPA 8020 was significantly lower than the result reported by EPA 8240. We have no explanation as to why EPA 8020 would tend to produce a lower analytical value than EPA 9240 in approximately 19% of the samples that do not contain elevated concentrations of gasoline compounds. Only 8 samples contained elevated concentrations of gasoline compounds (13TEX and/or B compounds at concentrations greater or equal to 200 ug/L) and low concena-arors of MT3E (20- 2