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compound may be different in a liquid-phase standard than in a vapor. validation is outside the 20%acceptance limit. This validation procedure is phase standard,especially when it is subjected to being forced into an strongly recommended regardless of the sample introduction technique being aqueous phase(in the sparger)and forced out into a gaseous phase used. This validation is not only technically sound,it will enhance the again(during the purge). Therefore,the calibration curve should be defensibility of the data. matrix-matched. Analytical laboratories should use vapor-phase standards to calibrate their instruments when employing EPA Method For laboratories that calibrate their analytical system with liquid standards 8260B/C for soil gas analysis. Gas-phase(or vapor-phase)standards expanded in vials or glass bulbs(headspace technique),validation of the used for ambient air testing are readily available and can be used for soil calibration curve should be conducted by injecting an aliquot of a vapor-phase gas analyses. standard(NIST traceable or equivalent)at a volume equal in concentration (this needs to be calculated based on the volume of the vaporized standard b. A modification using a liquid-phase standard for calibration that attempts injected and the concentration of the standard,etc.)to the mid-point of the • to matrix-match the standard to soil gas samples is also being used. The calibration curve to validate and to verify the accuracy of their standard technique entails injecting the liquid standard into an empty VOA vial preparation technique. The frequency of the validation,percent differences of through the septum or into glass bulbs and allowing the standard to validation check and reportable data should be the same as for liquid vaporize and equilibrate before taking an aliquot of the vapor and injecting standards. it into the gas chromatograph. This is essentially a headspace technique. This modification attempts to matrix-match the calibration standards to the If vapor-phase standards are used to prepare the calibration curve with EPA soil gas samples,but this technique may not be amenable to all VOCs Method 8260B,the validation mentioned.above is not necessary. However, since it is dependent on the vapor pressures of the target analytes and analytical laboratories should verify the accuracy of their vapor-phase how well they will vaporize in the vial or bulli. The few laboratories that standards periodically by comparing them to a secondary standard either use this technique are analyzing a limited number of analytes. from another source or to a different lot of standards from the same supplier. Laboratories using this approach should standardize their procedures(i.e., temperature range,time for equilibration,etc.)in preparing the calibration 4. Sample Size standards. Furthermore,laboratories using this technique should validate and verify the accuracy of their vaporized standards by comparing their The amount of sample to be analyzed is determined by the sample calibration with vapor-phase standards(see next section). introduction technique in conjunction with the data quality objective of the project. If lower reporting.limits are desired,then a larger volume of sample 3. Calibration Validation - needs to be injected. If the direct injection technique is employed,this would be limited since only a very small volume can be injected onto the GC. With If liquid-phase standards are used for calibration with EPA Method 8260B/C the sparger technique,a larger volume can be accommodated. Five(5)to for soil gas samples,then the analytical laboratory should validate their 250 milliliters(mL)of soil gas sample are the typical volumes used,and some calibration curve for soil gas analysis. Validation is done by analyzing a mid- laboratories have used up to 500 mL of sample.When a larger volume of level NIST(National Institute of Standard and Technology)traceable(where sampleis used,the sample is introduced into the sparger in aliquots. available or equivalent)vapor-phase standard(vapor-phase validation check - • standard)on a routine basis. At a minimum,the vapor-phase validation When an aliquot of soil gas sample is injected into a sparger filled with water, check standard should be analyzed and evaluated every time a calibration - the gas sample actually spends a very short time in the water. During the curve is generated. Routinely,a vapor-phase check standard should be "injection of the sample,the gas is forced through the water directly into the analyzed with each analytical batch to verify the validity of the liquid trap. The contact time with the water is minimal. However,the more water calibration curve. In addition,the vapor-phase validation check standard soluble compounds(e.g.,ketones,MTBE)will preferentially stay in the water should include all the target analytes in the calibration curve. The percent phase until purged out. difference of the validation check standard should be 20%or less in order to assume that the liquid-phase standard calibration curve is valid for soil gas As part of the validation process,laboratories should validate their injection samples. Soil gas sample results can still be reported for those compounds technique by injecting aliquots of vapor-phase standards into the sparger and where the percent difference of the validation standard check is greater than evaluate the recovery levels. Recommended recovery range is 70-130%for 20%provided that the results are clearly Flagged to indicate that the results most compounds. are biased because the calibration standards were not matrix-matched and 12/10/2009 -73- 12/10/2009 -74-