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since the traps may absorb target analytes. Other issues with TO-17 include F-4 MODIFIED GC METHODS interferences from sorbent artifacts(USEPA,1999,Method TO-17,Section 7.1). There is no single"universal"sorbent that can be used for all possible VOCs. The Regular gas chromatography(GC)methods are also being used for the analysis of soil choice of sorbent depends on the target VOCs. However,"multi-bed(sorbent)" gas samples. These methods should be used only for routine monitoring of VOC tubes are also available that can be used to sample for a wide range of target contamination at well-characterized sites and should not be used for the initial compounds. characterization of contaminated sites. Routine monitoring involves the analysis of sites where the contaminants,and to some extent their concentrations,are known. However, Method TO-17 should not be used with highly concentrated soil gas samples since there will be instances when new unknown compounds may be detected. In those there are no provisions for screening or diluting samples.The concentration of VOCs cases,any new compounds should be confirmed by analysis with a GC/MS method. or SVOCs should be predetermined by another active(or passive)soil gas method On a routine basis,at least 10 percent of the positive results from regular GC analysis prior to deployment. Otherwise,concentrated soil gas samples will saturate the MS should be confirmed by analysis with a GC/MS method. if completely desorbed into the GC. Field screening of soil gas,sample with EPA 5035/8260 or other available techniques where appropriate are recommended to Two primary GC methods associated with soil gas analysis are EPA Methods 8015 and provide information concerning the soil gas concentration encountered at specific 8021. Various versions of each method exist in the SW-846 manual(SW-846,1996). location. Laboratories should use the most updated versions of the method as they will contain the most up-to-date practices and information. Regardless of which version of the A, method is being employed,laboratories should clearly state in their analytical reports D. EPA Method TO-13A which version of the method was used. EPA Method TO-13A(Determination of Polycyclic Aromatic Hydrocarbons(PAHs)in A. EPA Method 8015(8015,8015A,8015B,8015C&8015D) Ambient Air Using Gas Chromatography/Mass Spectrometry(GC/MS))is designed for the sampling and analysis of polycyclic aromatic hydrocarbons(PAHs)in ambient EPA Method 8015(Non-halogenated Organics by Gas Chromatography)is used to air(USEPA,1999). Since there is no specific method designated for semi-volatile determine the concentration of nonhalogenated organic compounds(both volatile organic compounds(SVOC)in soil gas,Method TO-13A has been used for this and semivolatiles),triethylamine and petroletim hydrocarbons(i.e.,gasoline range purpose. Samples are collected/adsorbed onto a combination of filter and sorbent organics(C4-C12)and diesel/motor ail range organics(C12-C32))(SW-846,1996). cartridges followed by solvent extraction,cleanup(if needed)and concentration Samples are introduced into the GC by one of the following methods:(1)purge-and- before analysis by GC/MS. Again,since a soil gas sample is treated in the same trap;(2)equilibrium headspace; (3)direct injection;(4)injection of azeotropic manner as an ambient air sample,there should be no modification needed to distillation concentrate;(5)injection of vacuum distillation concentrate;or(6) analyze soil gas samples with this method, One consideration for this method is the injection of solvent extraction concentrate. A Flame ionization detector(FID)is used necessary volume needed to meet the required reporting limits considering the with this method, In order to apply this method to soil gas samples,the same types amount of vapor that can be collected from soil gas. The required flow rate of of modifications described for Method 8260B/C should also be used. Samples are sampling need to be calculated given the capacity of the sampling tube is also a either injected into a purge-and-trap sparger filled with water and purged or directly consideration. injected into the GC. E. Scan vs.SIM Mode B. EPA Method 8021 (8021,8021A&8021 B) With GC/MS Methods,there are two data acquisition modes: Scan and Selected Ion EPA Method 8021 B(Aromatic and Halogenated Volatiles by Gas Chromatography Monitoring(SIM). The most common mode is the Scan mode in which the detector Using Photoionization and/or Electrolytic Conductivity Detectors)is used to scans from high to low across a range of masses continuously. This mode provides determine the concentration of volatile organic compounds(both halogenated and the sample mass spectrum by which compound identification is made against a aromatic)(SW-846,1996). Samples are introduced into the GC by one of the spectral library. In SIM(Selected Ion Monitoring)mode,only a few selected ion following methods:(1)direct injection;(2)purge-and-trap;(3)headspace or(4) fragments(or masses)are monitored. Because the detector concentrates its time injection of vacuum distillation concentrate. Both a photoionization detector(PID) only on selected masses,the sensitivity is maximized. SIM mode has the and a Hall electrolytic conductivity detector(HECD)are used with this method in advantages of having lower reporting limits and overcoming background problems. either series or as a single detector. In order to apply this method to soil gas The only limitation of using the SIM mode is fewer compounds are identified. samples,the same types of modifications described for Method 8260B/C should also be used. 12/10/2009 -79- 12/10/2009 _80-