Thursday, July 7, 2011

Measurement of ppb Levels of Gasoline in Groundwater by Portable GC-PID

Contamination of groundwater and soils by leaking underground storage tanks continues to be a serious environmental problem. Twelve  years ago, the EPA was monitoring about 370,000 leaking underground storage tank sites in the United States. Only about 5% were scheduled for site cleanups in 2001.  Cleanups are funded by the EPA's LUST Trust Fund, which was funded at a level of about $70 million per year. The University of Colorado at Boulder has summarized the information on Leaking Underground Storage Tanks at the following link:
HNU, working with Jim Stuart, Professor Emeritus, University of Connecticut, has shown that a portable GC with a photoionization (PID) is an ideal tool for analyzing gasoline samples in groundwater and soil.  The recently introduced portable GC (Model 312 see photo) has an improved PID, improved range, is more lightweight & compact, is battery operated and will run off the 12 VDC in an automobile. Thus local power requirements are not necessary.

Photo GC312 Portable GC
Aromatics are among the most hazardous components of gasoline (the EPA maximum contaminant level in drinking water is 5 ppb of benzene). The PID has a higher sensitivity to aromatics than to any other hydrocarbons making this GC-PID ideal for cleanup of gasoline contaminated sites. EPA CLP methods as outlined in SW-846 include’ Method 3810, a headspace method for VOC’s, that provides the rapid screening of large numbers of samples.

Note that the detection limits for benzene, toluene, ethylbenzene, xylenes, TCE & PCE are all at the ppt level with just a 50 mL sample injection. The PID has a dynamic range of nearly 108 . The GC312 software has a 4 point calibration so that the results can be kept linear over a wide concentration range. Some typical response curves are shown in the Figure below.
Calibration Curves for benzene and toluene
The headspace method can be performed in the same 40 mL VOA vials that samples are collected in for lab analysis. After completely filling with sample, cap the vial with a teflon faced septum. To produce the headspace, remove 10 ml of the water via a syringe (filled with clean air) while keeping the vial inverted. Shake for about 2 minutes. Remove a 25-100 mL headspace gas sample for injection into the GC.  The Table below displays the excellent results for the field headspace and the purge & trap lab GC results.

For more information on our GC312, visit our website at  

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