KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

APPENDIX B
Field Analysis with a Portable Gas Chromatograph
This method entails analyzing the soil gas samples with a portable gas chromatograph set up in the field.
Portable units, such as a Photovac Model 10S PLUS, can be used. This analysis method has the inherent
advantage of giving the user immediate results, so the survey can be directed based on the incoming data.
This is the most common method for analysis of soil gas samples.
With the portable gas chromatograph, a gas sample is injected directly into the machine using a gas-tight
syringe. The GC then chromatographically separates and detects the organics present. The results are
recorded in the form of a chromatograph. Each peak on the chromatograph is indicative of a single
compound. The area under the peak is indicative of the concentration of that compound.
In most cases, the portable GCs use flame ionization detectors (FIDs) or PIDs for detection. FIDs detect
all hydrocarbons, but for that reason can suffer from interference. Photoionization detectors detect most
compounds except methane and halogenated alkanes. For sites where low levels of chlorinated
compounds are expected, electron-capture detectors are used. electron capture detectors are selective
detector allowing considerable lower detection limits for halogenated compounds than do the PID or FID.
Using a portable GC, the soil gas samples are analyzed anywhere from a few minutes to several hours
after collection. The results are then available to direct additional soil gas identification and
quantification of each compound present in the sample is possible. To be quantified, standards of the
compounds present in a sample must be included in the instrument calibration.
Calibration
Standards should be prepared each day and can consist of one or several of the compounds suspected to
be present at the site. An alternative to preparing standards on site is having gas standards prepared by a
specialty gas vendor. The instrument should be calibrated at the beginning of each day and the
calibration should be checked at the end of each day and at least every ten samples.
Standards can be prepared by several procedures, each of which is acceptable if performed properly.
Preferably, the standards should be prepared in glass bottles fitted with Teflon ® septa, but inert gas
sampling bags are also acceptable. The preferred standard preparation method is to inject calculated
volumes of spectroscopic-grade liquid reagents into an amber glass bottle that has been filled with
hydrocarbon-free air. The vapor from that bottle is used to produce serial dilutions in other bottles.
Another method is to collect headspace vapor above a liquid solvent of interest and inject it into a bottle
and again make serial dilutions. If the vapor is collected above a liquid solvent, the solvent must be kept
in a temperature-controlled bath. If a laboratory-type GC is used, standards can be prepared in methanol.
Methanol-based standards are prepared by serial dilution from spectroscopic-grade liquid reagents and
may be used for up to one week.
Calibration consists of injecting the standard into the gas chromatograph and setting the integrator to
recognize and quantify the peaks in the standard. Standards are run throughout the soil gas program to
check the calibration and the GC. Following the initial calibration, calibration should not be changed
unless the integrator is not recognizing or quantifying the compounds in the standard.
Analysis
Sample analysis consists of injecting the sample into the calibrated gas chromatograph. A chromatogram
is produced, and the integrator will identify the peaks in the standard. If the field chemist determines that
Kirtland AFB
SOPs for Field Investigations B-104 April 2004