Environmental consulting activities frequently require that on-the-spot decisions be made in the field concerning the presence or absence of volatile, or even semi-volatile, organic compounds (VOC's) in soil. There is nothing that can replace the value of field experience in 'making the right call' when the trucks are lined up, the backhoe operator is waiting, and the client is watching the hole in the ground get bigger and bigger. However, the application of basic knowledge and consistent methodology to your field procedures can assist you in the interpretation of field soil screening results.
First, are you using the right piece of equipment for the job? You have to understand the capabilities and limitations of the Organic Vapor Analyzer (OVA) you are using.
A Photo-Ionization Detector (PID) will be limited in its response by the electron voltage (eV) output of the lamp in the instrument. Most VOC's have a published Ionization Potential (IP). PID's are equipped with a 9.5 eV, 10.6 eV, or 11.7 eV lamp. The 10.6 eV lamp is the most common. In order for a PID to respond to a particular VOC, the IP of the compound of interest must be less than or equal to the eV output of the lamp. There are some compounds that a PID will not detect, most notably methane. In addition, methane can chemically mask the presence of VOC's. PID's are subject to poor performance in the presence of high humidity in moist soil. Perhaps the greatest advantage in using a PID is that it does not require a hazardous gas for operation. This can be a great consideration in logistics or cost when traveling or working in an isolated area.
A Flame-Ionization Detector (FID) will respond to most VOC's by nature of its destructive detector function. An FID will not be adversely affected by the presence of humidity in moist soil. An FID can be used in methane determination or differentiation by using a charcoal filter adapter. FID's are calibrated to methane. A charcoal filter adapter absorbs VOC's that are present in a sample. Therefore, the determination or differentiation of the presence of VOC's can be made in the presence of methane. Perhaps the greatest disadvantage in using an FID is the need for zero-grade or ultra-high purity (UHP) hydrogen for the detector's flame fuel source. Once again, this can be a great consideration in logistics or cost when traveling or working in an isolated area.
Whichever OVA you are using, current manufacture instruments provide sub-parts per million (ppm) digital performance and, usually, dynamic ranges of 10,000 ppm, or higher. Due to the differing responses instrument to instrument, do not change from a PID on one phase of work to an FID on another phase of work. From a practical perspective, both instruments are field-screening tools, not laboratory analysis equipment. If a soil sample exhibits a gross VOC odor that you cannot even stand to get near, you may actually harm the instrument you are using by analyzing such a sample. You may want to consider a company standard field note procedure for such samples rather than risking the contamination of the OVA's detector system.
Second, you have to understand the nature of volatile organic compounds and semi-volatile organic compounds as they relate to actual laboratory-testable product in the soil sample versus the amount of vapor in a soil sample headspace. The principle is simple. The headspace vapor-in-air concentration of a highly volatile compound may be higher than the actual laboratory-testable amount of the compound in the soil sample as you go from contaminated to clean soil. The opposite is true for semi-volatile organic compounds. The headspace vapor-in-air concentration of a semi-volatile compound may be lower than the actual laboratory-testable amount of the compound in the soil sample as you go from contaminated to clean soil.
Third, are you simply waving the instrument's sample probe over an open split-spoon, handful of soil, or open excavator bucket or are you head-spacing your soil samples?
There are too many uncontrolled factors present in simply waving the instrument's sample probe over an open split-spoon, handful of soil, or open excavator bucket. In most instances, your OVA will only respond to gross levels of contaminants when using this method. Additionally, outdoor ambient airflow will affect the sample that gets drawn into the OVA. In windy conditions, you might actually be detecting VOC's from a background source, such as heavy construction equipment or gas pumps. This method provides no controlled methodology to make a determination of what's happening sample to sample. It also allows no time for semi-volatile VOC's to release vapors that can be detected by an OVA.
Head-spacing soil samples provides for the best possible application of consistent and controlled methodology to your field soil screening procedures. The key word here is consistent. The container you use, amount of soil you collect, or amount of time the containerized soil sample sits are not so important as being consistent with each one. Glass jars with foil and zip-closure bags are commonly used for head-spacing procedures. Use the same type of container for all phases of work. Glass jars heat up faster and stay cold longer than a zip-closure bags. If using zip-closure bags, test a bag first to make sure that it does not in itself release VOC's that will be picked up by your OVA. Collect the same amount of soil with each sample. Most importantly, allow each containerized soil sample to sit for the same amount of time. A very common error is to collect samples over the course of the day and then check them all at the end of the day. One sample may have been sitting for five minutes while the first one has been sitting for five hours! It is usually sufficient to allow a sample to sit for a matter of minutes to provide for a practical indication of the presence or absence of VOC's.
The best possible scenario for controlled soil sample screening, using the headspace method, is to split samples between two containers. This applies to soil samples that are being screened for potential submission to a laboratory and to soil samples that are being screened for differentiation of VOC's and methane. In the first instance of screening, the sample to be screened should be containerized for head spacing. The potential laboratory sample can be tightly wrapped in a zip-closure bag, wrapped in foil, and placed in a cooler for later use, if necessary. In the second instance of methane differentiation, the sample should be split between two containers, and then screened individually using the FID on one and the charcoal filter adapter on the other. A single sample headspace will be depleted by the first analysis and leave an inadequate sample for the second analysis.
In closing, apply these basic principles and procedures to your field soil screening activities. You will find that your field data will provide you and your project professionals with more accurate data. Best of all, though, you'll find that you will more quickly gain the experience necessary to 'make the right call' when it counts most, in the field.