PIDs As HazMat Response Tools

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PIDs As HazMat Response Tools

Technical Note TN-118rev 3 wh.06-03Using RAE Systems PIDs For Measuring Soil HeadspaceAnd In Soil Vapor ExtractionsIntroductionPhotoionization detectors (PIDs) are often employedto measure VOC concentrations in water or soil. Forexample, hydrologists and environmental engineersoften use PIDs to monitor chlorinated solvents (e.g.,percholorethylene) and fuels (e.g., gasoline, diesel) insoil or groundwater during clean-up operations such asexcavations and soil vapor extractions. PIDs can alsobe used to monitor water stripper effluents and offgasesfrom wastewaters. Although PIDs cannotmeasure directly in these media, they can indirectlymeasure the vapors emitted from them. For example,Hewitt and Lukash* reported linear correlationsbetween headspace PID response and soil concentrationsof benzene, toluene, xylenes, dichloroethylenes,trichloroethylene and perchloroethylene.The concentration in the headspace, measured inppmv, does not equal the soil or water concentration,measured in mg/kg or mg/L. The vapor concentrationdepends on such factors as soil-to-headspace weightand volume ratio, soil permeability, affinity of thecompound to the soil, temperature, equilibration time,and dilution during the measurement procedure.Therefore, it is important that these factors arecontrolled as closely as possible if quantitative soil orwater concentrations are desired.Soil and Water Headspace ScreeningIn a typical procedure, a sample of soil or water isfilled approximately halfway into a jar with a ringtypelid. Enough air space (“headspace”) above thesample is allowed for sampling. A piece ofaluminum foil is placed over the mouth of the jar andheld in place with the lid ring. The jar and itscontents are brought to room temperature and shakento mix the soil sample with the air in the headspace.The influent probe and effluent line of the portablePID are then poked through the foil and the VOCconcentration measured in the headspace of the jar.A similar procedure can be performed using a plasticbag instead of a jar, but the headspace volume is notas reproducible, and thus the results are lessquantitative. A sampling septum can be made easilyfor virtually any hard container with aluminum foilheld in place by a rubber band around the rim of thevessel. When sampling with the PID, ensure that dirtand water are not sucked into the PID probe.Procedures for Optimum Performance of PIDsMeasuring organic compounds emitted frompotentially contaminated soil and water requiresspecial attention beyond that needed for typicalambient air monitoring. Stripper effluents and soilvapor extraction streams are typically near 100%relative humidity (RH), and soil samples are oftendusty and humid. Such conditions can cause high,drifting readings on many PIDs, if not properlymaintained. Interferences are usually traceable tocondensation in the sensor, causing a current leakageacross the electrodes and thus a false positive signal.This situation is exacerbated when the sensor iscontaminated by soil dust or condensed, high-boilingorganic compounds.1. Keep the sensor clean by using high-puritymethanol, preferably using an ultrasound bath. Alow-cost ( less than $150) ultrasonic cleaner canbe obtained from Cole-Parmer (part number H-08849-00; phone number: 800-323-4340), or usea jewelry cleaner from a local department store.Flush the residual solvent from the sensor with arapid stream of clean air, and clean the lamphousing area that contacts the sensor when it is inplace.2. On sensors with interdigital fingers, check thatthe metal electrode fingers are straight, parallel,and do not contact the Teflon sensor walls. Bendthem carefully if necessary. Replace the sensor ifthe electrodes are corroded.3. Keep the lamp surface clean by using high-puritymethanol. Never use acetone on 11.7 eV lamps.4. Use the 4.5 cm “water trap” filters as an extraprecaution, especially in dusty or moistenvironments where water mist may be present.Change filters frequently.5. Avoid situations in which the PID is colder thanthe soil being sampled, such as heating the soilsamples to increase the headspace organicconcentration, or bringing a cold PID into awarm room without allowing time fortemperature equilibration. If anything, try tokeep the PID warmer than the soil samples.1RAE Systems Inc.3775 N. First St., San Jose, CA 95134-1708 USAPhone: +1.888.723.8823Email: raesales@raesystems.comWeb Site: www.raesystems.com


Technical Note TN-118rev 3 wh.06-036. To obtain more stable readings, plumb theeffluent flow from the MiniRAE or ppbRAEback into the sample container to reduce thelosses. Use Teflon or metal tubing for thispurpose so as to prevent adsorption to Tygon orother plastic tubing. Losses will not be stoppedaltogether, but will be greatly reduced.7. If humidity problems persist, use a humidityfiltering tube (p/n 025-2002-010) to absorbmoisture. These tubes allow many organicvapors such as TCE and gasoline to pass almostunaffected, but they may absorb some heavy orpolar compounds. (See Technical Note TN-178).A lower cost but less effective means is to use theC-Filter to absorb dust and some moisture. (SeeTechnical Note TN-162). Perform frequentchanges of the C-Filter (daily to weekly,depending on usage and dirtiness).Response of PIDs to Semi-volatiles on SoilsOften repeated is the statement that PIDs do notrespond to semi-volatile organic compounds. Thisstatement seems to be an old piece of "commonknowledge" that may have been true at one time foran old PID used for soil headspace measurements,but is no longer true in general. The larger theorganic molecule (and thus less volatile), the lowerthe ionization energy and the greater the PIDsensitivity. However, there is a point of diminishingsensitivity when adsorption losses in the instrumentsample linesand filters begin to dominate over this sensitivityincrease. This is true for any instrument includingFIDs (flame ionization detectors) and PIDs if they arenot specifically designed to handle semi-volatilecompounds. New PIDs have higher flowrates andbetter sensor designs that reduce such losses, andtherefore most compounds up to a boiling point ofabout 300° C can be detected on the MiniRAE 2000or ppbRAE. Boiling points for fuel oils, diesels, andkerosenes range from about 170° C for #2 Fuel Oil to260° C for #5 or #6 fuel oils. As the oil weathers andthe light ends evaporate, the response time increasesand the overall response drops because less organicvapor is present. The response may drop to zerobefore all the oil is removed from a soil sample,because only non-volatile components remain.Again, this effect is the same for FIDs and PIDs. Insuch cases, a direct measurement of the oilcontamination on the soil may be needed–forexample, using a solvent-extraction procedurefollowed by laboratory gas chromatography. Forsuch high-boiling compounds, it is also important notto use any rubber or Tygon tubing to draw insamples, as several inches of such tubing cancompletely absorb heavy fuels. Teflon or metaltubing is preferred.* Hewitt, A.D. and J.E. Lukash: PID-basedestimation of the concentration of aromatic andchlorinated VOCs in soil. Field Analyt. Chem.Technol. 1999, 3(3):193-200.2RAE Systems Inc.3775 N. First St., San Jose, CA 95134-1708 USAPhone: +1.888.723.8823Email: raesales@raesystems.comWeb Site: www.raesystems.com

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