Industrial Hygiene News

July 2008

The PID is an ideal tool for IH surveys where chlorinated solvents are used. These compounds are heavier than air because of the embedded chlorine atoms and the highest concentrations are found near the ground. The PID can be used to find the problem areas, then charcoal tubes can be collected and analyzed in the lab in order to define the compounds present and their concentrations. This is the approach taken by OSHA industrial hygienists.

First Responders go to a site where little is known initially about the actual chemicals spilled so an analyzer with the broadest response & greatest flexibility is required. Here, the 11. 7 lamp is ideal since it responds to chlorine, low molecular weight chlorinated organics and other VOCs. The 102+ PID can also have 3 electrochemical sensors (Cl2, HCl & NH3). In addition, the 102+ has snap on heads so that a 10. 6 head can replace the 11. 7 head. The readout unit recognizes the head and sensors and retains the PID calibrations.

By J.N. Driscoll from PIDAnalyzer LLC in Pembroke, MA he can be reached at 781/709-2131

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References:

1. Driscoll and Spaziani, "A New Instrument for Continuous Monitoring of Odorous Sulfur Compounds," Paper 74-709, Anal. Inst. (1974)

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2. Driscoll and Spaziani, "PID Development Gives New Performance Levels,"

Research and Development, 27, 50 (1976).

3. Driscoll and Becker, "Industrial Hygiene Monitoring with a Variable Selectivity PID," Amer. Lab., 11, 69 (1979).

4. Driscoll, J. N., J. Hanby, and J. Pennaro, "Review of Field Screening Methodology for Analysis of Soil," pp 153-172 in Hydrocarbon Contaminated Soils ,P. Kostecki, E. Calabrese, and M. Bonazountas edited, Lewis Publishing, Ann Arbor, MI (1992)

Leak Detection- A required application for chemical manufacturing is leak detection and repair of valves, pumps etc to minimize leaks of monomers such as VCM (PEL = 1 ppm), EDC (PEL= 1 ppm). The VC is detected with a 10. 6 eV lamp but the intermediate material is ethylene dichloride that requires an 11. 7 eV lamp. An advantage of the PID is that a 10. 6 and 11. 7 Head can be used for the measurements in place of two instruments (PID & FID) and the portable FID can be eliminated. One additional advantage is that the ambient methane ( 3-5 ppm) is not detected by the 11. 7 PID.

Headspace Samples- The measurement of low concentrations (ppm levels) of organics in water or soil can be performed through the application of Henry’s Law which states that, at equilibrium, the solubility of a gas in a liquid or solid is proportional to the partial pressure of a gas in contact with a liquid or solid as given below:

TCA (aq) = K P_TCA

where TCA (aq) is the concentration of TCA in the liquid phase, K is the Henry’s Law constant which governs the solubility of gases in water or soil, and PTCA is the partial pressure of TCA in the gas phase.

As a result of the above equation, it can be seen that if the concentration of TCA in the gas phase and at equilibrium is measured, this is related to the concentration of TCA in the dilute aqueous solution by a proportionality constant (K) that can be determined by calibration.

The headspace method is easy to perform, requires a minimum of equipment, and requires only that the sample and standards be at equilibrium to obtain accurate results. A typical procedure involves weighing (or measuring) 1 g of soil into a weighing boat which is placed in a container of about 40 (VOA vial) -100 cc volume which can be sealed placed in an oven at 60°C for 15 minutes, cooled to room tempera-ture ( 8). The sample can also be left at ambient (in a trailer on site, for example) for a specified period of time ( approximately 1 hour). Standards bracketing the samples should be run at the same time and under exactly the same conditions. For a total hydrocarbon measurement, the vessel top is removed and a headspace reading is taken. With the Model 102, the headspace method is incorporated in the 102 software. Go to the headspace mode and press ENT, the peak height (maximum value) is measured automatically and displayed on the screen until the next reading is taken. This eliminates any operator error in the measurements. The concentration can then be determined by comparison to a calibration curve generated from standards. A typical calibration curve for 1, 1, 1 trichloroethane (TCA) in water is shown in Fig. 8. Note the linear response and sensitivity of this method. Robbins ( 9) described a headspace method using a polyethylene bag where samples are col-lected in a modified one quart bag. Twenty five grams of soil are added to the bag and the bag is inflated until taut. After three or four minutes of agitation, the bag is ready to be sampled. This method will also work with water samples.

5. Driscoll, JN, US Patent 4801841, “ Gas Discharge Lamp with Built in Resistance to Color Center Formation in UV Transmitting Window (1989)

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6. Driscoll and Duffy, "Photoionization Detector: A Versatile Tool for Environmental Analysis," Chrom. Forum., 2 #4, pp. 21-27 (May 1987)

7. Driscoll, “Photoionization”, CH 10 in Environmental Instrumentation Handbook by Randy Down and Jay Lehr, Wiley & sons 2005.

8. Driscoll and Becker, “Headspace Analysis of Food and Environmental Samples via a Portable PID”, Pitt. Conf. on anal. Chem., 1990

9. Robbins, “Lab in a Bag Method”, private communication, Univ. of Connecticut (1989)

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Fig. 3 Response Curve for 1, 1, 1 TCA in Water (headspace)

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3. Summary

A stable 11. 7 lamp is a key component for a PID since chloroalkanes are present in many environmental samples and cannot be detected with a 10. 6 lamp. The Snap-on heads improve the flexibility of the analyzer and minimize the equipment needed for field applications. With the expansion capabilities of the 102+, there are now nearly 40 different sensors that can be added to the multisensor head.