Gas Detection for VOC Measurement

PID-equipped instruments are generally the best choice for measuring VOCs at exposure limit concentrations.

• By Bob Henderson
• Oct 01, 2004

VOLATILE organic compounds (VOCs) are organic compounds characterized by their tendency to evaporate easily at room temperature. Familiar substances containing VOCs include solvents, paint thinner, and nail polish remover, as well as the vapors associated with fuels such as gasoline, diesel, heating oil, kerosene, and jet fuel. The category also includes many specific toxic substances, such as benzene, butadiene, hexane, toluene, xylene, and many others.

Increased awareness of the toxicity of these common contaminants has led to lowered exposure limits and increased requirements for direct measurement of these substances at their exposure limit concentrations. Photoionization detector-equipped instruments are increasingly being used as the detection technique of choice in these applications.

OSHA's permissible exposure limits (PELs) are designed to protect workers against the health effects of exposure to hazardous substances. The PEL is the maximum concentration of an airborne contaminant to which an unprotected worker may be exposed during the course of workplace activities. PELs are listed in 29 CFR 1910.1000, the Air Contaminants Standard, in the Z-1 Table. The table currently lists exposure limits for about 500 substances. These PELs are enforceable. Unprotected workers may not be exposed to a concentration of any substance listed in the Z-1 table that exceeds the permissible limit. It's up to the employer to determine that these exposure limits are not exceeded. In many cases, a direct reading gas detector is the primary means used to ensure the PEL has not been exceeded.

Gas detectors can be equipped with a number of different types of sensors. The type of sensor used is a function of the specific substance or class of contaminant being measured. Many toxic contaminants can be measured by means of substance-specific electrochemical sensors. Direct reading sensors are available for hydrogen sulfide, carbon monoxide, chlorine, sulfur dioxide, ammonia, phosphine, hydrogen, hydrogen cyanide, nitrogen dioxide, nitric oxide, chlorine dioxide, ethylene dioxide, ozone, and others. Although some of these sensors are cross sensitive to other substances, there is very little ambiguity when it comes to interpreting readings. When you are interested in hydrogen sulfide, you use a hydrogen sulfide sensor. When you are interested in phosphine, you use a phosphine sensor. In many cases, however, a substance-specific sensor may not be available.

VOCs are quite detectable, but usually only by means of broad-range sensors. Broad-range sensors provide an overall reading for a general class or group of chemically related contaminants. They cannot distinguish between the different contaminants they are able to detect. They provide a single aggregate reading for all of the detectable substances present at any moment.

The most widely used technique for the measurement of combustible gases and volatile organic contaminants continues to be use of a hot-bead pellistor type combustible gas sensor. Pellistor sensors detect gas by oxidizing the gas on an active bead located within the sensor. Oxidization of the gas causes heating of the active bead. The heating is proportional to the amount of gas present in the atmosphere being monitored and is used as the basis for the instrument reading. Most combustible gas reading instruments display readings in % LEL increments, with a full range of 0-100% LEL. Typically, these sensors are used to provide a hazardous condition threshold alarm set to 5% or 10% of the LEL concentration of the gases or vapors being measured. Readings are usually displayed in increments of ± 1% LEL. Hot-bead pellistor combustible gas sensors are unable to differentiate between different combustible gases.

The resolution provided by this sensing technique depends on the LEL concentration of the gas used to calibrate the sensor. As an example, consider an instrument that displays readings in ±1.0% LEL increments that is calibrated to methane. Methane has a lower explosive limit concentration of 5% volume in air. Below 5.0% volume methane, the concentration of methane to air is too low to form an ignitable mixture. Five percent is the same thing as 50,000 parts per million (ppm) methane. Because the instrument can display changes only in ±1% LEL increments, it can display only changes that are at least 1% of 50,000 ppm, or 500 ppm.

Hot-bead pellistor sensors that display readings in ±1.0% LEL increments are excellent for gases and vapors that are primarily or only of interest from the standpoint of their flammability. Many combustible gases, such as methane, do not have a permissible exposure limit. For these gases, using a sensor that expresses readings in percent LEL increments is an excellent approach. But many other combustible vapors fall into a different category. Although VOC vapors may be combustible and easily measured by means of a hot-bead sensor, they also may have a PEL that requires taking action at a much lower concentration.

Hexane provides a good example. The PEL for hexane in states that follow federal OSHA guidelines is an 8-hour time weighted average limit of 500 ppm. The NIOSH Recommended Exposure Limit (REL) followed by many states is an 8-hour TWA of only 50 ppm. The American Conference of Governmental Industrial Hygienists Threshold Limit Value® (TLV®) for hexane is also an 8-hour TWA of only 50 ppm. Many federal, state, and corporate health and safety standards require compliance with TLV® exposure limits. The lower explosive limit concentration for hexane is 1.1%.

Below 1.1% volume hexane, the concentration of hexane vapor to air is too low to form an ignitable mixture. The most commonly cited hazardous condition thresholds for combustible gas are 5% LEL or 10% LEL. Thus, with a properly calibrated combustible gas reading instrument, assuming the alarm is set at 10% LEL, it would take a concentration of 10% of 1.1%, that is, 0.11% volume hexane, to trigger an alarm. Because 1% volume equals 10,000 ppm, every 1% LEL increment for hexane is equivalent to 110 ppm. It would therefore take a concentration of 1,100 ppm hexane to trigger an alarm set to the standard 10% LEL hazardous condition threshold. Even if instruments are set to alarm at 5% LEL, it still would still require a concentration of 550 ppm to trigger the alarm.

Sensitivity Concerns
Using a combustible gas monitor to measure VOCs presents a number of other potential problems, as well. To begin with, most combustible sensors have poor sensitivity to the large molecules found in VOCs, fuels, and solvents with flashpoint temperatures higher than 100 degrees F. But even when the span sensitivity of a properly calibrated instrument has been increased sufficiently to make up for this inherent loss of sensitivity, an instrument that provides readings incremented in 1.0 percent LEL steps cannot resolve changes in concentration smaller than ± 1.0% of the LEL concentration of the substance being measured. Because percent LEL detectors are poor indicators for the presence of many VOCs, lack of a reading is not necessarily proof of the absence of hazard.

Reliance on hot-bead type LEL range pellistor sensors for measurement of VOC vapors means in many cases that the PEL, REL, or TLV® is exceeded long before the concentration of vapor is sufficient to trigger the combustible hazardous condition threshold alarm. When toxic VOCs are potentially present, it is necessary to use additional or different detection techniques that are better suited for direct measurement of VOCs at ppm toxic exposure limit concentrations. Photoionization detectors are becoming increasingly popular for this application.

It should be noted that other combustible gases and vapors may be present at the same time as toxic VOCs. Although catalytic bead sensors may have limitations with respect to the measurement of toxic VOCs at exposure limit concentrations, they are by far the most widely used and dependable method for measuring methane and other combustible gases and vapors with smaller, lighter molecules.

Recently Updated TLVs
Increasing concern with the toxicity of VOCs has led to a number of newly revised TLVs®, including those for diesel vapor, kerosene, and gasoline. Several others are currently under review. The TLV® for diesel vapor adopted in 2002 has proven to be particularly problematic and has led to the revision of numerous oil industry, maritime, and military health and safety monitoring programs. The ACGIH TLV® specifies an 8-hour time weighted average for total diesel hydrocarbons (vapor and aerosol) of 100 mg/m3. This is equivalent to approximately 15 ppm diesel vapor.

Diesel vapor always has been regarded as a potential fire hazard but largely ignored as a potential toxic vapor hazard. Diesel fuel is an NFPA Class II Combustible Liquid with a typical lower explosive limit of 0.6 percent (6,000 ppm). For diesel vapor, 1.0% LEL is equivalent to 60 ppm. Even if the instrument is properly calibrated for the detection of diesel--which is not possible for many designs--a reading of only 1.0% LEL would exceed the TLV® for diesel by 600 percent!

Because ACGIH recommendations are frequently more conservative than the OSHA PELs or NIOSH RELs, many corporate health and safety programs, especially those of multinational or prominent corporations, use the ACGIH TLVs&REG. Table 1 lists 15 common VOCs, their LEL concentration, flashpoint temperature, and their exposure limits per the OSHA PEL, NIOSH REL, and ACGIH TLV&REG. The table also identifies contaminants with toxic exposure limits lower than 5% LEL.

It goes beyond the scope of this article to argue how long it might be permissible to remain at 5% or 10% LEL without actually exceeding the 8-hour TWA or STEL. What is most striking about the list is how few of these common VOCs have 8-hour TWA exposure limits higher than 5% LEL. None of the VOCs on the list has exposure limits higher than 10% LEL.

Using Photoionization Detectors to Measure VOCs
Photoionization detectors use high-energy ultraviolet light from a lamp housed within the detector as a source of energy to remove an electron from neutrally charged VOC molecules. The electrically charged fragments are called ions. PIDs collect the charged particles on charged plates. This produces a flow of electrical current proportional to the concentration of contaminant.

The amount of energy needed to remove an electron from the target molecule is called the ionization potential (IP) for that substance. The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IP. Thus, in general, the larger the molecule, the easier it is to detect! This is exactly the opposite of the performance characteristics of the catalytic hot-bead type combustible sensor.

Photoionization detectors are easily able to provide readings at or below the TLV® for all of the VOCs listed in Table 1, including diesel.

Multi-Sensor Detectors with PIDs
Catalytic hot-bead combustible sensors and photoionization detectors represent complementary, not competing detection techniques. Catalytic hot-bead sensors are excellent for the measurement of methane, propane, and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by hot-bead sensors, even when they are operable in ppm measurement ranges.

The best approach to VOC measurement in many cases is to use a multi-sensor instrument capable of measuring all of the atmospheric hazards that may be present. Having a single instrument equipped with multiple sensors means no condition is accidentally overlooked.

In the past, photoionization detectors have tended to be bulky, temperamental, and expensive. This has changed dramatically during the past few years. Today, compact multi-sensor designs that include LEL, O₂, and electrochemical toxic sensors, as well as a miniaturized photoionization detector, have allowed this very useful detection technique to be included in many confined space, hazmat, and environmental monitoring programs.

This article originally appeared in the October 2004 issue of Occupational Health & Safety.