Q3 2009 / Photoionization for gas detection

A Photoionization Detector (PID) is a versatile gas detector used for measurement and detection of many Volatile Organic Compounds (VOCs) at ranges as low as parts-per-billion, or parts-per-trillion, using a specialized detector. PID configurations can range from micro-sensors (less than one square inch in size) used in portable instruments up to larger, fixed detectors used in on-line instrumentation such as gas chromatographs.

PID micro-sensors (Figure 1) are used primarily in portable battery powered instrumentation and display a total VOC reading. They do not distinguish one compound from another. These micro-sensors differ from gas chromatograph detectors in that they are used to monitor ambient air and therefore do not require a carrier gas.

Figure 1: A micro photoionization sensor. PID micro-sensors are used in portable battery powered instruments. They monitor ambient air and do not require a carrier gas. Actual size 17.6mm x 20.4mm

Portable instruments that include a PID are used for indoor air quality, hazardous waste and remediation sites, and as personal safety monitors. In these instruments, the micro-sensor is used alongside other micro-sensors using additional technologies to monitor many gas hazards. These other sensors compliment the PID by monitoring toxic and combustible gases. The most common models detect oxygen, carbon monoxide, hydrogen sulfide, and sulfur dioxide.

Large, fixed PIDs

The larger, fixed PIDs are used in field and laboratory analyzers like fixed total VOC monitors and gas chromatographs. However, like the micro-sensors, fixed VOC monitors display the total concentration of VOCs.

These larger analyzers offer many advantages over the micro-sensors. Larger analyzers are easier to integrate into existing systems, they are capable of analyzing multiple sample points, and can also have auto-calibration and data-output capabilities. 

Figure 2: Ionization potential examples. Different chemical ionization potentials are required by each industry.

PID-based gas chromatographs are used to analyze separated samples. They display specific chemical concentrations at low levels. In these units a carrier gas is required to move the sample through a chromatographic column to the detector. These PIDs are versatile using a broad range of carrier gases. This allows them to be more easily integrated into existing systems than other detector technologies.

Nitrogen and helium are the most common carrier gases, but hydrogen can also be used. Hydrogen’s advantage is that it is less expensive and readily available. Using a hydrogen generator can also eliminate the need for an additional gas cylinder.

How photoinozation works

Photoionization is the process by which a photo-excited electron absorbs enough radiant energy to be ejected from an atom or molecule. The Ionization Potential (IP) is the amount of energy required to eject the electron from the molecule and is measured in electron volts (eVs). Some examples of chemical IPs found in different industries are shown in Figure 2. 

Figure 3: A high-energy ultraviolet photoionization lamp.

High-energy ultraviolet (UV) lamps (105 to 147 nanometer wavelengths) (Figure 3) in a PID can provide energies ranging from 8.4eV to 11.7eV.

A UV lamp with energy greater than the IP of the molecule to be analyzed is required to eject the electron from the molecule. By choosing different energy UV lamps the analysis can be more selective or expansive. The electrons ejected from the molecule by the UV lamp are directed to a measuring circuit by a negatively-charged polarizing electrode. The measurement circuit senses the electron stream as an electric current that is proportional to the number of those molecules in the detector. This current is sent to a high-gain amplifier and then reported as a concentration by the analyzer (Figure 4). 

Figure 4: Different energy UV lamps produce analysis that can be either selective or expansive. Here the electron is shown ejected from the molecule, directed to a measuring circuit, and measured as a concentration.

Three common lamps

The first lamp is the 9.6eV. It is the most selective since many common VOCs have an IP greater than 9.6eV. The 9.6eV lamp is used in many portable VOC detectors when the target gas of choice needs to be isolated from other VOCs – such as when benzene (9.25eV) and acetaldehyde (10.22eV) are present together with benzene as the target.

The second lamp is the 10.6eV. This lamp is by far the most common lamp used in almost every setting. This is due to the fact that many common VOCs have an IP less than 10.6eV, while common background gases such as nitrogen (15.58eV) do not.

The third lamp is the 11.7eV. This lamp is capable of detecting many different compounds due to its high ionization energy. However, 11.7eV lamps tend to have a short operating service life which makes them impractical for most applications outside of a laboratory.

Each of these lamp’s energies is determined by the type of gas filling the lamp, and the type of material used in the window attached to the lamp. Different fill-gases provide different spectrums and different window materials block out different wavelengths of the spectrum emitted by the electrically-excited plasma.

Response factor

VOC analyzers equipped with a PID are used to detect a broad range of compounds. Total VOC instruments are typically calibrated at the factory or in the field with isobutylene. However, PIDs react with different levels of sensitivity to different chemicals. This means that since the IP of all chemicals are constants, a more accurate reading of a specific compound can be calculated without recalibrating the instrument for different target gases. In fact, the concentration can be adjusted using a Relative Response Factor (RRF) as seen in this equation. 

...

Sa = Actual sample concentration

Sx = Sample concentration reading

Ca = Actual calibration sample concentration

Cx = Calibration sample concentration reading

with 100 ppm of isobutylene, the PID produces a reading of 200 ppm. Using these numbers it is possible to calculate the RRF as follows: 

...

This shows that a PID is twice as sensitive to benzene as it is to isobutylene. In order to get an accurate measurement, simply multiply the displayed concentration by the appropriate RRF (200 ppm * 0.5 = 100 ppm).

This relative response factor adjustment is invaluable in the field since it is impractical to recalibrate the instrument for each target gas.

Detector comparision

For every application in gas analysis there is a choice of which detector is the right one for the job. The unique feature of a PID is that it is non-destructive and offers selective ionization of compounds. Some of the most common uses for a PID are to detect the groups of aromatics, alkenes, alkynes, and amines (Figure 5). 

Figure 5: A graph of benzene, toluene, ethylbenzene, and xylene analysis using a high-sensitivity photoionization detector.

Compared to other ionization type detectors, a PID is quite easy to maintain and only requires periodic cleaning of the lamp. If an analysis needs to be more selective or expansive, it is simple to replace the lamp with one having higher or lower energy to match the IP of the target compound.

When using a PID micro-sensor, high humidity tends to have a quenching effect on the PID since water absorbs UV energy. This UV absorption reduces the number of photons reaching the component of interest, reducing its response. This problem is easily avoided in gas chromatographs since the detector is only exposed to the carrier gas flow.

The adverse effects on PID micro-sensors can be lessened by regular maintenance and cleaning. Since the sample molecule is only ionized, not destroyed once it leaves the detector, it captures a free electron and is “reconstituted.” This allows, in a sealed detector design, “redetecting” the chemical with another type of detector like a Flame Ionization Detector (FID) or a Thermal Conductivity Detector (TCD) and can create several layers of selectivity.

FIDs ionize the carbon in a sample by combustion. Any sample containing carbon atoms can be detected using an FID. FIDs have less sensitivity than PIDs and also require hydrogen fuel, combustion air, and extra precautions due to the open flame. Also, since an FID destroys the sample, it cannot be run serially into another detector.

Helium Ionization Detectors (HIDs) use a DC-arc to create excited helium atoms that are capable of ionizing trace levels of noble and atmospheric gases that have IPs that are much higher than a PID is able to detect. However, HIDs can only be used in gas chromatographs, require helium as a carrier, and are much more expensive to maintain than PIDs.

TCDs analyze the sample by comparing its thermal conductivity to a reference gas via a wheatstone bridge without destroying the sample. TCDs are generally universal detectors, but they are much less sensitive than PIDs.

Mass spectrometers are extremely versatile detectors, but require more support equipment and maintenance than PIDs. Only recently has the price and physical profile of mass spectrometers decreased enough to make them feasible for field instrumentation.

Summary

The photoionization detector has become a valuable tool for chemical analysis due to the long list of compounds it is capable of analyzing and offers simple tools, which broaden its capabilities such as selective IP and RRFs – without the need for a fuel gas. In addition, they have minimal support gas requirements.

For all these reasons PIDs are now used in numerous industries. Soft drink manufacturers, for example, have high demand for PID-based gas chromatographs to monitor trace aromatics in their products (Figure 6). 

Figure 6: The Baseline-MOCON BevAlert Model 8900 Gas Chromatograph using PID monitors acetaldehyde, BTEX, methanol, vinyl chloride, and total sulfur content for the beverage industry.

The chemical and medical industries also use PIDs to monitor workplace exposure limits. PID micro-sensors in portable instruments are also used by the military and police to test for VOCs.

As air quality issues continue to grow in the public consciousness there is increasing demand for PID-based VOC monitors for schools and commercial buildings. Advances in PID technology continue to lower the detection limits, expanding the opportunities for PIDs in new applications. In conclusion, many of today’s industries have critical VOC monitoring needs that can be met with this versatile, easy-to-use, and cost-effective technology.

Ben Kahn

Ben Kahn is the Service Manager for Baseline-MOCON, Inc. located in Lyons, CO. He has over 20 years experience in both manufacturing and service of PID and FID-based gas chromatographs. Ben can be reached at: Ben.kahn@baselineindustries.com

Adam Gniewek

Adam Gniewek is the Sales and Marketing Coordinator for Baseline-MOCON, Inc., an industry leader in the development and manufacture of gas analyzers and PID micro-sensors. Gniewek earned a BS in Mathematics from the University of Northern Colorado in Greeley, CO. He can be reached at: adamg@baselineindustries.com