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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 1 square inch in size) used in portable instruments 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, not distinguishing one compound from another. The micro-sensor differs from the gas chromatograph detector as it is used to monitor ambient air and therefore does not require a carrier gas. Portable instruments that include a PID are used for indoor air quality, hazardous waste & remediation sites, and as personal safety monitors. In these instruments, the micro sensor is used alongside other micro sensors using other technologies to monitor many gas hazards. These other sensors compliment the PID by monitoring toxic and combustible gases; the most common detect oxygen, carbon monoxide, hydrogen sulfide, and sulfur dioxide.
Fig. 1: A micro photoionization sensor.
The larger fixed PIDs are used in field and laboratory analyzers such as fixed total VOC monitors and gas chromatographs. Like the micro-sensors, fixed VOC monitors display the total concentration of VOCs. There are many advantages to having the larger analyzer rather than the micro sensor. The larger analyzer is easier to integrate into an existing system, it is capable of analyzing multiple sample points, and could also have auto calibration and data output capabilities.
PID based gas chromatographs are used to analyze a separated sample and therefore display specific chemical concentrations at very low levels. A carrier gas is required to move the sample through a chromatographic column to the detector. The PID is very versatile and can use a broad range of carrier gases; this allows the PID to be more easily integrated into existing systems than many other detector technologies. Nitrogen and helium are the most common carrier gases, but hydrogen can also be used as it is less expensive and readily available. Using a hydrogen generator can also eliminate the need for another gas cylinder.
HOW IT WORKS / IP
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.
Fig. 2: Ionization potential examples
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.
Fig. 3: A photoionization lamp.
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).
Fig. 4: The electron is ejected from the molecule, directed to a measuring circuit, and measured as a concentration
The three most commonly used lamps are:
9.6eV Lamp – The most selective lamp 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 being the target.
10.6eV Lamp – By far the most common lamp used in almost every setting since many common VOCs have an IP less than 10.6eV while common background gases such as nitrogen (15.58eV) do not.
11.7eV Lamp – A lamp capable of detecting many compounds due to its high ionization energy. 11.7eV lamps tend to have a very short operating lifespan which makes them impractical for most applications outside of the 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 can be used to block out different wavelengths of the spectrum that is emitted by the electrically excited plasma.
RESPONSE FACTOR
VOC analyzers with a PID are used to detect a broad range of compounds. Total VOC instruments are normally calibrated at the factory or in the field with isobutylene, but PIDs react with different levels of sensitivity to different chemicals. Since IPs of chemicals are constants, a more accurate reading of a specific compound can be calculated without recalibrating the instrument for a different target gas. The concentration can be adjusted using a Relative Response Factor (RRF).
Sa = Actual sample concentration
Sx = Sample concentration reading
Ca = Actual calibration sample concentration
Cx = Calibration sample concentration reading
For example, measuring a 100 parts per million (ppm) benzene standard after calibrating with 100 ppm of isobutylene the PID produces a reading of 200 ppm. Using these numbers we can calculate the RRF:
This shows that a PID is twice as sensitive to benzene as it is to isobutylene; in order to get an accurate measurement, we 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 with each target gas.
COMPARISIONS
For every application in gas analysis there is a choice of which detector is the right one for the job. The distinct feature of a PID is its non-destructive and 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).
Fig. 5: 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 that is 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 an FID or TCD and can create several layers of selectivity.
Flame ionization detectors (FIDs) ionize the carbon in a sample by combustion. Any sample containing carbon atoms can be detected using an FID. FIDs have worse sensitivity than PIDs and also require hydrogen fuel, combustion air, and extra precautions due to the open flame. Since the 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. HIDs can only be used in gas chromatographs, require helium as a carrier, and are much more expensive up front and to maintain than PIDs.
Thermal conductivity detectors (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 started to go down to be feasible for use in 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, simple tools which broaden its capabilities (selective IP and RRFs) without the need for a fuel gas, and its minimal support gas requirements. It is for these reasons that PIDs are used in numerous industries. Soft drink manufacturers have high demand for PID based gas chromatographs to monitor trace aromatics in their products (Figure 6).
Fig. 6: The Baseline BevAlert Model 9100 Gas Chromatograph using a PID is used to monitor acetaldehyde, BTEX, methanol, vinyl chloride, and total sulfur content for the beverage industry
The chemical and medical industries use PIDs to monitor workplace exposure limits. PID micro-sensors in portable instruments are 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 industries’ critical VOC monitoring needs are met with a single versatile, easy to use, flexible, and cost-effective solution: PIDs.
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