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Early photoionization detector (PID) development is credited to research done in the U.K. in the mid 1960s. An open cell PID for gas chromatography (GC) was invented by Professor Lovelock at Cambridge University. It provided improved sensitivity in an era where the thermal conductivity detector was king and where gas chromatography systems operated under vacuum, allowing the open cell PID to operate. The advent of the flame ionization detector (FID) resulted in the demise of the open cell PID as GC systems began to be operated under pressure rather than vacuum.

In the early 1970s, J. Driscoll of HNU Systems introduced a sealed UV lamp that, in effect, revolutionized PID development and paved the way for the handheld PID we know so well today. It is no coincidence that all PIDs were generically known as “h-nu”s through the mid ’90s.

Portable instrument development

In the early ’70s, portable FIDs (flame ionization detectors) were used to check for leaks and ambient exposure to harmful chemicals in the workplace atmosphere.

The vinyl chloride crisis of the mid ’70s presented a challenge that the FID could not overcome. As the acceptable exposure levels were lowered in response to health concerns, the background level of methane exceeded the proposed threshold for vinyl chloride and the FID could not discriminate between the two. This led to the rapid development of the handheld PID, and an industry was born.

In the late ’70s and early ’80s, as the Superfund program got underway, the trusty PID went from being a tool to protect workers on-site from exposure to known and unknown health hazards to a screening tool for soil and water samples.

Before long, such products as portable GC/PIDs and handheld FID units were developed. The PID was now ubiquitous! By the 1990s the first ppb PID was introduced, as was the first PID/multi-gas unit for indoor air quality and confined space entry markets.

PIDs: the early days

Early PIDs were analog units that used meters to display concentration and span pots to set response factors, pages of which were provided with the unit. The development of a smaller lamp enabled the introduction of a one-piece unit. The advent of the microprocessor allowed designers to use software to perform most of the non-detection functions of the unit, resulting in more ease of use and enhanced datalogging and averaging capability. In addition, instead of manually inputting response factors, they began to be stored in the unit’s memory. The PID detection chamber itself became shallower in profile, enabling a greater sensing range. Corresponding loss in detection, which might accompany this design change, was offset by the development of brighter lamps and smoother signal amplification.

Despite innovations in lamp shape and size, the same basic lamp energies were available in the ’70s as are available today, determined by the deployment of only a few select excitation gases. Apart from size, modern PID lamps are typically electrodeless; excitation of the lamps is by high frequency (RF) excitation by external electrodes.

While it is true that smaller lamps drove PID development, it is also true that the range of compounds detectable by PID has not changed; nothing new can be detected.

Dealing with humidity

In summary, over the years PIDs became easier to use, their range was extended, sensitivity was improved so that ppb units were introduced, but one limiting factor was not addressed, which was and still is the biggest performance limitation: humidity.

Before we can discuss the solution to the humidity problem it helps to understand the detection mechanism behind PID.

Traditional cell designs — Historically, PID cells have utilized two internal electrodes to collect and measure the ion current produced from the impact of the photon energy on the analyte vapor. Cells, or ion chambers as they are sometimes called, were originally quite large. As lamps got smaller so did path lengths, and cell volumes decreased dramatically. The result was better linearity, faster response and clean down time. Ionization mechanism — Photons emitted from the lamp produce positive and negative ions within the cell. Positive ions move to the cathode, away from window, and negative ions move to anode. Current is produced that is directly proportional to concentration. Photons also strike the cathode, causing electrons to be emitted (Einstein’s Photoelectron Effect). These photoelectrons produce background current that is detected as signal and results in a “background.”

Typically, contamination occurs in a PID due to build-up of ionic material on the cell wall. The lamp produces ionic species and, while most recombine, some condense on the cell walls. The presence of humidity on this ionic contamination results in a path for current to leak from cathode to anode along the cell wall. This is known as the “Wall Effect.” The result is a small background current that can drift up and down, for example due to uptake of water from humidity in the air.

Fence electrode cell design — The fence electrode cell contains three electrodes, has a 100 microliter volume and a 1 mm path length.

Fence electrode technology

With fence electrode technology, three electrodes are arranged in a stack one above the other. The anode (polarizing electrode) is close to the lamp window, then comes the fence electrode and, finally, the cathode or collecting electrode.

The anode is at ground as is the fence electrode. The cathode is at -200V.

The fence electrode cell ionization mechanism is somewhat different. Photons emitted from the lamp produce positive and negative ions within the cell. Positive ions move to cathode, away from window, and negative ions move to the anode. Current is produced.

Photons also strike cathode and electrons are emitted (Einstein’s Photoelectron Effect). These photoelectrons produce background current. This current is “mopped up” by the fence electrode, resulting in a low background. Contamination cannot be prevented, and even in a fence electrode cell, contamination builds up on the cell wall. The lamp still produces molecular fragments (and low volatility gases), some of which may condense on the walls. The presence of humidity on this ionic contamination still results in a path for current to leak from cathode to anode along the cell wall. However, because the fence electrode is embedded in the cell wall, it “mops up” wall current and virtually eliminates the background current that causes drift. The result is a humidity-resistant cell with no wall effect and a more stable detector.

While stable background readings are always important, their importance is overwhelming in the detection of ppb concentrations. Fence electrode technology improves performance across the board, but it is particularly effective down in the ppb range. Low backgrounds can be expected even with relative humidity up to 99 percent.

Meaningful advantages

Fence electrode technology offers meaningful advantages in real field situations. Notwithstanding the benefits of this technology, it is still important for the operator to follow conventional procedures, especially with respect to the use of the external particulate filter.

The bottom line is that PIDs equipped with fence electrode technology are resistant to humidity without the use of external devices such as tubes, which may remove important analytes.