Photoionization Detector Lamp Improvements

By: Carl Carelli and Robert F. Wallace, Reporter US Volume 25.3

Carl Carelli1 and Robert F. Wallace2

1. Andrews Glass Co., Vineland, New Jersey, USA, e-mail: ccarelli@andrews-glass.com
2. Supelco, Bellefonte, Pennsylvania, USA, e-mail:bob.wallace@sial.com

Introduction

The photoionization detector (PID) lamp is a valuable tool for chromatographers and others investigating the presence of volatile organic compounds (VOCs). Major applications include laboratory analyses of air, water, and soil samples; on-site field monitoring of air, water, and soil; and personnel safety monitoring in confined spaces. In fact, it continues to be the preferred choice for detecting VOCs after nearly 50 years of use due to its fast response time and sensitivity; the PID can detect volatiles as low as 1 ppb. Recent improvement in both manufacturing processes and packaging should allow the PID to remain a top detector choice well into the future.


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How the PID Lamp Works

The PID is a glow-discharge lamp, which when excited, releases photons into an ionization chamber. The combination of krypton gas and a magnesium fluoride lens (typical config-uration) operates at wavelengths 116.6 nm and 123.6 nm, or the equivalent of 10.0 and 10.6 eV. An eV is an electron volt, which is the energy gained when a particle of one electronic charge is accelerated through a potential of one volt. The term eV is used as a convenient way to express the radiant strength of a PID lamp.

When the “lamp is lit” it radiates into the ionization chamber and passes between a pair of electrode plates. As the test gas moves into the radiated field between the electrodes, it is ionized and the free electrons are collected at the electrodes. This produces a current flow whose magnitude is in direct proportion to the gas concentration.

Each gas has its own ionization potential (IP), also expressed as an eV. Gases with an IP at or below the eV output of the lamp will be detected. The most popular PID is the 10.0/10.6 eV lamp, because it detects most VOCs, lasts longer, is easier to clean, and is less expensive.

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Recent Lamp Improvements

Andrews Glass Co., a recognized leader in specialty glass products, has acquired the PID lamp product line from Scientific Services Co., pioneers of the PID lamp. Coupling the innovativeness and proprietary fabrication processes of Scientific Services Co. with the craftsmanship of Andrews Glass Co. allows the continued supply of the first, the best, and the broadest range of PID lamps.

Andrews Glass Co. now manufactures PID lamps in their ISO-compliant facility, providing the highest possible quality at critical points in manufacturing. Every step in the manufacturing manufacturing process has precise documentation to ensure the highest standards of quality in every PID lamp produced. Precision cutting, glass-to-metal sealing, and glass sealing all provide excellent protection against leaks. The use of a state-of-the-art plasma welder to achieve the critical metal-to-metal seal is inherent in lamp construction. This helps to eliminate any possibility of microscopic voids in the seal that would result in the slow leak of the krypton or xenon gas from the lamp. Precise control of dimensions also ensures a proper fit in the GC instrument.

With stringent quality control standards each lamp is tested and inspected prior to packaging to ensure they are in excellent operating condition. Quality and longevity tests are continually conducted to ensure the level of performance is maintained. The longevity tests are conducted under conditions simulating normal PID operation.

In addition to the changes in manufacturing, the way the lamps are packaged has been changed. PID lamps are now placed into plastic bags that contain a desiccant. The bag is then packaged into a cardboard form instead of a foam insert. Both these changes are designed to minimize exposure to moisture during shipment and storage to help extend the longevity of the PID lamp

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Conclusion

Sensitivity and stability are the two primary areas of PID lamp performance that are most important to GC users. Recent changes in PID lamp production and packaging have been done to improve performance in these areas. PID lamps now being produced are very good and very consistent, from one lamp to the next.

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