PHOTOIONIZATION DETECTION
Photoionization detector (PID) is a well-known powerful analytical tool. The idea to use current, formed in gas mixtures under vacuum ultraviolet (VUV) light, for the detection of mixture components, was born as early as 50s of last century. At first, PID with undivided radiation and ionization volumes (so called windowless) where the noble gas passing through both volumes is used to provide VUV emission has been designed, Due to necessity of noble gas flow for VUV emission support, this type of PID finds a limited application only as gas chromatography (GC) detector. In the middle of 60s it was proposed to separate ionization and radiation volumes, and PID having sealed VUV-lamp and ionization chamber has been designed.
Nowadays thousands of PID based on this principle enjoy worldwide application. PIDs are used both in GC and gas monitors. These versions significantly differ from each other due to different operating conditions (frowrate, temperature etc.). This paper deals with PID for gas monitors.
OPERATION
The obligatory condition for PID use is
where Ei – component ionization potential;
Ep – energy emitted by VUV-lamp.
There is no difference what type of component is to be detected: organic or inorganic, but in so doing sensitivity of PID depends on chemical structure of components detected.
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Photoionization detection of impurities air in is based on the fact that may be called a ”present” made by nature to analysts. The fact is that energy of photons emitted by VUV-lamp is sufficient to ionize the majority of the most common air contaminants, but is insufficient to ionize the components of clean air (oxygen, nitrogen, water, argon, carbon dioxide). Consequently, when air flows through PID, ionization current is a measure of air contamination.
The principle of PID operation gives it the obvious advantages over the instruments using the other methods of detection.The photoionization occurs in the volume, the inner surface of ionization chamber is free of any chemical reactions which could affect on result of measurement or make PID inoperative. That differs PID from the themocatalytic and the semiconductor sensors, which suffer from, for example, sulfurous substances.
PID is highly tolerant of high concentration attack, even of PID is not limited by the total product concentration exposure in contrast to electrochemical sensors. For organics monitoring PID and flame ionization detectors (FID) have close characteristics ( besides C1 - C2). The advantage of PID over FID is a more simple design because PID is free of flame and additional gases.
APPLICATIONS
The detection limit of PID to the great number of compounds is at low ppm level. Such a sensitivity enables to use PID in Industrial Hygiene for monitoring hazardous compounds below MAK and PEL levels. òàáëèöà ñ ïîòåíöèàëàìè èîíèçàöèè. The PID fast response ( s) allows to locate leakage. The versatility of PID gives also the opportunity to use PID for waste monitoring. It is also conveniently to use PID to detect places for air sampling. Low power consumption and small size measurement unit allow to design hand-held PID instruments. PID monitors also are available in intrinsically save version.
Delusions
Despite a long-standing practice of PID use a number of delusion are widely believed. Delusion Nr1. PID significantly suffers from contamination because appearance of a film on the surface of the lamp window that reduces intensity of VUV output. Perhaps, that opinion came from gas chromatography, where surface of window periodically is to be cleaned. Nevertheless operating condition PID at GC are basically other, than in gas monitor. Photoionization lamp of GC PID operates in oxygen free atmosphere. Photoionization lamp of gas monitor operates in air. Ozone, formed in ionization chamber, takes part in oxidation processes and prevents appearance of organic film on the surface of window. It was shown that after 200 hours operating PID with CHROMDET 10.6 lamp in air containing 2000 ppm of kerosine the decrease of VUV output was no more than 10 %. No film on the lamp window was found with the microscope.
Delusion Nr.2. PID features elevated drift and needs a frequent calibration. This delusion sometimes is met in reviews. One of the possible causes of PID drift – the absorption of VUV-light by the film on the lamp window, was discussed above. The second cause is the variation of VUV output of the lamp in time, which is determined by VUV sources quality only. The variation of VUV output of CHROMDET lamps 10.6 is less than 5 x 10-3 %/hour. Our experience shows that it is enough to calibrate PID every 6 months
Delusion Nr.3 All the PID have large instrumental error at high humidity. Readings of some commercially available PID, indeed, greatly depend on humidity. That is because of VUV interaction with water vapor. In some instruments air humidity is measured to correct reading. Photoionization detector PHD-2 is free of that drawback. Humidity variation in the range between 0 – 85% gives reading deviation no more than 7%. No measurement of humidity and correction are necessary.
Delusion Nr. 4. PID offers low selectivity. This parameter is governed by emission spectra of VUV source and IP of the substances. The smaller is IP, the greater is the selectivity. Lamps, emitting photons having energy more than 11 eV , of course. can not provide high selectivity of detection. To provide different selectivity CHROMDET offers special lamps with wide variety of energies.
THE LAMPS
The PID lamps differ in operation mode, design, window material, filling gas. DC and RF modes of the lamp operation are in use. DC lamps contain electrodes. RF lamps are electrodeless. The emission spectra of gas discharge plasma are determined by gas filling and the window material. One of the main questions interesting for users is what type of lamp should be applied. For the adequate choice of the PID lamp or detector further we will clarify some features of lamps operation. Because analytical possibilities of detectors supplied with both DC and RF lamps are closely allied, other criteria , such as stability, life service and etc. are to be used. During lamp operation filling gas composition changes. That is due to gas absorption by and gas liberation from interior parts of the lamp. These phenomena deteriorate the lamp characteristics in particular stability, life service and the selectivity of PID as well. However DC and RF lamps have different degradation of gas composition. In DC lamps metal vapor appearing due to cathode sputtering absorbs molecular gases liberated from the glass wall of the lamp. Electrodeless lamps have no such a sputtering and molecular gases can change the emitting spectra.
PID available from CHROMDET uses only the DC lamps supplied with titanic electrodes. This ensures a high stability and constancy of spectra for a long period. Besides gas composition life service of PID lamps depends on window condition. The deposition of the metal vapor appearing due to cathode sputtering results in the decrease of window transparency. The intensity of sputtering and consequently the film rise off depends on discharge current and voltage. Because of this it is important to keep these parameters as low as possible. The discharge current of CHROMDET photoionization lamps usually is in the range from 0,1 to 0,3 mA. Operating voltage of the lamps available from CHROMET is only 250V. That ensures the low sputtering, which is however enough for absorbing of molecular gases. The intensity of the lamp having discharge current 0,15 mA decreased no more than 40 % after 9000 hour operation.
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