Photoionization detector operation

The analytes separated on GC column are determined by a halogen-specific detector, such as an electrolytic conductivity detector (ELCD) or a microcoulo-metric detector. An ECD, FID, quadrupole mass selective detector, or ion trap detector (ITD) may also be used. A photoionization detector (PID) may also be used to determine unsaturated halogenated hydrocarbons such as chlorobenzene or trichloroethylene. Among the detectors, ELCD, PID, and ECD give a lower level of detection than FID or MS. The detector operating conditions for ELCD are listed below ... 

Figure 2.19 Portable gas chromatograph. A lightweight (6.6kg), battery operated, isothermal GC. This miniature analytical engine using a capillary column and a photoionization detector is conceived for the analysis of gas and other volatile compounds (VOCs) (reproduced courtesy of Photovac). Below is an example of chromatogram obtained with such an instrument.

What is the basic principle of operation of a photoionization detector ... 

The above considerations, when translated into the practice of GC detection and quantitation mean that detectors such as the electron capture detector or the photoionization detector will greatly benefit from the columns of reduced flow-rates (provided that the detection cells can be manufactured correspondingly smaller). Further advantages of capillary columns include considerably reduced bleeding rates during the high-temperature operation as well as the already discussed column inertness. These practical gains may frequently be decisive in practical applications. 

At the temperatures and pressures generally used in gas chromatography the common carrier gases employed behave as perfect insulators. In the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged species is easily measured, providing the low sample detection limits characteristic of ionization based detectors [259]. Examples of ionization detectors in current use include the flame ionization detector (FID), thermionic ionization detector (TID), photoionization detector (PID), the electron-capture detector (ECD), and the helium ionization detector (HID). Each detector employs a different method of ion production, but in all cases the quantitative basis of detector operation corresponds to the fluctuations of an ion current in the presence of organic vapors. 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

For closed-cell detectors, including the photoionization detector, the electron-capture detector and the TCD, extracolumn band broadening can be excessive unless specially designed devices with small cell volume are used or the detector is operated at subambient pressure. At reduced column outlet pressure, the carrier-gas velocity in the detector is increased, and the cell is swept out more quickly. Extra gas, called makeup gas, can be introduced into the detector cell to sweep the cell more rapidly and reduce peak broadening and distortion. 

Photoionization detector, the photoionization detector ionizes analyte molecules with photons in the UV energy range, provides a concentration dependent signal. The photoionization detector is a selective detector that responds to aromatic compounds and olefins when operated in the 10.2 eV photon range, and it can respond to other materials with a more energetic light source. 

Pellistor type combustible sensors and photoionization detectors represent complementary, rather than competing, detection techniques. Pellistor sensors are excellent for the measurement of methane, propane and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by pellistor sensors, even when the catalytic sensor is operable in ppm measurement ranges. The best approach for VOC measurement in many cases is to use a multi-sensor instrument equipped with both a pellistor LEL sensor and a PID sensor. 

Detectors commonly used in GC and specified in the USPP include FID, alkali FID (NPD, TD), BCD, and TCD. A description of these detectors, including their operational principles and relative performance, was presented in a previous volume of this encyclopedia. Various other useful detectors for GC include photoionization (PID), flame photometric (FPD), electrolytic conductivity (BLCD), redox (RCD) and sulfur chemiluminescence (SCD), and helium ionization (HID).[4 1 Table 1 summarizes some of the features of detectors used in GC. 

The commercial softwares, initially developed by instrament manufacturers for open-access operation, were adapted to enable unattended data acquisition and automated data processing for large series of samples from an autosampler supporting the 96-well microtitre plate format, which is the sample format of choice in combinatorial synthesis. Initially, mainly Gilson 215 or 233 XL autosamplers were used, but other systems have become available from other instrument manufacturers. The complete system is under control of the MS data system. It consists of a 96-well-plate autosampler, an LC pumping system, eventually a UV-photodiode-array detector (DAD) in series and/or evaporative hght scattering (ELSD) detector in parallel, and the mass spectrometer eqnipped with ESI, APCI, and/or atmospheric-pressure photoionization (APPI). 

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