Detectors photoionisation

PIDs were originally developed in the 1960s but it was 1976 before the first commercial detectors were available. Considerable developments have 

Ionisation is initiated by absorption of a photon, hv, by the analyte molecule, A. Direct ionisation results in a one step formation of ions and indirect ionisation involves initial excitation of the carrier gas, G, which then transfers energy to the analyte molecules forming ions  

Successful ionisation of analytes depends on their ionisation potential and the energy of the lamp. The most useful source is the krypton lamp of 

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Ionisation detectors. An important characteristic of the common carrier gases is that they behave as perfect insulators at normal temperatures and pressures. The increased conductivity due to the presence of a few charged molecules in the effluent from the column thus provides the high sensitivity which is a feature of the ionisation based detectors. Ionisation detectors in current use include the flame ionisation detector (FID), thermionic ionisation detector (TID), photoionisation detector (PID) and electron capture detector (ECD) each, of course, employing a different method to generate an ion current. The two most widely used ionisation detectors are, however, the FID and ECD and these are described below. 

Figure 2.16—A miniature chromatograph. Instrument using a capillary column and a photoionisation detector. The instrument, weighing 4 kg including the carrier gas (C02), is mainly used for the analysis of volatile organic compounds (VOCs) in air pollution. The photoionisation detector, which is of limited use because of its variable sensitivity, is well suited for the analysis of hydrocarbons. The high powered UV source emits photons that have energies between 10 and 11 eV, ionising the compounds that exit the column, with the exception of the carrier gas. The ionic current generated is amplified using an electrometer and is proportional to the concentration of analytes (reproduced by permission of Photovac).

For the gas chromatographic method no detector is specified flame ionisation seems to be the best choice. Lack of selectivity in the method can lead to interference by compounds that are not completely removed by the clean-up. Higher selectivity can be achieved by use of a photoionisation detector. In addition some pairs of the PAH isomers are incompletely separated by the 15cm column used in this method, while the heavier PAHs often show tailing peaks. 

Vien and Fry [762] have reported a gas chromatographic determination of arsenic, selenium, tin and antimony in natural waters. The gaseous hydrides are generated, concentrated on a cold trap, and then injected into the gas chromatograph with the use of drying agents or carbon dioxide scrubbing. A specially conditioned Tenax column suppresses unwanted byproduct elution and separates the volatile hydrides at room temperature. A photoionisation detector was used and the authors reported a detection limit as low as 0.001 pg L ... 

Cutter and Oatts [806] determined dissolved sulphide and studied sedimentary sulphur speciation using gas chromatography in conjunction with a photoionisation detector. The... 

A detector that is especially sensitive to sulphur compounds is critical to the gas-chromatography system. There are three types of detectors that are sensitive to ppb concentrations of sulphur compounds the flame photometric detector (FPD) the Hall electrolytic conductivity detector (HECD) and the photoionisation detector (PID). The advantages and disadvantages of each are summarised in Table 8-II. 

Photoionisation detector (PID) Sample not destroyed by analysis Same sensitivity as FPD and HECD Not sulphur specific (responds to a variety of compounds)... 

Fig. 8-12. Schematic of high-temperature photoionisation detector (reproduced with permission of American Laboratory from Driscoll et al., 1978).

The photoionisation detector (PID) is used for the selective determination of aromatic hydrocarbons and unsaturated compounds such as aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics and some organometallics. It is relatively insensitive to saturated hydrocarbons and halocarbons. This device uses ultraviolet light as a means of ionising the analytes exiting the GC column and the ions produced by this process... 

Photoionisation detector, PID component molecules in the detector are ionised by photons from a high energy ultraviolet source, selectivity can be achieved by using a different source, e.g. 10.2eV low energy krypton lamp for aromatics and alkenes, 11.7eV argon lamp for alkanes, halogenated compounds as well as aromatics. Sensitivity is similar to a FID, 10 to 10- 2gs-. ... 

A paper has been published showing the use of the photoionisation detector (Sim and co-workers [6]. The photoionisation detector is to a certain extent specific in that only compounds that can be ionised by a ultraviolet (UV) lamp will give a response. The solvents used were dichloromethane and acetonitrile, both of which should have little response in the photoionisation detector. However, a clear sharp solvent peak was observed. 

The amount detected by this system (0.3 pg on column) was below the level which could have been determined using a flame ionisation detector. Initial indications show that the photoionisation detector may be a very useful detector for people who wish to detect lower levels on the SFC and cannot concentrate their sample. 

The flame ionisation detector (FID) has been used in analytical SFC and could be used in preparative SFC by by-passing a very small fraction of the mobile phase flow through to the detector. Obviously the FID could not operate in a largely carbon dioxide atmosphere and would respond to many modifiers. The photoionisation detector (PID) has been used in analytical SFC but is better suited to highly sensitive detection of specific types of compound. 

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