The Photoionization Detector PID

In the photoionization detector energy sufficient to ionize the compounds emerging from the GC is

Table 2 The electron absorption coefficients of various compounds and of classes of compound for thermal electrons

Electron absorption coefficienta Compounds and classes

Electrophores

1-10 10-100

100-1000

103-104

104-105

105-106

Aliphatic saturated, ethenoid, ethinoid and diene hydrocarbons, benzene, and cyclopentadiene Aliphatic ethers and esters, and naphthalene Aliphatic alcohols, ketones, aldehydes, amines, nitriles, monofluoro and chloro compounds

Enols, oxalate esters, stilbene, azobenzene, acetophenone, dichloro, hexafluoro and monobromo compounds

Anthracene, anhydrides, benzaldehyde, trichloro compounds, acyl chlorides

Azulene, cyclooctatretrene, cinnamaldehyde, benzophenone, monoiodo, dibromo, tri, and tetrachloro compounds, mononitro compounds

Quinones, 1,2-diketones, fumarate esters, pyruvate esters, diiodo, tribromo, polychloro, and polyfluoro compounds, dinitro compounds

None

None

Halogens

• CH:C • OH •CO•CO• Halogens CO•O•CO • Phenyl • CO • Halogens Halogens NO2

• CO • CH:CH • CO • Quinone structure Halogens

Source: After Lovelock. aValues are relative to the absorption coefficient of chlorobenzene.

supplied by a small, cheap UV lamp which can be changed over a range of wavelengths to give a certain amount of selectivity. The first commercial detector was designed to operate with packed columns but the large amount of stationary phase bleed from these columns caused rapid fouling of the quartz window of the cell facing the UV lamp and consequent deterioration of performance. Relatively simple design modifications reduced the dead volume to about 40 |L which is adequate for capillary columns of about 0.25 mm i.d. or greater. Use of capillary columns minimizes the fouling from stationary phase and also results in much sharper peaks; since the PID is a concentration-dependent detector this gives a considerable improvement in sensitivity. The main drawback of the commercial PID lies in the UV lamp sources available. Only one gives a single pure wavelength and this is of such low intensity that it offers poor sensitivity. The 10.2 eV lamp emits two wavelengths with an average energy of 10.2 eV. The inhomogeneity of the source blurs the selectivity of the detector although there is no doubt that it gives up to two orders of magnitude greater sensitivity for aromatics over aliphatic compounds with olefins somewhere in between. The lamp of higher wavelength (11.7 eV) is sufficiently energetic to ionize most compounds indiscriminately so the detector fitted with this lamp becomes more or less universal in response. The ideal version of this detector would be one with a tunable laser source and such a detector might rival the mass spectrometer in qualitative information. Unfortunately, the current cost of a tunable laser would render the detector prohibitively expensive and development on these lines is unlikely in the near future.

Solar Panel Basics

Solar Panel Basics

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