Fiame ionization detectors

Schematic diagram of a fiame ionization detector for gas chromatography.

The thermal chromatograph is the instrument of choice for the heating of small (mg) shale samples. These are heated under controlled conditions of temperature increase under a chosen atmosphere, and volatiles produced are carried directly through thermal conductivity and/or fiame ionization detectors. After trapping of all or part of the evolved species, GC may be carried out. A typical pyrolysis profile of the total volatiles produced with respect to temperature for both types of detection is shown in Figure 11. The characteristic twin-humped curve is seen with thermal conductivity detection. The latter hump, which is absent... 

Andreae described a method for the sequential determination of arsenate, arsenite, mono-, di- and trimethyl arsine, MMAA, DMAA and trimethylarsine oxide in natural waters with detection limits of several ng/1. The arsines are volatilized from the sample by gas stripping the other species are then selectively reduced to the corresponding arsines and volatilized. The arsines are collected in a cold trap cooled with liquid nitrogen. They are then separated by slow warming of the trap or by gas chromatography, and measured with atomic absorption, electron capture and/or fiame ionization detectors. He found that these four arsenic species all occurred in natural water samples. 

The gas chromatograph is equipped with a Ni electron capture detector mounted in parallel with a fiame ionization detector ... 

In this paper, only three kinds of detectors are discussed in detail. These are the fiame ionization detector (FID), the fiame photometric detector (FPD), and the alkah fiame detector (AFD). All three are highly sensitive systems which have been used extensively in trace analyses. The FID and the FPD represent the plain fiame type, in which the fiame conductivity (FID) or a selected fight emission (FPD) are monitored. The AFD is basically an FID doped with alkali, in which usually the conductivity, occasionally the emission (6), generate the detector signal. 

Solid-probe mass spectrometric analysis (31) showed that the benzene-ether extracts consist mainly of organic acids. Therefore, these extracts were deriva-tized with dimethylsulfate-de to yield methyl-da-labeled derivatives. The derivatives were analyzed by GCMS and high resolution MS using techniques that have been described previously (31). Authentic samples of phenolic acids deriva-tized with dimethylsulfate-dg or diazomethane were also analyzed by GCMS for reference. The distribution of the organic acids as methyl esters was determined by measuring areas of GC fiame ionization detector peaks with a correction for the effective carbon number for each compound. 

By monitoring a part (e.g. 20%) to the total ion current (TIC) it is possible to follow and record the elution of compounds from the column (Fig. 1). Such a recording is similar to a gas chromatogram achieved by a fiame ionization detector. When the compound appears as a peak on TIC-recording, it is just to start the scan. The scan can be done within a few seconds, which means that the change in concentration of the compound during the scan is negligible. 

Fig. 6. Schematic diagram of a fiame ionization detector, FiD. Reproduced from i.A. Fowiis, Gas Chromatography Analytioal Chemistry by Open Learning, 1995, 2nd edn, with permission from Her Majesty s Stationery Office. Crown Copyright.

Organosulfur compounds may be analyzed by various instrumental techniques, including GC, HPLC, and GC/MS. A fiame photometric or flame ionization detector is suitable for GC analysis. Akintonwa (1985) described reversed-phase HPLC separation of thiourea, thioacetamide, and phenobarbi-tone for screening of the purity of substances for toxicological evaluation. 

The FID is the most widely used GC detector, and is an example of the ionization detectors invented specifically for GC. The column effluent is burned in a small oxy-hydrogen fiame producing some ions in the process. These ions are collected and form a small current that becomes the signal. When no sample is being burned, there should be little ionization, the small current (10 a) arising from impurities in the hydrogen and air supplies. Thus, the FID is a specific property-type detector with characteristic high sensitivity. 

No detectors for LC are as universally applicable as the fiame ionization and thermal conductivity detectors for gas chromatography described in Section 27B-4. GC detectors were specifically developed to measure small concentrations of analytes in flowing gas streams. On the other hand, LC detectors have often been traditional analytical instruments adapted with flow cells to measure low concentrations of solutes in liquid streams. A major challenge in the development of LC has been in adapting and improving such devices. ... 

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