Microcoulometric detector

The methyl esters of herbicides are analyzed by GC using an electron capture detector. Microcoulometric detector or electrolytic conductivity detector may alternatively be used. GC/MS, if available, should be employed to confirm the presence of analytes. Other instrumental techniques include GC-FID and HPLC. The latter can measure the acids for which esterification is not required. 

Positive identification of low-ppb (pg/L) levels of endosulfan in human blood has been achieved by GC equipped with a microcoulometric detector (GC/MC) (Griffith and Blanke 1974). Although GC/MC is specific and nearly as sensitive as GC/ECD for detecting endosulfan in blood, GC/MC is more difficult to operate. Both isomers of endosulfan can be measured in blood using a method described by Guardino et al. (1996). According to the authors, endosulfan can be recovered and measured with an approximate limit of quantitation (LOQ) of 0.2 pg/L (sub-ppb). 

ECD = electron capture detector GC = gas chromatography HPLC = high-performance liquid chromatography MC = microcoulometric detector MS = mass spectrometry NICI = negative ion chemical ionization RSD = relative standard deviation SPE = solid phase extraction... 

Microcoulometric titration is used as the detection mode in some commercial sulfur-specific analysers. Sulfur in PP and waxes (range from 0.6 to 6 ppm S) were determined by means of an oxidative coulometric procedure [537]. The coulometric electrochemical array detector was used for determining a variety of synthetic phenolic antioxidants (PG, THBP, TBHQ, NDGA, BHA, OG, Ionox 100, BHT, DG) in food and oils [538],... 

GC/ECD and GC equipped with a microcoulometric detector have been used to determine heptachlor and heptachlor epoxide in a variety of human tissues, including the liver, brain, adrenals, lungs, heart, kidneys, spleen, and pancreas (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). Details of a sample preparation method were not reported for GC equipped with a microcoulometric detector (Curley et al. 1969). Sample preparation steps for GC/ECD include homogenization, extraction with petroleum ether or hexane, usually followed by a clean-up procedure (Klemmer et al. 1977 Radomski et al. 1968). Recovery, sensitivity, and precision data were not reported (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). 

The halogen selective microcoulometric and electrolytic conductivity detectors have been applied extensively to the analysis of halogenated compounds in drinking water. Both have adequate sensitivity for the application and sufficient selectivity to allow reasonably accurate identifications with the retention time data. However, these detectors are also best applied to well defined samples where the probability of unexpected compounds is low. 

Pesticides and Fungicides. Modern pure food regulations require that the food processor be responsible for their finished products. Since so many pesticides and fungicides are used in agriculture, their detection and quantitative analysis are difficult (5, 22). Organophosphorus and chlorinated hydrocarbons are the most common pesticides. When GLC is used for halogens, electron capture or microcoulometric detectors are used for phosphorus, a thermionic flame photometric detector is required. 

GAS CHROMATOGRAPHIC SIMULATED DISTILLATION (FIDl AND MICROCOULOMETRIC DETECTORS)... 

Most published methods are for analysis of crops and soil residues of the intact acaricides. Extraction has been done by stripping, blender or soxhlet. Extraction solvents have included petroleum ether, benzene, carbon tetrachloride, acetonitrile, diethyl ether, methanol and hexane/acetone. Clean-up steps have em -ployed liquid/liquid partitioning and adsorption on activated charcoal, activated charcoal/Florisil, Florisil, alumina and silica gel. Burke (14) reported that CB is not completely recovered from Florisil. Horn and coworkers (7) found that no clean-up was necessary when analyzing dog urine for CB using a Schecter-Haller procedure. For detection of residues, the colorimetric and UV methods have been replaced by gas chromatographic methods employing microcoulometric or electron capture detectors. 

Ramsteiner et al. [156] compared alkali flame ionisation, microcoulometric, flame photometric and electrolytic conductivity detectors for the determination of triazine herbicides in water. Methanol extracts were cleaned up on an alumina column and 12 herbicides were determined by gas chromatography with use of conventional columns containing 3% Carbowax 20m on 80-100 mesh Chromosorb G. 

At a concentration level of lppm sulphur compounds, the analytical error does not exceed 8% for the flame ionisation detector and 12% for the microcoulometric detector, of the given amount of the compound. When analysing solutions with concentrations of... 

The non-polar chlorinated hydrocarbon pesticides are routinely quantified using gas chromatography (GC) and electron capture(EC) detection. Alternate detectors include electrolytic conductivity and microcoulometric systems. Organophosphate pesticides which are amenable to GC are responsive to either the flame photometric detector (FPD) or the alkali flame detector (AFD). Sulfur containing compounds respond in the electrolytic conductivity or flame photometric detectors. Nitrogen containing pesticides or metabolites are generally detected with alkali flame or electrolytic conductivity detectors. 

Nitrogen a Dohrmann microcoulometric reactor being used in reductive mode was replaced with an Antek Model 771 pyroreactor which uses a chemiluminescent nitrogen detector. 

Extracts from all samples were analyzed by gas chromatography as described by Hindin, May, and Dunstan (5). After the residue had been taken up in a measured volume (0.1 or 1.0 ml.) of petroleum naphtha, a 10-/Jiter aliquot was injected into a Beckman temperature programmer coupled with a Dohramann microcoulometric titration detector. The column-packing material used for the separation of the insecticides was 5% by weight of EPON 1001 on 60/80-mesh acid-washed, flux-calcined diatomite (Chromosorb P). This material was packed in aluminum tubing % inch in o.d. by 3 feet. 

Various procedures described by Crippen and Smith (5) are available for identifying components in a chromatogram. Because of the limitations mentioned above, the choice of method was restricted. Peak identifications for this sample relied on cleanup and preliminary separation by column adsorption chromatography with activated alumina and Florisil, use of the chlorine- and sulfur-specific microcoulometric detector, and parallel injections onto three or more columns containing liquid substrates varying widely in polarity. 

Analysis by Gas Chromatography. All soil samples were analyzed by gas chromatography using a Dohrmann instrument equipped with a microcoulometric detector and an 18-inch, 5% Dow 11 column. This column did not separate the individual pesticides present, indicating only the total halide content of the sample (10). All results were calculated in terms of DDT and based on the dry weight of the sample. 

It may be mentioned that the concept of choosing a derivative with a particular detector in mind is quite frequently employed in residue analysis. And with the development of more diversified selective detectors, we are sure to see more of it. Thiophosphoryl derivatives of phenols for the flame photometric detector (59), nitrophenyl derivatives of amines and thiols (60) and brominated anilines for the EC detector (61), chloro-acetylated phenols for the microcoulometric detector (62), and many other examples (63) would be worth mentioning. The selectivity of a chemical reaction combined with the selectivity of a gas chromatographic detector can provide superior analytical eflBciency. 

Gas Chromatographic Measurement and Identification of Pesticide Residues with Electron Capture, Microcoulometric, and Electrical Conductivity Detectors... 

As the title of this paper indicates, it is concerned only with the use of the electron capture, microcoulometric, and electrolytic conductivity detectors for quantitation and identification of pesticide residues. Further background information on gas chromatographic detectors may be found in the paper by Westlake and Gunther (I). 

The latest production model of the microcoulometric detection system has a minimum detectable limit of about 1 ng of chlorine but, in the writer s opinion, the minimum for practical use is about 3 ng. This estimate is based upon responses obtained by Dohrmann Instruments personnel with standard solutions of pure lindane. These results were obtained in mode I operation (gas flow into the cell between the electrodes) rather than mode II operation (gas flow impinging directly on the sensor electrode), a more sensitive mode currently being used on last year s model detector in our laboratories. Mode I is preferred, for operating parameters are much less critical. 

The electrolytic conductivity detector for gas chromatography was developed by Coulson (15, 16, 17), who described modes of operation for the detection of chlorine, sulfur, or nitrogen, but did not establish the reliability of the detector for pesticide residue analysis or the minimum detectability for each molecular species. Cassil et al. (11) described the use of the detector for determining residues of carbamate pesticides and compared its response with that of the microcoulometric detector, as mentioned earlier, finding them equal in response and selectivity and usable over a range of 3 to 200 ng of nitrogen. An improved pyrolysis tube was described, and nickel wire or turnings was used as the catalyst... 

The electrolytic conductivity detector has been used to determine organic iodine with excellent success by Westlake (20), as well as chlorine in organochlorine pesticides, operating in the reducing mode to yield HI or HCl as the detected product. The minimum detectability for chlorine is approximately equal to that of the current microcoulometric detection system. Coulson (4, 15) compared the responses of the electrolytic conductivity, microcoulometric, and electron capture detectors for organochlorine compounds in various extractives and found the first two approximately equal and the electron capture detector unsatisfactory because of high background. 

Some techniques may offer selective screening as well as specificity —e.g., microcoulometric methods described by Coulson et al. (24). This technique consists of a combination of gas chromatography, combustion, and continuous coulometric titration for chlorine or sulfur. The development of the flame photometric detector offers a similar potential for the selective screening and specificity of pesticides which contain phosphorus or sulfur (25). Even so, one or more tests in addition to the initial... 

A selective thermionic specific detector (TSD) was developed by Albert (107) but it is more commonly referred to as a nitrogen-phosphorous detector (NPD). It is basically an alkali FID. Figure 13.36 compares the TSD and the microcoulometric detector. More resolution is obtained through the TSD with elimination of the mixing in the transfer line, reactor tube, and titration cell of the... 

FIGURE 13.36 Comparison of NPD and microcoulometric detector chromatograms of light catalytic cycle oil (LCCO). (Reprinted with permission from Reference 103, Analytical Chemistry, Copyright 1978, American Chemical Society.)... 

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