Thermionic detection

Instead of MS, other types of detection can be used in GC. The most widely used are thermal conductivity detection and flame ionization detection. In addition, various more specific types are available, e.g., electron-capture detection, thermionic detection, and flame photometric detection. A comparison of some characteristics is given in Table 3. On-line combinations with spectrometric detection, other than MS, are available, e.g., Fourier-transform infrared (FT-IR) and atomic emission detection (AED). 

Analysis for the butanals is most conveniendy carried out by gas chromatography. Trace quantities of -butyraldehyde (18 ppb) in exhaust gases have been determined employing a combination of capillary gas chromatography with thermionic detection (35). Sinulady, trace amounts of -butyraldehyde in cigarette smoke and coffee aroma have been determined by various capillary gc techniques (36,37). 

The alkah flame-ionisation detector (AFID), sometimes called a thermionic (TID) or nitrogen—phosphoms detector (NPD), has as its basis the fact that a phosphoms- or nitrogen-containing organic material, when placed ia contact with an alkaU salt above a flame, forms ions ia excess of thermal ionic formation, which can then be detected as a current. Such a detector at the end of a column then reports on the elution of these compounds. The mechanism of the process is not clearly understood, but the enhanced current makes this type of detector popular for trace analysis of materials such as phosphoms-containing pesticides. 

Analysis of methyl parathion in sediments, soils, foods, and plant and animal tissues poses problems with extraction from the sample matrix, cleanup of samples, and selective detection. Sediments and soils have been analyzed primarily by GC/ECD or GC/FPD. Food, plant, and animal tissues have been analyzed primarily by GC/thermionic detector or GC/FPD, the recommended methods of the Association of Official Analytical Chemists (AOAC). Various extraction and cleanup methods (AOAC 1984 Belisle and Swineford 1988 Capriel et al. 1986 Kadoum 1968) and separation and detection techniques (Alak and Vo-Dinh 1987 Betowski and Jones 1988 Clark et al. 1985 Gillespie and Walters 1986 Koen and Huber 1970 Stan 1989 Stan and Mrowetz 1983 Udaya and Nanda 1981) have been used in an attempt to simplify sample preparation and improve sensitivity, reliability, and selectivity. A detection limit in the low-ppb range and recoveries of 100% were achieved in soil and plant and animal tissue by Kadoum (1968). GC/ECD analysis following extraction, cleanup, and partitioning with a hexane-acetonitrile system was used. 

Using established extraction and cleanup methods, followed by GC/FPD and GC/thermionic detection, Carey et al. (1979) obtained detection limits in the ppb range and recoveries of 80-110% in soil and 70-100% in plant tissue. Good sensitivity and recovery were maintained in a simplified extraction procedure of sediments followed by GC/FPD analysis (Belisle and Swineford 1988). Bound methyl parathion residues that were not extracted with the usual methods were extracted using supercritical methanol by Capriel et al. (1986). They were able to remove 38% of the methyl parathion residues bound to soil, but 34% remained unextractable, and 28% could not be accounted for. 

Plants (cereals, cotton, potato, citrus, apple, pear, peach, grape, persimmon, apricot, chestnut), soil and water Gas-chromatographic determination [mass spectromet-ric detection (MSD), flame thermionic detection (FTD) or nitrogen-phosphorus detection (NPD)] for plant materials, water and soil... 

Nitenpyram and its metabolites. The metabolites of nitenpyram, CPMA and CPMF, are determined by HPLC under the same conditions as for the parent nitenpyram. The retention times of nitenpyram, CPMA, and CPMF are 9.2,7.9 and 5.3 min, respectively. However, these compounds are unstable and need to be derivatized to a more stable compound, CPF, prior to analysis. It is necessary to remove acetone from the extract before derivatization, because a by-product can be formed in the presence of acetone thus impacting the recovery of CPF. Nitenpyram is more effectively determined using HPLC, whereas CPF, as the analyte of nitenpyram and its metabolites, is more effective by gas chromatography/flame thermionic detection (GC/FTD). 

The sample is homogenized with acetonitrile. An aliquot of the extract is evaporated to dryness and the residual material is dissolved in ethyl acetate-toluene (3 1, v/v), and subjected to cleanup by gel permeation chromatography (GPC). After GPC, the sample is subjected to an alumina and Florisil SPE cleanup procedure. The concentrated eluate is analysed by gas chromatography/thermionic nitrogen-specific detection (GC/TSD). 

Transfer the concentrate into a 200-mL separatory funnel using two portions of 20 mL of n-hexane. Add 100 mL of saturated sodium chloride aqueous solution and extract twice with 100 mL of n-hexane by shaking for 5 min and allow the phases to separate. After dehydration of the n-hexane extract with 10 g of anhydrous sodium sulfate, concentrate the extract to dryness below 40 °C with a rotary evaporator. Transfer the residue with three portions of 5 mL of n-hexane into a glass column containing 10 g of Florisil (deactivated by water at a rate of 1%). Elute with 100 mL of n-hexane-ethyl acetate (9 1, v/v) and then with 100 mL of n-hexane-ethyl acetate (7 3, v/v). Concentrate the second eluate to dryness and dissolve the residue in 10 mL of n-hexane and analysis by gas chromatography/flame thermionic detection (GC/FTD). 

Plot the peak area against the injected amount of benfuracarb. The injection volume (8 iL) should be kept constant as the peak area varies with the injection volume in flame thermionic detection. Before each set of measurements, check the GC system by injecting more than one standard solution containing ca 5-10 ng of benfuracarb. Recommendation inject standard solutions and sample solutions alternately rather than constructing the calibration curve in advance. 

The TID design proposed Patterson consists of an alkali metal doped cerwlc cylinder, containing an embedded heater surrounded by a cylindrical collector electrode [100]. The ceramic thermionic emitter is biased at a negative potential with respect to the collector electrode, and it is heated to a surface temperature of 400-800 C, depending on the mode of detection. The response of the detector to different elements depends on the electronic work function of the thermionic surface (i.e., the... 

To improve the detectability of silyl ethers, silylation reagents containing an electron-capturing group [443,449-451,468] or cyano group for thermionic detection [469] have been prepared, the 2-cyanoethyldimethylsilyl derivatives are only marginally aore sensitive (ca. 5 fold) to the thermionic detector than to the flame ionization det ftpr which Units their usefulness. The... 

The depth distribution of light stabilisers in coatings has been studied in 1 - 3 mg microtomed slices, by means of SFE-GC with ToF-SIMS and nitrogen thermionic detection, as well as by direct ToF-SIMS analysis results were in good agreement [59]. As the SFE effluent... 

Shinohara et al. [299] have described a procedure based on gas chromatography for the determination of traces of two, three, and five-ring azarenes in seawater. The procedure is based on the concentration of the compounds on Amberlite XAD-2 resin, separation by solvent partition [300], and determination by gas chromatography-mass spectrometry with a selective ion monitor. Detection limits by the flame thermionic detector were 0.5-3.0 ng and those by gas chromatography-mass spectrometry were in the range 0.02-0.5 ng. The preferred solvent for elution from the resin was dichloromethane and the recoveries were mainly in the range 89-94%. 

Lores et al. [382] discussed the determination of the organophosphate insecticide fenthion in seawater. The method comprises a solvent extraction followed by silica gel clean-up procedure prior to determination by gas chromatography with thermionic detection. Detection levels of 0.01 pg/1 were achieved. 

PDMS = polydimethylsiloxane. PA = polyacrylate. CW = Carbowax. DVB = divinylbenzene. FID = flame ionization detection. NPD = nitrogen-phosphorus detection. TSD = thermionic-specific detection. LOQ = limit of quantitation. LOD = limit of detection. TCA = trichloroacetic acid. PICI-MS = positive ion chemical mass spectrometry. SIM = selected ion monitoring. 

GC is coupled with many detectors for the analysis of pesticides in wastewater. At the present time the most popular is GC-MS, which will be discussed in more detail later in this section. The flame ionization detector (FID) is another nonselective detector that identifies compounds containing carbon but does not give specific information on chemical structure (but is often used for quantification because of the linear response and sensitivity). Other detectors are specific and only detect certain species or groups of pesticides. They include electron capture,nitrogen-phosphorus, thermionic specific, and flame photometric detectors. The electron capture detector (ECD) is very sensitive to chlorinated organic pesticides, such as the organochlorine compounds (OCs, DDT, dieldrin, etc.). It has a long history of use in many environmental methods,... 

For amines absorption in an acid solution, or preferably adsorption onto an acid ion exchange column (acidified divinylbenzene-styrenesulfonic acid copolymer) is used. 10-50 1 of ambient air is sent over a wet 100 mmx 3 mm I.D. column the ion exchange polymer is put into a vial, made alkaline and the water solution is analysed on packed Carbowax-KOH GC-column with a thermionic selective detector (TSD), which is specific for nitrogen- and phosphorus-compounds. Trimethylamine is detected easily at 1 ppb. 

Flame ionisation. Flame Photometric and Thermionic Detection use a Perkin—Elmer Sigma 1 gas chromatograph, equipped with an S.G.E on-column injector. The column used was a 25 metrex0.31mm I.D. fused silica WCOT capillary coated with BP-1 (S.G.E), a methyl silicone. 

Ryan, D., Marriott P. (2006) Studies on thermionic ionisation detection in comprehensive two-dimensional gas chromatography. J. Sep. Sci. 29 2375-2382.D... 

Cl = chemical ionization CI-NI = monitoring negative ions in the chemical ionization mode GC = gas chromatogmphy HPLC = high performance hquid chromatography MS = mass spectrometry pmol = picomole SPE = solid-phase extraction TSD = thermionic specific detection... 

Air Air collected/derivatized through sodium hydroxide in ethanol neutralization derivatization with heptafluorobutyric anhydride solvent extraction GC/thermionic specific detection 10 pg/ L injected 100% (simulated sample) Skarping et al. 1988... 

GC/FPD or thermionic detection (P-mode) GC/MS for qualitative identifications recommended. (Method 1657)... 

Three standardized methods were found in the Official Methods ofAnalysis of the Association of Official Analytical Chemists (AOAC 1990). The first of these methods is based on the extraction of crops (kale, endive, carrots, lettuce, apples, potatoes, and strawberries) with ethyl acetate and isolation of the residue followed by a sweep codistillation cleanup prior to GC/thermionic detection (Method 968.24). The second of these methods utilizes Florisil column chromatography clean-up followed by GC/FPD (Method 970.53). In the third method (Method 970.52), the sample is extracted with acetonitrile, and the residue is partitioned into petroleum ether followed by Florisil clean-up and GC/KC1 thermionic detection. Identifications are based on combinations of gas, thin-layer, and paper chromatography. The recovery for diazinon in this method is stated to be greater than 80% no data on limits of detection were given. 

MS, Mass spectrometry El, electron impact Cl, chemical ionization MID, multiple ion detection PICI, positive-ion chemical ionization NICI, negative-ion chemical ionization SIM, selected ion nmonitoring TSP, thermospray PPINICI, pulsed positive ion-negative ion chemical ionization ECD, electron-capture detector NPD, nitrogen/phosphorous detector NSTD, nitrogen-selective thermionic detector FT-IR, Fourier transform infrared spectrometry. 

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