Sulfur cell detector

In some instances, a single peak was used to identify two pesticides however, the identification thus made can be justified, in every case, on the basis of the presence of additional peaks displayed by each compound. Ronnel was identified solely on the basis of a single match on the DC200 column. Additional evidence for this compound was obtained with the sulfur microcoulometer detector cell since the Ronnel molecule contains both chlorine and sulfur atoms. The identifications account for all peaks except one occurring at a relative retention time of 2.94 on the DC200 column. 

Universal sulfur-selective detectors for GC or supercritical fluid chromatography (SFC), that incorporate a cool hydrogen-rich flame or closed hydrogen/ air burner have been developed and are now commercially available. Combustion products are transferred via a connecting line to a low-pressure reaction cell, where they are mixed with ozone. Although sulfur monoxide is believed to be the common intermediate that reacts with ozone to form the excited sulfur dioxide emitter, it is not necessarily the product of the first conversion step, which has caused debate between instrument manufacturers. One proposed mechanism proceeds as shown in reaction [VI] and it was suggested that the reduced sulfur species could be H2S ... 

The sulfur chemiluminescence detector (SCD) for GC was developed by Benner and Stedman and is based on the formation of sulfur monoxide from sulfur containing compounds by combustion in a reducing hydrogen/oxygen flame (73). The effluent from a column enters a combustion tube with a stainless steel burner maintained at 800°C. The combustion process, however, can achieve temperatures of 1800°C. The products of combustion are transferred to a reaction cell under vacuum, and ozone is added to the reaction cell, resulting in a chemiluminescence reaction. 

Electrochemical detectors measure the current resulting from the application of a potential (voltage) across electrodes in a flow cell. They respond to substances that are either oxidizable or reducible and may be used for the detection of compounds such as catecholamines, carboxylic acids, sulfonic acids, phosphonic acids, alcohols, glycols, aldehydes, carbohydrates, amines, and many other sulfur-containing species and inorganic anions and cations. Potentiometric, amperometric, and conductivity detectors are all classified as electrochemical detectors. 

Portions of the 6 and 15% Florisil eluate were injected into the DC200 and DEGS columns with the coulometer equipped with the sulfur-detecting cell. No peaks were obtained from the 15% eluate injection. Of the several peaks that appeared on the DC200 and DEGS columns from the 6% eluate injection, one was identified as Ronnel DC200 reference 1.00, sample 1.00, DEGS, reference 1.00, sample 1.00, and concentration 0.2 p.p.m. Ronnel is chemically 0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate and thus should yield a peak with both the chloride and sulfur detectors. Identification was assured since a peak was found at the proper retention times with both detector cells one column was used for the chloride, and two columns were used for the sulfur detector cells. 

Trace quantities of hydrazoic acid at levels of 10 M can be determined by gas chromatography [22], The sample is freeze-dried in alkaline media on a vacuum line, sulfuric acid is added to the azide sample without breaking vacuum, and helium is then used to sweep the hydrazoic acid through a Linde No. 5 molecular sieve powder, binder-free, packed in a 2-mm glass capillary column. The retention time for a 6-ft column and a 50 ml/min helium flow rate at ambient room temperature is about 15 min. A thermal conductivity cell with gold-plated filaments is used as a detector. 

Equipment intended to obtain kinetic data by EGA methods must minimize delay between evolution and detection by reducing, as far as possible, the distance traveled and diffusive dispersal (65). Specific detectors for an identified product may operate continuously, e.g., electrolysis cells for water or sulfur dioxide (64), infrared absorption responding to a particular bond, MS operating to detect a particular mass, etc. Alternatively, MS can be used to scan repeatedly a selected range containing several products. GC necessarily analyzes samples at time intervals dictated by the longest-retained component, and the output can use MS as a detection method. 

Electrolytic Conductivity Detectors In the Hall electrolytic conductivity detector, compounds containing halogens, sulfur, or nitrogen arc mixed with a reaction gas in a small reactor tube, usually made of nickel. The reaction tube is kept at 85(FC-lO(X) C. The products arc then dissolved in a liquid, which produces a conductive solution. The change in conductivity as a result of the ionic species in the conductance cell is then measured. A typical delector is illustrated in Figure 27-11. 

An on-line analyzer must be packaged much more robustly than a laboratory instrument to withstand the process environment which, for example, may have an explosive atmosphere and significantly variable ambient temperature. It must also be capable of continuous, unattended operation over long periods of time. Clearly, the simpler the instrument the better. Of the methods listed in Table 1, WDXRF, polarized EDXRF, and Pyro-microcoidometry have not been adsqrted to on-line process instrumentation, whereas the other methods have. The relative simplicity of Pyro-EC makes it particularly suitable for adaptation to process instrumentation. The sulfur dioxide sensor is a small, plug-in, low cost electrochemical cell, easily replaceable and with an expected lifetime of over one year. The UV lamp, UV optics, and photomultiplier used in Pyro-UVF are not required. The X-ray tube (or radioactive source). X-ray detector, and X-ray optics used in all the XRF instruments are not required. 

A schematic block diagram illustrating an entire DP-SCD detection system is shown in Fig. 2. An analytical system consists of a gas chromatogr h equipped with a split/splitless iigector with the option of a Pressurized Liquid Injection System (PLIS), wifli or without low diermal mass gas chromatogr q)hy apparatus, for sample introduction and sulfur speciation (if required) an electrically heated burner with an interface that controls the burner gas flows and temperature and a detector that contains a chemiluminescent reaction cell, ozone generator, optical filter, amplifier, and electronics. Lastly, a vacuum pump is used to keep file reaction cell under low pressure conditions to prevent loss of chemiluminescent species and to reduce collisional quenching. 

As seen in Table 8.11, the absorptivities of C and H are almost identical. The method is therefore insensitive to changes in the C H ratio and is sensitive only to the sulfur content. In practice, a process stream passes through a flow cell where sulfur in the hydrocarbon matrix absorbs X-rays transmitted between the source and detector (Figure 8.61a). The recorded X-ray intensity is inversely proportional to the sulfur concentration that is. X-ray transmission decreases as sulfur concentration increases. The commercial process analyzer is shown in Figure 8.61b. The process analyzer can withstand pressures up to 1480 psig and temperatures to 200°C. 

Figure 8.61 (a) Schematic of the X-ray transmission NEX XT system showing the flow cell, source, and detector. (b) The Rigaku NEX XT on-line sulfur analyzer. (Courtesy of Applied Rigaku Technologies. Inc., TX. www.rigakuedxrf.com.)... 

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