Sulfur detection mode

The use of electrolytic conductivity detectors (HECDs, also known as Hall detectors) for the analysis of organic sulfur compounds is limited, probably because they require high maintenance. The electrolyte must be kept extremely clean and its sulfur specificity is limited by high concentrations of cotrapped carbon dioxide. Despite these problems, HECD performed well in the sulfur detection mode. ... 

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],... 

The use of the flame photometric detector in the sulfur-sensitive mode (attributed to the emission of S2 spectral species at 394 nm) is exemplified in measuring the sulfur-containing volatiles in physiological fluids [110], or breath of liver-disease patients [111]. A word of caution concerns the fact that co-eluting non-sulfur compounds may result in a diminished or quenched response of the measured species [112]. Hence, the need for maximum solute separation. The detector is responsive to nanogram amounts of sulfur-containing compounds, but the response increases with the square of sulfur content [112]. Merits of the flame photometric detector in the detection of phosphorus compounds is somewhat overshadowed by a similar capability of the thermionic detector. 

There are three principal detection modes in commercial detectors halogen mode to detect HX, sulfur mode for delecting SO2 or SO3, and nitrogen mode for detecting NH3. The detector response depends on the reaction conditions, the solvent, the pH, and the use of a postreaction scrubber. A schematic representation of a commercially available detector unit is shown in Figure 6.28. 

As discussed for Figure 10.3B, a large baseline signal is encountered in HPLC-PAD for the oxide-catalyzed detections of amino acids and sulfur compounds (Mode n). Furthermore, the large baseline current is frequently observed to drift to large anodic values, especially for new or freshly polished electrodes. This drift is the consequence of a slow growth in the true electrode surface area as a result of surface reconstruction caused by the oxide on-off cycles in the applied multistep waveforms. As listed in Table 10.2, Mode II detections performed with PAD are subject to a number of disadvantages because of the formation of surface oxide, which is required and concomitant with the detection of amine- and sulfur-based compounds. 

LaCourse and Owens (1995) have demonstrated the superiority of IPAD over PAD for the determination of thiocompounds using standard reversed-phase conditions. These results were as expected since sulfur detections arc Mode II. Figure 10.16 shows response of biotin (160 pmol injected) using microborc chromatography. 

The species S3 (absorbing at 420 nm) and S4 (absorbing at 530 nm) have been detected by reflection spectra in the condensate but the formation of S4 is unexplained [16]. S3 and SO2 have also been observed by Raman spectroscopy in such samples [15] (the expected S4 Raman line at 678 cm was probably obscured by the SS stretching mode of S2O at 673 cm but a shoulder at the high-frequency side of the S2O line indicates that some S4 may have been present). While the reddish colors turn yellow on warming at about -120 °C, the sulfur radicals could be observed by ESR spectroscopy up to 0 °C [10]. If the condensation of S2O gas is performed very slowly at -196 °C the condensate is almost colorless and turns red only if the temperature is allowed to increase slowly. Hence, it has been suspected that S2O is actually colorless like SO2. 

Ethylenethiourea (ETU) is a toxic decomposition product/metabolite of alky-lenebis(dithiocarbamates). This compound could be generated during processing of treated crops at elevated temperature. Different chromatographic methods to determine the residue levels of ETU have been published. After extraction with methanol, clean-up on a Gas-Chrom S/alumina column and derivatization (alkylation) with bro-mobutane, ETU residues can be determined by GC with a flame photometric detector in the sulfur mode. Alternatively, ETU residues can also be determined by an HPLC method with UV detection at 240 nm or by liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/tandem mass spectrometry (LC/MS/MS) (molecular ion m/z 103). ... 

Chlorpyrifos had an amount range of only 300 to 1. It was different from others cited by its flame photometric (sulfur mode) detection. It is interesting, however, that the power transformations for both chlorothalonil and chlorpyrifos were so similar. 

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