The Sulfur Chemiluminescence Detector SCD

Heteroatom selective (for organic compounds with S atoms only) destructive mass-flow detector more sensitive and greater dynamic range than S-FPD. 

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The chemiluminescent reaction of SO with ozone is the basis of the sulfur chemiluminescence detector (SCD) [23] discussed later in this chapter,... 

An alternative to FPD in the sulfur mode is the sulfur chemiluminescence detector (SCD) (48). This detector works by forming sulfur monoxide in a reducing flame. Sulfur monoxide is detected by its chemiluminescent reaction with ozone. The SCD is at least one order of magnitude more sensitive than most FPDs. It provides a linear response with high selectivity and does not suffer considerably from quenching. 

The operation of the sulfur chemiluminescence detector (SCD) is based on the combustion of sulfur-containing compounds in a hydrogen-rich/air flame of a flame ionization detector to form sulfiir monoxide (SO) (equation 1). Sulfur monoxide is then detected based on an ozone-induced, highly exothermic chemiluminescent reaction to form electronically excited sulfur dioxide (SO2 ) (equation 2). The excited sulfur dioxide, upon collapse to the ground state, emits light with a maximum intensity around 350 nm that is detected in a manner similar to that of the FPD (equation 3) (2,5),... 

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. 

Similarly, the sulfur chemiluminescence detectors (SCD), namely fluorine-induced SCD (FSCD), ozone-induced SCD (O-SCD) and redox CD (RCD), work by first oxidising the organosulfur compound to give a species which may either react with fluorine or ozone to form a chemiluminescence species (HF and SOj, respectively). Reaction equations for O-SCD at 800-1100°C, developed by ref. [555], are ... 

Many sophisticated analytical techniques have been used to deal with these complex mixtures.5,45,46 A detailed description is not possible here, but it can be noted that GLC, often coupled with mass spectrometry (MS), is a major workhorse. Several other GLC detectors are available for use with sulfur compounds including flame photometer detector (FPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED).47 Multidimensional GLC (MDGC) with SCD detection has been used48 as has HPLC.49 In some cases, sniffer ports are provided for the human nose on GLC equipment. 

The system described in the previous section has been extended with a sulfur chemiluminescence detector (SCD) for the detection of sulfur compounds (32). The separated fractions were thiols + sulfides + thiophenes (as one group), benzothio-phenes, dibenzothiophenes and benzonaphtho-thiophenes. These four groups have been subsequently injected on-line into and separated by the GC unit. Again, no overlap between these groups has been detected, as can be seen from Figure 14.20, in which the total sulfur compounds are shown and from Figure 14.21 in which the separated dibenzothiophenes fraction is presented. The lower limit of detection of this method proved to be 1 ppm (mg kg-1) sulfur per compound. 

Element selective detectors Element selective detectors applicable in pesticide residue analysis include electron capture detector (ECD), electrolytic conductivity detector (ELCD), halogen-specific detector (XSD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), pulsed flame photometric detector (PEPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED). To cover a wider range of pesticide residues, a halogen-selective detector (ECD, ELCD, XSD) in conjvmction with a phosphorus- (NPD, FPD), nitrogen- (NPD), and/or sulfur-selective detector (FPD, SCD) is commonly used. A practical approach is to spht the column flow to two detectors that reduces the number of injections however, the reduced amoimt of analyte that reaches the detector must be considered. 

Selective GC detectors aid in the detection and identification of compounds containing specific elements halogens with electron capture detector (BCD) or electrolytic conductivity detector (ELCD) nitrogen and phosjAorus with nitrogen-phosphorus detector (NPD) sulfur and phosphorus with flame photometric detector (FPD) and sulfur with sulfur chemiluminescence detector (SCD). The development of the atomic... 

SCD—The sulfur chemiluminescence detector shall meet or exceed the following specifications (1) greater than 10 linearity, (2) less than 5 pg S/s sensitivity, (3) greater than 10 selectivity for sulfur compounds over hydrocarbons, (4) no quenching of sulfur compound response, and (5) no interference from co-eluting compounds at the usual (X) sampling volumes. 

Figure 14.2 Schematic diagram of the cliromatographic system used for the analysis of low concenti ations of sulfur compounds in ethene and propene VI, injection valve V2, column switcliing valve SL, sample loop R, restriction to replace the column SCD, sulfur chemiluminescence detector.

ABSTRACT A novel measurement technique has been developed for the rapid determination of low levels of sulfur in hydrocarbon matrices. The technique employs low thermal mass gas chromatography (LTMGC) and a dual plasma sulfur chemiluminescence detector (DP-SCD). Highly sensitive total volatile sulfur measurement can be made in less than 30 s with detection limits in the 20-30 parts per billion range. Response is linear over at least three orders of magnitude sulfur with excellent repeatability. 

Other even more sjjccific detectors can also be coupled to GCxGC. Atomic emission detectors (AEDs), and more element-selective detectors, such as sulfur compound detectors (sulfur chemiluminescence detector, or SCD), have been reported in the oil characterization area [84,85]. In these detectors, the combustion of sulfur compounds by an energetic induced plasma produces sulfur oxides that further react to and produce light at a specific wavelength that... 

Sulfur Chemiluminescence Detector. Another sulfur selective detector based on photometric sensing of a chemiluminescent reaction has become commercially available (Sievers Research, Inc., Boulder, CO). Similar to the FPD, the detection scheme of the SCD incorporates combustion of the GC column effluent in a hydrogen rich flame. However, the sulfur species detected is an excited state of sulfur dioxide (SO2 ), not... 

A flameless sulfur chemiluminescence detector has also been described (74). The design uses an externally heated ceramic assembly that is operated at low pressme under the necessary fuel-rich conditions but they are out of the flammability limits of hydrogen in air. The hydrogen and air are mixed as the effluent reaches a high-temperature zone. This results in partial oxidation before the effluent reaches the highest temperature zone. It utilizes combustion at low pressure, which is thought to increase the production of sulfur monoxide. The flameless system reduces the effect of column bleed and shows improved detectability over conventional SCD by about one order of magnitude. 

Sulfur Chemiluminescence Detection—As sulfur compounds elute fiom the gas chromatographic column they are combusted in a flame ionization detector (FID). These combustion products are collected and transfer to a Ifur chemiluminescence detector (SCD). This detection technique provides a highly senative, selective, and linear response to volatile sulfur compounds and may be used simultaneously while the usual fixed gas and hydrocarbon determinations are being made. 

Detectors commonly used in GC and specified in the USPP include FID, alkali FID (NPD, TD), BCD, and TCD. A description of these detectors, including their operational principles and relative performance, was presented in a previous volume of this encyclopedia. Various other useful detectors for GC include photoionization (PID), flame photometric (FPD), electrolytic conductivity (BLCD), redox (RCD) and sulfur chemiluminescence (SCD), and helium ionization (HID).[4 1 Table 1 summarizes some of the features of detectors used in GC. 

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