The Chemiluminescent Detector

The chemiluminescent detector is a mass-sensitive detector, which is highly selective for either sulfur (SCD) or nitrogen (NCD), depending on the instrumentation. The mechanism of detection is a two-step process with initial combustion followed by low-pressure reaction with ozone. The oxidation products emit a characteristic light, which is measured. The detection limit is about 0.5pgSs and 3 pg N s and the linearity is 10 . One main use is the determination of sulfur compounds in petrochemical products. 

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Nitric oxide release from blood vessels was first detected by chemiluminescence (Palmer et ai, 1987). In the original adaptations of the nitric oxide detector, perfusates from isolated vessels were directly mixed in a reflux chamber containing acetic acid and iodine. The iodine in the reflux chamber served to reduce any nitrites or nitroso-containing groups to nitric oxide, which was stripped from the chamber by a continuous stream of nitrogen or helium that flowed to the chemiluminescent detector. Replacement of the acetic acid with the less volatile trichloroacetic acid reduces problems with contamination of the nitric oxide detector (Dr. D. Harrison, Emory University, Atlanta, Georgia, personal communication, 1991). While extremely sensitive, the use of the acid reflux chamber also reduces the specificity of the assay, raising questions as to whether nitric oxide or a nitrosothiol is EDRF (Myers et ai, 1990). 

The reaction between olefins and ozone produces light that can be measured and related to the concentration of the reactants. One of the preferred methods for measuring ambient ozone concentrations utilizes the chemiluminescence generated in the ozone-ethylene reaction for detection. Recently, Hills and Zimmerman (16) described the use of this detection principle for determining hydrocarbon concentrations. They utilized the chemiluminescence created when ozone reacts with isoprene for development of a continuous, fast-response isoprene analyzer. This real-time isoprene system is reported to be linear over three orders of magnitude and to have a detection limit of about 1 ppbv. Because the system doesn t include a preseparation of hydrocarbons, interferences from other olefins (ethylene, propylene, and so forth) could occur. Thus far the chemiluminescent detector has been used to monitor isoprene emissions under conditions in which the concentrations of olefins that could interfere are negligible compared to those of the biogenic hydrocarbon. 

We now report on the reactivity of carbonaceous deposits which can be laid down on the Cu/ZSM-5 catalysts by exposure to the reaction mixture at low temperatures. As in our earlier study [16], carbonaceous material referred to for simplicity as coke was deposited by exposing the catalysts to the reaction mixture at 473 K. The reactivity of material deposited was then examined by exposing the catalyst at 473 K to the same concentration of NO and oxygen as present in the reaction mixture, (but no propene), and then heating to ca 920 K at 10 K min. The conversion of NO was monitored by the chemiluminescent detector, which has a response time of < 1 s. 

While the chemiluminescence detectors have considerable selectivity for nitrosamines it must also be recognized that the possibility exists that any compound that can produce NO during pyrolysis will produce a signal (20). For example, TEA responses have been observed from organic nitrites, C-nitro and C-nitroso compounds (17,28) and nitramines (29). In the routine analysis of N-nitroso compounds, possible TEA analyzer responses to compounds other than N-nitroso derivatives normally do not represent a problem since the the identity of a compound can be readily established by co-elution with known standards on GC-TEA and/or HPLC-TEA systems (30-34). Additional confirmation could be provided when the sample can be chromatographed on both GC-TEA and HPLC-TEA (30,33). The technique accepted as the most reliable for the confirmation of N-nitrosamines is based on mass spectrometry (22, 35,36). Low-resolution mass spectrometry is satisfactory for the analysis of relatively simple mixtures and in those instances in which extensive clean-up of samples has been performed. However, complex samples require more sophisticated GC and MS procedures (e.g., high resolution-MS). 

Boduszynski and co-workers [114] and Andersson and Sielex [115] have used GC-AED to examine the sulfur-containing molecules in petroleum and its heavier processed products. Both groups found a preponderance of thiophenic sulfur compounds in processed materials, in contrast to petroleum. The unprocessed petroleum had mercaptans, sulfides, thiophenes, as well as other species containing other heteroatoms in addition to the sulfur. These two studies also showed that this detector has the selectivity for sulfur that is attainable with the two older commonly used sulfur detectors, the flame photometric detector (EPD) and the chemiluminescent detectors. The first of these, however, suffers from a very non-linear response. This effect is very compound-class dependent. This is due to the fact that the light-emitting species that is detected is a two-sulfur atom one that results from recombination of the sulfur atoms in the combusted sample peak. The efficiency of combustion and the rate of recombination are both dependent on the... 

Since the chemiluminescence detector system depends on the reaction of O3 with NO, it is necessary to convert NO2 to NO in the sample prior to analysis. This is accomplished by passing... 

The introduction of the chemiluminescence detector, known also as ther... 

Since the chemiluminescence detector system depends upon the reaction of O3 with NO, it is necessary to convert NO2 to NO in the sample prior to analysis. This is accomplished by passing the air sample over a thermal eonverter, which brings about the desired conversion. Analysis of such a sample givesNOx, the sum ofNO and NO2. Chemiluminescence analysis of a sample that has not been passed over the thermal eonverter gives NO. The difference between these two results is NO2. 

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