Chemiluminescence detector mechanism

Optimization strategies and a number of generalized limitations to the design of gas-phase chemiluminescence detectors have been described based on exact solutions of the governing equations for both exponential dilution and plug-flow models of the reaction chamber by Mehrabzadeh et al. [12, 13]. However, application of this approach requires a knowledge of the reaction mechanism and rate coefficients for the rate-determining steps of the chemiluminescent reaction considered. 

A typical chemiluminescence detector consists of a series-coupled thermal decomposition and ozone reaction chambers. The selective detection of nitrosamines is based on their facile low-temperature (275-300°C) catalytic pyrolysis to release nitric oxide. Thermal decomposition in the presence of oxygen at about 1000°C affords a mechanism for conversion of nitrogen-containing compounds to nitric oxide (catalytic oxidation at lower temperatures is also possible). Decomposition in a hydrogen-diffusion flame or thermal oxidation in a ceramic furnace is used to produce sulfur monoxide from sulfur-containing compounds. 

This reaction mechanism was first proposed by Halsted and Thrush (30) when studying the kinetics of elementary reactions involving the oxides of sulfur. Visible and UV spectroscopic studies (31) confirmed that the chemiluminescent emission was from SO2. Recently, it has also been confirmed that the sulfur- analyte molecule from the GC effluent is converted to SO in the flame of the SCD (32). Even though SO is a free radical, it can be sufficiently stabilized in a flow system under reduced pressure (33,34) to be sampled and transferred to a vessel to react with introduced O. Based on these operational principles. Burner and Stedman (33) concluded that SO produced in a flame could be easily detected. They modified a redox chemiluminescence detector (36) to produce what was termed a Universal Sulfur Detector (USD). A linear response between 0.4 ppb and l.S ppm (roughly equal to 3 to 13,000 pg of S/sec) was demonstrated with equal response to the five sulfur compounds tested. This detection scheme has been utilized as the basis for the commercially available GC 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. 

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

FIGURE 10.2 Mechanism for the chemiluminescent nitrogen detector. R = any group attached to a nitrogen. 

Thi,s chemiluminescent nitrogen detector for HPLC was first described in (23). The detection mechanism for nitrogen determination is shown below ... 

Figure 11.9 Mapping of bond cleavage in self-reporting chemiluminescent elastomers that are toughened ty sacrificial bonds, (a) Bis(adamantyl)-1,2-dioxetane breaks under a mechanical force, resulting in chemiluminescence. (b) Intensity-coloured images of polymer networks during crack propagation of notched samples and schematic depiction of bond breaking around the crack tip. SN, DN, TN label elastomers of different molecular architecture ( single network , double network and triple network ). The dashed line indicates the perimeter of the sample. Vertical lines are artefacts of the detector.

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