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Abstract
A mathematical model was developed for the temperature dependence of the flame/furnace infrared emission (FIRE) produced by the antisymmetric stretching vibration of carbon dioxide. The model can be used to predict the detector performance as a function of the temperature of the excitation source. With minor modifications, the model can be applied to other potentially important analyte species such as hydrogen chloride, hydrogen fluoride, and sulfur dioxide present in the flames. Comparison of the signal calculated theoretically and observed experimentally shows that the model should be of great value in improving and optimizing the detector performance.
Previous applications of FIRE system were limited to gas-phase samples. Liquid samples could be introduced into the FIRE system on a continuous basis in the form of a fine spray. Both thermospray and cross-flow pneumatic nebulizers were studied as a means of interfacing a FIRE radiometer to a high performance liquid chromatograph (HPLC). Due to the large quantities of liquid mobile phase introduced into the burner, the temperature of the H$\sb2$/air flame was changed significantly. The detector could be configured in carbon- or chlorine-specific modes. When the carbon-specific mode was employed, the mobile phase was restricted to deionized water. With the chlorine-specific mode, combustible organic mobile phases (free from chlorine) could be used and the detector could selectively determine mixtures of chlorinated hydrocarbons. Flame background caused by the combustion of carbon-containing mobile phases could be electronically subtracted by using dual channel FIRE radiometer. Typical detection limits for both kinds of interface (i.e. thermospray and cross-flow nebulization) are in the range of $\mu$g s$\sp{-1}$. Alternatively, the FIRE radiometer can be interfaced to an HPLC by means of post-column reaction to form dissolved carbon dioxide which is then purged by H$\sb2$ gas through a specially designed purge chamber. Sodium peroxydisulfate (Na$\sb2$S$\sb2$O$\sb8$), in the presence of silver nitrate, was employed as an oxidizing agent to oxidize carboxylic acids and sugars to carbon dioxide.
The specially designed purge chamber, having a cell volume less than 2 mL, is highly versatile. In addition to use as a detector for HPLC (or more specifically, ion-exchange chromatography), it can also be used in both process analysis and flow injection analysis (FIA) for the determination of total inorganic carbon, purgeable organic carbon in aqueous samples and the available chlorine in bleach liquors. Nonvolatile organics can also be determined by means of sample pretreatment with sodium peroxydisulfate catalyzed with silver at elevated temperature.
Hydrogen/nitrous oxide, which has a burning velocity similar to the hydrogen/air flame but has a flame temperature about 600$\sp\circ$C higher, was investigated as an alternative fuel/oxidant mixture.
