Abstract: An analytical cell assembly comprising a base with a radiation source mounted on one end, an enclosure on the other end, and a cell proper, with cell heating means, within the enclosure. The enclosure and cell have cooperative fluid flow connections whereby the cell is modular and may be simply lifted out of the enclosure. The enclosure also includes a collimating lens assembly interposed between the cell within the enclosure and the source outside the enclosure on the base. The entire assemblage of source, enclosure and cell can be moved as a unit, making it modular, or just the cell can be moved as a module leaving the enclosure and the source in place in an instrument using the invention cell assembly.

Abstract: A fluorescent gas analyzer to detect the concentration of a trace gas in another gas or a mixture of gases. The invention is particularly adapted to measure pollutant gases in the air, and still more particularly to measure SO.sub.2 in air, stack gases, and the like. The invention includes a removable and heated sample cell. The analytical determinations are made digitally by direct photon counting. The digital logic which handles the data produced by the cell includes means for continuous adjustment to accommodate changes in source energy and means to correct for dark current.