SIMPLE MEASUREMENT OF DEPOSITION VELOCITIES AND WALL REACTION PROBABILITIES IN DENUDER TUBES

The diffusion column in the Reversed-Flow Gas Chromatography (RF-GC) t echnique is replaced by a simple denuder tube and the theoretical anal ysis based on this experimental set-up is developed for measurement of deposition velocities and reaction probabilities of an injected gaseo us analyte on the denuder wall coating. The mathematical formulation o f the problem is based on a general mass balance equation of the analy te, which is solved by the method of Laplace transforms, under nonstea dy-state conditions. The final result gives the height of the sample p eaks of RF-GC method as a function of time of flow reversal, thus desc ribing mathematically the diffusion band for each analyte, in the form of a sum (or difference) of four exponential functions of time. A sim ple PC program, based on nonlinear regression analysis, permits the ca lculation of both the pre-exponential factor and the exponential coeff icient of all four functions from the experimental diffusion band. Fro m these factors and coefficients, five physicochemical parameters for the denuder operation are calculated in seven different ways by anothe r simple PC program. Consistent results are obtained by the seven meth ods of calculation, the mean values of the five parameters and their s tandard error also being printed. The physicochemical parameters are t he initial deposition velocity of the analyte on the tube wall, the ra te constant for its desorption, the rate constant for surface reaction , the final deposition velocity and the wall reaction probability. The method has been applied to propene in synthetic air depositing on sil ica gel, aluminium oxide and marble powder, and also to hydrogen sulph ide in air on silver foil.