DIFFUSION VAPOR TRANSFER MODELING FOR AN END-CAPPED ATOMIZER .1. ATOMIZER WITH CLOSED INJECTION PORT

For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (tau(R)) Of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%, The theoretically predicted tau(R) values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained tau(R) values for the end-capped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding tau(R) ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1-5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).