THE EFFECT OF CHEMICAL FORM UPON ABSORPTION - TIME PROFILES OF Pb AND Cd IN BIOLOGICAL SAMPLES EMPLOYING THE »TAPE SANDWICH« SS-ETAAS TECHNIQUE ^†

Janez Štupar and Franci Dolinšek
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia, e-mail: janez.stupar@siol.net

Abstract
Pb and Cd were determined in a variety of biological and plant samples by solid sampling electrothermal atomic absorption spectrometry (SS-ETAAS) employing both the slurry and direct solid (»tape sandwich«) introduction techniques. A laboratory assembled spectrometer and graphite cup atomizer with platform were employed for the measurements. The integrated absorbance was used as an analytical parameter. Calibration was made against aqueous standards. Simultaneously the absorption-time profiles were recordered. A reasonable agreement of the results in comparison to the certified or recommended values was obtained for Pb and Cd regardless of sample origin if the slurry sample introduction was employed. A low value was only observed in determination of Pb in a marine plant (Sea Lettuce) which was probably due to partly overlapping processes of Pb atom formation and matrix (NaCl) vaporization. Single peak absorption-time profiles were observed with different times of atom appearance and peak maxima characteristic of the element and sample origin.
Substantially  lower results for Pb and possibly for Cd ( statistically not significant) associated with the appearance of double peak absorption -time profiles were typical of vegetable samples from heavily polluted soil when the »tape sandwich« sample introduction was used. This feature was presumably the result of partial loss of the analyte in the ashing step and was found specific for the majority of vegetables but not for all plant samples and other biological materials investigated (hair, liver muscle). This phenomenon was obviously caused by the differences in ashing conditions between the sample introduction techniques employed. Namely, ashing of plastic tape disks (»tape sandwich« technique) in an oxygen atmosphere proceeds with considerable smoke formation. Simultaneous liberation of heat raises the temperature of the sample above that of the inner cup-platform (450 °C).
Differences in the shape and/or time of atom appearance in Pb and Cd absorption-time profiles using the »tape sandwich« introduction technique may be related to the various chemical forms of these elements in particular samples and not to the different location of the analytes in the samples. In this respect the environmental conditions and routes of Pb and Cd incorporation play a key role in plant and vegetable samples. Some evidence to support this hypothesis is presented.

Key words: Lead, Cadmium, determination, biological samples, absorption, tape sandwich technique, etass technique.