Estimation of the homogeneity of reference materials by SS-ETAAS and use of the tape-sandwich sample-introduction technique

The so called "tape sandwich", a novel sample-introduction technique developed for SS–ETAAS, has been employed in a study of the microheterogeneity of some biological CRM. Pb, Cd, and Cr were measured by use of a laboratory-assembled instrument with graphite-cup atomizer. Aqueous standards were found satisfactory for calibration. The average mass of sub-samples atomised was in the range 0.250–0.660 mg±30%. The relative sampling error found for the CRM investigated varied from 15–34%. Calculated relative homogeneity constants of the order of 4–21 mg1/2 were obtained, and were reduced considerably by reduction of the particle size. The minimum sample sizes representative of the CRM were, with one exception, much lower (11–75 mg) than those recommended in the documentation accompanying the standards (100–200 mg). In a Chinese hair reference sample Cr data were normally distributed but the presence of nuggets was observed for Cd and Pb; this was associated with severe radial and longitudinal gradients of these elements in hair.