Raporty IChTJ. Seria A nr 3/1997

A new Polish certified reference material Virginia Tobacco Leaves (CTA-VTL-2) for inorganic trace analysis including microanalysis has been prepared. Fresh tobacco leaves of the variety "Virginia" grown in Bulgaria were hand-picked, dried, comminuted and sieved in conditions preventing contamination with metals. The next step was homogenization by mixing in a plastic drum. Preliminary homogeneity testing was performed by X-ray fluorescence determining Ca, Fe and K content in several subsamples taken from the drum. The material was then distributed into PE bottles. Extensive homogeneity testing by neutron activation analysis revealed, that CTA-VTL-2 can be considered homogeneous for sample size of single milligrams. Thorough microscopic examination of CTA-VTL-2 has shown, that the most numerous were particles with the diameter within the range 15-35 µm. To assure long-term stability the whole lot of the material was sterilized by electron beam radiation. Certification of the candidate reference material was based on the world-wide interlaboratory comparison in which 60 laboratories from 18 countries, participated using various analytical methods and techniques. Data evaluation was performed by means of the new multifunctional software package - SSQC. The outlying results from the population of laboratory averages were rejected by concurrent use of the four criteria i.e. those of Dixon, Grubbs, coefficient of skewness and coefficient of kurtosis at significance level of 0.05, followed by calculation of overall mean, confidence intervals etc.
Several criteria formulated previously were employed to decide whether the overall mean can be given the status of "recommended" (certified) or "information" value. "Recommended" values were assigned for 33 and "information" values for 10 elements, respectively. The validity of "certified" values was confirmed for several elements using "very accurate" methods developed in this Laboratory.