Application of pressurized fluid extraction to determine cadmium and zinc in plants.

A procedure for the determination of Cd and Zn in plants is proposed. The metals are extracted by pressurized fluid extraction (PFE). Operational conditions are: pressure 1500 psi, temperature 75 degrees C, static time 5 min, flush volume 35%, purge time 60s, cycles 1 and 1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA) 0.01M at pH 4.5 as extracting solution. Determination of Zn is carried out by flame atomic absorption spectroscopy and depending on the concentration level, Cd content is determined by flame or electrothermal atomic absorption spectroscopy. Certified samples of Virginia tobacco leaves, tea leaves, spinach leaves, poplar leaves, a commercial spinach sample (Spinacea oleracea) and genetically modified Arabidopsis thaliana were analysed by the proposed procedure and also by microwave acid digestion and extraction with HCl-Triton X-100. Confidence intervals for Cd and Zn content obtained by the proposed procedure overlap with the certified values. The other procedures, however, provide inaccurate results for Cd. Recoveries obtained for a confidence level of 95% are 96+/-6% and 95+/-5% for Zn and Cd, respectively. Reproducibility of Zn by the proposed procedure is 7% (n=8), similar to the other tests and the detection limit is 2.6 microg. For Cd reproducibility is 8.5% (n=8), better than with HCl-Triton X-100 and similar to acid digestion, the detection limit is 3.5 ng of Cd.

Detection of bias errors in ETAAS Determination of copper in beer and wine samples.

A method that evidences changes in the shape of the absorbance profiles obtained by graphite furnace atomic absorption spectrometry (ETAAS) is proposed. The method is based upon the apparent content curves model previously described for molecular spectroscopy and it permits the detection of possible sources of bias errors. Moreover, a procedure that allows to detect the existence of constant and/or proportional errors is also described. Both models has been applied to the determination of copper in wine and beer samples with and without pre-treatment of the samples. Results obtained evidence the usefulness of the proposed models.

Characterisation of quaternary mixtures by the apparent content curves method: identification of tocopherols in vegetable oils.

A procedure for identification of the compounds in quaternary mixtures has been developed. The proposed procedure is based on the apparent content curves method. From these curves and using the Q parameter, work conditions are selected and quaternary mixtures treated as "pseudoternary" mixtures.A simple strategy to test matrix effects at working wavelengths has been developed, identification limits established and following the identification table quaternary mixtures were easily characterised. The procedure has been applied to the identification of phenols in mixtures by UV-visible spectrophotometry and tocopherols in edible oils by fluorimetry. Results obtained for edible oils are in agreement with the composition obtained by use of the reference method.

Spectrophotometric determination of the theophylline in plasma by the apparent content curves method.

A direct spectrophotometric procedure for the determination of theophylline in plasma is proposed. This procedure is based on the apparent content curves method and it is not necessary to know the nature and contents of interferent substances. Bias errors introduced by plasma and metabolites are avoided. The results obtained are in agreement with those provided by an immunofluoroassay used as a reference method.