Improvement of precision for pipetting blood serum samples into a graphite furnace

Graphite furnace atomic absorption spectrometry is a well-established technique for trace metal determination in blood and serum samples. For this kind of samples, a R.S.D. of up to 10% is considered acceptable, especially for those elements, the concentrations of which do not allow high dilution. Among the reasons that contribute to an increase of the S.D. is the retention of proteins (and analyte) in the sample dispenser capillary, because proteins might be adsorbed on polymeric surfaces. In the present work, the interaction protein/polymer was studied, considering the amount of protein that could be retained by the capillary, the influence of sample dilution, time of contact and the possible co-adsorption of metals. Serum proteins were retained in the capillary, depending on the material (Tygon>silicone rubber>poly(tetrafluorethylene)); dilution did not prevent the adsorption, and only 30 s of contact were enough for the adsorption to occur. Using a column filled with PTFE powder (60 mesh), it was possible to observe that metals were co-adsorbed to a large extent. Water, diluted nitric acid or aqueous solutions of Triton X-100 were not able to promote the complete desorption of the proteins retained by the polymeric materials. Total elution was achieved with methanol, and its use as rinse solution decreased the R.S.D. (n=10) for aluminium, manganese and chromium determination in serum to <10%.