Acid extraction of metals from proteins

Trichloracetic acid (TCA) has been used to release Fe from serum proteins for many years, yet it is only recently that this kind of preparation has been used for the analysis of other metals. A comparison of TCA precipitation with simple dilution as preparation for serum Zn analysis was reported by Boyde and Wu [23] and by Kelson and Shamberger [24], Both groups found that Zn was quantitatively recovered from serum using TCA and that the concentrations found were consistently higher than those obtained by direct dilution. The former group found a mean positive bias of +20% for the TCA preparation, but the latter found more consistent 

De-proteinisation of diluted whole blood, and acid extraction of lead from proteins with 2M HNOs is the preparation chosen for the National Bureau of Standards reference method for blood-lead analysis [28]. Centrifugation of blood samples treated in this way yields clear supernatant fractions that contain all of the lead present, and which are easily dispensed into electrothermal atomisers using auto-sampling techniques. This method gave results that compared very well with those obtained using anodic stripping voltammetry, r = 0.975, for concentrations of 10—900 pg l-1 of lead in blood [15]. 

Jackson et al. [29] reported a simple method for dissolving tissue samples using a 25% alcoholic solution of tetramethylammonium hydroxide which contained 2% m/v APDC. Clear homogeneous solutions were obtained when 100—300 mg weights of liver, brain, or muscle tissues were heated at 60°C with 1—3 ml of the base/APDC solution, and the cooled solutions were diluted with toluene. The solutions were then analysed for Zn, Cu, Fe and Mn. This procedure has also been applied successfully to the analysis of Cu in brain [30] and Pb in human placental tissue and animal liver [31]. The addition of APDC ensured the stability of solutions of exogenous metal ions added for calibration as well as those of endogenous metals from the tissues for at least 24 h after dilution with toluene. Gross and Parkinson [32] ommitted APDC from their solution of the quaternary ammonium hydroxide and used water rather than toluene for the final dilution. They were able to dissolve successfully 28 different types of human tissue, and measured Cu, Pb, Zn, Cd and Mn in liver by ETA—AAS with RSD s of 0.04—0.06. Quantitative recovery, 94—104%, was observed for all metals 

---