Abstract
Accurate analysis of metal ions, especially at trace levels, is one of the most difficult and complicated analytical tasks, since it requires the rigorous requirement of versatility, specificity, sensitivity and accuracy in the analysis. For determination of trace metal ions present in various samples like industrial materials and biological samples, direct determination using various instrumental methods is not possible owing to matrix effects and low concentration of metal ions in these samples. Thus, the need of separation and preconcentration arises. This results in simplification of the sample matrix and enhances the concentration of the sample to be analyzed. For the determination of trace metals in complicated KOH and microdialysate of extracellular fluid of anesthetized rat brain, two on-line analytical systems were developed to overcome all analytical obstacles standing in the way of micro-trace analytical works. In the first part of this study, a sequential injection on-line quantification procedure for the determination of trace impurities in KOH by using desalter coupled to ICP-MS is described. Due to trace elements and potassium are all cations, a selective chelating agent, named EDTA, was added to form anionic EDTA complexes with analytic ions. To keep the charge balance during the ion exchange mechanism, a lot of potassium ions can be removed to stripping solution and hydrogen ions entered the sample cell from the stripping solution. The process will reduce the pH of the sample solution from 13 to 2 and metal ions associated with EDTA will be substituted by hydrogen ions. Using 600mM NH4Cl as stripping buffer, NH4+ ion will cross the membrane to keep the charge of sample solution in balance instead of H+ ion. According to the results obtained, over 99.99% of 78000 mg/L KOH can be removed and the interferences caused by dissolved salts can be completely eliminated by the proposed analytical system. The robustness of proposed method is expected to be satisfactory for the analysis of high salt content samples and the recovery of analyte elements can maintain 100±10%. The applicability of proposed on-line desalter ICP-MS system has been proven by the determination of trace metallic impurities in high alkaline KOH solution. In the second part of this work, an on-line method for the determination of trace metals in microdialysis samples using flow injection combine with hollow tube preconcentration technique and inductively coupled plasma mass spectrometry has been developed and optimized. In this study, the effects of pH value of sample stream, and sequential injection variables related to reagent loading (tube precoating) and sample loading (analysis preconcentration) were separately optimized. Using the developed hollow tube preconcentration mechanism, selective preconcentration succeeded by way of forming metal complexes on-line with ethylenediaminetetraacetic acid (EDTA) adsorbed on the inner walls of a PVC tube, in a wide range of sample acidity (pH from 5 to 10). A quantitative elution was achieved using 20 mg/L EDTA at pH 10. Under the optimized conditions, the method can be successfully applied to the in vivo determination of Mn, Cu, Zn, Ni, Co, Cd and Pb in extracellular fluid anesthetized rat. With the use of the proposed microdialysis-hollow tube-ICP-MS system, the response time and temporal resolution can be controlled down to 30 min and 15 min, respectively. As the methodology described above was equipped with a 20-□L sample loop, the detection limits of low □g/L for almost all analyte ions could be reached based on three times the standard deviation of the baseline noise (n=7). In view of a prolonged monitoring might proceed to collect data from the anesthetized rats, a normal saline solution containing 30 □g/L of analytes was measured for investigate the long-term stability of proposed on-line method. The precision of the 5 measurements was within 10% RSD. This method has been successfully used to on-line monitor in vivo microdialysate from probe implanted in the brain of anesthetized rats. The consistence between obtained results and reported basal level of Mn and Zn in the extracellular fluid in the rat brain suggests that the proposed system is a potent in-situ, in vivo and continuous method for evaluating the dynamic variation of various metal ions in the tissues of anesthetized rat.