We report a fully automated online sample pretreatment system for ionic analytes that extracts the ionic analytes from the sample and largely removes the nonionic sample matrix and can preconcentrate the analyte. Sample pretreatment is a key analytical process; conventional pretreatment is conducted in a difficult to automate batchwise manner. The present system relies on the transport of ions induced by an electric field to a water acceptor. Cations and anions are simultaneously and separately collected into individual acceptor streams which can be directly introduced to a chemical analyzer. Common inorganic ions (≤10 meq/L) are quantitatively transferred from samples within a few seconds. Small nonionic molecules are transferred by 0.5-10%, and proteins are not transferred at all. The method has been successfully applied to drinking water, urine, and cow's milk with 3.7 ± 2.5, 3.8 ± 2.6, and 4.6 ± 2.6%, respectively, in variance (n = 10). Present results agreed well with those from conventional pretreatment methods. Interestingly, when calcium in milk is measured by the present method, the results correspond to the total calcium by conventional methods; i.e., it can extract calcium from its protein-bound form in milk.
Chromatographic determination of organic acids is widely performed, but the matrix often calls for lengthy and elaborate sample preparation prior to actual analysis. Matrix components, e.g., proteins, non-ionics, lipids etc. are typically removed by a combination of centrifugation/filtration and solid phase extraction (SPE) that may include the use of ion-exchange media. Here we report the quantitative electrodialytic transfer of organic acids from complex samples to ultrapure water in seconds using cellulose membranes modified with N,N-dimethylaminoethyl methacrylate, which essentially eliminates the negative ζ-potential of a regenerated cellulose membrane surface. The transfer characteristics of the ion transfer device (ITD) were evaluated with linear carboxylic acids. While the ion transfer efficiencies may be affected by the acid dissociation constants, in most cases it is possible to achieve quantitative transfer under optimized device residence time (solution flow rate) and the applied voltage. In addition, the transfer efficiency was unaffected by the wide natural variation of pH represented in real samples. The approach was applied to organic acids in various samples, including red wine, considered to represent an especially difficult matrix. While quantitative transfer of the organic acids (as judged by agreement with standard pretreatment procedures involving SPE) was achieved, transfer of other matrix components was <5%. The processed samples could then be chromatographically analyzed in a straightforward manner. We used ion exclusion chromatography with direct UV detection; in treated samples; there was a dramatic reduction of the large early peaks observed compared to only 0.45μm membrane filtered samples.
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