Sample pretreatment by UV photolysis for the ion chromatographic analysis of plant material

Information on the elemental content of leaves, juices and extracts of vegetables, fruits and plants is of great botanical, nutritional and environmental interest. Procedures are described for the ion chromatographic determination of total chlorine, bromine, phosphorus and sulfur (carbonate-hydrogen carbonate eluent), iron, copper, nickel, zinc and cobalt (pyridine-2,6-dicarboxylic acid eluent) and lead and cadmium (H2SO4-HCl-KCl eluent). Detection limits range between 20 and 100 mu g/l and the calibration graphs were linear up to 1000-2500 mu g/l. In order to dissolve samples prior to chromatographic analysis, oxidative UV photolysis was used; it has decisive advantages over other sample pretreatment methods, owing to the simple procedure and the minimal reagent addition requirement, resulting in minimal contamination. Depending on the type and amount of organic matter, the UV photolysis conditions can be adjusted as required. The homogenized sample is mixed with a small amount of hydrogen peroxide and/or nitric acid and subjected to UV photodegradation in a digester equipped with a high-pressure mercury lamp (500 W); the temperature of the sample was maintained at 85 +/- 5 degrees C by means of a combined air-water cooling system. Organic constituents are degraded and organic-bound metals and non-metals are set free and their quantification remains unaffected by UV radiation, except for nitrate, iodide and manganese. It follows that chlorine, bromine, phosphorus and sulphur, normally present in different organic compounds, can be quantified as total amount only, without any speciation. The dear-solution is injected directly into an ion chromatograph equipped with both a conductivity detector, for the determination of total chloride, bromide, phosphate and sulphate ions, and a postcolumn membrane reactor and variable-wavelength UV-Vis detector, for the determination of lead, cadmium; iron, copper, nickel, zinc and cobalt.