Microwave-assisted sample preparation in sequential injection: spectrophotometric determination of magnesium, calcium and iron in food

A sequential injection system for spectrophotometric analysis of foods and similar involving direct sample introduction as a natural suspension or as slurry is proposed. Sample and concentrated nitric acid are sequentially aspirated and directed towards a digestion bomb located inside a microwave oven where sample decomposition takes place. After digestion, aliquots of the processed sample and randomly selected reagents are sequentially aspirated towards the holding coil and pumped towards the detector. As application, magnesium, calcium and iron were determined in milli, soft drinks and similar by using o-cresolphthalein (with or without EGTA addition) and o-phenanthroline as colour forming reagents. The system is versatile, and yields reproducible results (r.s.d. <0.04 for 5.0-50.0 mg l(-1) Mg, 15.0-150 mg l(-1) Ca and 2.0-20.0 mg l(-1) Fe) in agreement with flame atomic absorption spectrometry involving the manual sample digestion procedure. (C) 2000 Elsevier Science B.V. All rights reserved.