Sample preparation in sequential injection analysis. Spectrophotometric determination of total phosphorus in food samples Original Research Article

A sequential injection (SI) procedure involving in-line sample preparation is proposed. A natural suspension or a slurry is transported together with nitric acid towards a home-made digestion bomb placed inside a microwave oven for subsequent digestion. The sample zone is stopped inside the oven and, after digestion, directed in reverse flow towards a holding coil and then towards detection. This is a good strategy because the SI system is versatile; the formed air bubbles are efficiently disposed of, thus avoiding the need for a debubbling unit, and the digestion bomb acts also as a mixing device promoting easy homogenization regardless of the number of required reagents. As application, the spectrophotometric determination of total phosphorus in foodstuffs based on the molybdenum blue method was selected. The proposed system is very robust and yields reproducible measurements, (r.s.d. usually <3%) for 20.0–400.0 mg l−1 P–PO4. Results are in agreement with a conventional spectrophotometric procedure involving manual sample digestion.