Flow injection systems for directly coupling on-line digestions with analytical atomic spectrometry. Part 1. Dissolution of cocoa under stopped-flow, high-pressure conditions

Abstract

A double flow injection manifold incorporating a resistively heated oven is under development for the direct coupling of the digestion of solid samples with an analytical spectrometric technique. The potential of the system was illustrated by the dissolution of cocoa powder and subsequent determination of the copper and iron content by flame atomic absorption spectrometry (FAAS). The cocoa powder was slurried in 10% nitric acid, injected into the manifold and digested under stopped-flow, high-pressure conditions. Gas–liquid separation was effected by a two-stage de-pressurization system. Copper and iron were determined by flow injection FAAS against acid matched standards. Values of 44 ± 19 and 144 ± 10 mg kg^–1 were obtained for the copper and iron content, respectively. The cocoa powder was also digested using an open-vessel hot-plate method and a closed-vessel microwave digestion method. The results for the copper content were 52 ± 2 and 50 ± 2 mg kg^–1, respectively. Both of these methods gave a value of 180 ± 10 mg kg^–1 for iron. The relatively large uncertainty in the copper result for the on-line method was owing to the higher dilution in this system producing a concentration in the digest nearer the detection limit of the flame procedure. The low results for iron were considered to be owing to incomplete digestion, as agreement with the results of the other methods was obtained if the solution was analysed by the standard additions method. The proposed procedure produces a clear solution, and allows increased sample throughput while minimizing sample contamination and decreasing sample and reagent consumption.