4.3: Sequential Injection Analysis (SIA): lab on a valve

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SIA, like FIA, is based on reproducible sample handling and controlled dispersion of sample and reagents into a carrier stream. Unlike FIA, it makes use of a computer-controlled multiposition valve and pump, usually peristaltic and operated synchronously with the valve.

A sequential injection manifold suitable for the determination of morphine by acidified potassium permanganate chemiluminescence.

Morphine is solvent-extracted from opium poppies Papaver somniferum on an industrial scale. Barnett et al. have used SIA to determine the drug in aqueous and non-aqueous process streams, with chemiluminescence detection involving oxidation with acidified potassium permanganate in the presence of sodium hexametaphosphate. Figure D3.1 shows a suitable SIA manifold for carrying out this determination. The process streams contain several related alkaloids and a range of other organic compounds as well as both dissolved and suspended solids. It is a good indication of the effectiveness of SIA-chemiluminescence that in these conditions the results correlated well with high performance liquid chromatography, a standard methodology that suffers the defect a much lower sample throughput.

In SIA, sample and reagents are aspirated into the holding coil by operating the pump in reverse so that carrier is returned to the reservoir. Restoration of forward pumping is synchronised with the opening of the valve port leading to the detector. The flow reversal leads to a mixing of the stack of sample and reagent zones to form a product zone which is transported to the detector. The pump tubing comes into contact only with the carrier, the samples and reagents being aspirated (instead of pumped) into the holding coil. This is a very useful characteristic of SIA when using samples /reagents that would attack PVC pump tubing, such as those containing non-aqueous solvents.