Studies of a microporous membrane for analyte preconcentration and separation

A dual phase gas diffusion-FIA system containing a tubular PTFE-membrane was studied as a mean of producing gas samples for routine 15N/14N isotopic ratio mass spectrometry. The method is based on Rittenberg's reaction; the ammonium sample is injected into a liquid alkaline stream containing hypobromite and the N2 gas produced in the reaction diffuses across a PTFE-membrane into a helium carrier stream which carries it to the detector. Initially here, the use of a tubular microporous PTFE-membrane as a device for the preconcentration of samples in aqueous solutions was investigated. The performance of such a membrane was studied under a variety of operating conditions. A qualitative model of the membrane mechanism was developed based on the diffusion transport of vapour away from the contained liquid surface through the connected pore space. The dispersion undergone by the sample in the GD-FIA system containing this preconcentration unit was also studied and this FIA system was applied as a practical device for the determination and speciation of aluminium in a river water sample. The procedure for generating nitrogen gas involved optimisation of the system parameters including the oxidation reaction step and the production on-line of the chemicals used. The nitrogen gas was generated easily and rapidly, allowing a sample throughput capability of the order of 20 h-1. The system was applied to the determination of total nitrogen content in agricultural sample prepared by the Kjeldahl digestion. The method offered precision and accuracy comparable to those of the standard distillationtitration procedure. Isotope ratios were determined with good precision and means for obtaining accuracy comparable with established techniques were developed. It was also shown that the DPGD-FIA system can be readily adapted to enable different forms of nitrogen e. g. N02-, N03- and NH4+ to be determined.