Investigation of carbon dioxide modified supercritical and near supercritical carrier streams for flow injection systems.

Since flow injection (FI) is a dilution technique, efforts have been undertaken to minimize online dilution or dispersion. Solutes in supercritical fluids exhibit increased diffusion coefficients which have been shown to decrease dispersion of the sample zone. This work investigates the use of supercritical fluids (or CO(2) modified fluids) as carrier streams for FI. Both a non-reacting tracer and an online chemical reaction were employed to investigate the behavior of solutes in supercritical and near critical systems. Further, these results are compared to those obtained in the system studied with a conventional carrier stream. Plots of peak response vs% CO(2) modifier increase with a sharp break at moderate modifier composition (20-30%). Plots of peak variance vs% CO(2) modifier show decreased variance with increasing % modifier. The system was also optimized with regards to temperature and pressure. The optimized system displayed improved limits of detection and decreased variance relative to 0% CO(2) modifier carrier streams.

Micellar catalysis in flow injection systems: the nitrosation reaction.

The use of aqueous micellar carrier streams in flow injection is shown to catalyse on-line reactions and thereby improve the sensitivity relative to non-micellar systems. The effect of several different micellar systems on the nitrosation reaction of N,N-diethylaniline in acidic solution was investigated. The mechanism of the micellar enhancement is discussed and the sensitivities of the method in both aqueous and micellar media are compared. The presence of the cationic surfactant, cetyltrimethylammonium bromide, is shown to improve the sensitivity of the analysis. The effect of the micellar solution on the dispersion characteristics of the system was also studied. The greater viscosity of the micellar phase increases dispersion for non-reacting systems, but when monitoring a reaction product the increased reaction rate can counteract this and the micellar carrier can show less over-all dispersion than the aqueous carrier.

Reduction of injection variance in flow-injection analysis.

In order to achieve maximum sensitivity in flow-injection analysis, sample dispersion must be kept to a minimum. This dispersion process, however, is not well understood. Studies of the dispersion process have concentrated on dispersion within the flow manifold while dispersion due to the injection process has been largely ignored. Here sample injection loops packed with inert glass beads and a Serpentine II (distorted) empty loop were constructed and compared to traditional empty sample loops. Digitization of the response curves and subsequent calculation of the statistical moments were used to compare the contribution of each sample loop type to the total system dispersion. Both packed and Serpentine II sample loops were shown to decrease dispersion and increase throughput in flow-injection systems. Plots of peak variance vs. injection volume show variance increasing 1.67 times faster with traditional open sample loops compared to packed loops. When combined with other peak width minimization techniques, this method should further lower concentration limits of detection.