Post-column concentration of anions using Nafion membrane tubing

In this tutorial review, the principle of on-line sample concentration by evaporation through a Nafion membrane tubing and its application to post-column concentration of anions by ion chromatography are described. As solution flows through the Nafion membrane tubing, drying nitrogen gas flows outside it to remove the solvent. The removal rate was increased with increasing sample residence time, drying gas flow rate, and device temperature. With the concentrator, the detection sensibility was improved, and the lower detection limit was obtained compared to that without the concentrator. The optimized system has successfully been applied to the determinations of perchrolate in atmospheric aerosol and orotic acid in urinary samples

Digital-movie-based flow colorimetry for pH measurement with universal indicators

A continuous, simple, and versatile pH monitoring method based on digital-movie-based colorimetry is proposed. A constructed flow system was of a two-channel configuration mainly composed of two peristaltic pumps, a digital microscope-based detector, a pH meter with a flow-through combination pH-reference electrode, and a laptop computer. While the total flow rate (FT) was held constant, the flow rate (FB) of a base solution for Britton-Robinson buffer containing a universal indicator (Yamada's indicator or Van Urik's indicator) was changed in proportion to the control signal (Vc) from the computer. An acid solution for the buffer containing the indicator was aspirated to the confluence point at the flow rate of FT – FB and mixed with the base solution. Thus, buffer solutions with arbitrary pH could be easily prepared. The image of the mixed solution was captured with the microscope downstream; the pH of the solution was measured with the pH meter at the most downstream. An in-house program written in Visual Basic .NET was developed to control the system, acquire and analyze the signals (image data and pH), and display the results automatically; the color of the image was expressed as tristimulus values (i.e., R, G, B), hue, and luminance. The relationships between these color-specific values and pH were analyzed after optimizing the Vc scan rate. Van Uik's indicator was superior to Yamada's regarding the applicable pH range (ca. 2.5 – 10). A sigmoid-like calibration curve was established between hue and pH, which was used to determine sample pH. The proposed method was validated by measuring the pH of different drugs and vinegar samples

Amplitude modulated flow analysis for speciation : Proof of concept by quantification of Fe2+ and Fe3+ ions

We propose a novel concept of flow-based analysis for spectrophotometric speciation based on flow rate modulation and fast Fourier transform (FFT). A redox reagent solution's and a sample solution's flow rates are varied by sinusoidal control signals with periods of T and 0.5T, respectively. Both solutions are merged with a color reagent, while the total flow rate is held constant. Downstream, the absorbance of the mixed solution is measured and acquired as the detector output voltage (Vd). The Vd is analyzed by FFT with the window's time length of T. One species that directly reacts with the color reagent contributes only to the amplitude (A2) of the second harmonic wave component in Vd. The other species that needs the redox conversion before the coloration contributes to the amplitude (A1) of the fundamental wave component, in addition to A2. The former species+' concentration can be estimated from A2 by taking the latter's contribution to A2 into account. The latter species’ concentration can be determined only from A1. The proposed concept was demonstrated by applying it to the speciation of Fe2+ and Fe3+ by an o-phenanthroline spectrophotometry, where Fe3+ was reduced to Fe2+ by L-ascorbic acid before the coloration