Development of flow systems incorporating membraneless vaporization units and flow-through contactless conductivity detector for determination of dissolved ammonium and sulfide in canal water.

Use of membraneless vaporization (MBL-VP) unit with two cone-shaped reservoirs is presented for on-line separation and detection of non-volatile species. A flow system comprising two sets of MBL-VP units with a single in-house capacitively coupled contactless conductivity detector (C4D) was developed for dual determination of ammonium and sulfide ions. Using the continuous-flow section, two zones (280µL) of a sample, either mixed with sodium hydroxide (for ammonium) or hydrochloric acid (for sulfide), are separately delivered into the donor reservoir of the MBL-VP units. The acceptor reservoir contains either 150µL of 15µM HCl solution (for ammonia) or pure water (for hydrogen sulfide), respectively. Vaporization and trapping of the ammonia or hydrogen sulfide gas from the donor reservoir into the liquid acceptor cone occur concurrently in the two separate MBL-VP units. After trapping the gas for 3min, the two 150-µL liquid acceptors are sequentially aspirated through the C4D flow cell for recording the changes in the conductivity. Linear calibrations were obtained for ammonium from 5 to 80µM (Volt = (0.0134 ± 0.0003) [NH4+] - (0.01 ± 0.01), r2 = 0.998) and for sulfide from 5 to 200µM (Volt = (0.0335 ± 0.0009) [S2-] - (0.13 ± 0.09), r2 = 0.996). Analysis time for both analytes is only 320s. Our method was applied to analyze canal water samples. The results agree well with membrane gas-diffusion flow injection techniques, using bromothymol blue for ammonium and methylene blue for sulfide. Recoveries ranged from 95% to 104%.

A Simple Microfluidic Electrochemical HPLC Detector for Quantifying Fenton Reactivity from Welding Fumes.

Development and characterization of a simple microfluidic electrochemical flow cell that can be coupled with HPLC to enable dual absorbance/electrochemical detection is described. Coupling absorbance and electrochemical detection increases the information that can be gathered from a single injection, but a second (typically expensive) detection system is required. Here, an inexpensive, customizable microfluidic electrochemical detector is coupled in series with a commercial HPLC/UV system. The microfluidic device is made from poly(dimethylsiloxane) and contains carbon paste electrodes. To demonstrate the utility of this dual-detection system, the reaction products of the radical scavenging agent salicylic acid and hydroxyl radical generated by Fenton chemistry were analyzed. The dual-detection system was used to quantify 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, and catechol produced by the addition of H2O2 to filter samples of welding fumes. Measurement recovery was high, with percent recoveries between 97-102%, 92-103%, and 95-103% for 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, and catechol, respectively, for control samples. The methods described in this work are simple, reliable, and can inexpensively couple electrochemical detection to HPLC-UV systems.

New membraneless vaporization unit coupled with flow systems for analysis of ethanol.

This work presents the development of a new design for a membraneless vaporization (MBL-VP) unit, called dual chamber MBL-VP for measurement of volatile compounds. With this unit, exact volumes of sample and reagent are introduced into their respective cone-shaped chambers from the base of the cones. Diffusion of volatile analyte then takes place. After an appropriate time interval, the acceptor solution is withdrawn from the chamber into the detector flow-cell, while the sample solution is withdrawn to waste. Unlike the previous MBL-VP design, problems with overflow of solutions are eliminated by precise control of the input volume to be less than the volume of the chamber. The developed flow system with the dual chamber MBL-VP unit was applied to the determination of the ethanol content of various liquid samples, using the oxidation reaction between potassium dichromate and the diffused ethanol. In addition, in order to accelerate the gas diffusion process, the donor chamber was aerated. As the result, relatively short analysis time of 144 s was achieved for ethanol content in the range of 5-50% (v/v). The proposed method was successfully validated against a gas chromatographic method for 17 alcoholic samples. Percentage recovery was in the range of 96-109%.