A low-cost light-scattering detector for the flow-injection nephelometric determination of sulfate.

A simple low-cost flow-through light-scattering detector was developed for determining the particle mass concentration in colloidal suspensions. Employing a laser pointer as a light source and a photodiode IC as a light sensor, the detector was shown to have good sensitivity, yet was small and battery operated. The detector was demonstrated to be effective for the flow-injection nephelometric determination of sulfate by precipitation as barium sulfate.     

A dynamic liquid-liquid interfacial pressure detector for the rapid analysis of surfactants in a flowing organic liquid

Design and development of a dynamic interfacial pressure detector (DIPD) is reported. The DIPD measures the differential pressure as a function of time across the liquid-liquid interface of organic liquid drops (i.e., n-hexane) that repeatedly grow in water at the end of a capillary tip. Using a calibration technique based on the Young-Laplace equation, the differential pressure signal is converted, in real-time, to a relative interfacial pressure. This allows the DIPD to monitor the interfacial tension of surface active species at liquid-liquid interfaces in flow-based analytical techniques, such as flow injection analysis (FIA), sequential injection analysis (SIA) and high performance liquid chromatography (HPLC). The DIPD is similar in principle to the dynamic surface tension detector (DSTD), which monitors the surface tension at the air-liquid interface. In this report, the interfacial pressure at the hexane-water interface was monitored as analytes in the hexane phase diffused to and arranged at the hexane-water interface. The DIPD was combined with FIA to analytically measure the interfacial properties of cholesterol and Brij^®30 at the hexane-water interface. Results show that both cholesterol and Brij^®30 exhibit a dynamic interfacial pressure signal during hexane drop growth. A calibration curve demonstrates that the relative interfacial pressure of cholesterol in hexane increases as the cholesterol concentration increases from 100 to 10,000 μg ml⁻¹. An example of the utility of the DIPD as a selective detector for a chromatographic separation of interface-active species is also presented in the analysis of cholesterol in egg yolk by normal-phase HPLC-DIPD.     

Evaluation of on-line preconcentration and flow-injection amperometry for phosphate determination in fresh and marine waters

Dissolved reactive phosphorus (DRP) was determined as orthophosphate (PO₄-P) in fresh and saline water samples by flow-injection (FI) amperometry, without and with in-valve column preconcentration. Detection is based on reduction of the product formed from the reaction of DRP with acidic molybdate at a glassy carbon working electrode (GCE) at 220 mV versus the Ag/AgCl reference electrode. A 0.1 M potassium chloride solution was used as both supporting electrolyte and eluent in the preconcentration system. For the FI configuration without preconcentration, a detection limit of 3.4 μg P l⁻¹ and sample throughput of 70 samples h⁻¹ were achieved. The relative standard deviations for 50 and 500 μg P l⁻¹ orthophosphate standards were 5.2 and 5.9%, respectively. By incorporating an ion exchange preconcentration column, a detection limit of 0.18 μg P l⁻¹ was obtained for a 2-min preconcentration time (R.S.D.s for 0.1 and 1 μg P l⁻¹ standards were 22 and 1.0%, respectively). Potential interference from silicate, sulfide, organic phosphates and sodium chloride were investigated. Both the systems were applied to the analysis of certified reference materials and water samples.     

Cost-effective flow injection spectrophotometric assay of iron content in pharmaceutical preparations using salicylate reagent

A new flow injection procedure for an assay of Fe(III) by using salicylate obtained from antipyretic powder, which is a cheap and easily available reagent, is proposed. A red complex was continuously monitored by a laboratory-made green LED colorimeter. A linear calibration was obtained in the range of 1–20 mg Fe l⁻¹ with a detection limit of 0.5 mg Fe l⁻¹ and R.S.D.s of 1.4–5.4% (n=3, for 1–20 mg Fe l⁻¹). The new procedure was applied to assay iron contents in pharmaceutical preparations. The results were in good agreement with those of the USP standard method.     

Stopped-flow injection simultaneous determination of phosphate and silicate using molybdenum blue

Kinetic information for the phosphate–molybdate–ascorbic acid reaction can be obtained by making use of a very simple manually operated stopped-flow injection (FI) system. Various parameters (concentrations of reagents, flow rate, mixing coils, and volume of flow cell) were investigated for determination of phosphate. A stopped-FI system should be arranged for low degree of mixing (of reactants) and low dispersion so that good signals of rate changes will be observed. Simultaneous determination of phosphate and silicate by the stopped-FI technique is proposed, using a laboratory-made semi-automatic stopped-FI Analyzer with LED-based photometer. It is based on kinetic separation of phosphate and silicate using molybdenum blue. The proposed procedure has been demonstrated for the application to water samples. The results obtained agree with that of a standard method.