Ziad Taha

Nitric oxide measurements in biological samples

The crucial role of nitric oxide (NO) in controlling many physiological functions in mammals is now established. To aid understanding this crucial role, sensitive and selective methods for its in vivo and in vitro detection are vital. The unique chemical and physical properties of NO set the tone for its detection strategies. This review summarizes different techniques and methodologies used in measuring NO in biological samples. Those include gas and liquid phase chemiluminescence, electron spin resonance spectroscopy, UV-visible spectroscopy, fluorescence, electrochemical sensors, and reporter cell assay. The principles, applications, merits, and limitations of each technique are discussed.

Catalytic–adsorptive stripping voltammetric determination of ultratrace levels of molybdenum in the presence of organic hydroxy acids

The adsorptive collection of the molybdenum complex with 3-methoxy-4-Hydroxymandelic acid was coupled with the catalytic current of the adsorbed complex to yield an ultrasensitive voltammetric procedure for measuring picomolar levels of molybdenum. Optimum solution conditions (particularly the solution composition) were established to give a detection limit of 4 ng dm–3(4 × 10–11 mol dm–1) of molybdenum (following a 2 min preconcentration). The relative standard deviation (at 0.5 µg dm–3) was 3.7%. Possible interferences were investigated and the applicability to assays of tap water is illustrated. Such coupling of catalytic and adsorptive collection processes holds great promise for the development of ultratrace voltammetric procedures for other analytes.

Batch injection with potentiometric detection

Abstract The characteristics and advantages of employing ion-selective electrodes (ISEs) as detectors for batch injection (BI) systems are evaluated. Sharp potential response peaks are obtained for the injection of microliter samples onto the nearby ISE surface. Such dynamic measurements performed in batch systems result in high sample throughput and reproducibility, similar to those of established flow-injection operations. For example, pH measurements can be performed at rates of up to 720 samples per hour. There is no observable carry-over (between samples of low and high concentrations) and the precision is typically 1–2% (relative standard deviations). The applicability of BI–ISE system to chloride and fluoride measurements and for assays of real samples is also illustrated. The effects of various experimental variables on the BI potentiometric response are described. Such use of highly specific detectors offers many prospects for future use of BIA systems. The absence of pumps, injection valves and flow cells greatly reduces the instrumentation costs for rapid automated measurements of discrete ions.

Electroanalysis at modified carbon-paste electrodes containing natural ionic polysaccharides

The strong affinity of natural ionic polysaccharides for certain metal ions is exploited in the design of a new class of voltammetric sensing devices. In particular, carbon-paste electrodes containing pectic and alginic acids are used for the nonelectrolytic collection and subsequent voltammetric determination of copper and lead, respectively. Cyclic and differential pulse voltammetry are used to quantify the accumulated ions. The response is characterized with respect to modifier loading, preconcentration period, metal concentration, reproducibility, possible interferences and other variables. Titrimetric experiments illustrate the potential of polysaccharide electrodes for speciation work. Preliminary data are also given for analogous measurements of copper at heparin-modified electrodes. Detection limits are 1 mug/ml and the relative standard deviation is 4.8%.

TRACE MEASUREMENTS OF RHODIUM BY ADSORPTIVE STRIPPING VOLTAMMETRY

A sensitive stripping voltammetric procedure for quantifying rhodium is described. The complex of rhodium with chloride ions is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated complex is measured during a negative-going scan. Cyclic voltammetry is used to characterize the interfacial and redox behaviors. The effect of pH, chloride concentration, accumulation potential and other variables is discussed. The detection limit is 1 x 10(-8)M ( approximately 1 ng/ml) with 5-min accumulation. A linear current-concentration relationship is observed up to 7 x 10(-7)M and the relative standard deviation (at the 2 x 10(-7)M level) is 3.0%. Possible interferences by co-existing metals are investigated.

Catalytic-adsorptive stripping voltammetric measurements of hydrazines

Hydrazine compounds can be determined by voltammetry, based on their hydrogen catalytic wave in formaldehyde-platinum-sulphuric acid media. Adsorption of the platinum-formazone complex on the hanging mercury drop electrode enhances the sensitivity, allowing convenient measurement of micromolar and submillimolar concentrations of simple hydrazines. The selectivity is better than that of conventional (anodic) measurements of hydrazines, as oxidizable compounds do not interfere. The sensitivity is also greatly improved. The hydrazine response is evaluated with respect to preconcentration time, concentration dependence, platinum and formaldehyde concentrations, reproducibility and other variables. The relative standard deviation (at 2 x 10(-5)M) is 1.6%.

Batch Injection Analysis with Thermistor Sensing Devices

Abstract Batch injection analysis (BIA) is a new non-flow technique involving the injection of microliter samples toward a nearby detector, immersed in a large-volume blank solution. This paper describes the characteristics and advantages of employing thermal sensing devices as detectors for BIA. Similar to analogous flow injection measurements, batch injection thermal analysis offers high speed, reproducibility and simplicity, while eliminating the need for pumps, valves and associated tubing. There is no observable carryover and the precision is typically 2% (RSD). The batch injection thermal analysis is illustrated for enzymatic reactions, as well as redox and acid-base reactions.

Stripping voltammetric determination of traces of catechol compounds preceded by adsorptive accumulation of metal complexes

Abstract Catechol compounds are quantified by controlled adsorptive accumulation of their metal complexes onto a hanging mercury drop electrode followed by stripping voltammetry. By using tin(IV) as a redox marker for quantifying the surface-bound species, selectivity can be improved relative to conventional oxidative methods; dopamine can be quantified in the presence of ascorbic acid. The method allows measurements of micromolar levels of catechol, dopamine, l -dopa, 3,4-dihydroxyphenylacetic acid and caffeic acid. The adsorptive stripping response is evaluated with respect to preconcentration time and potential, tin(IV) and analyte concentrations, stripping mode, reproducibility and possible interferences. Analogously, solochrome violet RS and dimethylglyoxime can be quantified after accumulation of their iron(III) and nickel(II) complexes, respectively. Detection limits are 7×10−9 M for solochrome violet RS and 5×10−8 M for caffeic acid (1- and 5-min preconcentration, respectively).