Nanomolar Concentration Level Research Articles

Cathodic stripping voltammetry of 2-thioorotic acid at the hanging mercury drop electrode

Controlled adsorptive accumulation of 2-thioorotic acid (6-carboxy-2-thiouracil) on the hanging mercury drop electrode provides the basis for the direct stripping measurement of that compound in the nanomolar concentration level. Differential pulse voltammetry, following 3 min preconcentration, yields a detection limit of 5.0×10 -10 M 2-thioorotic acid. The cathodic stripping response is evaluated with respect to experimental parameters such as preconcentration time and potential, bulk concentration and others. Best results are obtained using a 0.001 M NaOH electrolyte. Two different methods of cathodic stripping voltammetry can be proposed for the determination of 2-thioorotic acid and the reproducibility of these methods is studied.

Fluorometric determination of hydrogen peroxide in groundwater

The fluorometric scopoletin-horseradish peroxidase method has been modified for field determinations of hydrogen peroxide concentrations in groundwaters. Standard additions calibration compensates for background fluorescence and inconsistent stoichiometry of the fluorescence quenching reaction due to interferences by the matrix. The detection limit, defined as the blank plus three standard deviations, ranged from 3.6 to 44.6 nM. However, this limit was more an indication of the difficulty of preparing peroxide-free water than the actual limit imposed by the sensitivity of the method for the peroxide contamination introduced with the reagents. For 111 field determinations the weighted average (uncorrected) hydrogen peroxide concentration was 20.2 nM and the pooled standard deviation was 7.7 nM. The average of 45 field blanks was 7.8 nM with a pooled standard deviation of 5.2 nM. At nanomolar concentration levels, it is essential that samples are analyzed for H/sub 2/O/sub 2/ in the field. Storage periods exceeding 1 h caused serious errors and irreproducible results.

Chelate adsorption for trace voltammetric measurements of iron(III)

A very sensitive electrochemical stripping procedure for trace measurements of iron(III) is described. The chelate of iron with Solochrome Violet RS is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated chelate is measured by voltammetry. The adsorption and redox behaviours are explored by cyclic voltammetry. The height of the chelate peak, which is about 0.28 V more negative than the peak of the free dye, is shown to be proportional to the iron concentration. Optimal experimental conditions include a preconcentration potential of −0.40 V, solution pH of 5.1 and a linear scan mode. The sharp chelate peak, associated with the effective interfacial accumulation, coupled with the flat baseline, facilitates measurements at the nanomolar and submicromolar concentration levels using short preconcentration times. The limit of detection after 1 min preconcentration is 0.04 μgl−1 (7 × 10−10 M), and the relative standard deviation at the 10−7 M level is 4.7%. The effects of possible interferences, due to coexisting metal ions or organic surfactants, are evaluated. The ability of measuring iron(III) in the presence of iron(II) is illustrated. Actual analyses of sea and tap waters are reported.

Trace measurements of tetracyclines using adsorptive stripping voltammetry

The surface-active properties of tetracycline antibiotics are exploited for developing a sensitive adsorptive stripping method for trace measurements of these compounds. Controlled interfacial accumulation at the h.m.d.e. permits convenient quantitation at the submicromolar and nanomolar concentration levels. With 210 s accumulation, the method provides 28, 27, 26 and 23 signal enhancements for tetracycline hydrochloride, oxytetracycline, chlortetracycline and doxycycline, respectively. The adsorptive stripping response is evaluated with respect to accumulation time and potential, stripping mode, concentration dependence, electrolyte and pH, and other variables. Detection limits are 6 × 10 −10 M for doxycycline, 1 × 10 −9 M for oxytetracycline and chlortetracycline, and 2 × 10 −9 M for tetracycline hydrochloride with 300 s accumulation. The reproducibility of the determination (at the 1 × 10 −7 M level) expressed in terms of relative standard deviation, ranges from 0.8 to 2.0%.

Determination of bilirubin by adsorptive stripping voltammetry

A stripping method for the determination of bilirubin at the submicromolar and nanomolar concentration levels is described. The method is based on controlled adsorptive accumulation of bilirubin at the static mercury drop electrode followed by differential pulse measurement of the surface species. After 15 min preconcentration, a detection limit near 5 × 10 −10 M bilirubin is obtained. The adsorptive stripping response is evaluated with respect to concentration dependence, preconcentration time and potential, pH, ionic strength, the presence of surfactants and other variables. Best results are obtained using sodium acetate (pH 8.2) solution. A signal enhancement factor of 11 is observed using 5 min preconcentration (compared to the response of solution-phase voltammetry). The relative standard deviation at the 2 × 10 −7 M level is 2.7%.

Flow electrolysis at a porous tubular electrode with internal stirring

The design and characteristics of a porous tubular electrode in which effective mass transport is ensured by a stirrer rotating inside the tube are described. The electrode provides high analytical currents owing to the combination of the high surface area with high mass-transport rates. The dependence of the limiting current and the degree of conversion on stirring rate, flow rate, and electrode length are described. The completeness of the electrolytic process is as high as 100%. A new hydrodynamic modulation mode, based on measuring the current difference while switching the stirring on and off, is used to correct background contributions. Analytical utility of the cell is demonstrated by application to flow injection and continuous flow analyses. Caffeic acid, p-acetylaminophenol, dopamine, and hexacyanoferrate(II) were used as test systems to give detection limits at the nanomolar concentration level.

A porous-jet flow-through electrode

An electrochemical flow detector, based on a jet of solution directed at a thin porous carbon electrode, is described, along with its electrochemical characteristics and analytical applications. The electrode has a volume of 19 μl and surface area of 1.26 cm 2. The detector exhibits better sensitivity and detectability than a wall-jet detector. Dopamine and ferrocyanide were used as test systems to give detection limits at nanomolar concentration levels. Applications indicated include continuous-flow analysis (utilizing stopped-flow voltammetry) and flow-injection analysis.