Evaluation of immobilized redox indicators as reversible, in situ redox sensors for determining Fe(III)-reducing conditions in environmental samples.

An in situ methodology based on immobilized redox indicators has been developed to determine when Fe(III)-reducing conditions exist in environmental systems. The redox indicators thionine (Thi, formal potential at pH 7 (E(7)(0')) equals 66 mV), toluidine blue O (TB, E(7)(0')=31 mV), and cresyl violet (CV, E(7)(0')=-75 mV) have been immobilized to 40-60 mum agarose beads via an amine-aldehyde coupling reaction. These beads were packed into a flow cell to allow spectrophotometric monitoring of the redox state of simple solutions and wastewater slurries pumped from in a bioreactor. Fe(II), a product of microbial activity, at levels observed in real systems reduces both the free (non-immobilized) and immobilized redox indicator to different degrees for samples with pH 6.5 or higher. At pH 7, immobilized Thi and TB are significantly reduced at Fe(II) concentrations greater than 0.1 and 0.3 mM, respectively. CV, with the lowest formal potential, requires Fe(II) levels in excess of 10 mM. The degree of reduction of the indicators (i.e. the fraction of indicator oxidized) observed during titrations can be qualitatively modeled with a simple equilibrium model based on ferrihydrite or lepidocrocite as the Fe(III)-solid phase. The reversibility of Fe(II)-indicator reactions was also demonstrated by showing that the reduced indicator becomes re-oxidized when Fe(II) levels decrease.

Chemiluminescence during the oxidation of alcohols by permanganate: application to the determination of ethanol in gin.

Chemiluminescence is observed during the oxidation of ethanol with potassium permanganate under very acidic conditions. Factors affecting the chemiluminescence signal are discussed. Based on this chemiluminescence reaction, a method for the determination of alcohol in gin was developed. The method, which has a detection limit of 0.3% (v/v) and a relative standard deviation of about 1% at 40.0% (v/v), was used in the determination of ethanol in three different brands of gin with no sample preparation.

Optimization of atomic fluorescence measurements with a microcomputer-based time-multiplex multiple-slit spectrometer.

An automated multielement flame atomic fluorescence (AF) spectrometer is described. The instrument employs a multiple exit slit monochromator and a single detector. Each element is excited to fluoresce with a single-element hollow cathode lamp (HCL) and a time-multiplex mode is used for pulsing the HCLs and gated data acquisition. A microcomputer controls the pulsing of HCLs, sample introduction into the flame and data acquisition. Optimization of the HCL pulse width and peak current is shown to be critical. With an air/H(2) flame sheathed with Ar, the following single-element detection limits (in mug/ml) were obtained: Au, 0.2; Cd, 0.005; Co, 0.03; Cu, 0.003; Fe, 0.05; Mg, 0.0008; Mn, 0.007; Ni, 0.04; Pb, 1 and Zn, 0.02. Some multielement detection limits were a factor of 2-5 worse. At higher analyte concentrations, the signal to noise ratio decreased due to an increase in the relative analyte fluorescence flicker noise.

Kinetic fluorometric FIA determination of total ascorbic acid, based on use of two serial injection valves.

The kinetic determination of total ascorbic acid with a flow-injection analysis system is described. The sample is loaded into two serial sample injection valves and injected into a carrier system containing mercuric chloride and o-phenylenediamine. The formation of a fluorescent product is monitored. The single-point and double-point kinetic methods are compared by using both peak height and peak area as the analytical signal. A detection limit of 0.1 mu/ml for vitamin C was achieved with a sample throughput rate of 30 per hour.

Automated two-column ion exchange system for determination of the speciation of trace metals in natural waters.

A rapid method for determination of metal speciation based on an automated two-column ion exchange system is described. Two fractions of dissolved trace metal species are directly determined by on-line flame atomic absorption spectrophotometry after preconcentration by sequential columns of Chelex-100 chelating resin and AG MP-1 macroporous anion resin. A third fraction is determined by standard addition. Variables that affect the results obtained by the two-column system are studied by the use of model complexing agents. With a 10-mL sample loop, the sample throughput is 6 samples per hour and detection limits are 0.1 micrograms/L for Cu(II), 0.08 micrograms/L for Cd(II), and 0.2 micrograms/L for Zn(II). The method is used to determine the speciation of Cu(II), Cd(II), and Zn(II) in natural water samples.

Two-column ion-exchange method for the determination of copper-complexing capacity and conditional stability constants of copper complexes for ligands in natural waters.

Sample solutions titrated with Cu(2+) ions are passed sequentially through two ion-exchange columns in an automated flow system. The first column is packed with Chelex-100 resin and retains Cu(2+) ions that are free or derived from copper complexes that dissociate in the column. The second column is packed with AG MP-1 anion-exchange resin and retains negatively charged Cu(II) complexes. The retained copper species are then eluted from the columns and determined on-line with a flame atomic-absorption spectrophotometer. It is necessary to correct for a small fraction of free Cu(2+) ions that pass through the first column and are retained by the second column. The Cu(II)-complexing capacity of sample solutions is determined from plots of the concentration ratio of free Cu(2+) ions to Cu(II) complexes vs. the concentration of free Cu(2+) ions. Conditional stability constants of the copper complexes are also estimated from these plots. The complexing capacity of sample solutions is also determined rapidly by measuring the concentration of complexed Cu(II) after spiking the sample with an excess of Cu(2+) ions. The sample solutions tested were 4.0muM NTA, 4.0-mg/l. humic acid, and a river water.

Design and characterization of an intensified diode array data-acquisition system for spectrometric measurements.

A computer-based data-acquisition system for an intensified diode array (IDA) detector is described. A unique combination of hardware and software provides many data-acquisition and calculation options useful for multiple-wavelength spectrometric measurements. The data-acquisition system is used to evaluate critically many characteristics of the IDA detector, including the dependence of the light and dark signals and noise on experimental variables, and the linearity, memory effects and resolution.

Simple sample-cell positioner for reducing the imprecision due to placement of test-tube type sample cells.

The construction and evaluation of a sample-cell positioner for test-tube type cells in simple single-beam spectrophotometers are described. Use of the sample-cell positioner reduces the uncertainty due to cell-positioning, by about an order of magnitude.

A srial ASCII keyboard-printer.

The construction of a simple keyboard-printer is described. It is intended for use with single-board microcomputers such as the KIM and SYM which have a 20-mA loop ASCII interface.

Determination of cobalt by pyrogallol chemiluminescence.

The chemiluminescence reaction involving pyrogallol in alkaline hydrogen peroxide solution is described. With reaction conditions selected for the determination of Co(II), the detection limit for Co(II) was at least two orders of magnitude below the detection limit of all other species tested. The results suggest that Ca, Mg, Mn, and Fe are the most likely interferents for environmental and biological samples. The log-log calibration graph for Co(II) is linear from a detection limit of 0.5 ng/ml up to 10 microg/ml.

Simple and inexpensive microcomputer-controllable stepping motor and drive circuit.

The construction of a simple stepper motor and drive circuit made from commercial components is described. The use of the stepper motor for a monochromator wavelength drive is discussed. The rate and direction of scan can be controlled by either a hardwired timer circuit or by a microcomputer.

Construction of a microcumputer-based reaction-rate ratemeter.

The design and construction of a ratemeter based on a KIM-1 single-board microcomputer are discussed. The reaction monitor signal is digitized with a voltage-frequency converter and the resulting data points are stored in computer memory. Through software, the reaction-rate is calculated by the fixed-time computation method. A unique software-hardware combination is used for conversion from binary to binary-coded decimal format so that the results are printed in decimal notation. The characteristics of the ratemeter are compared with those of earlier ratemeters.

Application of an intensified diode array system to diagnosis of matrix effects and the rapid sequential determination of thiamine and riboflavin with fluorescence detection.

The unique capabilities of a multiple wavelength spectrofluorometer based on an intensified diode array detector are used for diagnosis of matrix effects and for rapid sequential determination of two analytes. This system is used to investigate problems due to background fluorescence, blank reactions, and scattering for the determination of thiamine in cereal and urine samples by a fluorometric kinetic procedure. A novel determination of thiamine and riboflavin in vitamin pills is based on monitoring the native fluorescence of riboflavin over one wavelength region, and the rate of formation of fluorescent thiochrome from thiamine over another wavelength region, after a computer-controlled change in the pH of the reaction mixture.