J.D. Ingle

Determination of the speciation of chromium with an automated two-column ion-exchange system

An automated two-column ion-exchange system was developed to determine the speciation of chromium in water samples. The system is based on retention of cationic Cr(III) species by Chelex-100 resin and anionic Cr(VI) species by an anion resin. The chromium species retained by the resins are eluted sequentially with stripping reagents and are detected by an on-line flame atomic absorption spectrophotometer. Operation of valves and pumps, data acquisition, and data manipulation are controlled by a microcomputer. The detection limit with a 2 ml sample loop is 2 ng ml−1 for Cr(III) and Cr(VI). The fraction of Cr(III) retained by the Chelex-100 varies with the pH of the solution. In the presence of humic acid, Cr(III) forms negatively charged complexes, some of which are retained by the anion resin.

Correction of Polychromatic Luminescence Signals for Inner-Filter Effects

Fluorescence (FL) and absorbance (ABS) data are acquired simultaneously with a multiple detector spectrometer. The measured ABS is utilized in mathematical equations for automatic correction of fluorescence signals for attenuation (inner-filter effects) caused by significant absorption of the excitation or emission radiation by the fluorophore or other species. Correction equations are presented which apply to situations in which the excitation or emission beam cannot be considered monochromatic, for collection geometries involving optical fibers, and for chemiluminescence measurements. The correction scheme was tested with quinine sulfate (QS) as the analyte in the absence and presence of other chromophores (gentisic acid and methyl red). Even when conditions are such that the FL signal is attenuated by a factor of 5, the correction scheme is accurate to 2% or better. These corrections extend the linear range of the QS calibration curve by a factor of over 100, and they can improve the accuracy up to a factor of ten in cases for which polychromatic effects are important.

Problems with sub-p.p.b. mercury determinations: preservation of standards and prevention of water mist interferences

Abstract Instrumentation, for the cold-vapor atomic absorption determination of mercury has been improved by several modifications including an a.c.—d.c. switching circuit; a single module for the light source, beam splitter, and reference PMT; and the use of an electrically heated glass tube to vaporize water mist, in place of the conventional magnesium perchlorate drying tube. Its use to study the effectiveness of nine common oxidizing agents as preservatives for sub-p.p.b. concentrations of mercury(II) in distilled water has shown that only combinations of nitric acid with potassium dichromate or gold(III) are suitable.

A chemiluminescence photometer for trace chromium(III) determinations

Abstract The design of a simple chemiluminescence photometer is described. The sample is injected into a spectrophotometric cell containing the reagents, and the resultant chemiluminescence peak is recorded along with the peak height and peak area. The instrument includes a temperature-controlled cell holder with stirring capabilities. The determination of p.p.b. levels of chromium(III) is described. Chromium(III) enhances the chemiluminescence reaction of luminol and hydrogen peroxide in basic solutions. Useful calibration curves are obtained from 4 · 10 -9 to 10 -4 M Cr(III); 5 · 10 -10 M is the detection limit. Chromium(III) is determined in natural water samples and NBS Orchard leaves.

Determination of humic acid by chemiluminescence

Abstract The analytical utility of the chemiluminescence resulting from the reaction of humic acid with permanganate is investigated. The chemiluminescence response curve rises sharply to a peak value at about 0.5 s after mixing and decays somewhat more slowly. The peak signal for a fixed humic acid concentration is shown to pass through a maximum near a permanganate concentration of 17 μmol l-1 and to increase continuously with potassium hydroxide concentration up to 2.0 mol l-1. Calibration plots of peak signal vs. humic acid concentration exhibit complex behaviour, being approximately linear up to about 20 mg l-1, curving slightly toward the concentration axis up to about 40 mg l-1, and then curving away from the concentration axis above 40 mg l-1. The detection limit for humic acid is about 0.7 mg l-1. No interference is observed for thirteen common inorganic species at typical levels in water samples. Substantial differences are observed for humic acid in selected samples determined by the chemiluminescence and visible absorption procedures.

Experimental and theoretical comparison of the precision of flame atomic absorption, fluorescence, and emission measurements

Theoretical equations and experimental evaluation procedures for the determination of the precision of flame atomic absorption, emission, and fluorescence measurements are presented. These procedures and noise power spectra are used to evaluate the precision and noise characteristics of atomic copper measurements with all three techniques under the same experimental conditions in an H2-air flame. At the detection limit, emission and fluorescence measurements are limited by background emission shot and flicker noise whereas absorption measurements are limited by flame transmission lamp flicker noise. Analyte flicker noise limits precision at higher analyte concentrations for all three techniques. Fluctutations in self-absorption and the inner filter effect are shown to contribute to the noise in atomic emission and fluorescence measurements.

Fluorimetric kinetic method for the determination of total ascorbic acid with o-phenylenediamine

Abstract A fluorimetric reaction-rate method for the determination of L -ascorbic acid (AA) in aqueous solution is presented. The technique is based on the rapid oxidation of AA by mercury(II) chloride to dehydro- L -ascorbic acid, which then reacts with o -phenylenediamine to form a fluorescent quinoxaline. The formation of the product is monitored fluorimetrically with a data acquisition system based on a microcomputer, a voltage-to-frequency converter and a timer-counter board. The initial rate is estimated with a fixed-time computational method. With a 20-s measurement time (after a 5-s delay from initiation of the reaction), the detection limit for AA is 0.02 μg ml −1 with a linear dynamic range extending to 10 μg ml − . The procedure is applied to the determination of the AA in vitamin pills and juice. The relative standard deviation is 1.9% or better.

Determination of free chlorine in water by chemiluminescence reaction with hydrogen peroxide

The analytical utility of the hydrogen peroxide—hypochlorite singlet oxygen chemiluminescence reaction for the determination of hypochlorite in water is investigated. Effects of pH and hydrogen peroxide concentration are discussed and interference data for over 35 species in the absence and presence of hypochlorite are provided. The limit of detection is 4 μg l-1 with a usable non-linear calibration curve up to about 200 μg l-1. The new method is shown to be relatively free from interferences and to give results for tap water comparable to a standard colorimetric method based on a reaction with N, N-diethyl-p-phenylenediamine.

Design and Performance of a Time Multiplex Multiple Slit Multielement Flameless Atomic Absorption Spectrometer

The design and performance characteristics of a new multielement flameless atomic absorption spectrometer are presented. Radiation from four hollow cathode lamps is combined by beam splitters and directed through a Varian model 63 carbon rod atomizer and then into a special monochromator with a separate exit slit for each element. The light exiting from all slits is directed to a single photomultiplier tube. Absorbance signals from different elements are distinguished with a time multiplex approach. Data acquisition and presentation is accomplished with a PDP-11 computer system. The absolute light levels are lower than those normally encountered with single element atomic absorption resulting in signal shot noise limited measurements. Detection limits (in ppb) of 0.2 (Cd), 2 (Mn), and 8 (Pb) were determined under multielement conditions. The measurement precision of moderate absorbance multielement solutions was determined to be 7% with manual injection of 5 μl samples.

Evaluation of ratemeters: application to an improved fixed-time instrument

Abstract Procedures and instrumentation for the evaluation of reaction ratemeters are discussed and applied to an improved fixed-time reaction ratemeter which employs digital integration. The improved ratemeter is compared with similar systems described previously. A procedure which allows evaluation of the computation circuitry with real noise from the reaction monitor is developed; this new procedure provides a much better evaluation of how the ratemeter will perform in real rate measurement situations. Results presented for synthetic slopes with and without artificial noise show typical relative standard deviations and relative errors of less than 0.3 %. However, with a significant amount of real noise, the precision of rate measurements is reduced.