Iain L. Marr

Inorganic and organic selenium compound speciation with coupled HPLC-MW-HG-AFS

Coupling between high-performance liquid chromatography, microwave-induced reduction, hydride generation and atomic fluorescence spectrometry (HPLC–MW–HG–AFS) has been used for selenium speciation. The concentrations of a redox mixture (HCl solution of KBr and KBrO3 aqueous solutions) used for both destruction of organic matter and reduction of the selenium species have been optimized, as well as the concentration of NaBH4 in the hydride generation step. Chromatographic separation of selenocysteine, selenite and selenate took place in 15 min, employing a polymer-based strong anion-exchange column. Calibrations with correlation coefficients r > 0.998 were obtained for the three compounds at concentrations between 0.5 and 10 µg l−1. Detection limits for selenite, selenocysteine (SeCys) and selenate were 0.2, 0.3 and 0.5 µg l−1, respectively. The coupling has been tested with spiked water samples (tapwater and seawater), obtaining recoveries in the range 94–104%. Copyright

Determination of chlorophyll a and b by simultaneous multi-component spectrophotometry

The mathematical technique of over-determined simultaneous equations has been applied to the analysis of chlorophyll extracts from plant materials. The choice of spectral range is important — it is shown that 50 data points in the range 500–700 nm give better results than a similar number in the range 400–700 nm, as they avoid interferences from carotenoids. Results agree with those calculated using Arnons method (1949), but have a better precision of 2% rsd compared with 4%. The computing procedure also allows testing of the correctness of the results.

Improvements to the ethylene blue method for the determination of hydrogen sulphide in air

The Ethylene Blue (EB) method, used in the determination of H2S in various environments, has given cause for concern due to poor precision and varying sensitivity. The reason for this has been shown to be poor pH control in the solution at the time of reaction. Both the rate of formation of the blue colour and the final absorbance obtained are affected by the amount of sulphuric acid added with the reagent (p-diethylaminoaniline). Good precision can be attained if the volumes of reagents added are strictly controlled, but the method has been improved further by incorporating a pH 1.6 maleic acid buffer system in the reaction medium before the addition of reagents. The new method is tolerant to wide variations in the amounts and concentrations of absorbent and reagents and has a greatly improved precision (RSD = 1.7% compared with 6% for ten samples containing 6.5 µg of S with absorbances ranging from 0.387 to 0.412). Extraction of the EB into 10 ml of chloroform can improve the precision further (RSD = 0.643% for ten samples containing 6.2 µg of S with absorbances ranging from 0.549 to 0.557).

Preparation and properties of unsymmetrical tetraorganotin compounds

Abstract Unsymmetrical tetraorganotins, R 2 R 1 R 2 Sn (R = Me, R 1 = Bu, R 2 = Pe (Pe = pentyl) or Ph; R = Bu, R 1 = Pe, R 2 = Ph or Me; R = Pe, R 1 = Bu, R 2 = Me or Ph), Bu n R 4 -nSn (n = 1-3, R = Me or Pe) and Pe 2 R 2 Sn (R = Me or Ph) have been synthesised. Various physical properties, including mass spectra, 1 H and 13C NMR spectra, are reported.

Systems theory in analysis—I definitions and interpretations in the basic terms of systems theory☆

The twelve basic terms of systems theory-system, element, relation, function, structure, organization, feedback, Black Box, model, input-output analysis, trial-and-error method, simulation-are given general definitions and are also interpreted in terms of chemical analysis. The distinction between a generally acceptable brief and precise definition and an interpretation peculiar to one particular specialized field avoids, in the case of universally used terms, a biased view and conception. Simple systems are deliberately chosen and the terms used are clarified by means of diagrams. A system-oriented approach is outlined, with feedback-coupled stages (defining the problems, setting the limits, designing the model, simulation) which has proved useful in practical systems analysis and in systems design. The working group will report, in subsequent publications, on specific analytical systems which are of considerable importance in automation. Those with special interests in these fields are warmly invited to communicate their critical opinions, suggestions, and examples of interpretations based on these definitions.

The influence of surfactants on the behaviour of heavy metals in the system n-octanol-water

The partition of heavy metals between octanol and water in the presence of surfactants has been investigated. It is shown that metal ions can be extracted from water into the organic phase, n-octanol, as a model for biological materials in the presence of various anionic surfactants. Anions such as chromate can be extracted with the help of certain cationic surfactants. It was found that the extent of extraction depends on various parameters, such as the concentration of the surfactant, ionic strength and pH. With some anionic surfactants, the K(ow) of lead or cadmium can reach values of 200 from waters of low ionic strength, decreasing as the ionic strength increases.

Measurement of carbon-13: carbon-12 ratios by Fourier transform infrared spectrometry

An approach to the measurement of 13C: 12C ratios in isotopically enriched samples is described. The carbon in samples is converted into CO2 gas by either combustion of organic material or acidification of carbonates. The gas is then measured by FTIR spectrometry. When spectra are recorded at 0.25 cm–1 resolution the magnitude of the isotopic shift is such that the areas of 12C and 13C rotational lines of the ν3 vibrational band for CO2 can be measured and the isotopic composition determined by reference to a standard calibration graph. The relative standard deviation at natural abundance is 1.2%, which gives an absolute limit of detection of 0.026 atom-%13C. Details of the decomposition procedures and a comparison of the results with those obtained by mass spectrometry are also presented.

Improved Infrared Spectroscopic Method for the Measurement of 13C: 12C Ratios

Pressure broadening of an infrared absorption band of gaseous CO2 makes measurement of a band intensity very much easier and also much more precise in comparison with the results obtained by measuring the intensity of a single line in a gas-phase spectrum. The isotopic shift of the v3 bands for carbon dioxide centered at 2350 cm−1 is sufficient to completely separate the P branch for 13CO2 from the two branches for 12CO2. The intensity ratios can be measured accurately on a low-resolution FT-IR spectrometer when the CO2 is pressurized to 10 bar (106N m−2) with N2, permitting isotope ratio differences as small as 0.02 atom % above the natural abundance of 1.11 atom % to be detected.

Determination of tin in poly(vinyl chloride) by atomic-absorption spectroscopy

A simple procedure is described for the determination of tin in PVC by atomic-absorption spectroscopy with an air-hydrogen flame, after wet digestion of the sample with sulphuric acid and hydrogen peroxide.

Determination of organochlorines in sea water: an assessment

Abstract Multi-matrix multi-determinand methods of analysis are needed when studying the fate and transport of trace organic compounds in the aquatic environment. These methods are cost effective and provide information about the distribution of the determinands within the sample. The current method of this laboratory for the determination of persistent organochlorine contaminants was extended to the analysis of large volumes of sea water (28 l) and suspended particles. The method was validated in each stage of the analysis by a series of experiments using spiked samples, reference materials and replicates. The results showed that the precision for the determination of the organochlorines in the aqueous and particulate fractions were similar, at the sub-nanogram per litre level, whilst the initial extraction from the sample matrix contributed over 80% of the total variance in the analysis.