G.F. Kirkbright

Spectroscopy in separated flames-IV: application of the nitrogen-separated air-acetylene flame in flame-emission and atomic-fluorescence spectroscopy.

The primary and secondary combination zones of an air-acetylene flame have been separated by a stream of nitrogen flowing parallel to the flame to prevent access of atmospheric oxygen to its base. The flame is very stable over a wide range of fuel-air mixture strengths, and organic solvents may be aspirated without difficulty. The low flame background enables thermal-emission and atomic-fluorescence measurements to be made with high sensitivity. Bismuth, for example, has been determined in the range 5-200 ppm by its thermal emission at 306.8 nm, with a detection limit of 2 ppm in aqueous solution, and in the range 1-10 ppm with a detection limit of 0.3 ppm in 50% ethanolic solution. Zinc and cadmium have been determined at 213.9 nm and 228.8 nm by atomic-fluorescence spectroscopy in this flame with detection limits of 2 x 10(-4) ppm and 5 x 10(-4) ppm respectively, vapour-discharge lamps being used as sources of excitation. The results obtained represent a considerable improvement over those available by the same methods in a conventional air-acetylene flame.

Emission spectra of nitrous oxide supported acetylene flames at atmospheric pressure

The emission spectra of a premixed flame of acetylene supported by nitrous oxide have been recorded under different fuel-gas mixture conditions. The emission spectra in these flames of a series of metals, for which it is difficult to obtain a significant population of ground state atoms for atomic absorption spectroscopy in more conventional flames, have also been studied. The red secondary zone which is present in the fuel-rich flames shows emission attributable to long-lived CN and NH species which form a strongly reducing atmosphere to inhibit refractory oxide formation from elements such as molybdenum, titanium and aluminium introduced into the flame. An attempt has also been made to explain some of the reactions which may occur between the flame species above the primary reaction zone.

Spectrofluorimetric determination of microgram amounts of thallium

Thallium(I) in a concentrated hydrochloric acid-potassium chloride medium shows a violet fluorescence with an excitation maximum at 250 mμ and a fluorescence emission maximum at 430 mμ. This has been applied for the spectrofluorimetric determination of thallium down to the range 0.01 to 0.08 ppm. The interference of 53 ions has been examined at the 100-fold molar excess level. Hydrogen peroxide oxidation to thallium(III) and extraction from 1.5M hydrochloric acid into diethyl ether, followed by sulphur dioxide reduction, separates microgram amounts of thallium quantitatively from all interfering cations investigated except antimony(V), bismuth, gold and platinum(IV). Large amounts (× 106) of sulphate do not interfere.

Temperature profiles in nitrous oxide supported acetylene flames at atmospheric pressure

Temperature profiles have been established at three mixture strengths of the nitrous oxide-acetylene flame used in atomicabsorption spectroscopy. Measurement of the electronic excitation temperature of the red-feather zone by the sodium line-reversal and iron two-line methods yields average maximum temperatures of 3070 and 3025 +/- 50 degrees K respectively. This is significantly lower than the only previously reported value, 3228 degrees K. Other temperature measurements obtained by studying intensity distribution of NH rotational fine structure and CN vibrational structure yielded less precise results, but suggest a state of thermal equilibrium in the flame. The temperature gradient within the flame shows a steady decrease with height above the primary zone. A study of CN spectra and the zones of persistence of free atoms and of metal oxide species suggests a mechanism of free atom production within the cyanogen zone whereby the removal of oxidizing radicals by CN promotes dissociation of metal oxide species previously formed in the primary zone of the flame.

Development and publication of new spectrophotometric methods of analysis

The method of study of factors affecting the development of a spectrophotometric method is reviewed. Various recommendations are made concerning data which should be reported in the publication of the results of these studies.

Some spectrofluorimetric applications of the cerium(IV)-cerium(III) system

Abstract Cerium(III) ions in dilute sulphuric acid medium exhibit a characteristic fluorescence which has its excitation maximum at 260 nm and its fluorescence emission maximum at 350 nm. By utilising the osmium-catalysed redox reaction between cerium(IV) and arsenic(III), microgram amounts of arsenic (7.5–37.5 μg) may be determined by spectrofluorimetric measurement of the ceriurm(III) produced. The principle may be applied to the determination of several other ions which cannot yet be determined by direct spectrofluorimetry, e.g. . Fe(II) (5.6–28 μg), oxalate (8.8–44μg). Osmium(VIII) (0.05–0.2 μg) and iodide (0.6–2.5 μg) may be determined by their catalytic action.

Shpol'skii luminescence spectroscopy of extracts of coal and coal-tar pitch

Shpolskii luminescence spectra in straight-chain-alkane matrices at 77 K have been measured from carbon-disulphide extracts of nine coal macerals and a cyclohexane extract of a coal-tar pitch. Twelve individual polycyclic hydrocarbons, containing from three up to ten rings, have been identified. The Shpolskii effect, which requires little pretreatment of the sample, is particularly sensitive (0.1 ppm) to the carcinogen benz [a] pyrene.

spectroscopy in separated flames—VI: The argon or nitrogen-sheathed nitrous oxide-acetylene flame in atomic-absorption spectroscopy

The separation of the premixed nitrous oxide-acetylene flame at a 50-mm slot burner by sheathing with argon or nitrogen is described. In comparison with the conventional flame, the interconal zone of the hot, slightly fuel-rich separated flames provides better conditions for the maintenance of free atoms of elements which form refractory oxides. Optimum conditions for the determination by atomic-absorption spectroscopy of the elements Al, Be, Ge, Mo, Si, Ti, V and Zr in both separated and conventional flames at the same burner have been established. Significant improvement in detection limits and sensitivities is obtained in the separated flames.

Spectroscopy in separated flames—III: Use of the separated nitrous oxide-acetylene flame in thermal emission spectroscopy

The separation of a premixed nitrous oxide-acetylene flame at a modified commercial burner is described. The reducing interconal zone of the fuel-rich separated flame exhibits low radiative background. The reducing atmosphere and high temperature of this flame result in an effective medium for the excitation of the atomic line spectra of the refractory elements. The use of the fuel-rich flame in the flame photometry of these elements has been investigated.

Carminic acid as a reagent for the spectrofluorimetric determination of molybdenum and tungsten—I: Development of procedures

Carminic acid forms red fluorescent complexes with molybdenum(VI) and tungsten(VI) which provide spectrofluorimetric methods for the determination of these metals in the range 0.1-0.9 ppm and 0.4-0.36 ppm at pH 5.2 and 4.6 respectively. The optimum conditions for the analytical procedures have been established, and the effects of a wide range of cations and anions have been investigated. The reagent forms 1:1 complexes with both molybdenum and tungsten, and the conditional stability constants have been evaluated.