Stanley Greenfield

Nebulization effects with acid solutions in i.c.p. spectrometry

Abstract The effect of high acid concentrations in the sample solution on signal intensity has been investigated with a high-power plasma as the excitation source. With mineral acids the intensity is reduced by a factor which correlates well with the expected reduction in sample uptake caused by increased viscosity. With organic acids such an expected reduction is outweighed by other enhancements which can be represented as a function of the viscosity, surface tension and density of the solution. Such a dependence of all the effects on solution properties suggests that they should be attributed to the aspiration and nebulization system rather than to the plasma source.

Automatic multi-sample simultaneous multi-element analysis with a h.f. plasma torch and direct reading spectrometer

Abstract The equipment described for automatic multi-sample simultaneous multielement analysis has been in everyday use for practical analysis for four years. Some important points in its design are discussed and reasons are advanced for using a high-powered, free-running oscillator to excite a plasma with two gas streams in addition to the stream carrying the sample aerosol. The frequently-used simple formulae for inductance, appropriate for long coils, were found inadequate; modified formulae, applicable to short coils, which agreed with experimental data, were used. Trace analysis and assay work can alike be performed with good accuracy and precision, and in a convenient way. Some typical results are given, with an indication of the versatility of the equipment.

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) with flow injection analysis (FIA)

Abstract Flow injection analysis is applied for solution introduction into an inductively coupled plasma discharge. Properties and advantages of transient emission signals, the analysis of Portland cement, and application of signal integration and multielement FIA-ICP are described. The possibilities of FIA-ICP for “instant” signal-to-background measurement, calibration by standard additions and exponential dilution, and separation methods are evaluated.

Calorimetric and dimensional studies on inductively coupled plasmas

Abstract The validity of the use of metallic loads to obtain formulae for the power dissipated in plasmas is confirmed by direct calorimetry on plasmas. As a rule of thumb, when high powers are used, about 40% of the power supplied to the generator reaches the plasma. The power in the plasma, as a function of plate kVA seems to be independent of plasma gas flow coolant. Measurements of plasma volumes show that those obtained with a nitrogen coolant are smaller than with an argon coolant; because of this, and in spite of higher losses to the coolant, nitrogen-cooled plasmas have higher power densities.

Investigation of decomposition products of microwave digestion of food samples

The involatile residues remaining after closed-vessel microwave digestion of various food samples, using nitric acid, with and without post-digestion treatment with hydrogen peroxide, have been studied. Decomposition products were found to include aliphatic and aromatic acids, nitro-compounds, oxalates and inorganic nitrates and phosphates. Measures of digestion completeness were provided by appearance, carbon content, infrared spectra and thin layer chromatograms of the residues, enabling a comparison of different digestion methods and sample types. Residual carbon levels varied linearly with the relative amounts of carboxylic acid and inorganic nitrate, as measured by infrared spectrometry of the residues. The formation of calcium oxalate was also a function of the degree of decomposition: the carboxylic acid: oxalate ratio increased in a logarithmic fashion with increasing residual carbon content. Particularly high levels of residual carbon from nitric acid digestion of milk powder, due largely to carboxylic acid residues, were substantially decreased by post-digestion treatment with hydrogen peroxide. However, nitrobenzoic acids, which proved major interferents in electrochemical analysis, were only removed by treatment with perchloric acid.

Twenty-five years of analytical atomic spectroscopy

Abstract Developments in atomic spectrometry are traced over the twenty-five years from 1960–1985. Although a few other methods are briefly mentioned, greatest emphasis is placed on emission, absorption and fluorescence techniques. Two emission approaches are considered in detail and are based on the high-voltage spark and on the inductively-coupled plasma, respectively. Atomic absorption progress is followed from its earliest introduction, through the trial-and-error improvement of instrumentation, to the recent availability of fully automated commercial systems. Finally, the youngest of the methods, fluorescence spectrometry, is described, its strengths and weaknesses reviewed, and its future potential assessed.

Comparison of alternating variable search and simplex methods of optimisation for inductively coupled plasma optical emission and atomic fluorescence spectrometry

The performance of several cyclic alternating variable search (AVS) optimisation methods are compared with two simplex methods with respect to the number of changes of variable required to search a model two-factor response space. The roles of the initial step size and of the variable step size are discussed, and the information produced concerning the shape of the factor space is evaluated. An AVS method which starts with a fixed step size and then changes to a variable step size on second and subsequent cycles is compared with a variable step size simplex for the optimisation of an inductively coupled plasma optical emission spectrometer and of the atomiser, source inductively coupled plasmas in atomic fluorescence spectrometry (the ASIA system). The order in which the variables are taken in the AVS method does not affect the value of the optimum eventually found. Both methods perform satisfactorily for the optical emission work, although the AVS method provides information about the shape of the factor space which is easier to interpret than in the simplex method. However, the simplex method was not always able to satisfy the conditions for termination in the case of the atomic fluorescence studies and was much slower to implement than the AVS method as the latter used direct visual feedback from the output of the lock-in amplifier as a measure of the figure of merit (total fluorescence signal).

Plasma spectroscopy comes of age

A “state of the art” review of ICP-OES is given in which the author attempts to give an impartial account of the technique with respect to detection limits, precision, interferences and the low power-high power controversy.

Atomiser, source, inductively coupled plasmas, in atomic fluorescence spectrometry (ASIA). Some recent work

Abstract The dual-plasma atomic fluorescence/atomic emission spectrometer (ASIA spectrometer) is described and the detection limits for several elements in both the fluorescence and the emission mode are presented and compared directly. The detection limits for the non-refractory elements are, in general, better in fluorescence than emission While for the refractory elements the converse is true, but all are in the ng ml−1 range. Growth curves for a refractory and a non-refractory element are presented and evaluated.

Communication. Liquid nitrogen cooling in microwave digestion

In an attempt to minimize the delay in opening Teflon pressure vessels following microwave acid digestion, and thus significantly reduce sample preparation time, various approaches to vessel cooling have been investigated. These include the feasibility of carrying out digestions with the pressure vessels immersed in liquid nitrogen and the use of liquid nitrogen as a pre- and post-digestion coolant. Liquid nitrogen cooling in the microwave unit was found to decrease digestion rates considerably, although the prevention of rapid and uncontrollable increases in pressure during digestion of organic material could be useful in some cases. Liquid nitrogen cooling subsequent to or between heating cycles was found to be very effective, especially where several heating, cooling and venting cycles were required. As well as enabling the vessels to be cooled rapidly and opened, it stopped any continuing increase in pressure which could otherwise cause the hot vessels to vent with potential loss of sample. Pre-digestion cooling also helped to delay the onset of rapid increases in pressure.