F.J.M.J. Maessen

Analytical aspects of organic solvent load reduction in normal-power ICPs by aerosol thermostatting at low temperatures

Abstract The effect of aerosol thermostatting on plasma stability and signal-to-background ratio has been investigated with an argon ICP as the excitation source. To distinguish between aerosol thermostatting required for the generation of stable plasmas and aerosol thermostatting aimed at improvement of signal-to-background ratio the parameters “maximum tolerable aerosol temperature” and “optimum aerosol temperature” have been introduced. Ignition procedures have been established for a number of solvents known to be difficult with respect to the generation of stable plasmas. Application of aerosol thermostatting over a wide temperature range enabled a choice of compromise aerosol temperatures for simultaneous multi-element analysis. Results of signal-to-background measurements indicate that detection limits attainable in aqueous and organic media are of comparable magnitude.

Preservation of accuracy and precision in the analytical practice of low power ICP-AES

Abstract In an inductively coupled argon plasma (ICAP) matrix effects caused by elements constituting major inorganic components in samples of widely different nature have been studied under experimental conditions not primarily aimed at achieving maximum detection power and minimum matrix effects but at optimum reproducibility of the analytical signals. Separate and combined matrix effects on thirteen ICAP analysis lines were investigated. From the results conclusions could be drawn of interest for the practice of ICP analysis. A close examination of acid effects revealed the appearance of an “adaptation” effect.

Electron temperatures and electron concentrations at low pressure in microwave induced plasmas

Abstract This investigation deals with measurements of electron temperature ( T e ) and electron concentration ( n e ) in monoatomic and polyatomic gas plasmas in the pressure region from 0.067 up to 2.67 mbar (0.05–2 Torr) in flow systems. Measurements have revealed that the T e reaches very high values in the low pressure region (0.067 mbar). T e and n e decrease at increasing pressure. The electron temperature appears to be independent of the applied microwave power; on the contrary the electron concentration increases with power.

Experimental control of the solvent load of inductively coupled argon plasmas and effects of the chloroform plasma load on their analytical performance

Abstract The solvent plasma load ( Q SPL ) of water, methanol and chloroform was established as a function of the liquid uptake rate ( Q L ) by using a continuous weighing method for recording the rate differences between the relevant liquid streams. The shape of the Q L vs Q SPL curves revealed that the liquid uptake rate is a parameter much too insensitive to serve as a criterion for assessing the stability of “organic” plasmas. The quantity “maximum tolerable solvent plasma load” is suggested as a more useful criterion. Effects of rf power, observation height and solvent plasma load on the properties of chloroform inductively coupled plasmas (ICPs) are reported. The measurement of the axial distribution of net line intensities of representative spectral lines showed that the behaviour of emission lines as to their “hardness” is essentially the same in ICPs loaded with chloroform or water. The chloroform plasma load was regulated by the use of a condenser of which the temperature was varied in a range between −50°C and +20°C. Analytical performance characteristics such as net line and background intensities, signal-to-background ratios, and relative standard deviations of the background signal are presented for ICPs with various chloroform loads. Two sets of experimental conditions were finally selected for simultaneous multielement analysis of chloroform solvent solutions, one with and one without aerosol cooling. In the case that aerosol cooling was applied, the detection limits were similar to those for aqueous plasmas. Without aerosol cooling the detection limits were up to an order of magnitude poorer. An attempt has been made to catagorize organic solvents on the basis of both volatility and their behaviour in ICP systems. For a better understanding of the consequences of solvent volatility in ICP-AES it is of importance to consider separately the properties that determine the volatility of liquids, viz. the evaporation rate and the saturation vapour pressure.

Organic solvent load of inductively coupled argon plasmas as a function of the liquid uptake rate and the inner gas flow rate

Abstract Solvent plasma loads of nine solvents, including water, have been determined for a wide range of nebulization conditions. The measurements were accomplished by making use of the so-called “continuous weighing method”. In addition, the tolerance of medium power inductively coupled plasmas (ICP) to solvent loading has been established. The organic solvents studied were selected on the basis of their significance for direct ICP analysis of organic solvent solutions using atomic emission spectrometry. The results show that the solvent saturation vapour pressure governs the solvent plasma load, whereas the evaporation factor dominates the distribution of the solvent over the liquid and vapour phase at the exit of the spray chamber.

Excitation conditions in a mixed-gas low-pressure microwave induced plasma

Abstract Double probe measurements and optical measurements have been carried out to determine the variation of electron temperature, electron density and emission intensities in low-pressure (0.1–0.5 torr) microwave induced plasmas (MIPs) in dependence of the composition of the binary gas mixture used to carry the discharge. For low concentrations of an added component the excitation parameters maintain values typical for the dominant species and the line intensities of the added component vary linearly with its concentration. When the percentage of the added component reaches 1% the excitation conditions start to change and the line intensities are no longer proportional to the concentration. Detailed analysis shows that the variations in line intensities observed in different mixed-gas discharges are closely related to the variations in electron temperature and density, with the former parameter exerting the major influence.

The influence of flow rate and pressure on the excitation conditions in low-pressure microwave induced plasmas

Abstract Using a double probe technique, electron temperatures and electron concentrations together with spectral line intensities have been measured in low-pressure microwave induced plasmas at various pressures and flow rates of monoatomic and polyatomic support gases. For two distinct pressures viz. 0.2 and 1.0 torr (0.267–1.333 mbar) the flow rate has been independently varied. Measurements of spectral-line intensities in the absence and presence of the probes demonstrate that the probes exert little or no influence upon the plasma conditions. The results show that when low-pressure microwave induced plasmas in flow systems are applied for quantitative analytical purposes exact specification of both flow rate and pressure of the carrier gas is required.

A comparative examination of sample treatment procedures for ICAP-AES analysis of biological tissue

Abstract The objective of this study was to contribute to the evaluation of existing sample preparation procedures for ICAP-AES analysis of biological material. Performance characteristics were established of current digestion procedures comprising extraction, solubilization, pressure digestion, and wet and dry ashing methods. Apart from accuracy and precision, a number of criteria of special interest for the analytical practice was applied. As a test sample served SRM bovine liver. In this material six elements were simultaneously determined. Results showed that every procedure has its defects and advantages. Hence, unambiguous recommendation of standard digestion procedures can be made only when taking into account the specific analytical problem.

An assessment of the laser microprobe analyzer as a tool for quantitative analysis in atomic emission spectrometry

Abstract The various sources of random error, occurring in laser microprobe analysis using cross-excitation have been examined. It is shown that the combination of laser sampling and cross-excitation constitutes the major source of random error. In an attempt to reduce this error, the spectral line intensity has been related directly to a reference line, to the crater depth, and to the crater volume. However, introduction of these parameters did not improve the analytical results. The crater depth formula proposed by Bar-Isaac and Korn appeared to be of little analytical use, not only because all numerical values of the physical constants involved have to be exactly known, but also because each laser mode gives rise to different removal processes. A reproducibility of the spectral line intensity up to 16% is met. Moreover, due to the small slopes of the analytical curves, the precision of the analysis is rather poor. The use of the laser microprobe (employing cross-excitation) in quantitative analyses is therefore to be dissuaded.

Critical examination of the borate fusion technique for spectrochemical trace analysis of geological materials using the d.c. arc

Abstract X-ray diffraction analysis permitted a systematic investigation of the influence of fusion temperature, fusion period, sample-to-flux ratio, and particle size on the isoformation of geological materials. Two rational experimental procedures are given. The favourable effect of flux addition on accuracy and precision of spectrochemical results is discussed in terms of a model for the exit of the analysis elements from the electrode cavity.