Richard F. Browner

Influence of water on conditions in the inductively coupled argon plasma

Abstract Aqueous media are used almost universally for sample introduction in both inductively coupled plasma atomic emission spectrometry (ICPAES) and in inductively coupled plasma/mass spectrometry (ICP/MS). In the process of aqueous sample introduction a substantial mass of water is introduced into the plasma as a combined aerosol/vapor mixture. In the present studies, the masses of water present as aerosol and vapor were controlled, in order to examine their separate influence on the key plasma properties of electron density ne and ionization temperature Tion. Water loading in the plasma was indeed found to have a major influence on ne and Tion, and plots of these parameters as a function of water loading are presented. Plasma viewing height and operating power were also found to be important variables in influencing the way in which water interacts with the plasma. The implications of water loading on background emission and noise level are also considered.

Evaporation characteristics of organic solvent aerosols used in analytical atomic spectrometry

Abstract A simple theoretical approach has been used to calculate changes in aerosol droplet diameters caused by evaporation in a solvent-vapor saturated atmosphere. The results for a range of solvents likely to be encountered in analytical atomic spectrometry are discussed, with particular reference to their possible effects on nebulization and atomization efficiencies, and hence on sensitivities attainable in nebulizer-based atomic spectrometry. The importance of droplet solvent evaporation is also discussed with respect to the aspiration rates which may be usefully employed when organic solvents are aspirated into an inductively coupled plasma.

Low-flow interface for liquid chromatography–inductively coupled plasma mass spectrometry speciation using an oscillating capillary nebulizer

The application of a novel nebulizer, the oscillating capillary nebulizer (OCN), is described for use in speciation studies. The nebulizer has certain features which make it very suitable for this application, without modification, at both micro-flows (1 µl min–1) and macro-flows (1 ml min–1). Short- and long-term precision at typical operating flows are comparable to a normal (1 ml min–1) concentric glass nebulizer. Column-to-nebulizer dead volume is approximately 1 µl. The narrow drop size distribution for the nebulizer at low flows leads to excellent sensitivity when coupled to a micro-LC column. Post-column peak broadening introduced by the interface is minor at flows 5 µl min–1, but widens the peaks noticeably at flows between 1 and 5 µl min–1. The very high efficiency of the nebulizer at flows <50 µl min–1 is exemplified by the fact that no drain is necessary at these flows in the open spray chamber, as no visible liquid condenses on the chamber walls. The ICP-MS response for the OCN (counts per ng of Se injected) does not change when water is replaced by methanol as solvent, whereas with a conventional nebulizer, a solvent change of this type inevitably results in a significant change in response. The OCN was used for the reversed-phase LC separation of a mixture of five organic Se compounds of pharmacological significance, at flows of 12, 50 and 400 µl min–1. With use of a 0.5 mm id column, a flow rate of 12 µl min–1 and a 60 nl injection, good peak separation was found, with an average efficiency of ≈ 10000 plates and a detection limit of around 30 pg.

Thermospray Sample Introduction to Atomic Spectrometry

Abstract Thermospray aerosols are generated by forcing a liquid sample through a capillary tube that is heated to partially vaporize the solvent, resulting in a blast of vapor that converts the remaining liquid to droplets. The droplet size character of thermospray aerosols can be electrically varied by changing the temperature and degree of solvent vaporization of the liquid stream. The primary droplets produced by thermospray under optimal conditions are smaller on average then those produced by pneumatic nebulizers, particularly of the types used for inductively coupled plasmas (ICPs). Solvent vaporization is enhanced for smaller particles and higher temperatures, with both aspects leading to faster size reduction due to solvent evaporation than would occur with pneumatic sample introduction at room temperature. As smaller droplets are more efficiently transported through sample introduction systems, the use of thermospray aerosol generation provides higher analyte transport, higher sensitivity, and lo...

Effect of analyte and solvent transport on signal intensities in inductively coupled plasma atomic emission spectrometry

Abstract Aerosols generated with several different nebulizers have been characterized by their major aerosol characteristics, namely the Sauter mean drop diameter, D3,2, the analyte mass transport, Wtot, the solvent mass transport, Stot, the analyte transport efficiency, ϵn, and the solvent transport efficiency, ϵs. These characteristic aerosol parameters have been compared with the magnitudes of analytical signals, Inet, generated in an inductively coupled plasma, in order to examine the possible existence of interrelationships between them. For three of the nebulizers tested, a reasonably linear correlation between Wtot and Inet was observed with water as solvent. For the fourth, a crossflow-type nebulizer, which produced a tertiary aerosol with a significantly larger mean drop size than the other nebulizers, the relationship between Wtot and Inet was not linear. For the organic solvents tested, methanol and n-butanol, solvent/plasma interactions were sufficiently strong as to make the relationship between Wtot and Inet quite complex.

Fundamental studies on pneumatic generation and aerosol transport in atomic spectrometry: effect of mineral acids on emission intensity in inductively coupled plasma atomic emission spectrometry☆

Abstract The mechanism of the mineral acid interference has been studied in ICP-AES. For this study five mineral acids have been evaluated (HCl, HNO3, HClO4, H2SO4 and H3PO4) in four concentrations (0, 0.5, 5 and 30%). In order to investigate this interference emission signal, sample uptake rate, primary and tertiary drop size distributions, total analyte transport rate and excitation temperature have been measured. From the results obtained, it seems that this interference is contributed by a reduction of the analyte transport rate and, also, by a decrease in the plasma temperature. The degree of the contribution to the interference of each one of these causes depends on the type of acid and sample uptake mode. The physical properties of the acid solutions are in the origin of the interference. These physical properties modify the sample uptake rate and/or the primary drop size distribution of the aerosols. The acids evaluated can be classified in two groups. The first group would consist of HCl, HNO3 and HClO4, and the second one of H2SO4 and H3PO4. In natural uptake mode the interference is mainly due to changes in sample uptake rate, and in controlled uptake mode to changes in primary drop size distribution of the aerosols. In both sample uptake modes a density-effect may appear on increasing acid concentration. All these factors tend to decrease the analyte transport rate and, hence, the emission signal. Finally, a cooling effect of the plasma due to a higher load of acids is superimposed to these causes. We think that from this study the mineral acid interference in ICP-AES, with pneumatic nebulization, should be better understood.

Study of the influence of water vapour loading and interface pressure in inductively coupled plasma mass spectrometry

The influence of water vapour loading on ion counts for singly charged, doubly charged, oxide and hydroxide species has been studied for ten elements (Li, Mg, As, Mo, Cd, Cr, W, Co, Ba and Pb) covering both a wide mass range from 7Li to 208Pb and a wide range of oxide and hydroxide species stabilities. Studies have been carried out using identical sample introduction systems for two different inductively coupled plasma mass spectrometry (ICP-MS) instruments (the VG PlasmaQuad and the Sciex Elan), and the behaviour of the two instruments has proved to be substantially different in this respect. Measurements of water loading as a function of spray-chamber temperature have shown that the nebuliser gas is precisely saturated with water vapour at the temperature of the spray chamber. Electron density and excitation temperatures of the plasma, measured as a function of water vapour loading in the plasma, have shown little variation with water vapour loading over the range examined. Studies of ICP-MS interface pressure as a function of water vapour loading indicate that variations in this loading may influence the partial pressures of species present in the interface sufficiently to alter both ion trajectories in the interface and ion movement in the ICP-MS lens system.

A method for investigating size distributions of aqueous droplets in the range 0.5–10 μm produced by pneumatic nebulizers

Abstract A novel method is described for the measurement of the droplet size distributions produced by nebulizers commonly employed in analytical atomic spectroscopy. It is shown theoretically that, at sufficiently high concentrations of dissolved sodium chloride, the evaporation of water from droplets as small as 0.5 μm in diameter may be reduced to a negligible level. When evaporation is reduced by the presence of a dissolved salt, a conventional cascade impactor may be used to elucidate the droplet size distribution. Empirical observations confirm that, at a sodium concentration of 10,000 μg ml −1 , evaporation is negligible: the method may be used to study particle size distributions over the size range 0.5–10 μm.

Empirical model for estimating drop size distributions of aerosols generated by inductively coupled plasma nebulizers

Abstract Drop size distributions have been determined for two inductively coupled plasma concentric nebulizers using laser diffraction measurements. The influence on drop size distributions of (1) gas and liquid flows (2) physical properties of several solvents and gases, and (3) principal dimensions of the nebulizers are reported. An empirical equation is developed which allows the prediction of two key parameters of primary aerosols (e.g. Sauter mean diameter and span). The equation has been checked by estimating Sauter mean diameters and spans for two additional concentric nebulizers, and good agreement has been obtained between experimental and estimated values. By comparison, estimates of Sauter mean diameters obtained from the Nukiyama and Tanasawa equation have been shown to differ markedly from experimental values. From the present study it may be concluded that gas velocity is the most critical variable in nebulizer operation, and that in order to obtain aerosols with small mean drop sizes and narrow distributions, a nebulizer with small gas cross-sectional area should be used, and operated with high gas flows. Organic solvents possessing low surface tensions are shown to lead to smaller mean drop sizes than aqueous solvents under all comparable operating conditions.

Investigation of Experimental Parameters with a Quadrupole ICP/MS

Preliminary studies have shown that signal behavior in ICP/MS is quite different from that in ICP-OES. This is particularly true for the manner in which ICP operating parameters affect the responses of elements detected, and results from behavior occurring both in the interface and in the mass spectrometer. This paper describes in detail the response of a commercially available instrument (VG PlasmaQuad) to variations in several important operating parameters. The effects of nebulizer flow rate, rf plasma power, and sampling depth on ion populations, on ratios of singly charged to doubly charged ions, and on singly charged ions to oxide and hydroxide ions are described for selected elements. A comparison is also made of the results obtained with the present instrument, with data published from other commercial instrumentation.