Ethanol loading in an inductively coupled plasma

Abstract

Inductively coupled optical emission spectrometers (ICP-OES's) that have been commercially available in South Africa have, until recently, been unable to determine elements in solutions containing more than 30 % ethanol or methanol. Inability of the plasma to be sustained while aspirating ethanolic solutions is a severe limitation to the analysis of certain elements as ethanol is an excellent solvent. Previously, ethanol had to be removed from samples prior to analysis using ICP-OES. This investigation was launched to determine the cause of the plasma being extinguished when higher ethanolic concentrations were nebulized. Fundamental characteristics of the plasma were investigated. These were temperature, electron density, drop size distribution, mass flow rate and hydrogen emission. Effects on the analytical parameters of sensitivity, detection limits and background equivalent concentration were then determined. Excitation temperature was determined by the "two line" method. The intensity of two lines of hydrogen (Ha and H6), at different positions in the plasma and while different concentrations of ethanol were being aspirated, was measured. For accurate answers to be obtained with this method the plasma must be in a state of local thermodynamic equilibrium (L TE). Sufficient evidence is available to indicate that ICP's do not reach L TE when aqueous solutions are aspirated, but for the purpose of this work this was not considered a problem. The aim of the investigation was to measure the changes that occurred to the temperature as the ethanol concentration being aspirated increased rather than to measure accurately the excitation temperature. Results showed that excitation temperature increased as ethanol concentration increased and reached a maximum of 10400 K when 15 % ethanol was nebulized. Electron density was determined using Stark broid~r.iI1g of the H6 line. It was found that electron density increased as the ethanol concentration increased. Furthermore, the major increase occurred on the region of the central channel, as measured after Abel inversion had been applied. The highest electron density determined was 8 x 1015 cm-3 , close to the reguirements for L TE. Drop size distribution was measured by means of a laser particle counter. The aerosol was diluted with argon and then passed through the measuring cell. Again, only the tendency of the drop size distribution was measured, as changes in the ethanol concentration occurred. Droplets increased in diameter as the ethanol concentration increased. This increase in drop size diameter was found to be accompanied by an increase in the mass of material entering the plasma as determined by gravimetry. It was discovered that the hydrogen content in the plasma increased as the ethanol concentration of the aspirated solution was increased. There was a fourfold increase in hydrogen emission signal (Ha line at 656.3 nm) as the ethanol was increased to 25 %. The effects that changes to the fundamental properties of the plasma would have on the analytical performance of the ICP-OES was then investigated. Calibration curves for Pb, Cd, Al, Cr, Fe, Na, Mn, Mo and V in a series of solutions containing increasing ethanol concentration were determined. For each element, the most sensitive results were obtained with 15 % ethanol solutions. Analytical parameters were found to vary according to the element analysed. Aerosol carrier gas flow rates were found to be more important for the ethanol containing solutions than for aqueous solutions. Optimization of aerosol carrier gas flow rates with aqueous solutions led to a doubling in emission intensity, while with 15 % ethanol solutions there was a fourfold increase in signal intensity. The introduction into South Africa of an ICP-OES capable of measuring 100 % ethanol, resulted in the investigation being extended to study higher concentrations of ethanol. The same elements were again investigated, but the highest concentration of ethanol which could now be used as solvent was 95 %. Using 1.5 kW power and optimum nebulizer gas pressure, the sensitivity of the determinations was found to increase throughout the ethanol range. Detection limits did not necessarily improve, as the BEC increased with increase in ethanol concentration. However, the advantage was that solutions where ethanol had been used as solvent could now be analysed. Electron density, as determined by the ratio of two Mg lines, was a maximum for 25 % ethanol. For that concentration of ethanol the ratio was 14, indicating that the plasma was close to being in a state of L TE, which made the determination of the excitation temperature with certain solutions more accurate. In addition to aqueous solutions, the effects of ethanol additions to an organic solvent were investigated. For this investigation, 30 and 45 % solutions of ethanol in xylene were prepared and Al, Pb, Fe, Cr and Cu were measured in a standard reference material using the ethanol/xylene mixtures as well as pure xylene. Simplex optimization was applied to achieve the greatest signal-to-background-ratio. Results obtained for the reference oil were close to those certified. Additions of ethanol resulted in severe degradation of the sensitivity and other analytical parameters. The electron density measured from the ratio of Mg lines also decreased drastically when the ethanol was added. The mass of material exiting the nozzle more than doubled for the ethanol solutions, and it is proposed that this led to a decrease in energy available to dissociate the sample. This investigation has led to a further understanding of the plasma and the processes that occur as the sample and matrix are injected into a region of high temperature. The significance of changes in fundamental properties for analytical work was described. A methodology was developed which utilized new technology so as to allow analysis of samples dissolved in ethanol.