Evaluation of slurry injection for the determination of metals in solid samples using inductively coupled plasma atomic emission spectrometry
It has been said that inductively coupled plasma (ICP) is the panacea for the determination of metals in environmental samples. The ease of sample introduction for liquids coupled with excellent limits of detection of this spectrometric method provide the analyst with the ability to perform rapid, multi-element determination of most elements in the periodic chart. However, when samples have to be introduced in solid form such as suspensions of finely powdered material (slurries), in order to avoid lengthy extraction procedures and the use of strong acids, the method must be modified so that it can handle solid, often refractory material.
Due to large size of the particles (≥ 10 μm), two problems are encountered: poor sample introduction efficiency in the conventional, concentric nebulizer; and poor vaporization efficiency at the argon plasma. The nebulizer tends to clog and a large fraction of particles is lost in the spray chamber due to their weight. The conventional argon plasma is not energetic enough to vaporize the analyte.
In this project, a clog free Babington nebulizer was used. A surfactant/thickening agent, polyethylene oxide (PEO), was added to alter such physical properties of the slurry as surface tension, viscosity, and aerosol droplet size. Mixed gas plasma containing small amounts of nitrogen were used. Results showed that, by adding about 5 ppm of PEO, the emission intensity of an analyte increased significantly. Further experiments demonstrated that the signal enhancement resulted from an increase in the nebulizer efficiency brought about by a slight increase in viscosity of the slurry.
The use of mixed gas plasma (Ar + 4% N₂) further improved the emission intensity. Temperature diagnostic measurements of such plasmas indicated that rotational and excitation temperatures are higher than those in a pure argon plasma. The improved temperature is believed to result from the higher thermal conductivity of molecular gases. Nitrogen added to the cooling gas works better than when added to the injector gas. Hydrogen does not seem to work as well as nitrogen, probably because its thermal conductivity is 14 times less than nitrogen. Further studies of the excitation temperature using Fe as the thermometric species, have been helpful in elucidating the mechanism of slurry vaporization in the plasma. There is evidence in this study that mass-transfer rather than heat-transfer is the limiting factor.
With these improvements in the sample introduction and atomization cell, slurries having particle diameters up to 7 μm have been successfully analyzed. This value is 3 times larger than particles injected into pure Ar-plasma without a surfactant. The percent recovery of Ca, Fe, Mg and Pb, are comparable to that obtained from the same samples analyzed as solutions following acid digestion.