The effect of acid digestion technique on the performance of nebulization systems used in inductively coupled plasma spectrometry

The pneumatic nebulization systems used in inductively coupled plasma spectrometry generally consist of a concentric glass (Meinhard type) or v-groove (Babington principle) nebulizer in combination with a cyclonic, cone or Scott double pass cloud chamber. The characteristic model of action of each nebulizer produces an aerosol, the droplet size distribution of which depends on the solution matrix. Before reaching the plasma the aerosol is partitioned by the specific mode of action of each cloud chamber and contains droplets less than 10 mu m in diameter. The partitioned fraction of solution that reaches the plasma determines the transport efficiency and hence the detection limits of the nebulization system. Nitric and nitric/perchloric acid digestion are the most commonly used procedures for the destruction and dissolution of the mineral components of organic and geochemical materials. However oxidation of plant material by nitric acid may be incomplete and the concentration and acid type used in the final digest solution may not match that of the standards used for calibration and optimization of the instrument. While the aerosol droplet size distribution is dependant on the mode of action of each component of the nebulization system, each of the digest parameters can also affect it. If the proportion of < 10 mu m droplets reaching the plasma varies between the standards and digest solutions, inaccurate analyses will result. Optimization of thenebulization systems for detection limit, background equivalent concentration and transport efficiency using standard solutions (in 0.1 M nitric acid) indicated the overall order of merit to be cyclonic > Scott double pass > cone. However, the cyclonic and Scott double pass cloud chambers were found to give erroneous results for NIST standard reference organic materials digested using either nitric or nitric/perchloric acids. The cone cloud chamber produced results comparable in accuracy and precision to the certified values. Analysis of reference sediment samples digested using aqua regia with microwave heating, for non-silicate matrix elements, produced superior recovery of analytes in less time and with less manipulation than other recommended procedures.