Automatic flow-injection system for the determination of heavy metals in sewage sludge by microwave digestion and detection by inductively coupled plasma-atomic emission spectrometry (MW-ICP/AES).

Results are shown obtained in the optimization of an automatic flow injection system that combines microwave digestion with atomic spectrometric detection (FAAS, ICP/AES) for the determination of heavy metals in sewage sludge. Digestion is performed by preparing a suspension of the sample in 1.5 mol/l HNO(3) and making it flow through a PTFE capillary tube placed inside a conventional microwave oven. The effects of the length and inner diameter of the capillary tube, as well as that of the pumping rate, have been studied in order to find the experimental conditions that allow a quantitative elemental recovery in the shortest period of time possible. The optimization study was carried out on a certified sample (BCR No. 146), and the elements determined were Zn, Cu, Pb, Cd, Ni and Cr. The experimental data (percent recovery vs. digestion time) have been fitted to a mathematical model in order to quantify the influence of each of the variables studied. The optimized procedure (MW-ICP/AES) has been applied to one ordinary and one certified sewage sludge sample. In comparison with the conventional methods of sewage sludge analysis, the one proposed is less time consuming, while being equally precise and accurate.

A microwave-powered thermospray nebulizer for liquid sample introduction in inductively coupled plasma atomic emission spectrometry.

A new thermospray nebulizer based on the absorption of microwave radiation (MWTN) by aqueous solutions of strong acids is presented for the first time. To this end, a given length of the sample capillary is placed inside the cavity of a focused microwave system. A small piece of a narrower capillary tubing is connected at the tip of the sample capillary, outside the microwave cavity, to build up pressure. Drop size distributions of primary aerosols are exhaustively measured in order to evaluate the influence of several experimental variables (microwave power, liquid flow, irradiation length, inner diameter of the outlet capillary, nature and concentration of the acid) on the characteristics of the primary aerosol that are related to the emission signal. These experiments have been performed mainly to increase our understanding of the microscopic process of this new type of aerosol generation. A standard Meinhard nebulizer was employed for comparison. Under the best conditions the entire aerosol volume is contained in droplets smaller than 20 µm compared with 45% of the volume of the aerosol generated by the Meinhard. Hence, higher analyte and aerosol transport rates are to be expected for the MWTN compared with the Meinhard nebulizer. As any highly efficient nebulizer, MWTN requires a desolvation unit. For solutions 0.75 M in strong acid, the new nebulizer improves sensitivity (1.0-2.8 times), limits of detection (1.2-3.0 times), and background equivalent concentration (0.9-2.0 times) as compared to the standard Meinhard nebulizer, features many of the advantages of the conventional thermospray nebulizer, and overcomes some of its drawbacks (MWTN does not show corrosion problems and works at lower pressure, the aerosol characteristics are not modified when the PTFE capillary is replaced).