Investigations on the on-line determination of metals in air flows by capacitively coupled microwave plasma atomic emission spectrometry

Plasma optical emission spectrometry with a capacitively coupled microwave plasma (CMP) operated with air has been investigated with respect to its po ssibilities for real-time environmental monitoring of combustion processes. The unique feature is the possibility to operate the CMP with air as worki ng gas, as is usually the case in exhaust gases of combustion processes. Th e CMP also is shown to be stable in the presence of large amounts of water and CO2, which makes this source ideally suitable for this purpose. The det ection limits obtained for the environmentally relevant elements Cd, Co, Cr , Fe, Mg, Ni and Pb show the possibility to monitor directly heavy metals i n air in an on-line mode and down to the 2-160-mug m(-3) level. These detec tion limits are generally lower than the threshold limit values of the 'Fed eral Law for Immission Protection' in Germany in the gaseous effluents of i ndustrial plants. In order to investigate the influence of the water loadin g, (32-222 g m(-3)) on the detection limits a comparison of results obtaine d with three different nebulizers (Legere nebulizer, hydraulic high-pressur e nebulizer and ultrasonic nebulizer) was made, with which aerosols with di fferent water loading are entered into the plasma. For the hydraulic high-p ressure nebulizer and the ultrasonic nebulizer no desolvation unit was foun d to be necessary. It was shown that especially for elements with lines hav ing high excitation energy (Cd) or for which ion lines are used (Mg II) the increase in water loading deteriorates the detection limits. The rotationa l temperatures (T-rot) and excitation temperatures (T-exe) in the case of d ifferent amounts of water are of the order of 3700-4900 K and 4700-7100 K, respectively. The temperatures show that changes in the geometry and temper ature distribution in the case of T-rot but also the values of T-exe themse lves are responsible for this increase in detection limits. Furthermore, di fferent amounts of CO2 mixed to the working gas (3-22%) while the total gas flow rate was kept constant at 1.2 l min(-1) were also shown to increase t he detection limits. (C) 2001 Elsevier Science B.V. All rights reserved.