Design and evaluation of an improved microwave-based thermal nebulizer for liquid sample introduction in inductively coupled plasma atomic emission spectrometry

Abstract

A re-designed microwave-based thermal nebulizer (MWTN) that overcomes all the drawbacks shown by previous prototypes (i.e. lack of robustness, poor heating efficiency) is presented. The new device employs an optimized TM₀₁₀ cavity and nozzle design. MWTN performance was evaluated in inductively coupled plasma atomic emission spectrometry (ICP-AES) and compared to that provided by a concentric pneumatic nebulizer. Current MWTN design improves sample heating efficiency and system robustness. As a consequence, a 10 times lower matrix concentration than previous designs can be employed. The minimum inorganic (i.e. acids and salts) and organic (i.e. alcohols) concentrations required to nebulize are 0.25 and 40% w/w, respectively. The influence of microwave power (180–290 W), matrix nature (acids, salts and organics), sample uptake rate (0.9–1.8 mL min^–1) and nebulizer critical dimensions, such as nozzle and PTFE capillary internal diameter (150–300 and 300–800 µm) and PTFE capillary length (50–150 cm) on the behaviour of the MWTN was studied. Optimum operating conditions for this nebulizer are obtained employing high microwave powers, matrix concentration and sample uptake rate, as well as nozzle and PTFE capillary with a narrow internal diameter. Although the MWTN behavior depends on the samples physical properties, aerosol drop size distributions and analyte transport rate for inorganic solutions above 2% w/w are independent of matrix composition. Current MWTN shows higher sensitivity and lower limits of detection (up to 12 times for acid solutions) than a conventional pneumatic nebulizer. The performance of the new prototype has been evaluated analyzing a certified bovine liver (CRM 185 R) and a spirit sample. Experimental results indicate that MWTN can be employed successfully for routine sample analysis.