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 TM 010 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 À1) and nebulizer critical dimensions, such as nozzle and PTFE capillary internal diameter (150-300 and 300-800 mm) 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.

Design and evaluation of a new fully microwave-assisted liquid sample introduction device for inductively coupled plasma atomic emission spectrometry

“…Previous results reported in the literature have shown that for both the MWTN and MWDS systems, the best analytical response is obtained when operating at high MW powers. 10,20 Due to this, the MW power used with the MASIS was fixed at the maximum available value, i.e., 290 W. In fact, it was observed that when operating at lower microwave powers, no reproducible aerosols were generated. It is interesting to note that the MW radiation power reflected in the cavity was of 40 W on average.…”

“…[6][7][8] Microwave radiation (MW) has also been employed as a promising alternative energy source in thermal nebulization. 9,10 Using these devices, the aerosol is generated as a consequence of the interaction between the sample vaporized inside the capillary and the remaining liquid. The aerosols generated by thermal nebulizers are finer than those produced by the conventional pneumatic ones, thus giving rise to higher plasma analyte transport rates (W tot ).…”

“…Details of both devices are found elsewhere. 10,20,24 MWTN was coupled to a home-made thermostated cyclonic spray chamber (inner volume 33 cm 3 ). The temperature was kept constant at À5 C using a thermostated bath (Haake F3-K, Haake Mess-Technik GmBH U Co, Karlsruhe, Germany).…”

“…The main component of the MASIS is the microwave cavity, the same TM 010 previously designed and reported for both the MWDS and the MWTN. 10,20 The only difference lies in two additional holes at the base of the cavity to introduce the PTFE capillary. The dimensions of the holes were calculated to avoid microwave leakages.…”

“…The aerosol emerging from the spray chamber (9) sequentially passes through a (10) Liebig (33 cm  1.3 cm id) and a (11) home-made Peltier (30 cm length  0.8 cm id) condensers where the solvent evaporated inside the spray chamber is partially removed from the aerosol stream. Liebig and Peltier condensers' temperatures are kept at 10 C and À15 C, respectively. The MASIS is coupled to the ICP by means of a silicone tube (40 cm length  1.0 cm id).…”