Abstract
The enhancement of laser-induced breakdown spectroscopy (LIBS) assisted with microwave radiation is demonstrated for an aqueous solution of indium using the 451.13 nm emission line. Microwave power was delivered via a near-field applicator to the LIBS measurement volume where the indium aqueous solution was presented as a liquid jet. The microwave enhancement effect was observed to decrease with increasing laser pulse fluence at 532 nm resulting in a maximum emission intensity occurring at a laser pulse fluence of 85.2 J∙cm(-2), independent of the microwave power used. The detection limits of indium in an aqueous solution were determined to be 10.8 ± 0.7 and 124 ± 5 ppm for the cases of microwave enhanced and standard LIBS, respectively. The 11.5-fold detection limit enhancement obtained in the liquid phase is of the same order of magnitude as that reported for other elements in solid samples, but lower than that obtained in solid phase utilizing a similar experimental setup. This establishes microwave enhancement as an effective technique for the detection of metals in aqueous solutions. In addition, the temporal evolution of plasma emission intensity was investigated and was found to be qualitatively similar to that of plasma produced from solid phase samples, which reveals the same coupling mechanism between laser generated plasma and microwave radiation.
Highlights
Metal detection in the aqueous phase in real time and in situ has many potential applications
MW-laser-induced breakdown spectroscopy (LIBS) has been demonstrated for the first time to be an effective technique for the detection of indium in an aqueous solution, increasing the signal emission intensity by up to 60 times and improving the limit of detection (LoD) by 11.5 times by extending the plasma lifetime from several microseconds to several hundred microseconds
The MW enhancement was observed to be significantly influenced by the laser pulse fluence used, with decreasing enhancement being obtained as laser fluence is increased, possibly due to the formation of a higher density plasma
Summary
Metal detection in the aqueous phase in real time and in situ has many potential applications. The lifetimes of plasma generated in bulk liquid and plasma generated on a solid surface in gas are on the order of 1 μs and 10 ms, respectively [8], with liquid surface plasma in gas having an intermediate lifetime of around 10 μs [3, 9] These difficulties have been partially overcome through performing LIBS on a flowing liquid sample [3, 10], a liquid jet [7, 11], a flat liquid sheet [12], a microdroplet [13] or a frozen sample [14]. It is desirable to improve limits of detection (LoDs) of LIBS in liquids for trace elements analysis
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