Quantitative metal detection by microwave assisted laser induced breakdown imaging and spectroscopy

Abstract

Real time, in-situ quantitative detection of the metals is important for many applications such as industrial processes for the quality control, mining for the quick scan of rocks samples, monitoring of the heavy metal contaminations for the pollution control and realtime analysis of the agriculture land for nutrients monitoring and fertilizer selection. Laser Induced Breakdown spectroscopy (LIBS) being able to offer quick response and multielemental analysis without sample preparation, can meet these requirements. As most of the mentioned applications involve detections of the trace metallic elements, thus LI BS is desired to deliver quantitative measurements with high sensitivity and improved limit of detection. Despite of inheriting some excellent features, LIBS suffers a few limitations such as low Signal to Noise Ratio (SNR), weak limit of detection and low sensitivity. Several methods have been suggested in literature to improve the performance of conventional LIBS, which are based on the concept of aiding LIBS by a secondary source of energy. Microwave-assisted laser induced breakdown spectroscopy (MW-LIBS) is one of these improvement methods, which has immense potential to be considered as a reliable analytical technique due to high sensitivity, improved SNR and limit of detection. However, further improvement in the performance of MW-LIBS is desired for the reliable quantitative metal detections at low concentration while offering high sensitivity. This research is amid to investigate ·the improvement of MW-LIBS using two different approaches. This first is to improve the plasma emission detection by single elemental imaging and the second is to improve the microwave injection by a well-designed near field applicator (NFA). Indium in solid matrix was used to investigate the improvement in emission detection by single elemental imaging. A narrow bandpass filter was used to select the elemental, indium emission at 451.13 nm. This narrow bandpass filter was attached with an ICCD camera to investigate the response of imaging based detection technique at various, laser ar\d microwave powers. Variation in image intensity at several concentrations of indium and evolution of plasma at various microwave powers, was also investigated. Spectral detection was carried out simultaneously with narrow-band imaging to study the extent of improvement in sensitivity. Outcomes demonstrated that imaging technique offers 14-fold improvement in sensitivity following enhancement by microwave radiation, as compared to spectral detection in LIBS with no microwave enhancement. Microwave injection devices such as NFA, being the main component to inject the microwave radiation into the laser ablated plasma, is the most important part of MW-LIBS system, hence defines the performance of the entire MW-LIBS setup. Therefore, having an efficient NFA can considerably improve the signal quality and detection capabilities of MW-LIBS. Considering the importance of an efficient NFA, four designs of NFAs were simulated using the characteristics of available setup, simple isolation techniques such as quarter-wave choke and a finite ground plane were used. These designs were fabricated and tested in the MWLIBS setup for the copper detection in a solid sample. Spectral detections and broadband plasma imaging were carried out simultaneously to investigate the effect of various NFA designs on the signa I quality, size of the plasma and improvement in the detection limit for the existing MW-LIBS setup. From the experimental results, it was concluded that the design D having a finite ground plane of 30 mm diameter performed better than the rest, using this design D a signal enhancement of 849 times was achieved. While 79-fold SNR at 2.6 mJ/pulse laser and 1.2 kWatt microwave power, was observed. By using design D of NFA, ore sample having certified copper concentration of 3.38 ppm was detected with the 166 SNR. The demonstrate high SNR, presents the possibility of detecting sub parts per million in future.