Role of geometric configurations in optimizing the LIBS signal enhancement

Abstract

The quest for improving the limits of trace elemental detection and reproducibility in Laser-Induced Breakdown Spectroscopy (LIBS) resulted in the development of novel methodologies for sample excitation and plasma collection. Efficient plasma collection and coupling to the detector is of utmost importance in LIBS studies for meaningful and logical data interpretations. Unfortunately, limited attention is paid to such critical aspects in comparison with the plethora of publications in advanced data analysis. In view of this, we probe the role of the different optical schemes on the collected plasma signal strength from three standard samples (Cu, Cd, Pb) while retaining all other experimental conditions such as laser energy, detector gain, gate width, and gate delay same. In addition, the role of coupling modalities on the plasma signal strength, an area not studied in depth, is investigated and found that the direct coupling scheme always provides better signal strength and repeatability. Moreover, the role of spectrographs in the measured signal strength is also investigated in detail for all the said standard samples. A comparative study elucidates that single shot experiments with low excitation pulse energies of the order of a few mJ are sufficient as compared to multi-short pulse laser excitation to provide the required limit of detection of the samples of interest which in turn makes the technique practically non-destructive. The efficacy of the optimized system was manifested by investigating idealistic and real-life samples viz. standard meals and teeth to differentiate between the normal and carious states. Our results reveal that the developed strategies may be useful in designing a robust system for fast ultra-trace elemental detection.