Laser induced breakdown spectroscopy with pulse width modulation microcontroller-based thermoelectric cooler for liquid samples

Abstract

Laser induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy technique that determines elemental analysis of solid, liquid or gas sample. Although LIBS has provided excellent results in quantitative and qualitative analysis of solid samples, less attention has been given to analyse the inner portion of the liquid bulk or on its surface as liquid samples were often associated with strong splashing and shockwave. Hence, a pulse width modulation microcontroller-based thermoelectric cooler (TEC) system was proposed as a sample pre-treatment method to freeze liquid samples prior to LIBS analysis. The TEC system was built to provide a user-friendly graphical user interface (GUI) for freezing and monitoring the temperature of the sample. The construction of this system was explained. The calibration results during the freezing process and maintenance of the samples at its freezing phase demonstrated excellent performance of the developed system. The effect of incorporating the TEC system with LIBS was studied and the effectiveness and shortcomings of the TEC were highlighted. A Q-switched Nd:YAG laser (1064 nm, 6 ns and 1 Hz) and a broad spectral range spectrometer LR1 were employed for laser induced breakdown spectroscopy study. Aqueous sodium chloride (NaCl) with different concentrations, and liquids categorized with different viscosities (44.07 to 16965.80 mPa.s) and types (paste, cream, gel and oil), were utilized as studied materials. Initially, direct laser irradiation of liquid and frozen NaCl samples were analysed and later the study was focused on laser irradiation of the frozen NaCl under different temperatures (0 to -5°C). The direct irradiation on aqueous NaCl samples were carried out at concentrations ranging from 0.2 to 2.5 mol/L. The irradiation of the frozen NaCl showed a higher signal-to-noise ratio (SNR) (3x), and lower detection limit (2.5x), relative standard deviation (around 5%), maximum relative error (2% to 9%) and root mean square error of prediction (0.04 mol/L) value. The analyses of the frozen NaCl with different temperatures led to the SNR optimisation as the temperature was kept constant at the freezing point of -1°C, -2°C and -3°C for 0.2, 0.5 and 1.0 mol/L frozen samples, respectively. The next set of experiments was carried out using liquids with different viscosities and types. The analyses on sodium component of the samples by direct laser irradiation of frozen samples showed emission enhancement and higher SNR as compared to that of liquids. Frozen samples also showed smaller craters diameter and higher energy fluence. The principle component analysis (PCA) is used to compare the principle component score separation and clustering pattern between frozen and liquid samples. The frozen samples showed a more established separation and clustering as compared to those acquired from liquid samples. The spectral signal quality was also optimised when the temperature was at its freezing phase. This work showed that the TEC pre-treatment method had improved the LIBS measurement of the liquid samples by maintaining its freezing state, thereby proving its ability to be used as an alternative sample preparation method. This simple and easy-to-assemble system is also significant for real-time and in-situ analysis as it is able to simultaneously freeze the sample while monitoring its temperature.