Analysis Of Trace Metal Elements In Aqueous Solution Samples Using Ultrasonic Nebulizer Assisted LIBS

Laser induced breakdown spectroscopy（LIBS） is a qualitative and quantitative analysis method for elements analysis which gets more and more popular in recent decades. When a high energy laser pulse is focused on the surface of sample, the high temperature plasma of sample will be induced. The atom or ion lines emitted by the plasma reflect the chemical composition of the sample. Now, LIBS attracts amount of attention as a quick, in‐time, remote, wide scope and in‐site chemical analysis method. But, there are many problems when the LIBS is employed in liquid sample analysis, such as the application in the fields of ocean detection and water environment monitoring. Base on a self‐developed LIBS enhancement method called ultrasonic nebulizer assisted LIBS （UN‐LIBS）, a lot of research works were carried out for improving quality of LIBS signal and sensitivity of detection. The study is composed of four parts, including the design and realization of the nebulizer assisted system, the optimization of experimental parameters, the detective ability of UN‐LIBS and the quantitative analysis ability of UN‐LIBS.Based on the problems LIBS faced in metal elements analysis for liquid samples and other problems appeared in signal enhancement method, this paper developed the idea of combining ultrasonic nebulizer with LIBS system. According the results from experiment and observation, a set of assistance system is established which is composed of a piezoceramics plate and a capacitance feedback oscillator circuit. Using this system, the sample of liquid can be converted into droplets with an average diameter of3um, and further on becomes a4mm diameter cylindrical aerosol formed by air dynamic system. After repeated test, the RSD of plasma peak intensity of metal elements could reach a maximum of0.0076. According to the test results, an optical path of emission and collection is set up and the operating procedure is designed specially for the UN‐LIBS experiments.Then, using the UN‐LIBS system, a series of experiments were carried out to understand the spectral characteristics of metal elements and the physical features of plasma. Taking the plasma of Mg element as example, the influences of the experimental parameters, such as laser energy power, vibration power of piezoceramics plate and the time revolution features, on the metal element lines of UN‐LIBS are also analyzed. Base on the above analysis results, the parameters is optimized. The Hα line is fitted with voigt function and then the FWHM of Lorentz profile can be determined. Then the electron density calculated by the Stark width is confirmed to be the magnitude of10¹⁶～10¹⁷cm^‐3 with a30mJ energy and a60mJ energy.24ion peaks in the iron plasma ranging between230nm and270nm is chosen to calculate the temperature assumption by the Boltzmann plot method. The temperature of plasma in UN‐LIBS obtained is1.24×10⁴K. The features of the electron density and temperature ensure that even with a lower emission energy, the plasma received under UN‐LIBS has a higher electron density and temperature compared with other methods, which could meet the LTE assumption needed for quantitative detection of different metal elements. This provides a firm theoretic support for the choose of parameters and quantitative analysis in future experiment.Further experiments has been carried out to analyze the evolution features and limit of detection of typical metal elements. Solution samples with8different metal elements including Mn, Cu, Pb, Zn, Fe, Mg, Ca and Na are subjected to the detection of UN‐LIBS with a30mJ laser pulse energy. The temporal evolution of peak intensity and signal to background ratio are analyzed. With the laser energy as low as30mJ and the best optimized parameters, the peak intensity of every elements with different low concentration are detected and the results are linearly fitted. The relative coefficient of linear fitting of most elements are higher than0.99except that of Ca which is0.982. The limit of detection of different elements are calculated with the3σ method. The results show that the LOD of7elements are lower than5ppm and the lowest LOD appears in the Na elements which is0.00596ppm that reaches the ppb level. In a comparison of UN‐LIBS and other LIBS enhancement methods, it is realized that the UN‐LIBS has an obvious advantage of high sensitivity. With very low emission pulse energy, the LOD of the UN‐LIBS method is about1to3orders of magnitude better than other LIBS enhancement methods.The element of lead, as a typical element, is employed to understand the quantitative analysis ability. The factors, such as exciting energy, peak intensity and standard deviation of noise are studied for the quantitative detection. Through some comparative analysis, the peak intensity is calibrated by the nearby background finally.12samples with different concentration from25.9ppm to10360ppm of lead are detected by UN‐LIBS and the linear scope by this regard varies from0‐4150ppm. The relative coefficient of linear regression function is0.99935and the LOD reaches2.93ppm. In the linear scope,6samples with different concentration of lead are separately analyzed and the error of quantitative estimate is between0.043%～7.1%. The error is relatively higher when the concentration of sample is higher and lower when the concentration is lower.At last, all the achievements of UN‐LIBS are summarized with a special focus on its innovations and the future direction to carry on the research is predicted.