Laser-induced breakdown spectroscopy in China

Laser-induced breakdown spectroscopy （LIBS） has been regarded as a future superstar for chemical analysis for years due to its unique features such as little or no sample preparation, remote sensing, and fast and multi-element analysis. Chinese LIBS comnmnity is one of the most dynamically developing communities in the World. The aim of the work is to inspect what have been done in China for LIBS development and, based on the understanding of the overall status, to identify the challenges and opportunities for the future development. In this paper, the scientific contributions from Chinese LIBS comlnunity are reviewed for the following four aspects： fimdamentals, instrumentation, data processing and modeling, and applications; and the driving force of LIBS development in China is analyzed, the critical issues for successful LIBS application are discussed, and in our opinion, the potential direction to improve the technology and to realize large scale commercialization in China is proposed.

Major elements analysis in bituminous coals under different ambient gases by laser-induced breakdown spectroscopy with PLS modeling

Three major elements, carbon, hydrogen, and nitrogen, in twenty-four bituminous coal samples, were measured by laser-induced breakdown spectroscopy. Argon and helium were applied as ambient gas to enhance the signals and eliminate the interference of nitrogen from surrounding air. The relative standard deviation of the related emission lines and the performance in the partial least squares （PLS） modeling were compared for different ambient environments. The results showed that argon not only improved the intensity, but also reduced signal fluctuation. The PLS model also had the optimal performance in multi-element analysis using argon as ambient gas. The root mean square error of prediction of carbon concentration decreased from 4.25% in air to 3.49% in argon, while the average relative error reduced from 4.96% to 2.98%. Hydrogen line demonstrated similar improvement. Yet, the nitrogen lines were too weak to be detected even in an argon environment which suggested the nitrogen signal measured in air come from the breakdown of nitrogen molecules in the atmosphere.

Development in the application of laser-induced breakdown spectroscopy in recent years:A review

Laser-induced breakdown spectroscopy(LIBS)has been widely studied due to its unique advantages such as remote sensing,real-time multi-elemental detection and none-to-little damage.With the efforts of researchers around the world,LIBS has been developed by leaps and bounds.Moreover,in recent years,more and more Chinese LIBS researchers have put tremendous energy in promoting LIBS applications.It is worth mentioning that the application of LIBS in a specific field has its special application background and technical difficulties,therefore it may develop in different stages.A review summarizing the current development status of LIBS in various fields would be helpful for the development of LIBS technology as well as its applications especially for Chinese LIBS community since most of the researchers in this field work in application.In the present work,we summarized the research status and latest progress of main research groups in coal,metallurgy,and water,etc.Based on the current research status,the challenges and opportunities of LIBS were evaluated,and suggestions were made to further promote LIBS applications.

Mechanism of signal uncertainty generation for laser-induced breakdown spectroscopy

Relatively large measurement uncertainty severely hindered wide application for laser-induced breakdown spectroscopy(LIBS),therefore it is of great importance to understand the mechanism of signal uncertainty generation,including initiation and propagation.It has been found that the Auctuation of plasma morphology was the main reason for signal uncertainty.However,it still remains unclear what mechanism leads to laser-induced plasma morphology fluctuation.In the present work,we employed three fast-imaging cameras to capture three successive plasma images fromn a same laser-induced Titanium alloy plasma,which enables us to understand more clearly of the plasma evolution process especially for the early plasma evolution stage when plasma and surrounding gases interact drastically.Seen from the images,the plasma experienced an increasing morphological fuctuation as delay time increased,transforming from a“stable plasma”before the delay time of 100 ns to a“fuctuating plasma”after the delay time of 300 ns.Notably,the frontier part of plasma showed a significant down-ward motion from the delay time of 150 ns to 200 ns and crashed with the lower part of the plasma,making the plasma fatter and later even splitting the plasma into two parts,which was considered as a critical process for the transformation of“stable plasma”to“unstable plasma”.By calculating the correlation coefficient of plasma image pairs at successive delay times,it was found that the higher the similarity between two plasma at early stage,the more similar at later stage;this implied that the tiny plasma fuctuation earlier than the critical delay time(150-200 ns)was amplifed,causing a large plasma fluctuation at the later stage as well as LIBS measurement uncertainty.The initation of slight fluctuation was linked with Rayleigh-Taylor Instability(RTI)due to the drastic material interpenetration at the plasma-ambient gas interface at earlier stage(before 50 ns).That is,the uncertainty generation of LIBS was proposed as:plasma morphology fluctuation was inevitably trigged by RTI at the early stage and the tiny fuctuation was amplified by the back pressed downward process of plasma frontier material,leading to severe morphology fluctuation as well as LIBS signal uncertainty.

On the improvement of signal repeatability in laser-induced air plasmas

The relatively low repeatability of laser-induced breakdown spectroscopy （LIBS） severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation （plasma evolution） and signal collection. Time- integrated spectra and images were obtained under different laser energies and focal lengths to inves- tigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 ram. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation （RSD） to increase with laser energies of 〉 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O（I）, N（I）, and H（I） lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.