Laser-Based Techniques for Industrial Analysis

July 8, 2015

Laser-induced breakdown spectroscopy (LIBS) is an ideal technique for elemental analysis in industrial applications because of its fast response and high sensitivity, and its ability to be used for real-time, noncontact analysis.

 

Laser-induced breakdown spectroscopy (LIBS) is an ideal technique for elemental analysis in industrial applications because of its fast response and high sensitivity, and its ability to be used for real-time, noncontact analysis. Yoshihiro Deguchi is a Professor in the Institute of Technology and Science at Tokushima University, and he and his group use LIBS and other laser-based techniques to analyze industrial samples such as fly ash from coal-fired thermal power plants, mercury in combustion gas, and trace-level species containing elements such as mercury, iodine, strontium, cesium, and arsenic. He recently spoke to us about this work.

In a recent study (1), your group used LIBS to examine size-segregated particle contents of fly ash generated by coal combustion. What makes LIBS a good choice for analyzing combustion products, including unburned carbon? Why is the information obtained in this analysis important?

The LIBS technique has been applied to various fields, including analyses in harsh environments such as coal combustion and other industrial fields. The most important feature of LIBS is that samples can be measured without any sample preparation. LIBS also has several other merits, such as fast response, high sensitivity, real-time and noncontact features, as well as multielemental capability.

Electricity is the world’s fastest-growing form of end-use energy consumption, and coal provides the largest share of world electricity generation. We should consider improving the efficiency of coal combustion by analyzing the coal and its combustion procedure. Measuring the content of unburned carbon and other materials after coal combustion is essential for adjusting boiler combustion in an effort to improve the economy and security of power generation.

What are the main challenges in using LIBS for fly ash analysis?

Calibration of the LIBS signal is necessary for quantitative analysis. However, the plasma induced by the LIBS process is complex and is not uniform. The plasma is affected by experimental parameters such as the optical setups, pressure, buffer gas composition, and so on. Therefore, appropriate correction factors and methods must be developed to obtain quantitative results. The temperature correction method is proposed in our study for its simplicity and real-time measurement capability.

Another paper from your group on monitoring industrial thermal systems (2) described the use of computed tomography tunable diode laser absorption spectroscopy (CT-TDLAS), low-pressure LIBS, and laser-breakdown time-of-flight mass spectrometry (LB-TOFMS) to measure species concentrations in engine exhausts and coal and fly ash contents. How do the low-pressure LIBS and LB-TOF-MS techniques differ from the normal LIBS and TOF-MS techniques, and what are their advantages?

The main advantage of low-pressure LIBS compared to normal LIBS techniques is its high sensitivity. The dominant phenomena in the laser-induced plasma process differ with changes in pressure, and the process can be controlled by the pressure. That is, interferences with the target signal, especially the continuum emission from the plasma itself (coexisting molecular and atomic emissions), can be reduced significantly at low pressure. At high pressure (atmosphere pressure), the electron diffusion and electron impact ionization processes are the major sources of plasma generation. When the pressure is reduced (by a few kilopascals), the effect of electron diffusion and electron impact ionization decreases; at low pressure, the multiphoton ionization process becomes the main influence.

TOF-MS with ionization processes such as laser ionization, electrospray ionization, matrix-assisted laser desorption–ionization (MALDI), and so on is an appealing technique for qualitative and quantitative analysis of atoms and molecules, and offers high sensitivity and rapid analysis. However these TOF-MS approaches require sample preparation. LB-TOF-MS, on the other hand, can directly detect the elemental composition of gas-phase samples and fine particles. Considering the features of TOF-MS, fragmentation and signal intensity are the important factors for sensitive measurement. The method of laser breakdown combined with TOF-MS was employed to eliminate or minimize the interference of fragmentation from the coexisting materials with the atomic ion signals.

A third publication (3) compared LIBS and LB-TOF-MS for measuring trace levels of mercury, iodine, strontium, cesium, and arsenic, which are elements sometimes found in materials released in environmental pollution incidents such as the Fukushima nuclear power plant accident. Were you able to obtain clear signals for the elements with both techniques? What notable results did you obtain in this study? Did you collect any surprising or unexpected data?

We have measured the trace species of mercury, iodine, strontium, cesium, and arsenic using low-pressure LIBS and LB-TOF-MS. In our current studies, the trace elements of mercury, iodine, strontium, and cesium provided clear emission signals using low-pressure LIBS under various conditions. When employing LB-TOF-MS, we obtained clear signals for trace elements of mercury, iodine and arsenic under different experimental conditions. The detection limit of trace species can be enhanced using low-pressure LIBS and LB-TOF-MS by controlling the laser-induced plasma process.

What are the next steps in your research?

We plan to study and discuss in detail the detection ability and detection characteristics of low-pressure LIBS and LB-TOFMS for different trace species. Low-pressure LIBS and LIBS with the temperature-correction method will be applied to thermal power plants (with conventional coal-fired boilers or using the integrated gasification fuel cell cycle) to measure the unburned carbon and trace species. These methods will be applied to the trace-species measurement of soils and liquids, as well as to gas-phase samples.

 

References

(1) Z.Z. Wang, Y. Deguchi, M. Kuwahara, T. Taira, X.B. Zhang, J.J. Yan, J.P. Liu, H. Watanabe, and R. Kurose, Spectrochimica Acta. Part B: Atomic Spectroscopy87, 130–138 (2013).

(2) Y. Deguchi, T. Kamimoto, Z.Z. Wang, J.J. Yan, J.P. Liu, H. Watanabe, and R. Kurose, Applied Thermal Engineering73(2), 1453–1464 (2014).

(3) Z.Z. Wang, Y. Deguchi, J. Yan, and J. Liu, Sensors15, 5982–0860 (2015).