Qualitative and Quantitative Comparative Investigation of Gas for Different Focusing Arrangements by Laser-Induced Breakdown Spectroscopy
Objective High-purity nitrogen and oxygen are essential as protective gases for many high-precision instruments,directly affecting their performance and detection sensitivity.It is crucial to develop methods suitable for industrial production processes,which have significant implications for scientific research and industrial applications.Laser-induced breakdown spectroscopy(LIBS),an emerging chemical analytical method,holds the potential for quantitative analysis in industrial detection due to its real-time,in-situ,and multi-element detection capabilities.However,due to field application constraints,standoff detection using LIBS has become ineffective for measuring industrial gases.The detection sensitivity of the system significantly decreases with increasing detection distance,leading to difficulties in obtaining effective plasma signals and performing quantitative analysis.Therefore,more effective excitation methods are needed.Recent studies on LIBS quantitative analysis have predominantly focused on liquid and solid phases,while plasma generation in air and its radiation characteristics are notably different.Effective plasma emission depends largely on an optimal focusing arrangement.We evaluate the breakdown threshold and signal stability for different focusing arrangements,analyzing the differences in line intensities among five focusing arrangements.Two typical focusing arrangements are analyzed to detail their quantitative performance for various gas mixtures,advancing the practical application of LIBS in measuring industrial gases.Methods A Nd∶YAG all-solid-state laser operating in Q-switched pulse mode at a fundamental wavelength of 1064 nm with a repetition rate of 20 Hz is used as the excitation source.The pulse width is 8 ns,and the laser beam diameter is approximately 6 mm.A portion of the laser energy is reflected through a beam splitter to an energy detector for real-time monitoring.The laser is focused on the gas sample using various focusing arrangements.The gas mixture,consisting of high-purity liquid oxygen and liquid nitrogen,is prepared using a custom-built mass flow controller.Spectral measurements are conducted with plasma emission collected by a system with a focal length of 50.8 mm and coupled into a delivery fiber.The spectrometer has a wavelength range of 200-900 nm and a spectral resolution of 0.3 nm.The laser operates in internal trigger mode,with the spectrometer synchronized to the laser pump signal.Five focusing arrangements—CDC1,CDC2,CDC3,CDC4,and SL1—are analyzed.Four arrangements use Galilean telescope systems comprising a combination of concave lenses(f=-50.8 mm)and biconvex lenses,while SL1 uses a single lens(f=200 mm)for comparison.Results and Discussions Time-resolved emission spectra under nitrogen-oxygen gas mixture conditions are obtained(Fig.2).The line intensity decreases with increasing delay.Given the lower background radiation at the early plasma stage for N and O atomic lines,an optimal detection delay of 100 ns is chosen.Typical spectra and peak areas for O Ⅰ 777.4 nm for the five arrangements at 100 ns are illustrated(Fig.3).Differences in line intensity among the five arrangements are attributed to the spot radius of the focusing lens and the depth of focus.Signal stability for different arrangements is calculated from 200 replicate spectra(Fig.4).The biconvex lens arrangement significantly improves signal stability compared to that of the single lens arrangement(SL1).Further analysis of spectral differences,considering the depth of focus and optical simulation(Figs.5 and 6),reveals that CDC3 achieves the highest line intensity due to its moderate focusing angle and shorter depth of focus,while SL1 has the longest depth of focus and root mean square(RMS)radius,resulting in the lowest line intensity.The simulation and calculation results for other arrangements are consistent with Fig.3.To evaluate LIBS quantitative performance,CDC3,and SL1 are used to measure oxygen-in-nitrogen and nitrogen-in-oxygen gas mixtures at various concentrations(Figs.7 and 8).The biconvex lens arrangement provides higher spectral line intensity and signal stability,enhancing detection sensitivity.Conclusions In this study,we investigate the influence of different focusing arrangements on the qualitative and quantitative performance of LIBS.Five focusing arrangements—CDC1,CDC2,CDC3,CDC4,SL1—are examined.Combining emission spectra acquisition with Zemax simulation,we explore line intensity differences among the arrangements.Results show that the biconvex lens arrangement offers a lower breakdown threshold than the single lens.The highest LIBS spectral line intensity and signal stability are achieved with the biconvex lens combination(CDC3)with focal lengths of 125 and 200 mm,due to its moderate depth of focus and focusing angle,as confirmed by Zemax simulation.In quantitative analysis,the combination of biconvex lenses can improve the correlation coefficient of calibration curves,while also enhancing prediction robustness and detection sensitivity.The stability for CDC3 is increased by over 1.5 times compared to SL1,and the root mean square error(RMSE)is reduced to less than 0.678,with the detection ability increased by more than 9.9 times.The CDC focusing arrangements can significantly enhance real-time industrial gas detection.
laser-induced breakdown spectroscopyoxygen-nitrogen gas mixturecomparison of lens combinationspectral radiation characteristicqualitative and quantitative analysis
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