Novel Method Based on Hollow Laser Trapping-LIBS-Machine Learning for Simultaneous Quantitative Analysis of Multiple Metal Elements in a Single Microsized Particle in Air

Abstract

Elemental identification of individual microsized aerosol particles is an important topic in air pollution studies. However, simultaneous and quantitative analysis of multiple constituents in a single aerosol particle with the noncontact in situ manner is still a challenging task. In this work, we explore the laser trapping-LIBS-machine learning to analyze four elements (Zn, Ni, Cu, and Cr) absorbed in a single micro-carbon black particle in air. By employing a hollow laser beam for trapping, the particle can be restricted in a range as small as ∼1.72 μm, which is much smaller than the focal diameter of the flat-topped LIBS exciting laser (∼20 μm). Therefore, the particle can be entirely and homogeneously radiated, and the LIBS spectrum with a high signal-to-noise ratio (SNR) is correspondingly achieved. Then, two types of calibration models, i.e., the univariate method (calibration curve) and the multivariate calibration method (random forests (RF) regression), are employed for data processing. The results indicate that the RF calibration model shows a better prediction performance. The mean relative error (MRE), relative standard deviation (RSD), and root-mean-squared error (RMSE) are reduced from 0.1854, 363.7, and 434.7 to 0.0866, 179.8, and 216.2 ppm, respectively. Finally, simultaneous and quantitative determination of the four metal contents with high accuracy is realized based on the RF model. The method proposed in this work has the potential for online single aerosol particle analysis and further provides a theoretical basis and technical support for the precise prevention and control of composite air pollution.

Multi-element Quantitative Analysis of Single Micro-sized Suspended Particles in Air with High Accuracy Based on Random Forest and Variable Selection Strategies

“…Compared to the three metal elements, the prediction performance of the VIP-RF calibration model for the Zn element is the best, which has the best correlation and the lowest values of MREP and RMSEP. After the smoothing spectral pretreatment and various variable selection methods, the performance of RF models is improved compared with our previous work …”

“…Chen et al combined standard normal variation (SNV) spectral preprocessing and feature variable extraction methods to establish an RF calibration model based on the mid-infrared (MIR) spectra and LIBS spectra for qualitative analysis of soil pH . In our previous research, we investigated the laser trapping–LIBS combined with RF to quantitatively analyze four metal elements in a single particle . However, being a single particle, in the process of LIBS signal acquisition, the size of the single particle and other factors will cause instability of the spectral signal.…”

“…At the same time, in the LIBS spectra of the single particle, there are plenty of noise signals, which affect the accuracy of quantitative analysis. While there is no correlation spectral pretreatment method in the existing work . Thus, the model’s prediction performance could be improved further.…”

Nanoparticles in analytical laser and plasma spectroscopy – a review of recent developments in methodology and applications

“…Most recently, Niu et al proposed an optical trapping approach based on the use of a hollow cw laser beam, and successfully demonstrated that OT-LIBS can be applied for the quantitative determination of metal concentrations in individual, 2 mm black carbon aerosol particles. 256 In other studies, nL volume droplets and 1-3 micrometer aerosol particles were also successfully trapped and individually analyzed by LIBS. Järvinen et al 257 used an electrodynamic, whereas Meneses-Nava et al 258 an acoustic levitator for the trapping of particles.…”

Atomic spectrometry update: review of advances in atomic spectrometry and related techniques

“…Laser trapping-LIBS and machine learning was investigated by Niu et al 115 for the determination of 4 elements absorbed in a single micro-carbon black particle in air. The authors employed a hollow laser beam for trapping, which was proposed as a possible method for online single aerosol particle analysis.…”