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A fast quantitative analysis method of soil potassium based on direct-focused laser ablation-laser induced breakdown spectroscopy (direct-focused LA-LIBS) was proposed and tested. A high single-pulse energy laser (200 mJ/pulse) beam was focused on the aerosols near the focus of the 10 kHz fiber laser to generate plasma spectra, and the analytical capability of the direct-focused LA-LIBS system was compared with traditional LIBS system using a high single-pulse energy laser (SP-LIBS). The result showed that for moist soil samples the data stability of the direct-focused LA-LIBS method was significantly improved and the R2 factor of the calibration curve improved from 0.64 to 0.93, the limit of detection improved from 159.2 μg/g to 140.9 μg/g. Three random soil samples from different areas of Beijing suburbs were analyzed by the direct-focused LA-LIBS method, and the results were consistent with AAS. The direct-focused LA-LIBS method proposed is different from the traditional double-pulse technology and laser ablation-assisted technology because it not only does not need carrier gas, but also can overcome the matrix differences better, especially the influence of moisture, which provides a new idea for the rapid detection of nutrient elements in field soils.

Rapid detection of soil nutrients has been the focus of research worldwide (Hussain et al., 2007; Gholizadeh et al., 2013). The potassium content is an important indicator of soil fertility (Dale, 1975; Assefa and Ledin, 2001; Mohammad et al., 2003), and is the most important trace element for plants (Toader et al., 2013; Wang et al., 2013). Rapid detection of potassium in soil provides guidance for rational fertilization and basis data for agricultural farmland census and production planning (Chen et al., 2013; Nikkhoo et al., 2013; Zhang et al., 2015; Fu et al., 2020).

Current methods to detect potassium in soil are based on laboratory analysis, such as inductively coupled plasma spectrometry (KOVACS, 2000), atomic absorption spectrometry (David, 1960), or titration (Haney et al., 2008), which are highly accurate, but inflexible and cannot meet the needs of large scale field detection (Jackson, 2015). Laser-induced breakdown spectroscopy (LIBS) is a popular, rapid detection technique with non-contact and real-time analysis (Fortes et al., 2013; Bol'Shakov et al., 2010), and many studies have used it for the detection of potassium in soil (Hussain and Gondal, 2008; Burakov et al., 2010; Khoso et al., 2021).

With different soil types and matrices, the accuracy of the analytical results may be affected (Bousquet et al., 2007; Bae, 2015; Nicolodelli et al., 2015), and therefore post-processing by algorithms is used to eliminate them (Li et al., 2020; Liu et al., 2022). Lu et al. designed an analytical model for potassium in soil based on convolutional neural networks (CNNs) with an R2 factor of 0.99 on the calibration curve, demonstrating that the LIBS technique can reduce the matrix effects through the algorithms (Chengxu et al., 2018). Yong He et al. used a combination of double-pulse LIBS technique and a partial least squares (PLSR) algorithm to analyze nutrients in 63 different soil samples and demonstrated that the PLSR could effectively improve the detection limit and analytical accuracy of the model (He et al., 2018). The drawback of this algorithm-based approach is how to find a sufficient number of representative samples, a difficult task for field soil (Islam and Chowdhury, 2002; Yang et al., 2015).

Recent studies have focused on reducing the interference of matrix effects by starting from the characteristics and production process of plasma. For example, Ciucci et al. proposed a method with calibration-free analysis (CF-LIBS). This was based on the relationship between spectral intensity, atomic transition energy, plasma temperature, and electron density, which do not need to establish a calibration curve based on standard samples, so the influence of the matrix can theoretically be avoided (CiucciCorsi et al., 1999). However, this method is very difficult to be applied due to the calculation of plasma temperature, electron density, the elimination of self-absorption effects, and the acquisition of the total element spectrum (Tognoni et al., 2009; Elsayed et al., 2022).

Another approach to reduce matrix effect is to separate the two processes of laser-ablation and plasmaization. The aerosol produced by laser ablation is transported to a new cavity through a carrier gas, and then the plasma is excited, collected, and analyzed. Hahn et al. investigated SP-LA-LIBS, DP-LA-LIBS, and LA-LIBS systems (Windom and Hahn, 2009; Pareja et al., 2013; Glaus and Hahn, 2014; Wang et al., 2021) and showed that the LA-LIBS systems, which require a carrier gas for transmission, can significantly reduce the matrix effect, but the whole system structure is complex, and the limit of detection is high. In addition, the influence of soil moisture on the spectral signal should not be neglected in the field analysis, but there is a lack of relevant studies using traditional LA-LIBS.

In order to improve the field analysis capability of LA-LIBS, this study tested a rapid analysis method by focusing a high single-pulse energy laser directly on the aerosol near the 10 kHz fiber laser focus point (direct-focused LA-LIBS) and compared the analytical performance with conventional LIBS system using one laser with high single-pulse energy (SP-LIBS). We demonstrate that the direct-focused LA-LIBS method can eliminate the effects of differences in soil type and water content within a limited range. This is the first successful study using LA-LIBS without a sample chamber and carrier gas to analyze soils, which will give a good impetus to the practical application of LIBS technology in the field.

A total of eight national standard soil samples were used in this experiment. The sample numbers and soil properties are shown in Table 1.

TABLE 1. Sample labels, locations, and properties used in the experiment.

These standard soil samples come from Northeast, North, Northwest, and Southeast China, with diverse soil types and strong representativeness.

1) making the first group of soil samples for the calibration of the characteristic spectral lines of potassium: three standard soil samples with the same weight (GBW07407A, 30g/piece) were mixed with three different contents of potassium chloride aqueous solution samples, and after air drying, grinding and tablet pressing, three calibration samples (sample1,sample2,sample3) of the same matrix type are formed, and the content of potassium in sample1 is 0.35%, in sample2 is 2.0%, in sample3 is 3.7%. The relevant characteristics of the first group of soil samples are shown in Table 2.

TABLE 2. Relevant characteristics of the first group of tablet samples.

2) Eight dry soil tablets were made as the second group. Take 30 g of soil from each of the eight standard soil samples, and then put them respectively into an aluminum box with a diameter of 40 mm and pressed into soil tablets, so eight kinds of dry soil tablets were formed: sample4 to sample11. The relevant characteristics of the second group of soil samples are shown in Table 3.

TABLE 3. Relevant characteristics of the second group of tablet samples.

3) Eight moist soil tablets were made as the third group. Similarly, 30 g of soil were also taken from each of the eight standard samples and placed into an aluminum box respectively with a diameter of 40 mm 1, 2, 3, 4, 5, 6, 7 and 8 ml of pure water were added to the aluminum box in turn. After stirring, the soil in the aluminum box was pressed into a set of soil samples with different water contents, so eight kinds of moist soil tablets were formed: sample12 to sample19. The relevant characteristics of the third group of soil samples are shown in Table 4.

TABLE 4. Relevant characteristics of the third group of tablet samples.

The experimental system is shown in Figure 1. There were two lasers in the system, one of which is a high single-pulse energy laser named Tiny200 series laser produced by Grace Laser Technology Co., Ltd (1,064 nm, 200 mJ/pulse, 10 Hz, 8 ns pulse-width), additionally a fiber laser with a wavelength of 1080 nm produced by Taizhou laser Co., Ltd. was customized. The maximum frequency of our customized fiber laser can reach 10 kHz, and the single pulse energy can reach 3 mJ/pulse, which is basically the upper limit of the single pulse energy transmitted by the fiber.

FIGURE 1. System structure diagram.

The generated plasma spectrum was coupled to several optical fibers and finally transmitted to the spectrometer produced by Avantes Ltd., which included four spectrometers with spectrum range of 180–760 nm, and typical resolution of 0.01 nm (FWHM).

The novelty of the system shown in Figure 1 is that SP-LIBS and direct-focused LA-LIBS can be realized respectively by adjusting the positions of several groups of lenses and using different lasers. In SP-LIBS, only the high single-pulse energy laser (200 mJ/pulse) is needed to work, the position of lens1 moves down, so that the focusing spot of the laser is on the surface of the soil sample, and in direct-focused LA-LIBS, fiber laser and the high single-pulse energy laser are required to work at the same time, the fiber laser is focused on the sample surface through lens2, but lens 1 needs to be moved up to focus the high-energy nanosecond pulse laser on the aerosol (about 2 mm above the ablation point of fiber laser) formed by the fiber laser ablation of the sample. Of course, the position of the spectrum collection lens needs to move up and down with the focus point of the high single-pulse energy laser to ensure that maximum plasma spectral intensity is obtained. The focal length of lens1 is 70 mm, and the focal length of lens2 is 100 mm. Whether in SP-LIBS or direct-focused LA-LIBS system, the ablation spot diameter of laser on the sample surface is 0.8 mm.

With reference to the National Institute of Standards and Technology (NIST) database and related references, three potassium characteristic spectral lines at K I 766.45 nm, K I 769.90 nm, and K I 518.36 nm were examined, and the characteristic peak at 766.45 nm was used as the basis of the quantitative analysis model (having the best signal-to-noise ratio).

For the selection of acquisition time of laser induced breakdown spectra, the signal-to-noise ratios at the emission peaks of potassium were observed at different delay times of 0, 0.5, 1, 1.5, 2.0, or 2.5 µs, at last the delay time was finally set to 1 μs. The integration time was set to 1 ms, which is the minimum exposure time of the CCD spectrometer. Combining with previous practical experience and considering that the interference of laser-induced breakdown spectrum mainly comes from bremsstrahlung

at the initial stage of spectrum generation (M1>M2, and 30 g of each sample was taken and placed in an aluminum sample box with a diameter of 40 mm, and other experimental parameters and spectral processing methods are consistent with the experiments above.

As shown in Table 6, the change in potassium detection results of the direct-focused LA-LIBS was consistent with those of AAS, indicating that the direct-focused LA-LIBS method effectively overcomes the matrix effect, especially the influence of moisture, and it does not require a carrier gas allowing for potential field application that requires rapid screening.

TABLE 6. Comparison of analysis results.

The results of direct-focused LA-LIBS detection fluctuated greatly compared with traditional laboratory methods, and relative errors of direct-focused LA-LIBS detection in different content sections were 5.4, 5, and 10.3%. In future studies, improvements are needed to make the analysis results more stable and accurate through adjusting different experimental parameters.

In this study, a LA-LIBS analysis method based on the direct-focused of high single-pulse energy laser on the aerosol produced by ablation was proposed and compared with SP-LIBS in the analysis of soil samples.

When using direct-focused LA-LIBS for spectral analysis, it is necessary to focus the high single-pulse energy laser on the aerosol rather than the surface of the soil sample, which results in lower limit of detection due to the lower background value of the signal. In addition, the difference in soil water content has always been one of the key factors that have plagued rapid field detection. Here soil samples with different water content, the R2 factor of the calibration curve of the direct-focused LA-LIBS analysis method was 0.93, which is better than 0.64 for conventional SP-LIBS. These results also showed that the direct-focused LA-LIBS is an effective analytical method to reduce the influence of soil matrix, especially that of moisture. Compared with the LA-LIBS system that requires carrier gas and a sample chamber, the LA-LIBS proposed here also had a better limit of detection and stability, giving a strong potential for field detection applications.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

P-CH: writing—original draft and editing. KY: Help with data processing. L-ZJ: investigation. H-CL: methodology, and conceptualization.

This work was supported by the National Natural Science Foundation of China (31972148), the Distinguished Young Scientists Program of Beijing Natural Science Foundation (JQ19023), and the Distinguished Scientist Development Program of Beijing Academy of Agriculture and Forestry Sciences (JKZX201904). The authors also thank AiMi Academic Services for the English language editing services.

P-CH, KY, and H-CL were employed by the BGRIMM Technology Group.

The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Keywords: LIBS, potassium in soil, matrix effect, laser ablation-assisted, field

Citation: Han P-C, Yang K, Jiao L-Z and Li H-C (2022) Rapid quantitative analysis of potassium in soil based on direct-focused laser ablation-laser induced breakdown spectroscopy. Front. Chem. 10:967158. doi: 10.3389/fchem.2022.967158

Received: 12 June 2022; Accepted: 12 August 2022;Published: 01 September 2022.

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Reviewed by:

Copyright © 2022 Han, Yang, Jiao and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Hua-Chang Li, [email protected]