Excess heavy metals in soil can seriously affect the growth of crops, among which chromium is considered to be one of the most toxic heavy metals. Excess chromium levels can lead to retarded plant growth and reduced yields, and can affect human health when consumed. Therefore, chromium detection in soil has become an important research element in the field of agriculture. Laser-induced breakdown spectroscopy (LIBS) can be used for the detection of heavy metals in soil. This technique has the advantages of simultaneous detection of multiple elements, online detection, and simple sample preparation methods. Conventional nanosecond laser-induced breakdown spectroscopy has poor reproducibility due to the influence of background continuum spectra and matrix effects during the detection process. Improved methods include filament-induced breakdown spectroscopy (FIBS) based on femtosecond filament, plasma-grating-induced breakdown spectroscopy (GIBS) based on the non-collinear superposition of two femtosecond filaments, multidimensional-plasma-grating-induced breakdown spectroscopy (MIBS) based on the superposition of three non-coplanar femtosecond filaments, and triple-filament interaction induced breakdown spectroscopy (TIBS) based on the coplanar and non-collinear superposition of three femtosecond filaments. All these methods do not require the introduction of additional equipment and complex sample preparation methods. Among these improved methods, GIBS and MIBS have been well-studied. However, studies on TIBS are still lacking, especially regarding the detection of heavy metals in soil.
In this study, a TIBS system based on the superposition of three coplanar and non-collinear femtosecond beams, a GIBS system based on the superposition of two noncollinear femtosecond beams, and a FIBS system based on one femtosecond beam are developed to study the heavy-metal detection capability of these three systems in soil. Standard soil samples are doped with various mass fraction Cr elements and pressed into sheets. To ensure that the position of the sample being excited remains the same, the sample sheet is placed on a three-dimensional translation table. The fluorescence signal generated by the excited sample is transmitted to a step spectrometer equipped with an intensified charge-coupled device (ICCD). In addition, we use three systems for the detection of standard soil samples doped with different mass fraction Cr elements and determine the detection limits.
We first compare the spectral line signal intensities of the FIBS, GIBS, and TIBS systems under the same experimental conditions. As shown in Table 2, the signal intensity of the TIBS system is enhanced by 2 times compared with that of the GIBS system and 7-11 times compared with that of the FIBS system. Then, we compare the changes in the spectral line intensity under the three systems by varying the sample position. As shown in Fig. 4, the TIBS, GIBS, and FIBS systems exhibit stable excitation in the spatial scale of 0.30, 0.35, and 0.15 mm, respectively. This implies that the plasma grating formed by the superposition of multiple filaments has more stable excitation. The spectral signal intensity decreases rapidly after the excitation is far from the corresponding region because the filaments are not superimposed in this case. Finally, we measure the calibration curves of the Cr samples in soil with the three systems and calculate the corresponding limits of detection. As shown in Fig. 5, the limits of detection for the FIBS, GIBS, and TIBS systems are 22.18×10-6, 8.68×10-6, and 5.06×10-6, respectively. The TIBS system exhibits higher detection sensitivity compared with the GIBS system, and the coefficients of determination of the calibration curves for all three systems exceed 0.99.
In the comparative analysis of the soil samples doped with various mass fraction Cr2O3 under the same experimental conditions, the spectral signal of the TIBS system is significantly enhanced compared with those of the GIBS and FIBS systems; specifically, the spectral intensity achieves an enhancement of 2 times and 7-11 times, respectively, which is due to the more intense electron-ion collision in the interaction region of three filaments than that of two filaments, thereby leading to a further enhancement of the fluorescence. Compared with the GIBS system, the TIBS system has similar excitation stability and maintains in the best excitation region when the sample is moved within 0.30 mm. The calibration curves of Cr in soil are established under FIBS, GIBS, and TIBS systems, and the detection limits are 22.18×10-6, 8.68×10-6, and 5.06×10-6, respectively. These results show that the TIBS technique can further improve the sensitivity of the detection compared with the GIBS system, and can be used as an effective method for the detection of heavy metals in soil.
