Laser-induced breakdown spectroscopy (LIBS) is a novel atomic-emission spectroscopic technique, which has many advantages such as multi-elemental detection, rapid response for online monitoring, and easy sample preparation for in situ measurements, and it has been applied in many fields. However, nanosecond laser-induced breakdown spectroscopy (ns-LIBS) still has some defects such as poor stability, low analysis accuracy, and matrix effects. Many methods have been proposed to solve these problems. Among them, filament-induced breakdown spectroscopy (FIBS) and plasma-grating-induced breakdown spectroscopy (GIBS) based on femtosecond laser excitation do not require the introduction of additional equipment and sample preparation steps if compared with other methods. In FIBS, a long and stable filament is used to ablate samples. And it has been proved that FIBS has very stable analysis results and can be used to realize remote sensing detection. And GIBS is based on the plasma grating formed by the interaction of two non-collinear filaments to excite samples. A significant increase in spectral line intensity has been successfully observed in the comparative experiment between FIBS and GIBS. However, the length of the plasma grating is greatly reduced compared with that of the filament, so it is necessary to investigate the analysis stability of the GIBS technology. In addition, the influence of the enhanced spectral line intensity brought by the GIBS technology on the detection limit is still to be studied.

We build a GIBS system based on a plasma grating formed by the interaction of two filaments intersecting at a small angle to ablate the sample and generate plasma. In order to better control the experimental variables, we realize the conversion to the FIBS system by blocking one of the light filaments in the GIBS system. The sample is placed on a three-dimensional translation table to avoid over-ablation and the position of the sample relative to the focal point of the lens is changed. We collect the light emission from the induced plasma and transmit it to a spectrometer by an optical fiber. The light dispersed by the spectrometer is detected with an intensified charge-coupled device (ICCD). The ICCD is set to an on-chip integration mode to realize 100 excitations for each spectral data. In addition, we also configure the standard soil samples with different concentrations of Cr to establish calibration curves and analyze the detection limits of these two systems.

We first compare the signal intensities of the spectral lines induced by the FIBS system and the GIBS system under the same conditions. An enhancement factor of around 2 is successfully obtained, which is very sensitive to the inter-pulse delay (Fig. 4). From this perspective, it can be clearly judged that the GIBS system we build is based on the excitation of the sample by the plasma grating formed by the interaction of two filaments. Then we study the influence of sample spatial position on these two systems to reflect the stability of the excited sample. When the laser energy is increased from 0.5 mJ to 1.5 mJ, the sample spatial position range corresponding to the obvious spectral line signal excited in the FIBS system increases from 1 mm to 4 mm, while in the case of GIBS system, such a range increases from 0.2 mm to 0.4 mm (Fig. 5). Unsurprisingly, the GIBS system has stricter requirements on the sample spatial location than the FIBS system. However, as laser energy increases, the optimal spectral line signal position in the GIBS system is stable around 0 mm, the position where the filaments intersect. In contrast, the optimal spectral line signal position in the FIBS system moves towards the lens as laser energy increases. In the last part, we compare the detection sensitivities of the FIBS system and the GIBS system by establishing a calibration curve for Cr element in the soil. In both systems, the coefficient of determination (R²) of the calibration curve exceeds 0.99 (Table 1), which means that both systems are very suitable for a quantitative analysis. But what is more, the limit of detection (LOD) in the FIBS system is calculated to be 55.09×10^－6, while in the GIBS system it is 29.96×10^－6. This shows that the GIBS system is more advantageous for the detection of trace elements.

We build a GIBS system to analyze soil samples, and the spectral signal enhancement by about 2 times that of the FIBS system is successfully obtained, which is closely related to the inter-pulse delay. By studying the influence of the spatial position of the sample on the spectral line signal under different laser energies, it is proved that although the sample spatial position range corresponding to the obvious spectral line signal excited in the GIBS system is reduced compared with that in the FIBS system, the optimal spectral line signal position in the GIBS system does not change with laser energy, as that in the FIBS system. In addition, in the quantitative study of the heavy metal element Cr in the soil, it is found that both FIBS and GIBS systems can be used to establish a linear calibration curve and the limit of detection in the FIBS system is 55.09×10^-6, while the limit of detection in the GIBS system is 29.96×10^-6, indicating that the GIBS technology has high detection sensitivity.