Laser-induced breakdown spectroscopy (LIBS) has the advantages of in situ, real-time, rapid detection, no sample pretreatment, and great potential for elemental analysis. However, when LIBS is used to detect liquid samples, water splashing, surface ripples, and plasma quenching reduce the detection sensitivity and accuracy, which limits the performance and development of LIBS in liquid detection. Conventional enhancement techniques for LIBS liquid detection require complicated experimental setups or sample preparation processes, which negates the advantages of LIBS. Previous studies have shown that the dispersed phase of wheat starch mixed with liquid samples can form a high-viscosity colloidal state, which effectively reduces the effects of water splashing and surface ripples and significantly improves detection sensitivity. The sample preparation process with this method is simple and only requires 30 s, which meets the requirements of LIBS liquid detection. To improve the detection performance of the dispersed-phase enhancement technique, the research problems related to enhancement characteristics, enhancement mechanism, and quantitative analysis techniques are analyzed and discussed in this study.

We first test the LIBS enhancement of various dispersed phases and investigate the enhancement characteristics of the dispersed phases through experiments to investigate the effects of dispersed phase mass fractions and atmospheric drying on the LIBS intensity. The enhancement mechanism of the dispersed phase is analyzed by diagnosing the plasma electron temperature and electron density of the liquid sample and by considering two samples with different dispersed phase mass fractions using the Saha?Boltzmann and Stark broadening methods. The local thermodynamic equilibrium state of the plasma is evaluated according to the McWhirter criterion to ensure the validity of the calculations of the plasma electron temperature. Then, an Al I 396.15 nm calibration curve is constructed to evaluate the detection performance of the dispersed-phase LIBS. The effect of drying time on the detection performance of dispersed-phase LIBS is also analyzed. Finally, a partial least squares regression model is constructed for the quantitative analysis of the dispersed-phase LIBS, and the effect of the generalized spectral method on the quantitative analysis of the partial least squares regression is investigated.

The results show that the spectral enhancement of the dispersed phase is more effective as the mass fraction of the dispersed phase increases under the premise of uniform mixing (Fig. 5). In addition, under atmospheric drying, moisture volatilization can enhance the spectral intensity. The structure of the high mass fraction dispersed-phase sample is more conducive to moisture volatilization, which leads to a rapid improvement in the spectral intensity of the detected element (Fig. 6). The time evolutions of the plasma electron temperature and electron density for the liquid sample and dispersed-phase samples with high and low mass fractions are consistent, which indicates that the electron temperature and electron density cannot explain the spectral enhancement of the dispersed phase, and the enhancement mechanism of the dispersed phase is related to the increase in the ablation of the detected elements. Compared to that of direct liquid detection, the detection sensitivity is improved by 6.03 times after the liquid is prepared as a high mass fraction dispersed-phase sample using wheat starch, and it increases to 17.26 times after 1 h atmospheric drying (Table 2). Unlike when constructing the partial least squares regression model with only spectral data, this study reduces the prediction error of the partial least squares regression model by utilizing the generalized spectral method with drying time as an additional feature dimension, with the average relative error and predicted root-mean-square error of the model reduced by 23.95% and 14.10%, respectively.

A simple and fast method for detecting metals in liquids using the dispersed-phase LIBS technique is presented in this study. The enhancement effect of dispersed-phase LIBS is investigated and determined to be related to the mass fraction of the dispersed phase. On the premise of uniform mixing, the structure of a mixed sample with a high dispersed-phase mass fraction is more stable, and the enhancement effect is better. In addition, the LIBS enhancement effect of the high mass fraction dispersed-phase sample can be further improved by atmospheric drying. The enhancement mechanism of dispersed-phase LIBS is analyzed by calculating the plasma electron temperature and electron density, and the results show that the enhancement mechanism is not related to the electron temperature and electron density but rather to the increase in the ablation of the detected element. Al as the target element in the liquid and wheat starch as the dispersed phase are used to evaluate the detection performance of dispersed-phase LIBS. Compared to that of direct liquid detection, the detection sensitivity is improved by 6.03 times using dispersed-phase LIBS and increases to 17.26 times after 1 h drying. A partial least squares regression model is constructed for the quantitative analysis of the dispersed-phase LIBS. Using the generalized spectral method, the drying time is introduced for partial least squares regression modeling as an additional feature dimension, and the average relative error and predicted root-mean-square error of the model are reduced by 23.95% and 14.10%, respectively.