| The pollution of trace heavy metals in water is a direct threat to life.The traditional detection methods have many problems,such as limited types of detection elements,complex pretreatment,lack of valence analysis,lack of multi-phase adaptability,and high equipment cost.In recent years,the novel detection method of laser induced breakdown spectrum(LIBS)has gradually replaced the traditional method,but the LIBS intensity of liquid directly excited by laser is low and the laser excitation ability is weak.In this paper,the laser surface treatment of aluminum target,combined with experimental research and modeling analysis,was used to obtain the laser surface treatment substrate with the best enhancement effect of LIBS lines,and the LOD(limt of detection)of trace heavy metals in water in the national standard document was reached.The main research contents are as follows:1.Theoretical study on the formation mechanism and spectral characteristics of laserinduced plasma provided the basis for the study of LIBS spectral enhancement mechanism.It was found that the fluctuation of electron temperature is more than 20% and the fluctuation of electron density is less than 3.7% by calculating the electron temperature and electron density of LIBS line under different laser processing parameters.It was concluded that the mechanism of line strength enhancement is due to the increase of the ratio of high ionized particles caused by the collision of high energy particles,and the contribution of neutral particles to low ionized state is small.2.Compared with the enrichment effect of traditional filter paper,laser surface treated aluminum target had better spectral enhancement effect.Compared with mechanically polished aluminum target,laser surface treated aluminum target improves the uniformity of element distribution.Combined with principal component analysis modeling,the influence of laser processing parameters on LIBS detection spectrum was obtained.3.The enrichment substrate of laser surface treatment with the best enhancement effect of spectral lines was obtained,and the orthogonal partial least squares(OPLS-DA)model is established to obtain the VIP value of each characteristic spectral line,3 spectral lines with high importance are selected as input to test multiple regression models,and the results were evaluated according to R2,MSE and other evaluation indexes.The results showed that the artificial neural network had the best fitting effect on the concentration of trace heavy metals in LIBS detection of aluminum target substrate.4.A scheme of enhancing sensitivity by multiple enrichment was proposed,the detection limit of trace heavy metals in water can be as low as 0.048mg/L.Aiming at the LOD of LIBS detection for heavy metals in water,the laser surface treated aluminum target was used as the deposition substrate,and the LOD of relevant elements in the fishery water quality standard documents was finally reached through qualitative analysis,mechanism research and quantitative analysis. |
