Comparison of laser-induced plasma spectroscopy of metal zinc in bulk water with different depths/thicknesses

Laser-Induced Plasma Spectroscopy (LIPS) is an atomic emission spectroscopy technique to detect multiple elements for the samples with different states rapidly and simply. However, the sensitivity of LIPS for liquids are significantly reduced compared to solids and gases. This makes it difficult to directly detect trace elements in liquids, which restricts its applications in some fields. To reveal the potential reasons for the reduction of sensitivity in liquid, the plasma emission spectra of metallic zinc was compared in bulk water with different water depths and thicknesses. The Zinc spectral lines of Zn(I) 328.23nm, Zn(I) 330.29nm, Zn(I) 334.59nm, Zn(I) 481.05nm and Zn(I) 636.23 nm excited under water layer could penetrate water layers over 10 mm, while the ones excited in water could only penetrate 1.2 mm. This suggests that the decrease in plasma emission through water does not prominently comes from water absorption. To further explain this result, the plasma electron density and the excitation temperature were analyzed, separately. It is found that electron density and excitation temperature decreased rapidly with increasing depth in water, while the decrease in those under the water layer looks steadier. The difference between plasma emission and parameters excited under water layer and in water might provide some clues to reveal the problem for the sensitivity reduction of LIPS in liquids.