Exploring the Use of Nanoporous Glass Substrates for the Quantitative Trace Microanalysis of Aqueous Samples by Surface-Enhanced Laser-Induced Breakdown Spectroscopy.

In this study, a novel laser-induced breakdown spectroscopy (LIBS) liquid sample presentation method is showcased based on drying microdroplets (2-3 μL) of aqueous solutions on nanoporous borosilicate glass (NPG) substrates possessing an average pore size of 4 nm and a porosity of about 30%. The combination of capillary effects and drying results in a fairly homogeneous deposition of dissolved matter inside the pore structure near the surface, where it is available for sensitive LIBS analysis. The nanoporous structure of NPG breaks Marangoni flows, thereby eliminating any coffee ring effects, and the pores allow for an effective coupling of laser light into the glass substrate, facilitating surface signal enhancement. The water absorption and drying dynamics of the NPG substrate were studied by imaging and gravimetric measurements. Laser ablation properties of the substrate using 266 nm wavelength nanosecond laser pulses were characterized by scanning electron microscopy and contact profilometry. The nanoporous glass was found to be robust enough to allow repeated ablation with even dozens of laser shots, which proved to be a powerful tool to boost the sensitivity. Spectral interference from the glass material was found to be mild and allowed for multielement trace analysis. Quantitative trace analysis was demonstrated for Ag, Ba, Cu, and Sr elements. Calibration plots showed a well-defined, quadratic trend between 1 and 1000 ppm, with LOD values in the low- to sub-ppm range.