Development of Dried Droplet Method for Trace Elements Analysis Using Laser-Induced Breakdown Spectroscopy


Student thesis: Doctoral Thesis

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Award date31 Aug 2021


Laser-induced breakdown spectroscopy (LIBS) is a versatile spectroscopic technique for elemental determination, whose basic principle is to record and analyze the optical spectrum of the plasma ablated by pulsed laser on sample surface. LIBS has many advantages such as small sample requirements, high sensitivity to low atomic number elements, and low-cost instrumentation. Because of those properties, LIBS has been widely used in many industries, including mining, plastics, biomedicine, food, and environment. In the past decades, most of LIBS works are conducted by ablating untreated sample or pelletized sample (convert solid to pellet). However, those no pretreatment and pellet-press pretreatment still have limitations such as matrix interference, which could result in inhomogeneous analyte distribution and relatively large sample requirements (1-2g). Herein, we develop the dried droplet method (DDM) as a novel sample pretreatment for trace elements analysis by LIBS. This method involves placing a droplet of solution onto substrate and drying for LIBS analysis. In this thesis work, the DDM was introduced in detail in chapter 1, assisted with the related knowledges to LIBS.

First of all, the DDM was applied for quantitative trace elemental analysis by LIBS. The pellet-press method was also used for comparison to verify the performance (accuracy and sensitivity) of the DDM in elemental quantification. Secondly, the DDM was combined with the ultrasound-assisted extraction (UAE) method to analyze the hazardous (available) heavy metals Pb and Cd in cosmetics. The UAE was applied to treat sample with complex matrix for improving the sensitivity of the DDM for heavy metals.

However, the DDM still has some technical limitations. For example, during droplet drying, the solutes would converge at the droplet edge and form a circular crater on the substrate surface. This phenomenon is called the coffee ring effect (CRE), which results in solutes enrichment at the edge, lowering stability and accuracy performance of the DDM. To suppress the CRE, two different methods were proposed to suppress the CRE by modifying the substrate, respectively. For the method one, electrochemical anodized nanoporous tin dioxide (SnO2) substrate was applied to suppress the CRE. For the method two, a superhydrophobic (SHB) substrate was synthesized for CRE suppression. Those two methods were demonstrated to suppress the CRE and improve the stability of LIBS analysis. Especially for the SHB substrate, it could enrich the analytes inside the droplet to a small area for laser ablation. The mechanism of CRE suppression by those methods were explained in the related chapter. Finally, the works in the formed chapter were summarized. All in all, the DDM is a facile and versatile sample pretreatment for stable and ultra-sensitive elemental analysis by LIBS.