Quantification\nof Lanthanides on a PMMA Microfluidic\nDevice with Three Optical Pathlengths Using PCR of UV–Visible,\nNIR, and Raman Spectroscopy

Microfluidic devices (MFDs) offer customizable, low-cost,\nand low-waste\nplatforms for performing chemical analyses. Optical spectroscopy techniques\nprovide nondestructive monitoring of small sample volumes within microfluidic\nchannels. Optical spectroscopy can probe speciation, oxidation state,\nand concentration of analytes as well as detect counterions and provide\ninformation about matrix composition. Here, ultraviolet–visible\n(UV–vis) absorbance, near-infrared (NIR) absorbance, and Raman\nspectroscopy are utilized on a custom poly(methyl methacrylate) (PMMA)\nMFD for the detection of three lanthanide nitrates in solution. Absorbance\nspectroscopies are conducted across three pathlengths using three\nportions of a contiguous channel within the MFD. Univariate and chemometric\nmultivariate modeling, specifically Beer’s law regression and\nprincipal component regression (PCR), respectively, are utilized to\nquantify the three lanthanides and the nitrate counterion. Models\nare composed of spectra from one or multiple pathlengths. Models are\nalso constructed from multiblock spectra composed of UV–vis,\nNIR, and Raman spectra at one or multiple pathlengths. Root-mean-square\nerrors (RMSE), limit of detection (LOD), and residual predictive deviation\n(RPD) values are compared for univariate, multivariate, multi-pathlength,\nand multiblock models. Univariate modeling produces acceptable results\nfor analytes with a simple signal, such as samarium cations, producing\nan LOD of 5.49 mM. Multivariate and multiblock models produce enhanced\nquantification for analytes that experience spectral overlap and interfering\nnonanalyte signals, such as holmium, which had an LOD reduction from\n7.21 mM for the univariate model down to 3.96 mM for the multiblock\nmodel. Multi-pathlength models are developed that maintain model errors\nin line with single-pathlength models. Multi-pathlength models have\nRPDs from 9.18 to 46.4, while incorporating absorbance spectra collected\nat optical paths of up to 10-fold difference in length.