Separating ionic and analyte spectral contributions for quantitative FTIR in complex aqueous systems.

Fourier transform infrared (FTIR) analysis of analytes in aqueous systems is fundamentally challenged by the strong and variable spectral influence of dissolved ions, which distort baselines and obscure analyte-specific features. A systematic FTIR-based analysis workflow is presented that explicitly models and removes ionic matrix contributions to enable robust quantification of glucose, lactate, and pyruvate in ion-rich aqueous solutions. The approach uses FTIR spectra of ion-rich water with region-specific baseline estimation anchored in an ion-sensitive spectral window, followed by interpolation of the predicted ionic baseline into the analyte-relevant region and subsequent baseline correction. Using controlled evaporation-based preconcentration experiments, ion preconcentration factors are shown to provide an internal reference for the overall preconcentration factors of the solutions. With this internal reference, baseline-corrected analyte spectra enable concentration determination based on infrared absorbance peaks. Comparable overall preconcentration factors are observed across ion-rich aqueous solutions of glucose, lactate, and pyruvate under identical evaporation conditions. By disentangling ionic and analyte spectral contributions, this framework establishes a pathway for quantitative FTIR analysis in complex, ion-rich aqueous environments and provides a foundation for extending infrared-based sensing to realistic biological and physiological systems.