Complex-Valued Chemometrics in Spectroscopy: Inverse Least Squares Regression.

Journal: Applied spectroscopy
Published Date: Aug 6, 2025

Abstract

Inverse least squares (ILS) regression is an advancement of classical least squares (CLS) regression, enabling the calculation of concentrations without requiring prior knowledge of the number of components in a mixture. Complex-valued ILS further enhances the performance of ILS by incorporating the complex refractive index function, as demonstrated in the thermodynamically ideal mixtures of benzene-toluene and benzene-cyclohexane. In both systems, the mean absolute error can be reduced by over 50% using the leave-one-out cross-validation (LVOOCV) scheme with complex-valued ILS. Additional error reduction is achievable by leveraging correlations between the errors and the imaginary components of the concentrations or volume fractions. Since the complex refractive index function can be conveniently determined using conventional infrared spectroscopy through the Kramers-Kronig relations, we believe that complex-valued machine learning has the potential to significantly advance analytical applications.

Authors

  • Thomas G Mayerhöfer
    Leibniz Institute of Photonic Technology (IPHT), Jena, Germany.
  • Oleksii Ilchenko
    The Danish National Research Foundation and Villum Foundation's Center for Intelligent Drug Delivery and Sensing Using Microcontainers and Nanomechanics (IDUN), Department of Health Technology, Technical University of Denmark, Lyngby, 2800 Denmark.
  • Andrii Kutsyk
    Lightnovo ApS, Blokken, Birkerød, Denmark.
  • Jürgen Popp
    Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany. christoph.krafft@leibniz-ipht.de iwan.schie@leibniz-ipht.de and Institute of Physical Chemistry & Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany.

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