Resonant Mie scattering in infrared spectroscopy of biological materials – understanding the ‘dispersion artefact’

TLDR

Infrared spectroscopic cytology could be a powerful clinical tool, but extracting a pure absorption spectrum is hindered by confounding factors, notably the dispersion artefact that appears as a sharp absorbance decrease on the high‑wavenumber side of the amide I band. The study aims to fully understand this artefact so that strategies can be developed to deconvolve scattering and recover the true absorption spectrum, thereby overcoming a key barrier to clinical spectroscopic cytology. The authors employ synchrotron‑radiation FTIR micro‑spectroscopy to record spectra of monodispersed PMMA spheres of varying size, showing that the observed spectral distortions arise from resonant Mie scattering.

Abstract

Infrared spectroscopic cytology is potentially a powerful clinical tool. However, in order for it to be successful, practitioners must be able to extract reliably a pure absorption spectrum from a measured spectrum that often contains many confounding factors. The most intractable problem to date is the, so called, dispersion artefact which most prominently manifests itself as a sharp decrease in absorbance on the high wavenumber side of the amide I band in the measured spectrum, exhibiting a derivative-like line shape. In this paper we use synchrotron radiation FTIR micro-spectroscopy to record spectra of mono-dispersed poly(methyl methacrylate) (PMMA) spheres of systematically varying size and demonstrate that the spectral distortions in the data can be understood in terms of resonant Mie scattering. A full understanding of this effect will enable us to develop strategies for deconvolving the scattering contribution and recovering the pure absorption spectrum, thus removing one of the last technological barriers to the development of clinical spectroscopic cytology.