Chemical mapping using two-dimensional Hadamard transform Raman spectrometry

A Raman chemical mapping investigation was conducted in the visible spectral region using an automated Spex 14018 double monochromator in concert with a moving two dimensional (2D) Hadamard encoding mask. The 2D Hadamard encoding mask combined with conventional Raman spectrometry was used to create chemical maps of heterogeneous liquid and solid samples. The 2D Hadamard encoding mask is based on the cyclic ${\mathrm{S}}_{\mathrm{255}}$ matrix as a $\text{15×17}$ array. It is mounted vertically and is moved from one encodement pattern to the next by an automated translation stage in concert with scans by the Spex monochromator. The moving 2D mechanical mask was used to spatially encode incident laser radiation or the Raman scattered radiation collected from a sample area illuminated by an argon ion laser operated at 514.5nm. The encoded radiation was decoded by a fast Hadamard transform that resulted in the assignment of a conventional Raman spectrum for each of the 255 “pixels” comprising the mask area. The Raman scattering intensity, plotted with respect to pixel coordinates, resulted in a chemical map of the illuminated sample area. The data demonstrates the utility of the moving 2D Hadamard encoding mask for Raman chemical mapping.