Compositional mapping of mixed gels using FTIR microspectroscopy

We have developed a technique to produce compositional maps of phase-separated protein/polysaccharide mixed gels using Fourier transform infrared (FTIR) microspectroscopy. The maps plot out the composition of either the protein, the polysaccharide or the water as a function of position in the sample and are presented in the form of two-dimensional contour plots. Our technique is completely general in nature, since it simply relies on there being some measurable spectral difference between the two biopolymers. However, in this paper we use our technique to study the particular case of aqueous gelatin/amylopectin gels. Semi-quantitative compositional maps were generated in the first instance by simply plotting the area of the infrared amide II absorption band from the gelatin. Fully quantitative compositional maps, in terms of actual weight percentage concentration of gelatin, amylopectin and water, were also produced by analysing a particular region of the spectra with the method of partial least-squares (PLS). We recently showed how PLS analysis can be used in conjunction with FTIR spectroscopy to plot the phase diagram of bulk phase-separated solutions which are held above the gel temperature of both components. Thus, our mapping technique allows the concentrations in a gel to be directly compared with those reached at equilibrium in the bulk phase-separated solution, using the same molecular probe, namely, infrared radiation. Copyright (C) 1996 Elsevier Science Ltd