Visualising Chemical Composition and Reaction Kinetics by the near Infrared Multispectral Imaging Technique

Abstract

A new multispectral imaging spectrometer which is capable of simultaneously recording spectral images in the visible and near infrared has been developed. In this instrument, an acousto-optic tunable filter (AOTF) is used to diffract an unpolarised incident light into two diffracted beams with orthogonal polarisation. One of the beams is detected by a silicon camera for the visible region while the other is detected in the near infrared (NIR) region (from 1000 to 1700nm) with an NIR camera. The imaging spectrometer is sensitive, inexpensive and field deployable because it is based on a camera using the recently available InGaAs focal plane arrays which are low-cost and can be sensitively operated at room temperature. The imaging spectrometer was used for measurements which previously were not possible. These include the kinetic determination of curing of an epoxy resin by amine, and the detection of reaction products from solid-phase peptide synthesis. Rates of reactions between epoxy and amine were found to be very inhomogeneous. Because of this kinetic inhomogeneity, differences in the degrees of cure at different positions within the sample can be as high as 37% when data from only a single pixel were used for calculation. The inhomogeneity was not observed if the average of a large number of pixels was used. The NIR imaging spectrometer was also used for the kinetic determination and identification of products formed during the solid phase peptide synthesis of glycine, alanine and valine mediated by aminomethylstyrene resin beads. Because this NIR imaging spectrometer can measure spectra at different positions within a sample, it was used for the first demonstration in which reactions of three different solid-phase peptide syntheses (in a three-compartment cell) were simultaneously monitored. Since relatively good spectra can be obtained by using data recorded by a single pixel and because the NIR camera has 240×320 pixels, this NIR multispectral imaging technique is not limited to three-compartment cell used in this study but rather can be used as the detection method for solid-phase peptide synthesis in combinatorial chemistry.