Near infrared spectroscopy: the practical chemical imaging solution

Chemical imaging spectroscopy is an exciting new analytical advance that answers commonly asked questions such as what chemical species are in a sample, how much of each is present, and most importantly, where are they located? Through the fusion of traditional infrared spectroscopy with powerful microscopic and macroscopic imaging capabilities, chemical imaging spectroscopy answers all these questions simultaneously, in a single rapid measurement.1 Chemical imaging enables the researcher to obtain spatial and spectral information characterising samples with unprecedented ease, speed and spatial and spectral resolution. The methodology is aimed at providing a comprehensive analysis of complex heterogeneous samples. Chemical imaging has advanced significantly with the commercialisation of infrared focal plane arrays (FPAs), which are cameras composed of many thousands of individual infrared detector elements. When coupled with infrared optics and a means of wavelength selection, these instruments provide an image where varying contrast is derived from the unique infrared chemical signature characteristic of each component within the sample. Powerful statistical and chemometric tools can enhance these chemical maps by performing valuable data reduction steps to identify and extract the most analytically useful information. This can be invaluable to winnow the large volumes of data collected by this technique. Infrared and near infrared (NIR) imaging, in particular, has the experimental flexibility to characterise a wide variety of samples chemically using transmission and reflectance measurements, while its ability to tackle samples ranging in size from microns to kilometers using microscopes or satellite-based remote sensing systems, makes it truly unique. The large format infrared cameras currently available, particularly in the NIR, provide high image fidelity and coupling these arrays to precise infrared wavelength selection devices with narrow bandpass capabilities including Fourier transform spectrometers1,2 and liquid crystal tunable fi lters3 make the technique fully hyperspectral. In fact, most commercially available spectral imaging systems have spectral resolutions comparable to those of conventional infrared and NIR spectrometer systems. This type of instrumentation has already started to supplant time-consuming mapping techniques, particularly in the infrared spectral region, in which data is recorded one spectrum at a time and the image is constructed by moving the sample in an x,y pattern under the spectrometer optics. It is likely that infrared array-based imaging approaches will completely replace single-point mapping techniques in the future. Although we have applied this general hyperspectral imaging approach to tackle a number of biological and industrial problems in the past, and have developed a variety of instruments and methods for mid-infrared, NIR and Raman imaging we will focus in this paper on industrial chemical imaging problems using NIR diffuse reflectance only. We believe that this spectral interval strikes a perfect balance between sensitivity, flexibility, simplicity and ruggedness that makes it an ideal industrial imaging tool. In addition, we will illustrate through several examples the unique features that chemical sensing with array detectors affords. With the application of quantitative data analysis in these examples, we will show it’s not just about pretty pictures. Foundations of chemical imaging