Fourier Transform Infrared (FTIR) spectroscopy is a common technique employed to identify a material and relate its properties the molecular structure. Measurements are primarily conducted in the 2-15 micrometer region of the electromagnetic spectrum. Absorption of radiation in this region results in the excitation of vibrational, rotational and bending modes of constituent functional groups, while the molecule itself remains in its electronic ground state. The combination of a microscope with an infrared spectrometer results in a powerful instrument that allows for the elucidation of chemical structure from a specific spatial region. Hence, microspectroscopy (microscopy + spectroscopy) is a powerful technique to measure both the chemical and the morphologic structure of materials.

Combining spatial specificity with information on its chemical constitution, a chemical species map may be constructed for the whole spatial area. This typically involved collecting the IR spectrum of the sample at a point, moving the sample to another location and collecting the spectrum at the second point and so on. In this manner, the whole area is mapped point-by-point and the technology was termed “point mapping”. In recent years, the use of CCD-type detectors has allowed the measurement of large sample areas simultaneously, providing a large multichannel detection advantage. This technology is termed “FTIR spectroscopic imaging”. In the time required to measure a spectrum by mapping, thousands of spectra may be acquired simultaneously by imaging.

The Concept of FTIR Imaging

The state of the art in FTIR microspectroscopy instrumentation today is the combination of a Focal Plane Array (FPA) detector with a conventional interferometer. This configuration allows for the imaging of a wide field of view in a single collection. The concept of imaging using an FPA is illustrated below. The beam of the interferometer is diverted through standard microscope optics with the FPA at the end of the optical train.

The premise behind the imaging experiment is that each pixel on the FPA corresponds to a unique spatial region on the sample. The time required to collect a spectrum for a single pixel is the time required to collect the whole image. Multiple channels of information are simultaneously collected (the multi-channel advantage).

Imaging Vs. Mapping

As opposed to image build-up by point-by-point FTIR mapping, FTIR imaging is used to describe the collection of a chemical profile of the sample area in a single experiment. The major instrumental difference between a mapping and an imaging instrument is the incorporation of a Focal Plane Array detector at the end of the optical train in the microscope. Average chemical information (i.e. the infrared spectrum) of a specific spatial area on the sample can be uniquely correlated to the output of a pixel in the FPA. Hence, no apertures are required to limit the sample area examined. Moreover, the sample does not need to be moved as a given field of view is imaged in a single collection experiment. It may be immediately seen that the collection time is decreased by a factor of n2, where n is the number of spatial resolution elements in one direction of a square sample area imaged (n is the number of steps in a mapping experiment considered equivalent to a pixel in the imaging experiment). Moreover, the practical spatial resolution limit in the mapping technique is close to 10 microns. For imaging, the resolution can be wavelength limited. Hence, the imaging technique has allowed for the collection of images in faster time with higher resolution. Some phenomena have also been examined in real time using FTIR imaging.