Chemical imaging
As high-speed electronics and sophisticated computers became more commonplace, and infrared cameras became readily commercially available, laboratory chemical imaging systems were introduced.
The rapid acceptance of the technology in a variety of industries (pharmaceutical, polymers, semiconductors, security, forensics and agriculture) rests in the wealth of information characterizing both chemical composition and morphology.
Hyperspectral imaging is most often applied to either solid or gel samples, and has applications in chemistry, biology,[4][5][6][7][8][9] medicine,[10][11] pharmacy[12][13] (see also for example: food science, biotechnology,[14][15] agriculture and industry.
The release of active ingredient is controlled by the presence of this barrier, and imperfections in the coating, such as discontinuities, may result in altered performance.
Chemical imaging shares the fundamentals of vibrational spectroscopic techniques, but provides additional information by way of the simultaneous acquisition of spatially resolved spectra.
Interferometer-based chemical imaging requires that entire spectral ranges be collected, and therefore results in hyperspectral data.
Some words common in spectroscopy, optical microscopy and photography have been adapted or their scope modified for their use in chemical imaging.
As with their bulk spectroscopy counterparts, each imaging technique has particular strengths and weaknesses, and are best suited to fulfill different needs.
The MIR absorption bands tend to be relatively narrow and well-resolved; direct spectral interpretation is often possible by an experienced spectroscopist.
The absorptions in this spectral range are relatively strong; for this reason, sample presentation is important to limit the amount of material interacting with the incoming radiation in the MIR region.
Mid-infrared chemical imaging can also be performed with nanometer level spatial resolution using atomic force microscope based infrared spectroscopy (AFM-IR).
In these spectral regions the atmospheric gases (mainly water and CO2) present low absorption and allow infrared viewing over kilometer distances.
Absorption is one to two orders of magnitude smaller in the NIR compared to the MIR; this phenomenon eliminates the need for extensive sample preparation.
[23][24] NIR imaging instruments are typically based on a hyperspectral camera, a tunable filter or an FT-IR interferometer.
External light source is always needed, such as sun (outdoor scans, remote sensing) or a halogen lamp (laboratory, industrial measurements).
Both organic and inorganic materials possess a Raman spectrum; they generally produce sharp bands that are chemically specific.
Fluorescence is a competing phenomenon and, depending on the sample, can overwhelm the Raman signal, for both bulk spectroscopy and imaging implementations.
The conditions required for a particular measurement dictate the level of invasiveness of the technique, and samples that are sensitive to high power laser radiation may be damaged during analysis.
Fluorescence emission microspectroscopy and imaging are also commonly used to locate protein crystals[25] in solution, for the characterization of metamaterials and biotechnology devices.
Imaging a liquid or even a suspension has limited use as constant sample motion serves to average spatial information, unless ultra-fast recording techniques are employed as in fluorescence correlation microspectroscopy or FLIM observations where a single molecule may be monitored at extremely high (photon) detection speed.
Raman imaging may be able to resolve particles less than 1 micrometre in size, but the sample area that can be illuminated is severely limited.
FT-NIR chemical/hyperspectral imaging usually resolves only larger objects (>10 micrometres), and is better suited for large samples because illumination sources are readily available.
Because a bulk spectrum represents an average of the materials present, the spectral signatures of trace components are simply overwhelmed by dilution.
