In the NIR, the bands are overtones -- which are broad -- and the spectra are often overlapping, requiring the use of complex statistical techniques to obtain analytical results. In the mid-IR, the bands are much sharper and better separated, and they are more strongly absorbing, giving lower detection limits and greatly simplifying the task of spectral interpretation. Analysis can frequently be performed by measuring a well defined peak height or peak area, even in mixtures. Raman is a light-scattering technique that is prone to fluorescence problems and surface-enhancement effects which make calibration difficult. The spectral peaks for Raman are derived from fundamental vibrations, so they are similar to IR spectra in their sharpness, but the selection rules are different. IR absorptions are allowed when there is a change in the dipole moment of a molecule and Raman spectra are allowed when there is a change in the polarizability. This means that the Raman and IR spectra of the same molecule are usually different. The Raman spectrum is dominated by non-polar C-C bonds in the backbone of the molecule and the IR spectrum is dominated by reactive functional groups such as C=O, C-OH, C-Cl, C=N.
With the ATR probe aqueous solutions are easy to do. Under good conditions we can measure concentrations of organic molecules in water in the tens of millimoles range.
The standard laboratory probes will withstand up to about 10 bars. The high pressure reactor configuration will work at over 200 bars (3000 psi) in media such as supercritical CO2.
Remspec's standard laboratory immersion probes can withstand temperatures up to 60 C without cooling. With the high-temperature cooling jacket or high temperature/high pressure probe head temperatures over 200 C can be used. For low temperatures down to below -100 C we have a low-temperature extension that allows both ATR and transmission measurements.
900 wavenumbers to 5000 wavenumbers, or 2 to 11 microns.
The fiber in our systems is an arsenic-selenium-tellurium glass which is clad in an arsenic-selenium-sulfur glass and coated with a polymer buffer for mechanical protection. Other glasses such as germanium-arsenic-selenide and germanium-antimony-selenide can also be used for mid-IR fibers but the composition we are using provides the best balance of cost and performance.
For full spectral coverage the cables should be about 2m long (1m out, 1m back). However for specific analyses where the bands occur in a high transmission window for the fiber, distances much greater can be contemplated.
A Remspec system can be coupled to most FTIR spectrometers. All that is required is a collimated beam output port and an external detector plug for the return of signal to the spectrometer. We have systems working and in active use on Bio-Rad, Bomem, Bruker, Midac, and Thermo Nicolet benches. The coupling is straightforward for Analect (ie Orbital, ie Hamilton Sundstrand) and most Perkin-Elmer systems