Accessories provide the interface between the sample and the spectrometer. They control the overall exposure of the sample to the light beam and ensure that a good signal is obtained from all the parts of interest.
On a practical level, accessories have the power to simplify workflows and give greater control over the experimental conditions, such as sample temperature or gaseous environment.
Types of Accessories
ATR-FTIR is nowadays the most common infrared spectroscopy method but you can also use transmission and reflectance methods, depending on the desired goal of the analysis.
What follows is a short overview of FTIR spectroscopy techniques. Click through to the relevant pages to read more.
ATR
A complex technique to explain, but ATR (attenuated total reflectance) usually offers the simplest and quickest measurement of a solid, powdered, or liquid sample. It takes advantage of a little-known optical phenomenon called evanescence to effectively ration the quantity of light reaching the sample. This makes it possible to analyse most non-gaseous samples in their natural or as-provided state without loss of signal due to over-absorbance of the light.
Transmission
Opposite to ATR, transmission is simple in concept – simply pass the light straight through the sample – but can present difficulties in practice. The sample is placed between two flat windows which have a thin spacer around their perimeter to hold them a fixed distance apart. The gap, or pathlength, needs to be small – usually on the order of 100 µm. It’s good for liquid samples and for solids too – as long as they are diluted in a non-absorbing matrix such as KBr.
In-situ analysis
It is possible to boost the capability of FTIR accessories with heaters for temperature control and/or sealed chambers for the introduction of gases or liquid reactants. This enables reactions to be studied by FTIR in-situ, under relevant reaction conditions.
Reflectance
Finally we have two external reflectance techniques. These reflect light directly off the external surfaces of the sample. We can design the optics to pick up either the direct, specular reflection, or collect the indirect diffuse reflection instead. These techniques are useful for studying chemistry on reflective surfaces or where there are gas species adsorbed on non-reflective surfaces, such as catalyst materials.
Microscopy
All the techniques of infrared spectroscopy barring gas analysis can be carried over into infrared microscopy. They work the same way but the accessories themselves are specifically adapted to the small physical working area of a microscope system.
Gas transmission
Analysis of gases requires much longer distances for the light to pass through the sample, up to several meters in many cases. For this we build multi-pass ‘White’ cells, which bounce the IR light several times between two mirrors to maximise the distance travelled through the gas.
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