About infrared (IR) microscopy
Infrared or FT-IR microscopy is the exciting combination of conventional light microscopy and a unique chemical identification by FT-IR spectroscopy.
Individually, both techniques are already quite powerful, but together they offer the possibility to examine smallest objects chemically, combining spectral characterization with spatial resolution.
That being said, there are some technological hurdles, as the usual optical microscopy use glass lenses, will will not allow IR light to pass freely, which is needed to analyze samples by infrared spectroscopy.
Thus, special lenses using IR transparent materials or cassegrain objectives must be used.
About sampling in FT-IR microscopy
Typical examples of the application of µ-FT-IR are particles and smallest product damages, coatings on metal surfaces, single crystal studies and much more.
In general, the same methods can be used in IR microscopy as for macroscopic samples, i.e. transmission, reflection and ATR.
However, for measurements in transmission or transflection the samples must be very thin (<15 µm) or be available as KBr pellets, which can be quite a challenge during sample preparation.
As in spectroscopy, ATR offers decisive advantages in microscopy, which have made this non-destructive analysis method the standard.
About ATR in microscopy
ATR stands for attenuated total reflectance and is applied by pressing a crystal with a very fine tip on the sample. The Infared light is passing the crystal and interacts with the sample beneath it, yielding an IR spectra.
It should be noted, that ATR produces high quality
FT-IR data of almost any sample type without prior preparation. Furthermore, it also gives you the edge when it comes to spatial resolution.
The germanium crystal acts as a solid immersion lens, improving spatial resolution by factor 4, when compared to transmission and reflection measurements. This way, you easily analyze samples as small as a few microns.
Above we described the basics of how µ-IR is used as a "point-and-shoot" method and this is the common approach for simple applications or research studies. As you can imagine, the smaller certain particles are, the harder it is to obtain a nice infrared spectrum.
This is exactly, why high-sensitivity detectors are used for these kind of applications. Among those you there are so-called single-element and imaging detectors. As this page deals with microscopy, we will focus on single-element detectors, hence: DLaTGS, TE-MCT and LN-MCT.
You want more basic information on FT-IR spectroscopy?
Deuterated Lanthanum α Alanine doped TriGlycine Sulphate (DLaTGS) detectors exhibit the most effective pyroelectric effect known and are versatile detectors that don't need external cooling to produce high quality spectra. However, as soon as the aperture (and samples) gets smaller and less and less light reaches the detector the spectra's quality quickly diminishes.
Below 50 µm, it is best to chose a cooled mercury cadmium telluride (MCT) detector, that offers increased sensitivity on low-light scenarios. Using a thermoelectrically cooled MCT has become the standard solution as it is continuously cooled and doesnt require maintenance.
But still, for the smallest samples below 10 µm in size, liquid nitrogen cooled MCTs (LN-MCTs) are the best option but, of course need some time to cooldown and/or may need refilling with liquid nitrogen during prolonged use.What is still missing is the last but most powerful way to do FT-IR microscopy:
Focal-Plane-Array (FPA) imaging.
If you want to perform highly detailed chemical analyses in spatial resolution, there is no way around focal plane array (FPA) detectors.Compared to rather cheap solutions using line-array detectors, FPAs are characterized by the fact that you create an infrared image of the selected field of view by a single measurement in a few seconds (not unlike a digital camera).
In these so-called chemical or FT-IR images, each pixel holds of a complete infrared spectrum. By interpreting that FT-IR data, the nature of the sample can then be precisely evaluated!The advantage of using FPA detectors is only the extremely high resolution (especially for ATR measurements). Compared to line array experiments they are faster, more precise and laser calibrated.
For more information about FT-IR imaging we have created a separate page.
Whether we are talking about microplastics or technical cleanliness. Infrared microscopy is the method of choice to detect smallest particles not just visually but also by subsequent chemical identification.
There are basically two approaches. The first and most simple one, is to take your sample (e.g. a surface showing contamination) and directly subject it to µ-ATR analysis. This clean and quick method will even work for particles embedded in complex matrices like plastic contained in a river sediment. This is mainly applied in failure and root cause analysis.
When water or air samples are investigated, it is best to use special filter materials that consist of a material that will let the IR light freely pass, as standard materials (e.g. Nitrocellulose) will absorb a significant part of the IR beam. Such filters are then analyzed by transmission IR. This is especially used in particle analysis
1. What is FT-IR microscopy?
It is the application of an FT-IR measurement to a microscopic sample. Therefore, it combines traditional microscopy and chemical analysis into one tool. It is ideally used in failure analysis and material science.
2. Why does an FT-IR microscope need apertures?
As in IR microscopy very sensitive detectors are used, it is important to avoid saturating the IR detector. Additionally, Apertures allow to fit the measurement spot to the size of the sample to acquire a much better spectrum. Imagine a 10 µm polyethylene flake embedded inside a PET matrix. If in that case you would use a 30 µm aperture instead of a fitting 10 µm one, the resulting spectrum would contain much more contribution of the PET matrix, than of the PE contamination.
3. What's the smallest object FT-IR microscopy can analyze?
This depends on the microscope, detector and measurement technique used. But an HYPERION, equipped with a FPA detector and using ATR microscopy can analyze objects at the diffraction limit of IR light, thus ≤ 1 µm.
3. Why does an Germanium-ATR crystal increase the resolution?
Germanium has (compared to many other ATR materials) a very high refractive index. As it is in direct contact with the sample, this means it acts as a solid immersion lense. This increases spatial resolution by a factor of 4 (refractive index) compared to standard transmission measurements.
4. What is FT-IR imaging?
FT-IR imaging is one way to create said spatially resolved chemical images. Each pixel of these images consists of a whole IR spectrum. By interpreting the individual spectra, interesting sample regions can be detected and evaluated.