Purpose and strength of the technique
- Analysis under ambient conditions
- Analysis of molecular structure and identification of organic compounds
- Characterization of solid materials, powders, films or liquids
- Identification of contaminations or residues
- chemical analysis of particles
- Chemical Sensitivity: Molecular Groups (limited inorganic information)
- Detection Limit: ~0,1 - 1wt %
- Information Depth: ~0,1 - 3 µm
- Quantification: by comparism to standards (if available)
- Lateral Resolution: ~20 - 30 µm (IR-Microscopy)
- Identification of contaminations on surfaces
- Analysis of fiber or particle contaminations
- Material identification of organic compounds
- Analysis of organic inclusions inside of coating layers
- Analysis of degradation or cross linking processes in polymers
Fourier transform infrared spectroscopy (FTIR) is a physical analysis method which works with infrared radiation (wave lengths between 800 nm to 1 mm). It allows the identification of molecular groups. Substances can be identified by comparison of spectra with databases.
Principle of Operation
Infrared spectroscopy is based on the excitation of energy states in molecules. From spectroscopic point of view differences are made between the
- Near Infrared (NIR, wave number: 12500 - 4000 cm-1 )
- Middle Infrared (MIR, wave number: 4000 - 400 cm-1)
- Far Infrared (FIR, wave number: 400 - 10 cm-1)
In molecular spectroscopy there is an absorption or scattering of the irradiating light. This is characteristic for specific molecule fragments (molecular groups). These fragments absorb specific portions of the incident light and are thereby excited to different molecular vibrations. The spectra recorded exhibit specific absorption bands which can be correlated with specific molecular groups. Different organic materials can therefore be identified by their absorption spectra.
1. Transmission IR Spectroscopy
When analyzing a sample in transmission mode, the sample has to be irradiated by infrared radiation. The radiation which is detectable after passing the sample shows the spectific absorbtion bands of the material. These signals give us Information about the molecular groups and the chemical composition of the sample. Depending on the sample form (gaseous, liquid, solid) there are special measurement techniques differing mainly in the nature of the sample preparation and the measurement cell / sample holder.
2. Attenuated Total Reflection (ATR)
ATR uses a property of total internal reflection resulting in an evanescent wave. An ATR-IR measurement is made by guiding the infrared light through an optical crystal (e.g. germanium, diamond). The light is totally reflected at the surface of this crystal. When this crystal is pressed onto a surface, the radiation can interact with the uppermost layers of the sample. The sample specific absorption of the infrared radiation can be detected. By comparison to databases the absorption bands can be be evaluated and the chemical composition of the surface can be figured out. The Information depth depends on the optical properties of the sample and ATR-crystal. It is typically between 0,1 and 3 µm. ATR-IR is therefore much less surface sensitive as XPS (~10 nm) or TOF-SIMS (~1 - 3 monolayers).
3. ATR-Microscopy/ IR-Microscopy
A special form of ATR-IR-Spectroscopy is used in IR-Microscopy. Here, an ATR-IR spectrometer is combined with an optical microscope to perform local ATR-IR measurements. The lateral resolution of these devices is typically in the range of 5 - 30 microns. Thus, individual particles or fibers may be examined.