ASTM E1252 recommends techniques for performing Qualitative analysis by Infra-Red (IR) spectroscopy while ASTM E168 deals with quantitative aspects including computer-based and manual calculations. Molecules that absorb infra-red radiation can be detected by their characteristic IR spectra.
Infra-red spectroscopy uses the fact that molecules in certain solids, liquids, and gases absorb IR radiation at characteristic frequencies. Molecules in a sample can be identified by subjecting test samples to varying frequencies and correlating absorption peaks with a standard database of IR absorption spectra. Converting raw time-domain absorption data into frequency domain spectra is performed using a mathematical technique called Fourier Transformation. Hence IR spectroscopy is popularly known by the acronym FTIR. The four common spectroscopic techniques that exist for making the IR beam interact with the sample are:
Attenuated Total reflection method (ATR)
Diffuse reflection method (DRIFTS)
Specular reflection method.
ASTM E1252 is applicable to all the above methods in the IR frequency range from 4000 cm-1 to 50 cm-1. The recommendations of ASTM E1252 are also useful in the near infra-red region above 4000 cm-1. The preparation, operation, and calculation techniques suggested for quantitative IR analysis in ASTM E168 are also generally applicable.
FTIR can be used for the identification of unknown molecular species and functional groups for which standard IR spectra are available in spectral databases. It can also detect contamination of solid, liquid, and gas samples, hence it is used in quality control. Since it provides molecular identification rather than atomic identification it is very useful in the analysis of polymers and other complex molecules.
Transmission spectroscopy involves placing the test sample is placed in the path of an incident IR beam with frequencies being varied so that resonant absorption by molecules results in percentage transmission being detected in the FTIR detector.
In attenuated total reflection, the sample is placed on the surface of a special crystal. The IR beam then enters the crystal and interacts with a sample that is on the surface of the crystal then exits the crystal reaching the FTIR detector.
In diffuse reflectance, the IR beam interacts with the sample placed in a loosely packed special powder and then passes on to the FTIR detector.
In specular reflection, the IR beam passes through a test sample that may be reflective or is on the surface of a reflective surface before reaching the FTIR detector.
These techniques have their individual merits and demerits and are applicable for specific sample types.
While specimen size depends on the field of view and is instrument-dependent, each technique is suitable for specific types of samples as described below.
The transmission method can handle samples in the following forms:
Organic powders in pellet or mull form
Thin polymeric films
Regular and irregular-shaped polymers (with preparation)
Liquids and gases (free-flowing or viscous)
ATR is ideal for relatively thick solid samples including the following:
Solids that can be ground into powder
DRIFTS is ideal for organic as well as inorganic samples that can be powdered to less than 10-micron size for mixing into an inert powder matrix. Typical sample types include:
Soft powders and powder mixtures
Specular reflectance is applied to the analysis of organic and inorganic samples possessing flat, reflective surfaces. Specimens for this technique can be as follows:
Thin films on reflective substrates
Laminated materials on metal
The FTIR result is typically presented as a spectrum of frequencies on the abscissa and percentage transmission on the ordinate. A standard database of spectra is needed for species identification.