Basics of ATR FTIR


There are various FTIR methods that can be used to identify foreign material or contaminants. FTIR microscopy is used for foreign material where the size is in the order of few microns, but when a sample with foreign material can be observed by the naked eye, the particle type is typically greater than several hundred microns and does not require an FTIR microscope.  An ATR FTIR method may be a suitable alternative and offers advantages such as minimal sample preparation, non-destructive measurement, and easy handling.

This is a reflection measurement method based on the penetration of infrared light into a sample, where the light is totally internally reflected by a prism. Measurements can be performed simply by placing the sample in close contact with the prism. When measuring acidic and alkaline solutions, use a diamond prism that will not corrode.

Types of ATR-FTIR Crystals

ATR method is used to analyze the sample surface within 1~2 μm in depth. The measurement is collected when the ATR prism is in contact with the sample. In the ATR method, the penetration depth of light into the sample for total reflection depends on the RI of prism and sample, incidence angle, and wavelength. In addition, measurement range and durability depend on the material of the prism. Features of each prism are shown as follows.

PrismRI (n1)Penetration Depth dp*1
RI of Sample (n2)*2
for Total Reflection
Measurement Limit
in Low Wavenumber*3
ZnSe2.4~ 2.0 μm<= 1.7~550 cm-1
Diamond2.4~ 2.0 μm<= 1.7~400 cm-1 *4
Ge4.0~ 2.0 μm<= 2.8~650 cm-1
PrismFeature of
Suitable SampleNon-Suitable SampleNotes
General organic
Hard powder, acidalkaline,
high RI
In case of hard
powder or hubbly
sample, diamond is
Hard powder, General
organic substance
High RI samplePoor S/N ratio in the
region around 2000
cm-1, due to internal
GeAnalysis for
high RI sample
Sample including carbonHard powder, acidalkalineWeak absorption
due to small depth of