Attenuated Total Reflectance

1. Submitted By :
Arisha Sharma
Dept. Of Analysis

2. INTRODUCTION
Attenuated Total Reflectance (ATR) accessories
remove the need for transmission cells and KBr
pellets when performing measurements on liquid,
semi-solid and solid materials. The ATR
spectroscopy works especially well for opaque
samples or samples that are too thick for
transmission measurements. The technique gives
information about the surface properties and
condition of materials and can be useful for both
qualitative and quantitative measurements.

3. BASIC PRINCIPLES
ATR spectroscopy uses the phenomenon of total
internal reflection. A beam of radiation passes
through a crystal (there are a variety of crystals
used for ATR measurements) and undergoes total
internal reflection through the crystal. An
evanescent wave is created which protrudes only
a few micros beyond the crystal surface. The
distance that the wave extends from the crystal
surface depends upon the material being used.
The evanescent wave which is formed decays
rapidly from the crystal surface, consequently, a
good contact between the crystal and the sample
must be maintained when performing ATR
measurements.

4. Hence, many ATR accessories employ a pressure
arm to do this. The radiation that penetrates a
fraction of a wavelength beyond the surface of the
crystal enters the sample that is placed on its
surface. Part of the radiation is absorbed by the
sample which will then produce an absorption
spectrum.
The depth of penetration for ATR is a function of
the wavelength, refractive index of the crystal
being used and the angle of incident of the beam.

7. TYPE OF MATERIALS USED FOR ATR
CRYSTAL
The crystal used in an ATR cell needs to have a
high refractive index. In general materials used for
a cell need to have a higher refractive index than
the sample being measured. The table below
shows typical physical properties of the materials
that are commonly used in ATR cells.

9. DIAMOND: Diamond has excellent chemical and
physical properties when used as ATR material. Its
hard scratch resistant properties make it suitable for
a wide range of applications. It can withstand highly
acidic and basic samples and does not react with
strongly oxidizing or complex agents. Diamond ATRs
can be used to analyze hard powder solid samples.
The only disadvantage with diamond is it’s relatively
high absorbance in the region of 2,500cm-1 to
1,650cm-1.
GERMANIUM: Due to it high refractive index,
germanium is excellent for highly absorbing samples
e.g. rubber materials containing carbon black. Due to
it low depth of penetration germanium ATR is useful
for analysis of a single layer in laminate samples.

10. KRS-5: Thallium-bromoiodide has the widest
spectral range of commonly used ATR crystals,
however, it is soft and can be easily damaged.
KRS-5 is insoluble in water but will form a surface
bloom which makes it unsuitable for use with
aqueous samples. Care must be taken when using
KRS-5 due to its high toxicity.
ZINC SELENIDE: This material has the
advantage of being resistant to many commercially
available which makes it easy to clean and ideal
for wet or aqueous samples. Zinc selenide has a
similar refractive index to that of diamond, which
make it a cost effective alternative for acids and
alkalis and is water resistant.

11. SILICON: Silicon has a high refractive index
which makes it applicable for high absorbing
samples. Silicon is also scratch and water
resistance. With its low depth of penetration,
silicon is ideal for the analyses of thin film
samples. However, the material is affected by
strong acids and alkali and also has a limited
spectral range.
AMTIR: This crystal is manufactured from
selenium, arsenic and germanium and is insoluble
in water. It has a similar refractive index to that of
zinc selenide and is mainly used in measurement
with strong acids.

12. TYPE OF ATR
• Horizontal ATR (H-ATR)
• Universal ATR (UATR)
• Diffuse Reflectance Infrared Transform
Spectrocopy (DRIFTS)

13. ADVANTAGES
• Faster sampling with no preparation.
• Excellent sample-to-sample reproducibility.
• Minimal operator-induced variations.
DISADVANTAGES
• The ATR crystals absorb energy at low energy
level.
• If the sample does not have good contact with
the crystals, the data will not be accurate.
• Most ATR crystals have pH limitations.

14. • Identification of drug substances and functional
group.
• Used in identification of paints and varnishes.
• Useful for substances that are difficult to handle
by conventional sampling technique.
• Used in forensic and biomedical field.