This document provides an overview of Attenuated Total Reflectance (ATR) Fourier Transform Infrared (FTIR) spectroscopy. It discusses how ATR addresses challenges with traditional transmission FTIR sampling methods, such as difficulties preparing homogeneous solid and liquid samples of a consistent thickness. The document describes how ATR uses an internal reflection element to produce an evanescent wave that interacts with the sample. It explains that ATR provides faster, more reproducible sampling compared to transmission by avoiding issues with sample thickness. Finally, it summarizes that ATR spectroscopy has revolutionized solid and liquid FTIR sampling through improved speed, reproducibility, and consistency between users.

1. F T- I R S P E C T R O S C O P Y
T E C H N I C A L
FT-IR Spectroscopy
Attenuated Total Reflectance (ATR)
N O T E
Sample in contact
with evanescent wave
To Detector
Infrared ATR
Beam Crystal
Introduction many cases some form of sample Key Features
Mid-Infrared (IR) spectroscopy preparation is required in order
Faster sampling
is an extremely reliable and to obtain a good quality spectrum.
Traditionally IR spectrometers Improving sample-to-sample
well recognized fingerprinting reproducibility
method. Many substances can have been used to analyze solids,
be characterized, identified and liquids and gases by means of Minimizing user-to-user spectral
transmitting the infrared radia- variation
also quantified.
tion directly through the sample. Higher quality spectral databases
One of the strengths of IR spec- Where the sample is in a liquid for more precise material verification
troscopy is its ability as an ana- and identification
or solid form the intensity of the
lytical technique to obtain spectra spectral features is determined by
from a very wide range of solids, the thickness of the sample and
liquids and gases. However, in typically this sample thickness
w w w. p e r k i n e l m e r. c o m

2. cannot be more than a few tens of thoroughly with about 350 mg of thickness or pathlength. A constant
microns. Figure 1 displays a typical ground KBr. The mixture is now pathlength is highly desirable when
IR transmission spectrum. transferred to a die that has a barrel performing quantitative analyses.
diameter of 13 mm (Figure 2). This is
The technique of Attenuated Total Overall, sample preparation is easier
then placed in a suitable press and
Reflectance (ATR) has in recent for liquid transmission studies
pressed (evacuation is optional) at
years revolutionized solid and when compared to solid transmis-
around 12,000 psi for one to two
liquid sample analyses because sion sampling but both suffer from
minutes. Re-crystallization of the
it combats the most challenging inevitable reproducibility issues
KBr results in a clear glassy disk
aspects of infrared analyses, given the complexity of the sample
about 1 mm thick. This disk is now
namely sample preparation and preparation methods. In addition,
ready to be analyzed by transmission.
spectral reproducibility. preparation can be very messy and
time consuming and is further com-
Issues surrounding plicated by difficulties in getting
traditional transmission sample to matrix ratios correct and
homogenous throughout the sample.
sample preparation
The materials involved are fragile and
The two most common forms of hydroscopic and the quality of meas-
sample preparation for solids both urements can be adversely affected if
Evacuable KBr Die
involve grinding the material to a handled or stored incorrectly. The
fine powder and dispersing it in a Figure 2. Potassium bromide 13 mm die.
technique of Attenuated Total
matrix. The ground material can be Reflectance addresses these issues.
dispersed in a liquid to form a mull.
The most commonly used liquid is
Principles of ATR
mineral oil (nujol). Typically no
more than 20 mg of solid is ground An attenuated total reflection
and then one or two drops of nujol accessory operates by measuring
are used to create a paste which is the changes that occur in a totally
then spread between two Mid-Infra- internally reflected infrared beam
red transparent windows e.g. NaCl, when the beam comes into contact
KBr, CaF2. The sample is now ready Figure 3. Sealed (l) and demountable (r) with a sample (indicated in Figure 4).
liquid cells. An infrared beam is directed onto
to be placed in the spectrometer for
analysis by transmission. an optically dense crystal with a
Liquids are traditionally analyzed
high refractive index at a certain
Potassium bromide (KBr) is prob- as thin films in cells (Figure 3), a cell
angle. This internal reflectance
ably the most widely used matrix consists of two IR transparent win-
creates an evanescent wave that
material. Between 1 and 3 mg of dows. A Teflon® spacer is generally
extends beyond the surface of the
ground material needs to be mixed used to produce a film of the desired
crystal into the sample held in con-
tact with the crystal. It can be easier
to think of this evanescent wave as
a bubble of infrared that sits on the
surface of the crystal. This evanes-
cent wave protrudes only a few mic-
rons (0.5 µ - 5 µ) beyond the crystal
surface and into the sample. Conse-
quently, there must be good contact
between the sample and the crystal
surface. In regions of the infrared
spectrum where the sample absorbs
energy, the evanescent wave will be
attenuated or altered. The attenuated
Figure 1. A Mid-Infrared spectrum of Hexene. energy from each evanescent wave
2

3. is passed back to the IR beam, which deformable materials, and also with Germanium has a much better work-
then exits the opposite end of the fine powders but many solids give ing pH range and can be used to
crystal and is passed to the detector very weak spectra because the con- analyze weak acids and alkalis.
in the IR spectrometer. The system tact is confined to small areas. The Germanium has by far the highest
then generates an infrared spectrum. effects of poor contact are greatest refractive index of all the ATR
at shorter wavelengths where the materials available which means
For the technique to be successful,
depth of penetration is lowest. that the effective depth of pene-
the following two requirements must
tration is approximately 1 micron.
be met: The issue of solid sample/crystal
For most samples this will result
contact has been overcome to a great
• The sample must be in direct in a weak spectrum being produced,
extent by the introduction of ATR
contact with the ATR crystal, however, this is an advantage when
accessories with very small crystals,
because the evanescent wave or analyzing highly absorbing materi-
typically about 2 mm across. The
bubble only extends beyond the als; carbon black filled rubbers are
most frequently used small crystal
crystal 0.5 µ - 5 µ. typically analyzed using Germanium
ATR material is diamond because it
• The refractive index of the crys- ATR accessories.
has the best durability and chemical
tal must be significantly greater inertness. These small area ATR crys- Diamond is by far the best ATR crys-
than that of the sample or else tal top-plates generally provide only a tal material because of its robustness
internal reflectance will not oc- single reflection but this is sufficient, and durability. The original purchase
cur – the light will be transmitted given the very low noise levels of cost is obviously higher than that of
rather than internally reflected in PerkinElmer’s modern FT-IR spec- other crystal materials available, but
the crystal. Typically, ATR crystals trometers. Much higher pressure with over the instrument’s lifetime re-
have refractive index values be- limited force can now be generated placement costs should be minimal.
tween 2.38 and 4.01 at 2000 cm-1. onto these small areas. A much The same cannot be said of Zinc
It is safe to assume that the majority smaller area of contact is now re- Selenide or Germanium, both of
of solids and liquids have much quired in comparison to the HATR which can scratch and break with
lower refractive indices. units. As a result, spectra can be improper use.
obtained from a wide variety of
ATR accessories As with all FT-IR measurements, an
solid materials including minerals.
infrared background is collected, in
The traditional ATR design in which
this case, from the clean ATR crystal.
a thin sample was clamped against Crystal materials The crystals are usually cleaned
the vertical face of the crystal has and cleaning by using a solvent soaked piece of
been now been replaced by a hori-
There are a number of crystal mate- tissue. Typically water, methanol or
zontal design. In horizontal ATR
rials available for ATR. Zinc Selenide isopropanol are used to clean ATR
(HATR) units, the crystal is a paral-
(ZnSe) and Germanium are by far crystals. The ATR crystal must be
lel-sided plate, typically about 5 cm
the most common used for HATR checked for contamination and carry
by 1 cm, with the upper surface ex-
sampling. Zinc Selenide is a rela- over before sample presentation,
posed (Figure 4). The number of
tively low cost ATR crystal material this is true for all liquids and solids.
reflections at each surface of the
and is ideal for analyzing liquids
crystal is usually between five and
and non-abrasive pastes
ten, depending on the length and
and gels but it is not
thickness of the crystal and the angle Sample in contact
particularly robust with
of incidence. with evanescent wave
a working pH range of
When measuring solids by ATR, it 5-9. ZnSe scratches quite
is essential to ensure good optical easily and so care must
To Detector
contact between the sample and the be taken when cleaning
crystal. The accessories have devices the crystal. It is recom- Infrared ATR
Beam Crystal
that clamp the sample to the crystal mended that lint free
surface and apply pressure. This tissue is used. Figure 4. A multiple reflection ATR system.
works well with elastomers and other
w w w. p e r k i n e l m e r. c o m 3

4. Analyzing liquids After the crystal area has been The strongest bands here extend
After the crystal has been cleaned cleaned and the background col- beyond 20% T with relatively little
and an infrared background has lected, the solid material is placed force applied; the force gauge is
been collected, the liquid is simply onto the small crystal area (Figure 6). registering 80 N. A lot more pres-
poured onto the crystal (Figure 5). Experience has shown that ideal sure would need to be applied when
The whole crystal must be covered if results from powder samples have analyzing high density polymers and
performing a quantitative or qualita- been achieved by placing just coatings on metal surfaces. Once the
tive analysis. The crystal is recessed enough sample to cover the crystal user is satisfied with the spectrum
into the metal plate to retain the area. The sample height should not shown in this ‘Preview Mode’, the
sample. Pastes and other semi-solid be more than a few millimeters. data is then collected in the normal
samples are readily measured by manner. The force should not be
Once the solid has been placed on the
spreading them on the crystal. Hori- adjusted when co-adding the final
crystal area, the pressure arm should
zontal ATR units are often used for spectrum. Unlike transmission
be positioned over the crystal/sample
quantitative work in preference to measurements, ATR sampling does
area. When using the Spectrum 100
transmission cells because they are not produce totally absorbing
Series’ Universal ATR accessory,
easier to clean and maintain. spectral bands because the effective
the pressure arm locks into a precise
path-length is controlled by the
position above the diamond crystal
crystal properties thereby minimiz-
(Figure 7). Force is applied to the
ing sample re-preparation time.
sample, pushing it onto the dia-
mond surface. After the spectrum has been col-
lected, which should typically take
no more than 32 seconds, the user
must return to the ‘Preview Mode’.
This mode is now used to check
that the crystal area is clean before
placing the next sample on the cry-
Figure 5. Using a pipette to add a liquid stal. A 100% T line with no spectral
sample to a ZnSe HATR trough plate.
features (Figure 9) should be seen if
the crystal is clean, if spectral fea-
Analyzing solids tures are seen, the crystal should
Solids are generally best analyzed be cleaned again using a solvent
Figure 7. Applying pressure to a solid sample
on the single reflection ATR acces- on the Universal diamond ATR top-plate. soaked tissue. The next sample can
sories; diamond being the preferred be placed on the crystal area once
choice for most applications because the 100% T line has been displayed
PerkinElmer’s revolutionary and the sampling steps are repeated.
of its robustness and durability.
Spectrum™ FT-IR software utilizes
a ‘Preview Mode’ which allows the
Conclusion
quality of the spectrum to be
monitored in real-time while fine ATR is an IR sampling technique that
tuning the exerted force. It is good provides excellent quality data in
practice to apply pressure until the conjunction with the best possible
strongest spectral bands have an reproducibility of any IR sampling
intensity which extends beyond technique. It has revolutionized IR
70% T, namely from a baseline at solid and liquid sampling through:
100% T down to 70% T. Good
• Faster sampling
sample/crystal interface contact has
been achieved once this rule has • Improving sample-to-sample
been satisfied. This is very easy to reproducibility
achieve with soft samples and fine • Minimizing user to user spectral
Figure 6. Placing a powder sample onto the
Universal diamond ATR top-plate. powders, this is shown in Figure 8. variation
4

5. Most importantly, the improved spec-
tral acquisition and reproducibility
associated with this technique leads
to better quality database building for
more precise material verification
and identification. ATR is clearly
an extremely robust and reliable
technique for quantitative studies
involving liquids.
Figure 8. ‘Preview Mode’ screenshot taken from PerkinElmer’s Spectrum
v6 FT-IR software.
Figure 9: ‘Preview Mode’ displaying a 100%T line, namely a spectrum
of a clean diamond ATR crystal.
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