A review article related to principles of Reflectance IR and more specificaly focussed on ATR Techniques

1. ATR & MICRO ATR
2013
A review on the most advanced form of attenuated reflection technique with
emphasis on principle, instrumentation & working of both traditional &
advanced ATR techniques.
AL AMEEN COLLEGE OF PHARMACY
Bangalore

2. TABLE OF CONTENTS
Contents
Introduction___________________________________________________________________________________________1
Reflection______________________________________________________________________________________________1
Types-Instruments (Based on Reflectance IR) _____________________________________________________2
History_________________________________________________________________________________________________3
ATR – (Intro, Principle, Factors, Types) _____________________________________________________________3
IR Elelment ____________________________________________________________________________________________5
Types – ATR Instruments_____________________________________________________________________________6
Applications & Limitations – ATR ___________________________________________________________________8
Micro ATR _____________________________________________________________________________________________9
Applications – Micro ATR __________________________________________________________________________ 10
References___________________________________________________________________________________________ 10

3. AACP SURAJ C.
INTRODUCTION
• CONVENTIONAL IR:
 Infrared spectroscopy is study of the interaction of radiation with molecular vibrations
which can be used for a wide range of sample types either in bulk or in microscopic amounts
over a wide range of temperatures and physical states.
• IR with RELECTANCE THEORIES:
 Aside from the conventional IR spectroscopy of measuring light transmitted from the
sample, the reflection IR spectroscopy was developed using combination of IR
spectroscopy with reflection theories.
 In the reflection spectroscopy techniques, the absorption properties of a sample can be
extracted from the reflected light.
NOTE: Reflectance techniques may be used for samples that are difficult to analyze by the
conventional transmittance method.
• DIFFERENCE BETWEEN TRANSMISSION & REFLECTANCE:
1. Transmission:
 Excellent for solids, liquids and gases
 The reference method for quantitative analysis
 Sample preparation can be difficult
2. Reflection
 Collect light reflected from an interface i.e., air/sample, solid/sample,
liquid/sample
 Analyze liquids, solids, gels or coatings
 Minimal sample preparation
 Convenient for qualitative analysis
REFLECTION
• INTRODUCTION:
 Reflection is defined as the bouncing back of a ray of light into the same medium, when it
strikes a surface.
 It occurs on almost all surfaces - some reflect a major fraction of the incident light while
others reflect only a part of it and absorbs the rest.
 Reflection of light from surfaces is governed by the ‘2’ Laws of Reflection:
1. The incident ray, reflected ray and normal at the point of incidence lie on the same
plane.
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4. AACP SURAJ C.
2. The angle which the incident ray makes with the normal (angle of incidence) is equal
to the angle which the reflected ray makes with the normal (angle of reflection).
• TYPES OF REFLECTION PHENOMENA:
 Reflectance techniques may be used for samples that are difficult to analyze by the
conventional transmittance method.
 In all, reflectance techniques can be divided into two categories:
1. Internal Reflection
2. External Reflection
a. Specular (Regular)
b. Diffuse
 Internal refers to reflection from smooth, polished surfaces like mirror, and the latter
associated with the reflection from rough surfaces.
TYPES OF INSTRUMENTS – BASED ON REFLECTION TECHNIQUES
• Internal Reflection Spectroscopy:
 Attenuated Total Reflection (ATR)
• External Reflection Spectroscopy:
 Specular Reflection (smooth surfaces)
• Combination of Internal and External Reflection:
 Diffuse Reflection (DRIFTs) (rough surfaces)
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HISTORY
• Internal reflectance Spectroscopy (IRS) date back to the initial work of Jacques Fahrenfort &
N.J. Harrick.
• Internal reflection Spectroscopy is often termed as attenuated total reflection (ATR)
spectroscopy.
• ATR became a popular spectroscopic technique in the early 1960s.
ATR – Attenuated Total Reflectance
• INTRODUCTION:
 Attenuated total reflectance (ATR) techniques are well established in FT-IR spectroscopy
for the direct measurement of solid and liquid samples without sample preparation.
 The technique requires good contact between the sample and a crystal made from a
material which transmits IR radiation and has a high refractive index.
 When the IR beam enters the crystal at the critical angle, internal reflection occurs.
 At each reflection, IR radiation continues beyond the crystal surface and enters the
sample.
• PRINCIPLE:
 Attenuated total reflection spectroscopy utilizes total internal reflection phenomenon.
1. When a beam of radiation enters from a more dense medium (with a higher refractive
index, n1) into a less-dense medium (with a lower refractive index, n2), the fraction of
the incident beam reflected increases as the angle of incidence rises.
2. When the angle of incidence is greater than the critical angle θc (where is a function
of refractive index of two media), all incident radiations are completely reflected at the
interface, results in total internal reflection.
 In ATR spectroscopy a crystal with a high refractive index and excellent IR transmitting
properties is used as internal reflection element (IRE, ATR crystal) and is placed in close
contact with the sample.
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 The beam of radiation propagating in IRE undergoes total internal reflection at the
interface IRE- sample, provided the angle of incidence at the interface exceeds the critical
angle θc.
 Total internal reflection of the light at the interface between two media of different
refractive index creates an "evanescent wave" that penetrates into the medium of lower
refractive index.
 The “evanescent field” is a non-transverse wave along the optical surface, whose
intensity decreases with increasing distance into the medium, normal to its surface,
therefore, the field exists only at the vicinity of the surface.
 The exponential decay evanescent wave can be expressed by Eq. (1):
Iev = I0 exp (-Z/dp) …………….. (1)
 Where, z = the distance normal to the optical interface,
dp = the penetration depth (path length), and
I0 = the intensity at z = 0.
• REQUIREMENTS OF ATR:
 Infrared beam reflects from Na interface via total internal reflectance :
1. Sample must be in optical contact with the crystal
2. Collected information is from the surface
3. Solids and powders, diluted in a IR transparent matrix if needed
4. Information provided is from the bulk matrix
5. Sample must be reflective or on a reflective surface
6. Information provided is from the thin layers
• FACTORS AFFECTING ATR:
 Factors influencing ATR analysis
1. Wavelength of IR radiation λ
2. Refractive indexes of sample and IRE nsmp, nIRE
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3. Angle of incidence of IR radiation θ
4. Depth of penetration (pathlength) dP
5. Sample and IRE contact efficiency
 DEPTH OF PENETRATION:
- The depth of penetration, dp, is defined as the distance from the IRE- sample boundary
where the intensity of the evanescent wave decays to 1/e (37%) of its original value), is
given by Eq. (2):
dp = λ /{ 2π n1 [sin2 θ − (n2/n1)2]1/2 } ……………. (2)
- Where λ = the wavelength of the radiation, n1 is the refractive index of the IRE
n2 = refractive index of the sample, and θ is the angle of incidence.
- Factors Affecting Depth of Penetration:
 the refractive indices of the crystal and the sample,
 the angle of incidence of the beam, and
 the wavelength of the IR radiation.
Ex: For a germanium crystal, the penetration depth (for a sample of refractive index
1.4) between 3000 and 1000 cm-1 ranges between approximately 0.2 and 0.6 μm,
allowing good spectra to be collected from optically thick, non-reflecting samples.
 CRITICAL ANGLE:
- Depends on nIRE and nsmp
 increasing nIRE - decreasing θ and dP
 high values of nIRE needed
IRE ELEMENT
• The ATR crystals are made from materials that have –
 a very high refractive index and
 low solubility in water.
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1. Zinc Selenide (ZnSe)
- Preferred for all routine applications, limited use with strong acids and alkalis, surface
etched during prolonged exposure to extremes of pH, complexing agents (ammonia and
EDTA) will also erode its surface because of the formation of complexes with the zinc.
2. AMTIR
- As a glass from selenium, germanium and arsenic, insolubility in water, similar refractive
index to zinc selenide, can be used in measurements that involve strong acids.
3. Germanium Ge
- High refractive index, used when analyzing samples have a high refractive index.
4. Silicon Si
- Hard and brittle, chemically inert, affected only by strong oxidizers, well suited for
applications requiring temperature changes as it withstands thermal shocks better than
other ATR materials, hardest crystal material offered.
- Except for Diamond, which makes it well suited for abrasive samples that might otherwise
scratch softer crystal materials, below 1500 cm-1 usefulness limited.
5. Diamond
- For analysis of a wide range of samples, including acids, bases, and oxidizing agents, scratch
and abrasion resistant, expensive, intrinsic absorption from approximately 2300 to 1800
cm-1 limits its usefulness in this region (5% transmission).
TYPES OF ATR INSTRUMENTS
Single-Bounce ATR Multi Bounce ATR
Classification based on Instrumental Setup
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• It can also be classified based on shape of the crystal used, i.e.,
1. Traditional ATR
2. HATR (Horizontal ATR)
3. Cylindrical ATR
1. Traditional ATR
 In the traditional ATR, a thin sample is clamped against the vertical face of the crystal.
 This design has been replaced by more modern designs, horizontal and cylindrical designs.
2. HATR
 In horizontal ATR (HATR), the crystal is a parallel-side plate (typically 5 cm by 1 cm) with
the upper surface exposed.
 The number of reflections at each surface of the crystal depends on length and thickness
crystal as well as the angle of incidence (usually between five and ten).
NOTE: The traditional design is used for continuous surface such as sheets and horizontal ATR
(HATR) cells are suitable for liquids and pastes as well as soft powder and sheets films.
3. Cylindrical ATR
 Application of cylindrical ATR cell is limited to mobile fluids.
Cylindrical ATR
Traditional
HATR
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APPLICATIONS OF ATR
• ATR technique is used commonly in the near –infrared for obtaining absorption spectra of thin
films and opaque materials.
NOTE: However, ATR spectra can be obtained using dispersive IR instruments, but the higher–
quality spectra are obtained using FTIR spectrometers.
• ATR is one of the most versatile sampling techniques that requires little or no sample
preparation for most samples. It only requires that the sample is placed in intimate contact
with the IRE crystal, which achieved by pressing the solid onto the crystal with a high pressure
clamps.
• It is an ideal method for liquids and oils, because the contact between the crystal and a liquid
is inherently close and hence without requiring the high pressure clamp, liquids are applied
directly onto the crystal.
• As a conclusion,
1. ATR is a non-destructive technique for a variety of materials including soft solid
materials, liquids, powders, gels, pastes, surface layers, polymer films, samples solutions
after evaporation of the solvent.
2. It is an ideal technique for thick and dark colored materials which often absorb too
much energy to be measured by IR transmission.
• LIMITATIONS:
1. Lack of a good contact between the sample and IRE can lead to non-accurate results.
2. Also, there are a few IRE crystals to be compatible with the samples properties,
especially from pH point of view.
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MICRO – ATR
• Advanced form of ATR Technique, with advantages like:
 Least sample requirement
 Much quicker analysis
 Much automated (auto clamping or pressure check valves)
 Permanent alignment of all the components possible.
 Comparatively much more reproducible results.
• Instrument – Perkin Elmer Micro ATR:
 For the analysis of micro samples using ATR, a small crystal allowing a single reflection
is incorporated into the cassegrain objective of the PerkinElmer® microscopes.
 This forms the unique PerkinElmer multimode objective in which the crystal has two
on-axis positions, raised and lowered.
 When the crystal is raised, the sample can be viewed, brought into focus and centered in
the field of view.
Lower view
Side view
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 The crystal is then simply lowered to contact the sample, and a spectrum can be
collected.
NOTE: Importantly, when raised, the crystal does not obscure the optical path through the
cassegrain.
NOTE: This allows the other modes of data collection, transmission and external reflectance, as
well as viewing the sample, to be carried out without either removing the micro-ATR assembly
or switching to another objective.
NOTE: Permanent alignment of all components in the multimode objective is therefore
maintained ensuring reproducibility.
APPLICATIONS OF MICRO-ATR
1. Measuring thick samples in Combinatorial Chemistry
2. Measuring the FTIR spectrum of Non-reflecting surfaces.
3. Contamination studies
4. Determining the release kinetics from permeable membranes
5. Determining impurities in the chemical process.
6. Monitoring the rate & function of a chemical reaction.
7. In determining the genuinity of age old paintings.
REFERENCES
1. Applications and Design of a Micro-ATR Objective; Perkin-Elmer Final Report, 2004.
2. Mustafa Kansiz Blog; Spectroscopy: Sample Preparation - Free Micro ATR FT-IR
Chemical Imaging of Polymers and Polymer Laminates; 2012.
3. Reflectance FTIR; Perkin-Elmer Final Report, 2004.
4. Joseph L, et.al; Vibrational Spectroscopy; Organic Structural Spectroscopy; CRC
Press, 2010.
5. Zahra M.K.; Reflectance IR Spectroscopy; Payame Noor University, Department of
Chemistry; Intecho Open Source, 2012.
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