This document discusses the technique of micro-attenuated total reflectance (micro-ATR) infrared spectroscopy. It begins by explaining conventional infrared spectroscopy and reflection techniques. It then covers topics like total internal reflection, attenuated total reflection (ATR), factors that influence the ATR process, types of internal reflection elements that can be used, and experimental setups for single-bounce and multi-bounce ATR. Micro-ATR is described as using a small crystal in a microscope to collect ATR spectra from microsamples without obscuring other modes of data collection. Applications of micro-ATR include depth profile studies, analyzing contamination spots, and determining the authenticity of ancient paintings.

1. MICRO-ATR
Suraj Choudhary
M.Pharm
2013
1

2. PPT. Package……..
Theme Questions
Reflection
Total Internal Reflection
Theory
ReflectionTechniques
Flashback
ATR
Requirements
Mechanismof ATR
Micro-ATR
Applications
2

3. Theme question
1. Conventional IR
2. Combination of IR with Reflectance Theories.
3

4. REFLECTION
4

5. REFLECTION
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. Others reflect only a part of it, while absorb the rest.
Reflection of light from surfaces is governed by the two Laws of Reflection:
1. The incident ray, reflected ray and normal at the point of incidence lie on
the same plane.
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).
5

6. TYPES OF REFLECTION
Reflectance techniques may be used for samples that are difficult to analyse
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.
6

7. TYPES OF REFLECTION
7

8. TOTAL INTERNAL
REFLECTION
8

9. Total internal reflection
θc
TOTAL INTERNAL REFLECTION
9

10. THEORY
10

11. Transmission:
Excellent for solids, liquids and gases
The reference method for quantitative
analysis
Sample preparation can be difficult
Reflection:
Collect light reflected from an interface
air/sample, solid/sample, liquid/sample
Analyze liquids, solids, gels or coatings
Minimal sample preparation
Convenient for qualitative analysis
THEORY
11

12. REFLECTION
TECHNIQUES
12

13. 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)
REFLECTION TECHNIQUES
13

14. FLASHBACK
14

15. FLASHBACK
Internal reflectance Spectroscopy (IRS) date back to the initial work of
Jacques Fahrenfort & N.J. Harrrick.
Internal reflection Spectroscopy is often termed as attenuated total reflection
(ATR) spectroscopy.
ATR became a popular spectroscopic technique in the early 1960s.
15

16. ATR
16

17. ATR
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.
17

18. ATR
The depth of penetration depends on
the refractive indices of the crystal and the sample,
the angle of incidence of the beam, and
the wavelength of the IR radiation.
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.
18

19. REQUIREMENTS
OF
ATR
19

20. REQUIREMENTS FOR ATR
Infrared beam reflects from a interface via total
internal reflectance :
Sample must be in optical contact with the crystal
Collected information is from the surface
Solids and powders, diluted in a IR transparent
matrix if needed
Information provided is from the bulk matrix
Sample must be reflective or on a reflective surface
Information provided is from the thin layers
20

21. MECHANISM
OF
ATR
21

22. MECHANISM OF ATR
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 (Figure 3).
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 22

23. MECHANISM OF ATR
Where:
IRE – internal reflection element = ATR crystal
23

24. MECHANISM OF ATR
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.
24

25. FACTORS AFFECTING
ATR
25

26. FACTORS AFFECTING ATR PROCESS
Factors influencing ATR analysis
 Wavelength of IR radiation λ
 Refractive indexes of sample and IRE nsmp, nIRE
 Angle of incidence of IR radiation θ
 Depth of penetration (pathlength) dP
 Sample and IRE contact efficiency
26

27. FACTORS AFFECTING ATR PROCESS
DEPTH OF PENETRATION
Depends on λ, nsmp, nIRE, θ
dP typically < 10 mm
The effective pathlength of the spectrum collected varies with the wavelength
of the radiation:
- longer λ -> greater dP: dP lower at higher wavenumbers
- ATR intensities decreased at higher wavenumbers
compared to transmission spectra dP typically < 10 mm
ATR correction accounts for this variation in effective pathlength by scaling
the ATR spectrum accordingly.
27

28. FACTORS AFFECTING ATR PROCESS
CRITICAL ANGLE
Depends on nIRE and nsmp
- increasing nIRE -> decreasing θ and dP
-> high values of nIRE needed
28

29. TYPES OF
IRE ELEMENT
29

30. TYPES OF IRE ELEMENTS
30

31. TYPES OF IRE ELEMENTS
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
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 31

32. TYPES OF IRE ELEMENTS
Germanium Ge
- high refractive index, used when analyzing samples have a high refractive
index
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 then 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
32

33. TYPES OF IRE ELEMENTS
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)
33

34. EXPERIMENTAL
SETUP
34

35. EXPERIMENTAL SETUP
 Broad sampling area
provides
- greater contact with the
sample
- use for weak absorbers or
dilute solutions
• Small sampling area - use
for strong absorbers -
solid samples, liquids
Single-Bounce ATR Multi-Bounce ATR
35

36. EXPERIMENTAL SETUP
Single-Bounce ATR
36

37. EXPERIMENTAL SETUP
Multi-Bounce ATR
37

38. ATR spectra
EXPERIMENTAL SETUP
38

39. ATR spectra
EXPERIMENTAL SETUP
39

40. ATR spectra
EXPERIMENTAL SETUP
40

41. SUMMARY
41

42. SUMMARY
Versatile and non-destructive technique for variety of materials – soft solid
materials, liquids, powders, gels, pastes, surface layers, polymer films, samples
after evaporation of a solvent …..
Requires minimal or no sample preparation
Useful for surface characterization, opaque samples Attenuated Total
Reflection (ATR)
Limitation: sensitivity is typically 3-4 orders of magnitude less than
transmission
42

43. Micro-ATR
43

44. Micro-ATR
44

45. Micro-ATR
45

46. Micro-ATR
For the analysis of microsamples 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.
The crystal is then simply lowered to contact the sample, and a spectrum can
be collected.
46

47. Micro-ATR
Importantly, when raised, the crystal does not obscure the optical path
through the cassegrain.
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.
Permanent alignment of all components in the multimode objective is
therefore maintained ensuring reproducibility.
47

48.  Depth profile studies
Micro-ATR
48

49.  Contamination spots on paper
Micro-ATR
49

50.  Ancient paintings determination
Micro-ATR
50

51. Applications
51

52. Measuring thick samples in Combinatorial Chemistry
Measuring the FTIR spectrum of Non-reflecting surfaces.
Contamination studies
Determining the release kinetics from permeable
membranes
Determining impurities in the chemical process.
Monitoring the rate & function of a chemical reaction.
In determining the genuinity of decade old paintings
Applications
52

53. References
53

54. Applications and Design of a Micro-ATR Objective; Perkin-Elmer Final
Report, 2004.
Mustafa Kansiz Blog; Spectroscopy: Sample Preparation - Free Micro ATR
FT-IR Chemical Imaging of Polymers and Polymer Laminates; 2012.
Reflectance FTIR; Perkin-Elmer Final Report, 2004.
Joseph L, et.al; Vibrational Spectroscopy; Organic Structural
Spectroscopy; CRC Press, 2010.
Zahra M.K.; Reflectance IR Spectroscopy; Payame Noor University,
Department of Chemistry; Intecho Open Source, 2012.
REFERENCES
54

55. Thank you
55