Micro ATR

MICRO-ATRSuraj ChoudharyM.Pharm2013

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PPT. Package……..

Theme Questions

Reflection

Total Internal Reflection

Theory

Reflection Techniques

Flashback

ATR

Requirements

Mechanism of ATR

Micro-ATR

Applications2

Theme question

1. Conventional IR

2. Combination of IR with Reflectance Theories.

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REFLECTION

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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).

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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.

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TYPES OF REFLECTION

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TOTAL INTERNAL REFLECTION

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Total internal reflection

θc

TOTAL INTERNAL REFLECTION

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THEORY

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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

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REFLECTION TECHNIQUES

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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

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FLASHBACK

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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.

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ATR

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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

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

REQUIREMENTSOF ATR

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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 layers20

MECHANISMOF ATR

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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

MECHANISM OF ATR

Where:

IRE – internal reflection element = ATR crystal

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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

FACTORS AFFECTINGATR

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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

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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

FACTORS AFFECTING ATR PROCESS

CRITICAL ANGLE

Depends on nIRE and nsmp

- increasing nIRE -> decreasing θ and dP

-> high values of nIRE needed

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TYPES OF IRE ELEMENT

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TYPES OF IRE ELEMENTS

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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

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 usefulness32

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)

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EXPERIMENTALSETUP

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EXPERIMENTAL SETUP

Broad sampling area provides

- greater contact with the sample

- use for weak absorbers or dilute solutions

• Small sampling area - usefor strong absorbers -solid samples, liquids

Single-Bounce ATR Multi-Bounce ATR

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EXPERIMENTAL SETUP

Single-Bounce ATR

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EXPERIMENTAL SETUP

Multi-Bounce ATR

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ATR spectra

EXPERIMENTAL SETUP

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ATR spectra

EXPERIMENTAL SETUP

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ATR spectra

EXPERIMENTAL SETUP

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SUMMARY

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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

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Micro-ATR

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Micro-ATR

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Micro-ATR

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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

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.

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Depth profile studies

Micro-ATR

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Contamination spots on paper

Micro-ATR

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Ancient paintings determination

Micro-ATR

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Applications

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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

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References

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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

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Thank you

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