ATR Theory
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Transcript of ATR Theory
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ATTENUATED
TOTALREFLECTION
SPECTROSCOPY
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Outline
Introduction to ATR technology
Evanescent Waves
Crystal Composition
Data Collection
Advantages & Disadvantages
Sample Analysis
Challenges
Future Applications
Conclusion
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Attenuated total reflection
(ATR) spectroscopy is aversatile and powerful
technique for infraredsampling
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How an ATR accessory works
Internal reflection spectroscopy passes infrared radiationthrough an infrared-transmitting crystal of high refractiveindex, allowing the radiation to reflect in the crystal oneor more times
An attenuated total reflection accessory measures thetotally reflected infrared beam when the beam comes in
contact with a sampleIn this way, an evanescent wave penetrates into thesample in contact with the crystal, producing a spectrumof the sample
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Evanescent Waves
The infrared radiation interacts with the samplethrough a series of standing waves, called evanescentwaves
An evanescent wave is a penetrating electromagnetic
field whose intensity quickly decays as it moves awayfrom its source
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ATR Data Collection
When collecting ATR data, samples are run as %
Reflectance or log (1/R)
% Reflectance is equivalent to % Transmittance
Log (1/R) is equivalent to Absorbance
In summary, % Reflectance is similar to Absorbance
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DisadvantagesATR vs. Transmission
The ATR crystal absorbs energy at lower energy levels
If the sample does not have good contact with thecrystal, the data will not be accurate
Most ATR crystals have pH limitations
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Crystal Composition
Crystals are typically made of:
Zinc Selenide (ZnSe)Germanium (Ge)
Zinc Sulfide (ZnS)
Silicon (Si)
Diamond
AMTIR: Germanium, Arsenic, Selenium (GeAsSe)
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AdvantagesATR vs. Transmission
When an ATR accessory is used, most samples can berun neat, which means in their natural state
ATR sampling is fast and easy because little or nosample preparation is required
Other techniques, such as infrared transmission, oftenrequire the sample to be heated, pressed or ground inorder to collect the spectrum
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Solid analysis
ATR is an excellent technique for measuring thecomposition of solids
Some examples of solids are films, fabrics, paper,hard polymer sheets, glass, rubber
ATR is an ideal technique for measuring dark colored
materials which often absorb too much energy to bemeasured by IR transmission
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Liquid Analysis
ATR is an ideal technique for analyzing liquids
Sample preparation is minimal
Cleanup is easy and fast
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Liquid Analysis
ATR can be used to analyze non-aqueous solutionssuch a lubricants, oils, paints, glues, solvents, inks anddyes
Gels and pastes can also be analyzed
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Powder Analysis
Powders are easier to run by ATR than by IR
transmission, because little or no preparation is required
This category includes pure samples and mixtures thatare available in powdered from
Some examples of pure samples and mixtures are
pharmaceuticals and pigments
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Challenges
Alignment of the beamsplitter
Technique will change with sample type
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Future Applications
Forensic Investigation
Biomedical applications
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Attenuated Total Reflectance is an easy-to-use, fast,
and versatile technique for infrared sampling
Solids, pastes, gels, liquids and powders can beanalyzed with little or no preparation
Conclusion
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References
Thermo Electron Corporation
http://www.brukeroptics.com/applications/microscopy.html