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