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Outlook
Internal Reflection Spectroscopy Infrared beam reflects from a interface
via total internal reflectance.
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Edge over Traditional methods
Little or no sample preparation Very thin sampling path length Depth of penetration of the IR beam into
sample Sample analysis- faster (less than 1
minute for thick samples)
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Factors
Wavelength of IR radiation – λ Refractive indexes of sample and IRE*- n(s), n(IRE) Angle of incidence of IR radiation- θ Depth of penetration(pathlength)- d Sample and IRE contact efficiency
*Internal reflection element
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CRYSTAL MATERIALS
MATERIAL ATR SPECTRA RANGE
REFRACTIVE INDEX
DEPTH OF PENETRATION
USES
Germanium 5,500-675 4 0.66 Good for most samples. Strong absorbing samples, such as dark polymers
Silicone 8,900- 1,500 and 360-120
3.4 0.85 Resistant to basic solutions
AMTIR 11,000-725
2.5 1.77 Very resistant to acidic solutions
ZnSe 15,000-650
2.4 2.01 General use
Diamond 25,000-100
2.4 2.01 Good for most samples. Extremely caustic or harder samples.
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Disadvantages
The ATR crystal absorbs energy at lower energy levels
If the sample does not have good contact with the crystal, the data will not be accurate
Most ATR crystals have pH limitations
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Applications
Solid AnalysisLiquid
AnalysisPowder Analysis
excellent technique for measuring the composition of solids
analyze non-aqueous solutions
easier to run by ATR than by IR transmission, because little or no preparation is required
ideal technique for measuring dark colored materials which often absorb too much energy to be measured by IR transmission
Cleanup is easy and fast
pure samples and mixtures that are available in powdered from
films, fabrics, paper, hard polymer sheets, glass, rubber
lubricants, oils, paints, glues, solvents, inks and dyes,Gels and pastes can also be analyzed
pharmaceuticals and pigments
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End Notes
Versatile and non-destructive technique for variety of materials- soft solid materials, liquids, powders, gels, pastes, surface layers, etc.
Useful for surface characterization, opaque samples
Limitations: sensitivity is typically 3-4 orders less than transmission
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References
http://www.azom.com/article.aspx?ArticleID=5958
Perkin-Elmer: Technical Note SpectraTech: Technical Note