1. Infrared Sampling Methods
Transmission
detector
source
Internal Reflection
Attenuated Total Reflection (ATR)

2. sample
sample
radiation
radiation
Transmission Mode
Reflection Mode

3. Transmission Mode IR transparent Salt Plates
To obtain an IR spectrum in the transmission mode, the sample must be placed in a “container” or cell that is transparent in the IR region of the spectrum.
Sodium chloride or salt plates are a common means of placing the sample in the light beam of the instrument.
IR transparent Salt Plates

4. Dessicator Transmission Mode Water-free Environment for
Water-sensitive
Salt Plates.

5. Preparing a “Neat” IR liquid Sample
Transmission Mode
Preparing a “Neat” IR liquid Sample

6. Transmission Mode

7. Transmission Mode

8. Transmission Mode

9. Transmission Mode

10. Transmission Mode
Light Path
(shown by red line)

11. Preparing a KBr Disk for solid samples
Transmission Mode
Preparing a KBr Disk for solid samples 1 2 3 5 4

12. Reflectance mode Attenuated Total Reflectance (ATR)
sample
Infrared energy propagates through an internal reflection crystal (IRE), usually ZnSe, Ge and KRS-5 (mixture of thallium iodide and bromide) and recently we use diamond as the internal reflection element.
Although the infrared radiation is “completely” internally reflected, an evanescent wave extends beyond the surface of the crystal.
Any sample that is in intimate contact with the crystal will interact with the infrared energy, reducing the internal reflectance (attenuated).

13. FT-IR Sampling Traditional transmission experiment
Employs alkali halide (salts) matrix
True transmission spectrum is collected
Attenuated Total Reflection
Simple and faster to use.
Minimal sample preparation (simply place the sample in contact with the ATR element)
Often 1 step process
Easy to clean-up
Reflectance-type spectrum is collected.
solids/liquids/films.
Spectral features are usually weaker than those of transmission spectrum.

14. Traditional ATR Disadvantages (using ZnSe as IRE)
Chemically vulnerable
Clean only with a non-corrosive solvent
Physically susceptible
Soft nature of internal reflectance element (IRE) restricts the application of force
Surface can be scratched
Not suitable for:
Corrosive liquids
Harder powders

15. Diamond ATR Advantages
Diamond ATR overcomes the limitations of traditional ATR
Small ATR element size allows intimate contact with samples and greatly reduces sample size requirements.
Reproducible results are achievable because, by filling the ATR element every time, path length and sample area are constant.
Mechanical strength of the diamond ATR element allows for compression of the sample for intimate contact.
Resistant to all corrosive and abrasive solvents and samples.
Cleans up easily because of the low friction coefficient of diamond.
Control sensitivity.

16. Diamond ATR Disadvantages
Diamond interferes (absorbs IR) between 2200 cm-1 to 1900 cm-1
Not major; however, some cyano group information could be lost
Spectrum usually weaker than traditional transmittance experiments
due to the small penetration depth of the evanescent wave into the absorbing medium, typically around 10 µm in the infrared

17. Infrared Spectra (KCl matrix versus ATR)
Transmittance / Reflectance
KBr matrix
ATR
4000
3000
2000
1200
1600
800
400
Wavenumber (cm-1)