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

sample sample radiation radiation Transmission Mode Reflection Mode

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

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

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

Transmission Mode

Transmission Mode

Transmission Mode

Transmission Mode

Transmission Mode Light Path (shown by red line)

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

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).

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.

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

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.

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

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