In FT-IR/ATR the spectrum depends upon the depth of penetration dp which, in turn, depends upon several factors.
Mathematically, this is given by
where λ is the wavelength of the light, θ is the angle at which the light strikes the surface of the crystal, and nS and nC are the index of refractions of the sample and crystal, respectively.
As per the formula the most critical factor is the Index of Refraction.
In the Table below you can find the main properties for ZnSe, Diamond and Germanium that are the most common crystal associate with ATR techniques
|ZnSe||20,000-500||2.43||2.00||120||5 - 9|
|Diamond||45,000-10||2.40||1.80||8000||1 - 14|
Practically speaking, the selection of the crystal material provides a valuable means of controlling the sampling depth and ensuring we meet critical angle requirements for ATR analysis.
At 45 degrees angle of incidence and a sample refractive index of 1.5, the depth of penetration of the IR beam into the sample varies from 0.66 microns for the germanium (Ge) ATR crystal to 2.0 microns for the diamond and ZnSe crystals as measured at 1000 cm-1.
These differences are useful to optimize the results for thin coatings or for analysis of samples with high refractive index such as a carbon filled rubber sample.
The spectra in figure below are similar relative to the major absorbance bands but a close inspection of the data reveals derivative shaped absorbance bands in the spectrum collected using the ZnSe crystal due to critical angle requirements. This problem is not found in the spectrum produced using the higher refractive index Ge ATR crystal.
Note: the baseline shift at long wavelength in both of these spectra is expected due to wavelength dependent absorbance by high carbon content.
So finally, high throughput, single reflection ATR accessory configured with an appropriate ATR crystal is capable of providing excellent FTIR spectra of solids, liquids, polymers and many other sample types.