ATR is the abbreviation for a spectroscopic technique called attenuated total reflection (ATR). In this technique, the IR beam of an IR spectrometer is guided in an IR transparent crystal by total reflection. Due to quantum mechanical properties of the IR light, the electromagnetic field may extend beyond the crystal surface for about one micron as a so-called evanescent field. By applying sample directly onto the surface of the crystal, it is sensed by this evanescent wave and contributes to the absorption of the IR beam.
An IR spectrum of a sample recorded with an ATR spectrometer is not identical to a transmission spectrum recorded with default IR spectrometer. The ATR technique introduces relative shifts in band intensity and absolute shifts in the frequency. The relative intensity shift is well described and can be easily corrected, whereby the absolute shift in frequency domain is more difficult to correct. Therefore the frequency shift is often neglected. The ATR algorithm in the software only corrects the relative shifts.
Relative ATR shifts will be corrected according to the penetration depth of the IR beam of the spectrometer.
The penetration depth is proportional to the wavelength. Under measurement conditions, the penetration depth strongly depends on the instrument configuration, but as a first approximation it is assumed to be equal to a single wavelength.
A quantitative description of the penetration depth dp is given in the following equation:
Legende:
dp
Penetration Depth
λ
Wavelength
n2
Refractive index of the crystal
n1
Refractive index of the sample
Ɵ
Incident Angle
The penetration depth has to be calculated for every point of the spectrum. The incident angle is limited because of the danger of a total internal reflection. For further calculation the penetration depth is needed to calculate the intensity.
Legende:
A
corrected datapoint intensity
n2
Refractive index of the crystal
n1
Refractive index of the sample
Ɵ
Incident Angle
E0
Electric field of the evanescent wave
α
Absorption coefficient
In case E0 and Alpha are unknown, a calculation with a reference spectrum can be performed.
Perform a base line correction first.
The results of an ATR correction strongly depend on a good base line of the spectrum. So please perform abaseline correction first to obtain good results.
References:
Simon Nunn, Ph.D., Koichi Nishikida, Thermo Electron Scientific Instruments LLC, Madison, WI, USA, "Advanced ATR Correction Algorithm"
The full article is also available online at: Thermo - Advanced ATR Correction Algorithm
