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:
refractive index of the crystal
refractive index of the sample
corrected angle of incidence
The penetration depth must be calculated for each data point of a spectrum.
In practice, the intensities Ii of each data
point will be recalculated instead according to the estimated relative
shift using the following equation:
corrected intensity of a data point
original intensity of a data point
wave number of a data point
reference wave number
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 a baseline
correction first to obtain good results.