ATR correction details

What is ATR?

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.

Why must an IR spectrum be corrected?

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.

ATR correction algorithm

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:

Legend:

Dp	penetration depth
n	wave number
nc	refractive index of the crystal
ns	refractive index of the sample
a	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:

Legend:

Icor	corrected intensity of a data point
Ii	original intensity of a data point
ni	wave number of a data point
ns	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.