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Below is a spectrum of a monolayer of DPPC lipid vesicles adsorbed onto the surface of a ZnSe IRE. If this is the case the depth of penetration will be greater at higher wavelengths, and one will be probing deeper into the medium rather than at the interface of the IRE.
![how to do atr correction using essential ftir how to do atr correction using essential ftir](https://www.mdpi.com/polymers/polymers-13-02879/article_deploy/html/images/polymers-13-02879-g006.png)
It is good to keep in mind that the if working with systems that have more than one monolayer of adsorbed species that the depth of penetration is dependent on the wavelength of the incoming radiation. This depth is known as the depth of penetration and the governing equation is shown below. The infrared radiation that interacts with the adsorbed species is limited to a given depth, and is dependent on the wavelength and angle of the incoming radiation, refractive index of the IRE, and the refractive indices of the interacting layers at the IRE interface. One way for a user to overcome the disadvantage that multiple reflections brings is to use a single reflection IRE rather than a multiple reflection IRE. One of the advantages is that the user is able to observe stronger absorbance by the sample due to the multiple points that the radiation is interacting, however the disadvantage of using a multiple reflection element is that the light absorbance is hard to quantify due to scattering that occurs with each reflection. There are advantages and disadvantages of using a multiple reflection element. It is called a multiple reflection IRE because the radiation makes more than one reflection within the element prior to exiting towards the detector. The one shown above is an image of a multiple reflection internal reflection element. This setup allows for the infrared radiation to probe molecules at the interface of the IRE. It shows the internal components found within a specific ATR setup which allows the infrared radiation to enter from the bottom of the ATR base and redirected by five different mirrors before the radiation enters then exits the internal reflection element (IRE) and continues towards the the detector. Experimental Setup To the left is an image of what a typical ATR set-up includes.