This is the main accesorry XA-511 for air-water interface measurements build by Bruker Optics. It is attached to an EQUINOX55 FT-IR. The EQUINOX55 has a sample chamber that can host an ATR accessory with 25 reflections at 45° or a specular reflection-absorption unit. The XA-511 accessory for air-water interface can be used for InfraRed Reflection-Absorption Spectroscopy measuring both s and p polarized spectra through a polarizer at various angles (usually 30° to 75° in our lab). The equipment can measure angles between 25° and 90°. The limitations that we are using are given by the fact that with the MCT detector (red in the picture above) that is liquid nitrogen cooled mounted on the arm at angles smaller than 30° the nitrogen will spill. The limitation for higher angles than 75°

is given by the fact that at higher angles the IR beam is no more spherical, a certain ellipticity appears which can lead to erroneous band intensities than expected.


  • Water vapor spectrum measured at the interface can be diminished using several approaches. Additional water baths present (blue color) that would saturate the environment with water vapors thus keeping the water vapor concentration constant. As an alternative nitrogen gas can be purged before and during measurements thus minimizing the water vapor contribution to the IR spectrum which is important mainly for the region 1400 - 1800 cm-1 where amides, carbonyls, peptides and enzymes have there IR bands.

  • A different setup can be made using a Photoelastic Modulator thus leading to PM-IRRAS. The main additions to the equipment are shown below:


  • A much more simple solution can be used for diminishing water vapor spectrum which is the use of D2O instead of water which has the IR band in a different position in the spectrum thus not interfering with the 1400 - 1800 cm-1 region. It is important to mention that D2O its expensive and 50 mL is needed for a single set of measurements.