Air Water Interface
External reflection spectroscopy can provide information concerning conformation, tilt angle and head group structure, as well as information about protein secondary structure and orientation.
The entire accessory is enclosed in a PMMA cover maintaining a saturated water vapor atmosphere above the sample, placed at the water surface. A motorized shuttle system provides the switch between the sample and the reference well of the Langmuir trough. As the shuttle and the optical arms are computer controlled, completely automated measurements at multiple incidence angles (20-85°) are allowed. The focal length of the mirrors above the sample film is 250 mm to minimize the error on the angle of incidence. In comparison to a collimated beam the reflected intensity is significantly increased due to the focussing at the sample film.
The dimension of the unit allows the use of troughs sufficiently large to prevent the influence of the sample meniscus on the angle of incidence. To control the incident polarization a computer controlled KRS-5 polarizer is available. To avoid depolarization by reflection from mirrors this is inserted as the last optical element before reflection from the water surface. To minimize water vapor absorption outside the sample chamber the optical arms and the detector housing are purge able.
Moreover, the MCT-detector can be refilled with liquid nitrogen during the experiment, without disturbing the water vapor atmosphere within the PMMA cover. As an alternative to the shuttle technique, which allows measurements in a constant water vapor atmosphere, a photo elastic modulator (PEM) can be used. To avoid mirror induced polarization artifacts is installed between the polarizer and the water surface. For the same reason the detector can be optionally mounted on a second position (marked on the beam path schematic). In this configuration angles of incidence between 40° and 80° may be used. The use of a cutoff filter is recommended, to reduce heating effects due to O-H (or O-D) stretching absorptions. This heating can cause the monolayers to be squeezed out of the peam path. The use of a vibration-damped table is also recommended, since even small vibrations in the water surface cause substantial noise in the IRRAS spectrum.