Methods and apparatus for vacuum ultraviolet (VUV) or shorter wavelength circular dichroism spectroscopy
A highly efficient vacuum ultraviolet circular dichroism spectrometer is provided; the spectrometer suitable for laboratory use or for integration into a beam line at a synchrotron radiation facility. In one embodiment, a spectroscopic circular dichroism instrument is provided; the instrument configured so as to enable circular dichroism data to be simultaneously obtained for multiple wavelengths of light. The instrument may be further configured to operate in at least a portion of the vacuum ultraviolet wavelength region.
1. A spectroscopic circular dichroism instrument, comprising:
a light source which generates a multi-wavelength light beam comprised of light of a plurality of wavelengths;
a region of the spectroscopic circular dichroism instrument for the placement of a sample from which a circular dichroism measurement is to be obtained by exposure of a sample to the multi-wavelength light beam;
a constant phase difference compensator optically coupled to the multi-wavelength light beam, the constant phase difference compensator introducing a phase difference between two orthogonal polarization components of the multi-wavelength light beam, the constant phase difference compensator being configured to be rotatable to allow rotation between the constant phase difference compensator and an optical axis of the multi-wavelength light beam;
an optical element coupled to the multi-wavelength light beam, the optical element selecting a linearly polarized component of the multi-wavelength light beam;
a detector coupled to the multi-wavelength light beam to provide a circular dichroism measurement of the sample, the instrument being capable of providing the circular dichroism measurement simultaneously for multiple wavelengths; and
an optical path of the spectroscopic circular dichroism instrument, the optical path being configured to allow the multi-wavelength light beam to pass through the compensator and the optical element after exposure of the sample to the multi-wavelength light beam;
wherein the spectroscopic circular dichroism instrument configuration is capable of allowing circular dichroism spectra to be extracted from dc and 2ω components of a detected intensity signal wherein ω is an angular frequency of the compensator rotation.
2. The spectroscopic circular dichroism instrument of claim 1 , where the constant phase difference compensator provides a circular polarization component to the multi-wavelength light beam.
3. The spectroscopic circular dichroism instrument of claim 1 , wherein the spectroscopic circular dichroism instrument configuration is capable of further allowing linear dichroism spectra to be extracted from the detected intensity signal.
4. The spectroscopic circular dichroism instrument of claim 1 , wherein the sample is a protein solution.
5. The spectroscopic circular dichroism instrument of claim 4 , wherein the spectroscopic circular dichroism instrument is configured to measure the circular dichroism of spectra of the protein solution, the spectroscopic circular dichroism instrument further configured to analyze the circular dichroism spectra to obtain a protein secondary structure.
6. The spectroscopic circular dichroism instrument of claim 1 , further comprising a depolarizer coupled to the multi-wavelength light beam.
7. The spectroscopic circular dichroism instrument of claim 1 , further comprising a second compensator and second optical element, the second optical element selecting a linearly polarized component of the multi-wavelength light beam.
8. A method for performing a circular dichroism measurement, comprising:
generating a light beam comprised of multiple wavelengths;
exposing a measurement sample with said light beam and passing the light beam through the sample;
inducing a phase difference between orthogonal polarization components of an emerging light beam emerging from the sample by use of an optical element;
selecting a polarization component of the emerging light beam;
allowing rotation of the optical element to provide rotation between the optical element and an optical axis of the light beam;
detecting an intensity of a polarization component output light beam at multiple wavelengths; and
extracting a circular dichroism measurement from the intensity of the polarization component output light beam for each of the multiple wavelengths, the circular dichroism measurement being a circular dichroism spectra extracted from dc and 2ω components of the detected intensity wherein ω is an angular frequency of the optical element rotation.
9. The method of claim 8 , further comprising extracting a linear dichroism measurement from the intensity of the polarization component output light beam for each of the multiple wavelengths.
10. The method of claim 8 , wherein the the optical element is an optical compensator.
11. The method of claim 10 , wherein the optical compensator is rotated during measurement, generating time- and wavelength-dependent intensity data.
12. The method of claim 8 , wherein the sample is a protein solution.
13. The method of claim 12 , further comprising analyzing the circular dichroism spectra to obtain data regarding a protein secondary structure.
14. The method of claim 8 , wherein the polarization component is selected by use of an analyzer/polarizer.