[RASMB] RE: measuring refractive index in the near IR, for light scattering apps

[Richard Kingston]

To give the background to my question, I've been considering using  
dynamic light scattering to monitor protein folding, adding the   
osmolyte trimethylamine oxide (TMAO) to push an unstable protein  
toward the folded state.  Quite high concentrations of TMAO (1 -  2M)  
are required to achieve full protein folding, and this will shift the  
refractive index (and the solution viscosity) a decent amount.

For most compounds the dispersion in refractive index seems to be  
small, so it's probably acceptable to measure the refractive index at  
589 nm and neglect the wavelength dependence. As John Philo points  
out, a more significant source of error may be in measuring the  
solution viscosity. However as those errors are likely to be random,  
I wasn't so bothered by them.

Mattia Rocco's idea of using wavelength filtration to convert a  
standard Abbe refractometer into a device capable of measuring the RI  
at different wavelengths was new to me.  However it seems that  
extrapolating far beyond the region 488-632 nm might not be robust.  
As 840 nm light lies outside the visible spectrum, some further  
adaption of the instrument (like using a CCD detector in place of the  
human eye) would be required in this case.

Richard.