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

[Richard Kingston]

Greetings all,

I am seeking advice about measuring the refractive index of a  
solution, which crops up when analyzing light scattering data.

Most who are familiar with the theory of dynamic light scattering  
will  know that the relationship between the measured auto- 
correlation function and the diffusion constant  involves the  
refractive index of the medium. To get a real number for the  
diffusion constant of a protein from DLS, you need the refractive  
index. Yet the laser light sources used on commercial instruments   
(from Wyatt or the now assimilated DynaPro) often operate in the near  
IR (e.g.  830-840 nm). Measuring the refractive index here is non- 
trivial. Most lab refractometers are calibrated to give the  
refractive index at 589 nm (The Sodium D line).

The options seem to be ... in  order of increasing correctness and  
decreasing ease.

1. Pretend your solution is water, and look the numbers up in a table.

2. Measure the refractive index of the solution using a standard Abbe  
refractometer  and ignore the dispersion (wavelength-dependence).

3. Directly measure the refractive index at the wavelength in question.

Option 1 starts looking bad in some circumstances (e.g. when adding  
salt or osmolyte to the solution)

Option 2 is not difficult, but I'm not certain how large an error is  
introduced by  ignoring the wavelength dependence of the  refractive  
index (say between 589 and 840 nM). Looking at data for pure solvents  
(water, ethanol, glycerol) it seems like the error introduced could  
be small.

Option 3 is made difficult by a lack of cheap, accessible  
instrumentation. Recently I came across an interesting idea which  
would allow measurement of refractive index of solutions in the near  
IR without the need for expensive equipment. The reference is ...

J Rheims, J Köser and T Wriedt (1997) Refractive-index measurements  
in the near-IR using an Abbe refractometer. Meas. Sci. Technol. 8  
601-605

It's easy to find the full text of the article using Google. The  
basic concept is to replace the light source on a conventional Abbe  
refractometer with an IR laser, and attach an IR sensitive CCD to the  
refractometer eyepiece tube. Then the instrument must be re- 
calibrated to make sense of the readings.

This is where it becomes problematic for owners of old Bausch and  
Lomb Abbe-3Ls (surely the most common refractometer ever produced ?)  
The re-calibration requires knowledge of the glass used in the  
refracting prism. Yet generally, all you have to go on is the prism  
series number - engraved on the prism, and on the left of the  
instrument scale. In my case it reads "538". Although I have a copy  
of the Bausch and Lomb glass catalog, the company long ago got  
swallowed by others, and I don't know a way of telling what glass was  
used in a refracting prism of any given series. That's likely true  
for most of the Bausch and Lomb Abbe-3Ls except those in current  
manufacture. So this route is blocked.

There doesn't seem to be a lot of discussion of the general problem  
in the literature, which might indicate that measuring the refractive  
index in the near IR is unimportant. Or it might not.  I'd appreciate  
any comments from light scattering specialists or others who might  
have thought about this issue.

Thanks,

Richard

Richard Kingston, PhD.
School of Biological Sciences
The University of Auckland
Private Bag 92019
Auckland
New Zealand.



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