<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><DIV>Greetings all,</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>I am seeking advice about measuring the refractive index of a solution, which crops up when analyzing light scattering data. </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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).</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>The options seem to be ... in order of increasing correctness and decreasing ease.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>1. Pretend your solution is water, and look the numbers up in a table.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>2. Measure the refractive index of the solution using a standard Abbe refractometer and ignore the dispersion (wavelength-dependence). </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>3. Directly measure the refractive index at the wavelength in question.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Option 1 starts looking bad in some circumstances (e.g. when adding salt or osmolyte to the solution) </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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 ...</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">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</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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.</DIV><BR>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.<DIV><BR class="khtml-block-placeholder"></DIV><DIV>Thanks,</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Richard</DIV><DIV><BR></DIV><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV>Richard Kingston, PhD.</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">School of Biological Sciences</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">The University of Auckland</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Private Bag 92019</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Auckland</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">New Zealand.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BR class="Apple-interchange-newline"></SPAN> </DIV><BR></BODY></HTML>