[RASMB] densimetry and refractive index question

[John Sumida]

Dear RASMB,

 

We've been measuring the density and refractive index increments of proteins
and nanoparticles and we notice that in general density of both materials
scale with concentration.  That is as one decreases the concentration so
also the RI and density of the solution decreases, I believe this to be
generally correct and in agreement with previous observations by Stouthart
et. al. and more recently by Huang et. al. (Spectrochimica Acta, Part A, vol
75 2010 pp 1031-1335)

 

However it is curious to me that this does not appear to hold up at
concentrations close to and below 0.1mgs/ml where there appears to be
inverse relationship between density and concentration, (we've observed same
also for RI and concentration); for CdSe qdot this effect seems to be quite
large.  Naturally it is possible that our initial worry was that our
instrumentation was out of calibration but after calibrating the DMA5000
with degassed milliQ water and air, and successfully measuring vbar and
dn/dc for BSA at 0.73 and 0.186 respectively I am beginning to wonder if
this effect is artifactual.

 

Wondering if anyone may have any comments on this observation and if so if
it would be possible to be pointed at any relevant references on the matter?

 

Thank you all for your attention and any help, advice or comments you may
have.

 

Best regards,

 

John Sumida

Analytical Biopharmacy Core

Dept. of Medicinal Chemistry

University of Washington

 

 

 

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