Approx. 1.38

Ref: Index-of-refraction-dependent subcellular light scattering observed with organelle-specific dyes (spiedigitallibrary.org)

Oihana Gordobil If this is your continuous phase then you can measure it easily with an Abbe refractometer. See this webinar (free registration required):

Uses of an Abbe refractometer

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W120626AbbeRefractometer.html

As this is a cell culture medium containing salts (Earle's) for buffering then the RI is likely to be very similar to water (1.33). To get an increase to 1.34 takes a huge amount of dissolved solids. Measurement is the proof.

For DLS the most robust parameters are the z-average and polydispersity index (see the international standards - ISO and ASTM - in this area).

The attached may also be helpful.

Oihana Gordobil: Do you have the culture fluid? Use a common Abbe refractometer to measure it (as Alan suggests). These inexpensive instruments can easily provide you with the RI for YOUR sample (vs. one from a vendor which may not be exactly the same due to differences in the formulation and purity).

Oihana Gordobil One further point. Pictures of BME commonly show it colored red. This may some effect on attenuating the laser beam but with the commonly used He-Ne at 632.8 nm then this should not be an issue. What is the type of material you envisage measuring in BME?

William Letter I bought an Abbe on eBay for $2.60 (a sugar refractometer that can easily be used to get RI values). Details in the indicated webinar above.