If a refractometer is not available then an alternative method of determining the density of gradient fractions is to measure the absorbance (optical density) of the fractions. 

All iodinated density gradient media absorb strongly in the UV. If the absorbance is measured at approx 244 nm (the absorbance maximum for Nycodenz® and iodixanol)

The gradient samples will need to be diluted with water (table below gives a few values for iodixanol solutions) to get an absorbance value that can be measured accurately. The need to dilute the solution also means that any interfering material will be diluted out at the same time.

 Alternatively, if the absorbance is measured at a higher wavelength, dilution is not required. The table below gives a few absorbance values for Nycodenz® solutions at 350 nm.