4-3) Fluctuation in Refractometer Baseline Due to Changes in Dissolved Air Content

Fig. 18Effect of Degassing on Refractometer Baseline

The refractive index of liquids varies depending on the dissolution of not only solids and liquids, but gases as well. Therefore, changes in the solubility of air due to changes in liquid temperature can cause the refractometer baseline to drift or undulate. If measured with THF as the mobile phase and 8 × 10^-6 RIU as full scale, a 1 % change in dissolved oxygen results in a baseline change of more than a 10 % full scale.   Appropriate corrective measures include inhibiting changes in dissolution by keeping the mobile phase temperature constant or purging the air with helium, which has a dissolution level that is minimally dependent on temperature.
As indicated in section 4 above, when performing analyses, keep in mind the detrimental effects dissolved air (oxygen) can have on detection.