A method is described for determining brain osmolality with a vapor pressure osmometer. This instrument measures dew point temperature depression of a solution in vapor equilibrium in a closed chamber. The principles of vapor pressure osmolality measurement suggest that it may have some advantages over freezing point depression methodology for analysis of tissue samples. Standard solutions (sodium chloride, 290 mOsm/kg water) of 10–20 μl and at temperatures considerably lower than ambient temperature may be delivered to the osmometer and measured without effects on recorded osmolality. Quick-frozen tissue specimens that were dissected while frozen into the shape of a thin slice (0.5–1.0 mm in thickness, 4–5 mm in diameter) and delivered to the machine while still in the frozen state resulted in osmolality values with high reproducibility. With this method, the osmolality of the cerebral hemispheres of pentobarbital anesthetized rats is 305.86 ± 0.74 mOsm/kg water, a value that is significantly higher than plasma values from the same animals (297.6 ± 0.72 mOsm/kg water).

The findings of this study suggest that with the use of a vapor pressure osmometer, small samples of brain tissue can be measured for osmolality with speed and high reproducibility and without the need for dilutions, weighings, calculations, and external determinations of tissue water content.