Measurements of solutions and contact lenses with a vapor pressure osmometer

To investigate the influence of sample volume and contact lens material on osmolality measurements made with a Wescor vapor pressure osmometer.

Accuracy: Sample volumes of 0.8, 2.0, and 10.0 microL were tested with 290, 320, and 1000 mmol/kg after the osmometer was calibrated with the intended sample volume. Influence of sample volume: Sample volumes ranging from 0.5 to 1.1 microL (0.1 steps) were applied with solutions of 290, 320, 500, and 1000 mmol/kg after the osmometer was calibrated with 0.8 microl, independent of the intended sample volume. Influence of contact lens material: Lens discs of 3.4 millimeters were trephined from the center of Lotrafilcon B, Nelfilcon A, Balafilcon A and Etafilcon A lenses, and equilibrated in phosphate buffered saline with 290 mmol/kg after dehydration for 16 h. The osmometer was calibrated with 0.8 microl and lens discs were inserted into the small sample holder of the osmometer.

There were no significant differences between the nominal and measured osmolalities for each sample volume and solution combination (all p > 0.05). Influence of sample volume: Differences of more than 0.1 microl between the calibration and sample volume significantly affected osmolality readings, with sample volumes larger than calibration volume resulting in lower readings, and smaller volumes resulting in higher readings. Influence of contact lens material: Measured osmolalities of Lotrafilcon B (358.8 +/- 45.4 mmol/kg) and Balafilcon A (356.7 +/- 38.7 mmol/kg) were not significantly different to each other (p = 0.999) but were significantly higher than Etafilcon A (298.2 +/- 15.9 mmol/kg) and Nelfilcon A (281.2 +/- 12.2 mmol/kg, p < 0.05). There was no significant difference between Etafilcon A and Nelfilcon A lenses (p = 0.056). The main factors associated with measured osmolality were water content, sample volume and their interaction (r(2) = 0.716).

Osmolality readings varied with calibration and sample volume, and with different contact lens materials.