Osmometry (non-GMP)

Introduction to osmometry

The osmometer measures the freezing point depression of an aqueous solution caused by the presence of solutes (ions, organis molecules, macromolecules etc.).  The freezing point depression is directly proportional to the total number of molecules and ions present, irrespective of their size and molecular weight, a large protein molecule will have the same effect as a single Na+ ion, and a single molecule the same as an agglomarate of the molecules.

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Osmometry can be used for:

  • Development of pharmaceutical formulations: Salts or non-electrolytes (e.g. glycerin) are often added to protein formulations to achieve an isotonic solution.  Further, intrathecal and epidural injection drugs require very precise specifications for the product to be isotonic. Osmolality testing provides these stringent assurances
  • General Quality Control: Osmolality is a rapid and accurate QC measurement for a number of pharmaceutical and biopharmaceutical applications, including: finished product testing, infant formulas, ophthalmic solutions, electrolyte solutions, tissue cultures, culture media, IV solutions and other parenterals.
  • Examination of aggregation: When macromolecules aggregate in solution it will give rise to a decreasing osmolality. Further, interactions between macromolecules and various excipients in relation to aggregation properties can be studied.

Instrument and measuring principle

In practice the sample is supercooled to a predetermined temperature, lower than the expected freezing point. Freezing is initiated by a physical shock. This produces a water-ice mixture which remains at the freezing point plateau for long enough to measure the freezing point temperature. The osmometer is calibrated at two points against accurately made up salt solutions with a known freezing point and then checked with a control solution with a known value around 290 mOsmols.

Technical info
Instrument Micro Osmometer Model 3MO plus from Advanced Instruments
Operating range 0-2000 mOsm
Temperature RT
Sample amount > 20 ml aqueous solution