Clinical Chemistry, Vol 14, 38-46, Copyright © 1968 by the American Association for Clinical Chemistry

An Automated Osmometer

¹ Departments of Pathology, Evanston Hospital, Evanston, Ill. 60201 and Mercy Hospital. Chicago, Ill. and the Department of Biochemistry, Northwestern University Medical School, Chicago, Ill. 60201.

A freezing-point apparatus, which measures osmotic pressure by determining freezing-point depression, was modified by incorporating a slowly rotating circular tray with space for 30 sample vials, and a sample pick-up device. The instrument is designed to automatically pick up a cuvet containing a 2-ml. specimen of serum, urine, spinal fluid, or any biologic fluid, and to cool, supercool, freeze, and record the osmolality of each sample. The pick-up device, consisting of a thermistor bridge null-point detector unit and 2 stirrers in an adapter unit, is automatically rinsed and dried after each determination. A sample reject system is incorporated in the event samples freeze improperly. A direct reading recorder is attached to the osmometer and gives a linear response in milliosmols per kilogram. The average time for individual sampling and recording of final data is 4 min. Thirty samples can be set up and run automatically with minimum direct supervision in 2 hr.

The reproducibility of this instrument showed an error not greater than ± 4 mosm./kg. The results of normal and abnormal physiologic osmolalities are reported and the effect of protein and carbohydrate on urine osmolality and specific gravity is examined.