Measurement of colloid osmotic pressure in submicrolitre samples

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Abstract
A colloid osmometer for submicrolitre samples was constructed from solid poly-methylmetacrylate and acrylnitrilmethylmetacrylate blocks, exposing a 0.85 mm diameter area of a Diaflo PM-30 ultrafiltration membrane. The unknown sample, contained in a 1-μl glass micropipette, was applied to the membrane by suction, providing minimal exposure to air. The lower limit for successful application was 0.1-0.2 μl. The accuracy of colloid osmotic pressure (COP) measurement depends strongly on the effective compliance of the pressure transducer. We tested three different systems: (i) A Hewlett-Packard 1280 ‘medical’ transducer gave acceptable measurements on 1-μl samples. In smaller samples (0.1-0.5 μl) COP was underestimated, especially at COP greater than 10 mmHg. The equilibration time was 10–30 min. (ii) As (i), but with air pressure applied to the sample by a servoregulated pump, minimizing fluid transport through the membrane. Accurate measurements on 0.2-,μl samples were obtained in the course of 2–3 min, but the system required special instrumentation and some operating experience, (iii) An ‘industrial’ transducer, SensoNor AE-880, with very low compliance, gave accurate measurements in the course of 1–3 min on samples as small as 0.1-0.2 μl and COP up to 37 mmHg. We recommend system (iii) for samples smaller than 1 μl

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