Colorimetric determination of blood lipides.

Extraction-As the oxidation is essentially non-specific, the plasma extract must be as free of non-lipide reducing substances as possible. All solvents should be tested occasionally for reducing residues remaining after evaporation, and redistilled if necessary. Ether should be maintained peroxide-free. The determinations reported here followed a conventional alcohol-ether extraction and subsequent reextraction in petroleum ether. Although Folch and Van Slyke (2) have shown that this procedure carries through significant amounts of urea, it has been found that urea, even in amounts considerably in excess of what might occur, produces no color change in the dichromate reagent. Methods of extraction based on primary elution of water-soluble substances (Folch and Van Slyke (3); Ahrensl) have been used recently and offer definite advantages. Reagent-20 gm. of K2Crp01, c.p., are powdered in a mortar and added slowly with shaking to 1000 ml. of HzS04, c.p. (sp. gr. 1.84) maintained at a temperature not exceeding 100”. There should be no undissolved residue. If the reagent is protected from contamination and from exposure to direct sunlight, it darkens only very slowly with age. This does not affect the calorimetry, however, as the reagent itself is used as the reference blank. The addition of catalysts, including Hg, Pd, and Ag, had no effect on the oxidizing power of the reagent. Silver, recommended by Bloor (I), should not be used with biological extracts, as the latter may contain enough chloride (4) to interfere with the calorimetry. Oxidation-In dealing with extracts of normal human plasma, an aliquot representing from 0.1 to 0.7 ml. of plasma is transferred to a 25 ml. volumetric flask. The solvent is evaporated under nitrogen (water-pumped)