The Estimation of Non-Protein Nitrogen in Blood

Of the methods proposed for the estimation of non-protein nitrogen in blood, only that proposed by Folin and Denis' has come into general use.The simplicity of this method has commended it to all workers, particularly to those called upon to ma!:e a large number of determinations.Nevertheless, no critical study of the method has appeared.Very recently Bock and Renedict,2 in an examinat'ion of part of the technique employed, pointed out cert'ain sources of error and suggested certain modifications for the purpose of increasing the accuracy of the method.The distillation method of Bock and Benedict has recently been employed in this laboratory.It is more convenient and more accurate than the aeration method of Folin and Denis.Duplicate determinations show behter agreement and the time required is much less.The method employed by Folin and Denis for the separation of the protein and non-protein nitrogenous constituents of the blood is open to serious objection.Methyl alcohol is, upon a pn'ori considerations, not a good solvent for the extraction of substances, such as some amino-acids, t'hat are comparatively insoluble therein.Indeed, Folin and Denis state that crcatine, nsparagine, and tyrosine added to blood could not be quantit: recovered.Moreover, methyl alcohol extracts a varying amount of lipoid nitrogen from blood.It is true that this is nonprotein, but it certainly does not belong in the same category as urea and amino-acid nitrogen.The amountS, though small, is appreciable.We have evaporated, in vucu~, the filtrates obtained 1 0. Folin and TV.