Ultra-microdetermination of Arginine by a Compound Microbiological Assay Method. ∗

Abstract

Summary

It has been shown that the sensitivity of a microbiological assay method may be compounded by employing a secondary microbiological assay to determine the D-lactic acid component of the products obtained from the response of the original assay. Sensitivities of up to several thousand times those realized by the standard microbiological procedures have been shown to be possible theoretically in compound assays. The method described for the compound assay of arginine, designed purposely to be of relatively low sensitivity, was approximately 144 times as sensitive as the standard microbiological method for this amino acid. The average percentages of arginine found in casein (3.49%), gelatin (9.04%), and silk fibroin (1.02%), respectively, by the compound assay were found to be approximately the same as those (3.67%, 8.78%, and 1.02%, respectively) by the standard method, although the probable errors ranged from 3.2% to 6.1% for the individual assays by the former method compared to 0.7% to 1.2% for those made by the latter method. The sample weights required per assay (each including triplicate determinations at each of 4 or 5 sample levels) were: casein 45 μg, gelatin 18 μg, and silk fibroin 142 μg. One hundred times these amounts were required for each assay by the standard microbiological method.