Advantages and Limitations of the Ninhydrin Test for Analysis of Historic Wool Fibers

Abstract

There are very few quantitative tests suitable for measuring the degradation of historic wools and silks. Most tests are qualitative and results are confounded by variations in fiber diameter. The ninhydrin test, in which ninhydrin (triketohydrinene hydrate) reacts with primary amino groups and produces Ruhemann's purple, is a well-known test for amino acids in solution. The test has been modified by researchers in order to give a quantitative measure of primary amino groups in wool and silk fibers. The primary amino group content of new wool is compared with wool degraded by moist heat and several archaeological wools. Some of the advantages of the ninhydrin tests include: 1) an analytical procedure for quantitative work; 2) small sample size; and 3) equipment commonly available in an analytical laboratory. One limitation of the test is the fact that ninhydrin reacts not only with (alpha + epsilon) amino groups but also with ammonia nitrogen and other free amines. In addition, archaeological and historic protein fibers are often contaminated with soil residues which are difficult to remove and will influence test results.