Description General description Undecanoic acid is a medium chain fatty acid and the most fungitoxic compound commonly used in the treatment of superficial mycoses in humans. Application Refer to the product′s Certificate of Analysis for more information on a suitable instrument technique. Contact Technical Service for further support. Undecanoic acid may be used as an internal standard for the determination of medium-chain fatty acids in human fecal water, fecal fermentation supernatants, and human plasma by gas chromatography-mass spectrometry method.