Combined use of filtered and edited 1H NMR spectroscopy to detect 13C‐enriched compounds in complex mixtures

In conventional metabolism and pharmacokinetic studies, radioactive isotopes are used to identify and quantify the breakdown products of xenobiotics. However, the stable isotope ¹³C provides a cheaper and less hazardous alternative. Metabolites of ¹³C‐enriched xenobiotics can be detected, quantified and identified by ¹³C‐filtered NMR spectroscopy. However, one obstacle to using ¹³C is its 1.1% natural abundance that produces a background signal in ¹³C‐filtered NMR spectra of crude biological extracts. The signal makes it difficult to distinguish between ¹³C‐enriched xenobiotics resonances from endogenous metabolites unrelated to the xenobiotic. This study proposes that the ¹³C background signal can be distinguished from resonances of ¹³C‐enriched xenobiotics by the absence of a ¹²C component in the xenobiotic. This is detected by combined analysis of ¹³C‐filtered and ‐edited NMR spectra. The theory underlying the approach is described and the method is demonstrated by the detection of sub‐microgram amounts of ¹³C‐enriched phenacetin in crude extracts of hepatocyte microsomes. Copyright © 2012 John Wiley & Sons, Ltd.