Species assignments of UDP- N -acetyl- D -glucosamine (UDP-GlcNAc) isotopologues in Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The same crude extracts used for NMR were analyzed following re-exchange of 2H back to 1H. Analysis conditions are stated in the text. With correction to an internal reference, all of the isotopologues were assignable at better than 1 ppm mass accuracy, with most better than 10 ppb mass accuracy. The molecular formulae were assigned using Xcalibur software with elemental limits set to CHONP and allowing up to 17 occurrences of 13C. The combination of the ultra-high resolution with extreme mass accuracy resulted in high confidence that only 'pure' 13C isotopologues were quantified for the moiety modeling.