Orbitrap measurements of acetate.

Acetate (C2H3O2) plays a central role in the metabolism of many -- arguably most -- organisms. It is produced by the aerobic respiration of sugars, by anaerobic fermentations, and from CO2 by autotrophic acetogenesis. It is also readily consumed by a wide range of microbes doing aerobic or anaerobic respiration. And of course it is the required precursor for building many lipids. Given this central role, knowing its C and H isotope composition should be very informative about how all those processes are competing to produce or consume acetate, and thus how the system as a whole works. But it has historically been very hard to measure acetate: its nonvolatile and won't go through a GC until you derivative it, at which point its too volatile to handle easily. Its typically present at very low concentrations so can't be easily isolated in sufficient quantity for EA-based measurements, and existing LC-IRMS methodologies struggle to separate it from complex samples. Elliott Mueller, a Ph.D. student in my group, has now shown that he can simultaneously measure δ13C and δD of acetate using an Orbitrap mass spectrometer with electrospray ionization. The measurement achieves performance metrics (accuracy and sensitivity) that are better than all preceding methods, in ~20 minutes, without having to derivatize the acetate. The methodology should be applicable to other small, water-soluble organic molecules and promises to profoundly improve our ability to examine these substrates and intermediates of metabolism in natural samples. The work is published in the January issue of Analytical Chemistry.