THE EMERGING FIELD OF ‘ISOTOMICS’, AND THE POWER OF GEAR-HEAD INNOVATION IN GEOCHEMISTRY (Invited Presentation)

John Valley’s career provides an object lesson in the power of technical innovations to transform isotope geochemistry. I will describe one body of work inspired by this example: The emergence of high resolution mass spectrometry as a platform for characterizing molecular isotopic structures (i.e., the constellation of site-specific and multiply substituted isotopic forms that contribute to a material’s overall isotopic content).

The mass spectra of molecules commonly provide dozens to hundreds of constraints on their isotopic structures, and thus can serve as the basis for a large number and diversity of geochemical tools. In principle, such measurements could have a richness and specificity rivaling established branches of ‘omics’ (proteomics, metabolomics, etc.). However, realizing this goal demands analytical methods and instruments having mass resolution and band-width greatly exceeding any well-established tool for isotope chemistry, along with exceptional sensitivity and precision, and the capacity for direct analysis of diverse compounds and complex mixtures.

Two new classes of analytical instruments have emerged to meet these demands: high-resolution, multi-collector, gas source, sector mass spectrometers, and Orbitrap-based systems, either with an electron impact gas ion source coupled to gas chromatography sample introduction, or electrospray ion source coupled to liquid chromatography sample introduction. The new sector instruments are best suited to measurements that demand high precision and abundance sensitivity, but are generally restricted to highly volatile compounds and masses ≤300 amu and a few isotopic properties per molecule; Orbitrap-based systems are broadly applicable to nearly any type or size of compound and can return data for dozens of independent isotopic constraints, but typically have precisions of ~0.5-2 ‰ and currently available instruments have limits in the dynamic range of ion abundances that can be usefully studied. These technologies and their uses will be described using illustrative applications to diverse hydrocarbons, amino acids, sterols and PAH’s.