The MAT 253-Ultra: A New High-Resolution Gas Source Isotope Ratio Mass Spectrometer

Stable isotope geochemistry provides tools for understanding hydrocarbons and their source rocks, but interpretations are often under constrained because of the small number of measurable isotopic properties—δ13C and δD values. These isotopic values reflect the biosynthetic origin of the source material and fractionations during diagenesis, catagenesis, mixing and transport. The combination of these influences makes it challenging to reach unique interpretations of data.

Stable isotopes could yield a far larger set of compositional constraints on organic compounds. For example, there are 10 isotopologues of methane, each with distinctive properties. The information recorded by this isotopic diversity is lost to conventional measurements because the analyzed molecules are destroyed through pyrolysis or combustion prior to measurement.

We have developed with Thermo Scientific a new gas source isotope ratio mass spectrometer — the MAT 253 Ultra — capable of resolving isotopologues of hydrocarbon molecules having the same cardinal mass (e.g., 13CH4 from 12CH3D), enabling direct isotopic analysis of a large subset of naturally occurring species of hydrocarbons, their fragments, and adducts. This machine combines the dual and carrier gas inlet systems and ion source of a MAT-253 gas source mass spectrometer with the analyzer and collector array used on the Triton and Neptune thermal and plasma source mass spectrometers. The resulting hybrid is built, functional, and has demonstrated mass resolutions greater than ~25,000. The mass range is 2-280 amu. The instrument has 7 detectors (switchable between faraday and electron multipliers). Precisions in isotope ratio measurements are ~0.1 ‰ for weak ion beams (< 104 counts per second) measured on electron multipliers. Taken together, these capabilities will allow for unprecedented exploration of the isotopic anatomy of organic compounds.

We are exploring the multiply substituted isotopologues of methane, N2, N2O and longer chain alkanes (e.g., ethane and propane). To prepare for these measurements we have undertaken a set of theoretical (DFT) studies of both methane and 5-pentanone. We predict equilibrium enrichments in 13C-D bonds over a random distribution by factors of 1.006 and 1.004 for methane and 5-pentanone in the gas phase at 25 C. Such equilibrium predictions provide a basis for new types of thermometers and a reference frame for interpreting kinetic isotope effects in organic species.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California