Memory Effects in Compound-Specific D/H Analysis by GC/P/IRMS

The reactor is modeled as a linear series of discrete gas parcels moving past and interacting with a single, stationary box representing surface adsorption sites in graphitic carbon (Fig. S-1). Each gas parcel equilibrates with the graphite surface for one residence time (τ, typically 0.5 s), after which the gas parcels shift sequentially and the next gas parcel equilibrates. The molar amount and isotopic composition of hydrogen in the graphite phase are retained from one gas parcel to the next. Our model is thus conceptually similar to simple models of chromatographic partitioning between mobile and stationary phases 1. Concentrations and isotopic compositions are assumed to be homogeneous within each gas parcel, and within the graphite 'box'. This approach is justified because at a typical GC flow of 1.2-1.5 ml/min, each gas parcel takes < 0.5 s to go through a pyrolysis reactor of 30 cm length, in which the pyrolytic carbon covers only 5-8 cm near the upstream end. Thus the timescale for interaction between H 2 gas and adsorption sites within the reactor is much shorter than typical peak width (~20 s). For similar reasons, diffusion within and between gas parcels and the graphite surface is ignored in the model. Potential consequences of these simplifications are discussed in the paper. Figure S-1. Schematic description of the model, showing the equilibration of discrete gas parcels with the graphite surface pool. Also shown are sample input and output chromatograms, plotted as n g (i) vs. i. In the model, the molar amount of hydrogen (n) and fractional deuterium abundance (F = [D]/[D+H]) are tracked for each gas parcel (n g (i), F g (i)) and for the graphite surface (n a , F a). n g is calculated from the product of the parcel volume (each equal to the pyrolysis reactor volume) and P H2 Page S-3 due to analytes plus background hydrogen. These parameters can be adjusted to simulate GC peaks of varying size, shape, and temporal separation superimposed on a background of fixed size. F g is calculated by mass balance from the stipulated δD values of the analyte (assuming no chromatographic separation of isotopes) and background hydrogen. The number of total graphite adsorption sites (N a) is constant in the model, and is calculated from typical parameters for graphite as described in the Methods section. As each gas parcel equilibrates with the graphite surface, partitioning of both H …