Composition Determination of Low-Pressure Gas-Phase Mixtures by 1H NMR Spectroscopy

H-1 NMR spectroscopy was used to analyze gas phase mixtures of methane and propane at pressures near 0.1 MPa. The mixtures were prepared gravimetrically and had low uncertainty in their composition. The primary mixture used for this work had a methane mole fraction of x(methane,grav) = (0.506875 +/- 0.00019) and a propane mole fraction of x(propane,grav) = (0.493125 +/- 0.00019). NMR samples were prepared in two types of commercially available sample tubes that seal with a PTFE piston. Sample pressures ranged from 0.02 to 0.5 MPa. An analysis of measurement uncertainty for the NMR method resulted in combined standard uncertainties that decreased from 0.0082x to 0.0010x, as the pressure increased from 0.02 to 0.5 MPa. The larger uncertainties at lower pressures were primarily caused by uncertainties associated with phasing and baseline correction. A key difficulty in working with gas-phase samples, especially at lower pressures, is that the spectral peaks are inherently broad. Consequently, peak overlap was problematic, and it was not always possible to integrate a high percentage of a peak's intensity. However, with corrections to the integrated areas, based on the assumption of ideal Lorentzian peak shapes, excellent agreement between the NMR analyses and the gravimetric composition was observed across the entire pressure range. These experiments demonstrate the potential of H-1 NMR for quantitative composition determinations of low-pressure gas-phase mixtures.