COMPOSITION DETERMINATION OF LOW-PRESSURE GAS-PHASE MIXTURES BY 1H NMR SPECTROSCOPY
Chris Suiter, Mark O. McLinden, Thomas J. Bruno, Jason A. Widegren
1H NMR spectroscopy was used to analyze vapor-phase mixtures of methane and propane at low to moderate pressures. 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 xmethane,grav = (0.506875 ± 0.00019) and a propane mole fraction of xpropane,grav = (0.493125 ± 0.00019). The gravimetrically prepared mixtures were used to prepare NMR samples in two types of commercially available sample tubes that seal with a PTFE piston. All samples were prepared and measured at 25 °C. Sample pressures ranged from 0.02 MPa to 0.5 MPa. An analysis of measurement uncertainty for the NMR method resulted in combined standard uncertainties that decreased from 0.0082·x to 0.0010·x as the pressure increased from 0.02 MPa 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. These two difficulties were surmounted with corrections to the integrated areas based on the assumption of ideal Lorentzian peak shapes. With the corrections, excellent agreement was observed across the entire pressure range. Without these two corrections, however, the combined standard uncertainty interval for the NMR-determined mole fractions did not overlap the uncertainty interval for the gravimetrically determined mole fractions in any of the measurements done at lower pressures. These experiments demonstrate the potential of 1H NMR for the composition determination of low- to moderate-pressure gas-phase mixtures.
, McLinden, M.
, Bruno, T.
and Widegren, J.
COMPOSITION DETERMINATION OF LOW-PRESSURE GAS-PHASE MIXTURES BY 1H NMR SPECTROSCOPY, Analytical Chemistry, [online], https://doi.org/10.1021/acs.analchem.8b04955, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926503
(Accessed January 29, 2022)