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

Abstract

1 H 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 1 H NMR for quantitative composition determinations of low-pressure gas-phase mixtures.

Nuclear Magnetic Resonance (NMR) Spectroscopy for the In Situ Measurement of Vapor–Liquid Equilibria

“…Invariably, the small contaminant peaks, which increased over the course of days, were found to have the same spectral features as the mixture that was previously in the same sample tube. The likely explanation is that a small amount of the previously stored mixture was desorbing from the PTFE piston valve . The advantage of the NMR method is that, by monitoring this effect in real time, it was easy to avoid any impact on VLE data.…”

“…Finally, a multipoint baseline correction was performed before signal integration. Signals were integrated numerically, and the integral values were then corrected for integral width and for peak overlap based on the assumption of Lorentzian peak shapes . This process for spectral acquisition and analysis is expected to give mole fractions with uncertainties between 0.01 x and 0.001 x for spectra with well-resolved peaks. − ,− ,,, …”

“…Corrections for overlapping spectra also contribute an uncertainty; this arises from errors in determining the peak width and from asymmetries in the peaks. With high-quality spectra the composition uncertainty due to this effect can be as low as 0.1%, as discussed by Suiter et al, but these contributions varied widely. The (R125 + R143a) system presented the best case among the mixtures studied here.…”

“…NMR spectroscopy is a well-established technique for the quantitative analysis of liquid-phase samples. − Although much less common, NMR spectroscopy has also been used for the quantitative analysis of vapor-phase samples, − and Suiter et al provide a thorough analysis of the uncertainty in vapor-phase compositions . NMR has been used to probe the composition of multiphasic liquid mixtures, although this type of work is uncommon and typically semiquantitative. − NMR spectroscopy has been suggested , as a method for VLE measurements. However, we are aware of only a single publication that describes a full VLE experiment by NMR spectroscopy, and that work was hampered by poor quality spectra and high measurement uncertainties.…”

“…Fifth, it is challenging to achieve good peak shapes (a necessary condition for high-accuracy signal integration) when the sample contains multiple phases. Finally, VLE measurements at pressures <0.1 MPa are challenging because of broadening of the vapor-phase signals. , …”