Quantification of dissolved O2 in bulk aqueous solutions and porous media using NMR relaxometry

Abstract

Effects of dissolved paramagnetic oxygen (O2) in water on 1H nuclear magnetic resonance (NMR) Carr-Purcell-Meiboom-Gill (CPMG) experiments is evaluated at a 1H Larmor frequency of 2 MHz. Dissolution of O2 into water significantly reduces the 1H transverse relaxation coefficient (T2). For deoxygenated water, T2 is 3388 ms, water at ambient atmospheric conditions (7.4 mg/L O2) exhibits a T2 of 2465 ms, and dissolution of 2710 mg/L O2 further reduces T2 to 36 ms. The results were fit with an empirical model to facilitate prediction of T2 times for bulk water as a function of paramagnetic oxygen concentrations in solution. Dissolved O2 also greatly influences 1H NMR CPMG experiments of confined water in a model system composed of Berea sandstone. For this system, 90 mg/L O2 in H2O enhances T2 relaxation of bulk water such that the relaxation time is comparable to physically confined water in the sandstone pores. Given the sensitivity of NMR T2 coefficients to paramagnetic oxygen, low-field NMR-based characterization of fluid and porous media structure requires control of dissolved oxygen, as geospatial variation in the partial pressure of O2 alone is expected to perturb fluid and pore relaxation times by up to 60 and 36%, respectively.

Zero- to low-field relaxometry of chemical and biological fluids

“…To summarize, it is demonstrated that the presence of paramagnetic oxygen molecules has a crucial impact on ZULF NMR spectra and it should be taken into account in quantitative ULF relaxometry experiments. This finding also suggests the potential use of ZULF NMR relaxometry in the evaluation of paramagnetic gas impurities 41 , 42 .…”

Nuclear magnetic resonance relaxometry to monitor chromium (VI) reduction by hydrogen peroxide, ascorbic acid, and aluminum powder

“…Dissolved oxygen was not removed from water by nitrogen bubbling or other techniques, even if its presence leads to an increase of the relaxation rates. [31,32] Indeed, the influence of O 2 was considered as rather small when compared with other sources of errors in the measurement of relaxation rates as sample temperature fluctuations. The experiments were performed at three different pHs using different buffers: at pH 2 (KCl/HCl), at pH 3 (NaCl/Glycine/HCl), and at pH 4 (sodium acetate/acetic acid) or (sodium citrate/HCl).…”

Earth's field NMR relaxation of pre-polarised water protons for real-time detection of free-radical formation

“…From the catalysed H 2 O 2 decay stable paramagnetic O 2 is generated in a 1 : 2 ratio relative to the H 2 O 2 concentration (eqn ( 1) and ( 2)), resulting in PREs for nearby 1 H nuclei. 17 radical species produced in this process are extremely shortlived 16 and do not generate PREs within the experimental timeframe (Fig. 3).…”