Oxygen removal from samples

In the absence of a better competing mechanism, small amounts of paramagnetic impurities can determine the spin- 

in cases where long T s are expected from intrinsic relaxation mechanisms, care must be taken during sample preparation to remove as much of these impurities as possible. An especially common paramagnetic impurity in liquid as well as condensed gas samples is molecular oxygen. 

If you can work with the sample only in the liquid state, you must use the freeze-pump-thaw technique or some variant of it. In this technique, the sample chamber is pumped down and then the sample frozen usually with liquid Ng to displace oxygen. The sample is then thawed, frozen, and pumped on again. Five or six such freeze-pump-thaw cycles is usually sufficient. Then the sample is sealed, while frozen, with a torch either under a vacuum or a partial atmosphere of an inert gas like nitrogen. A variation, useful for pure liquids, is vacuum distillation. The sample is distilled back and forth on a vacuum line under low pressure. 

The freeze-pump-thaw technique is difficult to use with aqueous samples which commonly break the tube when they freeze. An alternative procedure is to bubble nitrogen through the sample to displace the oxygen and then to quickly 

It may be difficult to determine whether sufficient oxygen has been removed. In general, there are no independent tests, but conscientious application of the above procedures should reduce the dissolved oxygen concentration sufficiently to lower its contribution to the relaxation rate below that of any competing mechanisms. Common sense clearly dictates that you should not stop whatever procedure you are using if the last application of it lead to an improvement, that is longer T. Keep doing it until the T becomes independent of the treatment. 

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The CO oxidation reaction has been extensively studied in M Sri. NiOs (M = Pr, Sm, and Eu) perovsldtes [47]. Sr-substituted samples were found to be more active than the unsubstituted M Ni03 counterparts. A close relationship between the CO oxidation activity and the reducibility was observed, indicating that the facilitation of oxygen removal from crystal lattice is the rate-determining step [47,49]. 

It is worth briefly pointing out the difference between in-line and online analysis. In-line analysis does not involve removal of the sample from the reaction vessel, for example in determining water or oxygen content. On-line analysis does involve removing a sample, usually as a side stream, which adds to the complexity of the plant since the sample off-take equipment will often need to be built to the same integrity as the plant. There are four common techniques employed ... 

The phosphorescence lifetimes have been examined for many protein systems as a function of temperature. In the early work oxygen was not removed from the sample.(72,73) In these works the lifetimes are dominated by quenching by oxygen, and so the temperature dependencies probably represent temperature-dependent oxygen diffusion. 

Adsorbed water was observed to have a large effect on the F spin-lattice relaxation time for fluorine-doped aluminas in the dilute and intermediate concentration range of fluorine (0.3-8 wt. % F). An increase in Ti by a factor of 2 to 3 was observed in these samples when adsorbed water was removed from the solid by heating between 200-300°. The effect was completely reversible addition of oxygen-free water back to the solid resulted in recovery of the original (shorter) relaxation time. This effect was observed by the measurement of the in phase and ir/2 out of phase components of the dispersion derivative at resonance dx /6Ho at high rf power, from which effective values of Ti may be calculated 46). Values of Ti were also obtained by saturation of the resonance absorption derivative. 

Transfer from the preparation bell jar to the sample chamber was accomplished by first backfilling the system with dry nitrogen or argon for several minutes. When the bell jar was removed, the sample was quickly moved to the chamber while continually blowing the inert gas across it. While this process does involve brief exposure to air, our results show that the effect of exposure to oxygen in the dark was largely reversible. Thus, we believe that any effects due to this transfer are small. 

A 350-mL volume of filtered middle distillate fuel is aged at 203 °F (95°C)for 16 hours while oxygen is bubbled through the sample at a rate of 3 L/hour. After aging, the sample is cooled to approximately room temperature before filtering to obtain the filterable insolubles quantity. Adherent insolubles are then removed from the oxidation cell and associated glassware with trisolvent. The trisolvent is evaporated to obtain the quantity of adherent insolubles. The sum of the filterable and adherent insolubles, expressed as milligrams per 100 mL, is reported as total insolubles. 

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