Sample preparation moisture removal

The swelling of cellulose in the presence of moisture is a well known property of this material. Other protic and nonprotic solvents such as methanol, ethanol, acetonitrile, and acetone also have the capacity to swell microcrystalline cellulose. However, solvents such as benzene, toluene, or dichloromethane do not promote this effect. Therefore it is possible to control adsorption of probes on microcrystalline cellulose, either on the surface or entrapped within the natural polymer chains. After the removal of the solvent used for sample preparation, and for a swelling solvent, a chain-guest-chain interaction is promoted, replacing the previous chain-solvent-chain interaction. 

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

Once a bulk sample is selected, a laboratory sample must be prepared for analysis (Figure 28-2). A coarse solid sample should be ground and mixed so that the laboratory sample has the same composition as the bulk sample. Solids are typically dried at 110°C at atmospheric pressure to remove adsorbed water prior to analysis. Temperature-sensitive samples may simply be stored in an environment that brings them to a constant, reproducible moisture level. 

Later, Cattanach, Wu, and Venuto did an elaborate thermogravi-metric study on the calcination of ammonium zeolite Y and the resulting products (19). They found that the hydrogen zeolite reacted with anhydrous ammonia to yield an ammonium zeolite identical in ammonia content with the initial ammonium zeolite. Further, these workers reported that after loss of chemical water ( dehydroxylation according to Uytter-hoeven, Christner, and Hall or decationization according to Rabo, Pickert, Stamires, and Boyle) the sample became amorphous when exposed to moisture. This observation conflicted with the statement of Rabo et al. (16) in which they emphasized the extreme stability of their decationized Y. The data of Cattanach, Wu, and Yenuto prove, beyond any doubt, that they obtained the expected normal hydrogen zeolite Y prior to the loss of chemical water above 450°. Rabo et al., however, did not prove that the material from which they removed chemical water, was in fact, the hydrogen zeolite. They probably prepared, unknown to them at the time, the ultrastable zeolite described below. 

Moisture in coal takes three forms (l)free or adherent moisture, essentially surface water (2) physically bound or inherent moisture (thai moisture held by vapor pressure and other physical processes) and (3) chemically bound water (water of hydration or combined" water). The ASTM defines total moisture as a loss in weight in an air atmosphere under rigidly controlled conditions of temperature, time, and air flow. Total moisture represents a measurement of all water not chemically combined. Total moisture is determined by a two-slep procedure, involving air-drying for removal of surface moisture from the gross sample, division and reduction of Ihc gross sample, and determination of residual moisture in the prepared sample. An algebraic calculation is used to obtain the total moisture value. 

Approximately 0.5-1.0 ml. of NTO is run into a sample tube, and the tube is quickly capped with a 12/30 outer joint glass cap. Next, the tube is immersed in LN2 to freeze the NTO. The cap is removed, and the taper joint is attached to a vacuum line as quickly as possible. Still frozen, the tube is pumped out and sealed off with a torch at the junction between the NMR tube proper and the taper joint Samples so prepared may be stored indefinitely without deterioration or accumulation of moisture. 

We monitored the residual moisture of a protein product at different temperatures to evaluate its dependence on the temperature of storage. The stoppers were prepared by our normal manufacturing process, which had previously been optimized for occluded water removal. The product was placed at 5°C, the normal storage condition, and 25°C, an accelerated stability condition. Residual moisture in the 5°C samples was monitored for 36 months in the 25°C samples, for 12 months. The data are plotted in Figure 2. Residual moisture increased slowly in the 5°C samples from the initial value of 0.5% to about 1.0% in 36 months. In contrast, residual moisture rose much more rapidly at 25°C, attaining a value of 1.8% in 12 months. The data were fitted to a straight line and the slopes of the curves were calculated. Whereas the slope of moisture uptake at 5°C was only 0.02% per month, that at 25°C was 0.11 % per month. In other experiments (data not shown), we have found that moisture is gained at an even faster rate at 37°C (approximately 0.50% per month). 

Initial experiments involved the irradiation of XVIII in the presence of chloranil in methylene chloride and the irradiation of XIX in the presence of N,N-dimethylaniIine in methylene chloride. Samples were prepared on the vacuum line with pure dry materials and with careful removal and exclusion of moisture and oxygen. However, no trace of photocross I inking was observable in either system. 

Depending in part on the preparation method, humic substances may be subject to these problems. However, since oxygen is a major constituent of humic substances, it is important to know its true value. As discussed above, careful accounting of the moisture content of the sample is critical. One should also have information on the composition of the ash so that its effect on the oxygen analysis can be considered. If HF has been used to remove ash constituents, the sample should be analyzed for fluorine residues. 

Figure 2 shows typical drying curves for commercial p-hemihydrate that was preconditioned in three different environments to prepare samples with different amounts of free moisture. The curves exhibit a definite plateau after only a few minutes of exposure to microwaves, which is a strong indication that the weight loss is due to free water removal only. 

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