Mass impurities

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

The purities in mass%, declared by worldwide commercial suppliers, range from 99.9% for alkaline-earth metals and rare earths metals, to 99.999% for Al, Cr, Mn, Fe. .., to 99.9999 for Au, Ag, Pt. In certain cases, these values can be misleading since they refer to the metal contents, disregarding the non-metallic impurities contents (interstitial H, C, N, O, etc.). Moreover, the concentration is expressed in mass% and, for low atomic mass impurities, such as H or O, this can result in a much lower at.% purity value. The elements for special use, such as for spectrog-raphy, may require very high purities, which can be attained only by specialized laboratories. 

The purified Pc was vacuum deposited by thermal evaporation onto the AlO, films at different substrate temperatures from -17 °C to 93 °C. The deposition rate of Pc was 5-7 nm/min (except for sample D, 1 nm/min). The shutter was closed during the heating of the evaporation cell in order to avoid deposition of low mass impurities, degassing from the Pc evaporation cell, onto the samples. The evaporation temperature of the cell was between 260 °C and 310 °C. From optical polarisation microscopy we derived that all Pc films were polycrystalline with no preferential lateral orientation. The thicknesses of the Pc films <7pc afm were determined from the averaged height AFM profiles and ranged from 90 nm to 155 nm (see Table 8.1). The film thicknesses of the Pc films could be verified independently by X-ray diffraction (XRD) measurements which are described below in detail. 

The heat capacity measurements of Wallace [1960WAL] from 298.15 to 1273 K merge smoothly with the low temperature data of [1953GRI/SKO]. However, Nakamura et al. [1980NAK/TAK] measured the heat capacity of thorium containing 0.05 mass% impurities using a laser-flash technique. This a valuable study, as not only... 

When the fused mass (impure potassium iodide) has cooled, dissolve it in two pints of boiling distilled water, filter through paper, wash the filter with a little boiling distilled water, unite the liquids, evaporate the whole till a film forms on the surface, and set it aside to cool and crystallize. Drain the crystals, and dry them quickly with a gentle heat. More crystals may be obtained by evaporating the mother-liquor and cooling. The salt should be kept in a stoppered bottle. Two decompositions take place in this process —... 

From SEM studies, it can be observed that the untreated sisal fiber has a network structure and includes waxes and other low molar mass impurities whereas sisal fiber gets thinner after treatment. It is possible that treatment leads to microfiber fibrillation. The surfaces of the treated sisal fiber become smoother as compared to those of untreated sisal fiber. The effective surface area of fiber available for contact with the matrix also increases in composites while also reducing the diameter of sisal fibers and thereby increasing their aspect ratio. This may offer better fiber-matrix interface adhesion and improve stress transfer. These will give rise to improvement in mechanical properties [78]. 

Online separation technique is not necessary for production of end group fingerprint spectra. This provides the advantage of simplicity and no need of compromising ESI condition and separation conditions. However, the presence of any low mass impurities in the polymer sample causes complications in spectral interpretation. Low mass impurities can be removed by offline separation by LC prior to tandem mass spectrometry (MS/MS) experiments. 

Mes and coworkers compared TDA, DLS, HDC, and SEC and showed that all four methods can be used effectively to determine diffusion coefficients of systems with low polydispersities by measuring a series of styrene acrylonitrile (SAN) copolymers. Although these are polymeric systems, it is possible to apply the findings to supramolecular ensembles. The characterization of samples of low polydispersity was achieved best with TDA and DLS, since they both allow the rapid and absolute determination of the diffusion coefficient. However, TDA has the disadvantage that it is subject to interference due to the presence of low-molecular-mass chromophoric compounds. DLS, on the other hand, is influenced much more by the polydispersity of the sample than TDA. Furthermore, the use of DLS enables direct measurements of the Z-average diffusion coefficient of a polydisperse sample but requires a relatively large amount of the sample and is concentration dependent. Unlike TDA, DLS is especially suited for the analysis of high-molecular-mass systems, such as supramolecular systems, and is not disturbed by the presence of low-molecular-mass impurities. 

Osmotic pressure can be used to determine the average molar masses of macromolecules and polymers. As Example 7.15 showed, significant osmotic pressure effects do not require a large concentration. Relatively dilute solutions can show measurable osmotic effects, which allow one to calculate the molality of the solution and, stepwise, the molar mass of the solute. Of course, if the high-molar mass polymer is even slightly impure, the number of presumably lower-molar mass impurities will dramatically affect the final determination. Again, this is because osmotic pressure is a colligative property, which depends only on the number of molecules, not their identities, in the solution. 

TABLE 3.5 Theoretical effects of low molar mass impurities upon the value of M as calculated using Equation (1.6)... 

Purity is of great importance when evaluating because low molar mass impurities can give rise to major errors in the measured value of A , as is shown by the data in Table 3.5. These errors are reduced in MO due to equilibration of the impurity on either side of the membrane and in VPO by evaporation of volatile impurities from the solution drop. Thus the effects of low molar mass impurities upon measurement of M can be negligible but should never be ignored. 

The presence of low molecular mass impurities in the initial oligomers changes the stoichiometric balance and these impurities often behave as chain-termination agents. For instance, Thuillier [14,15] showed that many telechelic oligoethers contain significant amounts of 1-butanol and butyrolactone. 

Due to the good recovery of die polymers from the column, RP-HPLC was used with success for the detection of small molecular mass impurities and for the small-scale purification of branched chain polypeptides. Chromatograms are illustrated in Figure 2. 

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