Chromatogram linear

A solute s capacity factor can be determined from a chromatogram by measuring the column s void time, f, and the solute s retention time, (see Figure 12.7). The mobile phase s average linear velocity, m, is equal to the length of the column, L, divided by the time required to elute a nonretained solute. 

A small forerun of 2,4-pentanedione, b p 32-100° (19 mm.), is obtained The purity of the product may be demonstrated by gas chromatography on a 2-ft column packed with silicone gum rubber (F and M Scientific Co, Avondale, Pennsylvania) programmed linearly from 100° to 300° The chromatogram obtained is a single sharp peak The three conceivable impurities, 2,4-pentanedione, 3-butyl 2,4-pentanedione, and 6,8-tridecanedione, would have been observed under these conditions if they had been present. 

Artifact removal and/or linearization. A common form of artifact removal is baseline correction of a spectrum or chromatogram. Common linearizations are the conversion of spectral transmittance into spectral absorbance and the multiplicative scatter correction for diffuse reflectance spectra. We must be very careful when attempting to remove artifacts. If we do not remove them correctly, we can actually introduce other artifacts that are worse than the ones we are trying to remove. But, for every artifact that we can correctly remove from the data, we make available additional degrees-of-freedom that the model can use to fit the relationship between the concentrations and the absorbances. This translates into greater precision and robustness of the calibration. Thus, if we can do it properly, it is always better to remove an artifact than to rely on the calibration to fit it. Similar reasoning applies to data linearization. 

Quantitation is possible in many cases [6-15]. However, the activation reaction does not always yield a single reaction product (check by SRS method ), so the dependence of the linear response interval on temperature and duration of heating must be checked for each product. It can be taken as a rule of thumb that there will be a linear response between measurement signal and amount applied over the range 10 to 100 ng substance per chromatogram zone [5]. 

FIGURE 5 Gas chromatogram of degradation products of a linear polymer prepared from 3,9-bis-(ethylidene-2,4,8,10-tetraoxaspiro[5,5]undecane) and l,6-hexanediol. (From Ref. 16.)... 

The calibration technique used in conventional SEC does not always give the correct MWD, however. The molecular size of a dissolved polymer depends on its molecular weight, chemical composition, molecular structure, and experimental parameters such as solvent, temperature, and pressure ( ). If the polymer sample and calibration standards differ in chemical composition, the two materials probably will feature unequal molecular size/weight relationships. Such differences also will persist between branched and linear polymers of identical chemical composition. Consequently, assumption of the same molecular weight/V relation for dissimilar calibrant and sample leads to transformation of the sample chromatogram to an apparent MWD. 

For illustration consider SEC chromatograms obtained for two polymers on the same chromatographic system. One sample is a linear homopolymer while the other is a branched polymer with the same chemical composition. In the latter sample assume that the polymer components of different molecular weight have uniform branching characteristics so that all have similar molecular size/weight relationships. 

Compare molecular size/weight characteristics of branched and linear species eluting at V in each chromatogram. Under the universal calibration formalism branched and linear components have the same hydrodynamic volume at V ... 

Since our indirect method produces both the linear (b=0) and branched intrinsic viscosities across the chromatogram, it is possible to estimate several LCB parameters as a function of elution volume or number average molecular weight. The branching factor G(V) can be written as... 

Finally, an officially updated definition of the retardation factor, R, issued by lUPAC is important to the whole field of planar chromatography (the linear and the nonlinear TLC mode included). The importance of such a definition has two reasons. First, it is promoted by the growing access of planar chromatography users for densitometric evaluation of their chromatograms and second, by the vagueness of the present definition in the case of skewed concentration profiles with the samples developed under mass overload conditions. 

Isocratic linear development is the most popular mode of chromatogram development in analytical and preparative planar chromatography. It can be easily performed in horizontal chambers of all types. The mobile phase in the reservoir is brought into contact with the adsorbent layer, and then the movement of the eluent front takes place. Chromatogram development is stopped when the mobile phase front reaches the desired position. Usually 20 X 20 cm and 10 X 20 cm plates are applied for preparative separations, and this makes the migration distance equal to about 18 cm. Due to the fact that the migration distance varies with time according to the equation Z, = (Z, c, and t are the distance of the solvent front traveled, constant,... 

The rows of X are mixture spectra and the columns are chromatograms at the p = 20 wavelengths. Here, columns as well as rows are linear combinations of pure factors, in this example pure row factors, being the pure spectra, and pure column factors, being the pure elution profiles. 

Qualitatively equation (7.15) is adequate to describe tiM f influence of layer quality, selectivity, and zone position in the 1 chromatogram upon resolution for a single unidimensional development under capillary flow controlled conditions. The variation of R, with Rf is not a simple function as can be seen from Figure 7.6. The resolution increases with the layer efficiency in a manner that depends linearly on the R, value. — Relatively small changes in selectivity have an enormous impact on... 

The Rf values for a substance measured in linear, circular and anticircular chromatograms, for which the flow conditions vary, can be related to each other by equation 7.18... 

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