Molecular weight columns

Although we often describe these as molecular weight columns, the separating parameter actually is their Stokes radius, the major axis of the molecule in its current configuration. The shape and folding of a protein molecule under differing solvent conditions affect their maximum radius and, therefore, their retention times. Only when extreme conditions are used to force all molecules into the same shape are we able to obtain a direct molecular weight relationship. 

To use a data filter on molecular weight (column B), click the arrow button in that column. Excel displays a drop-down list of all values in the column, plus "Custom...". To display all records that match one of the values in the selected field, you can select it from the list. To perform other logical comparisons, choose Custom... to display the Advanced Filter dialog box (Figure 6-6). 

Figure 21.24. High-speed exclusion chromatogram of polystyrene samples with various molecular weights. Column Toyo Soda TSK ... 

Figure 5. Internal water (V.) that is accessible to a solute as a function of molecular weight. Column packings were cotton (X) and mercerized cotton ( ).

Only those solutes in the third group will be separated from one another, and their retention volumes are directly proportional to the logarithm of their relative molecular mass (RMM molecular weight). Columns can be cahbrated with standards of known RMM before analyzing unknowns. 

All of the tables contain a column relating solubility information. This column generally lists the solvents that have been reported for the polymer. If nothing was said about the solubility of the polymer in the original article, this column is left blank. If a polymer was described as being insoluble, this has been reported in the table. If data reported in the Molecular Weight column were obtained by solution methods, the first solvent listed under Solubility is the solvent that was used. 

Separation of low-molecular-weight materials. Low-molecular-weight materials are distilled at high pressure to increase their condensing temperature and to allow, if possible, the use of cooling water or air cooling in the column condenser. Very low... 

The characteristic of a relational database model is the organization of data in different tables that have relationships with each other. A table is a two-dimensional consti uction of rows and columns. All the entries in one column have an equivalent meaning (c.g., name, molecular weight, etc. and represent a particular attribute of the objects (records) of the table (file) (Figure 5-9). The sequence of rows and columns in the tabic is irrelevant. Different tables (e.g., different objects with different attributes) in the same database can be related through at least one common attribute. Thus, it is possible to relate objects within tables indirectly by using a key. The range of values of an attribute is called the domain, which is defined by constraints. Schemas define and store the metadata of the database and the tables. 

In a chromatographic analysis of low-molecular-weight acids, butyric acid elutes with a retention time of 7.63 min. The column s void time is 0.31 min. Calculate the capacity factor for butyric acid. 

The first and second columns of Table 1.4 give the number of moles of polymer in six different molecular weight fractions. Calculate and for this polymer and evaluate a using both Eqs. (1.7) and (1.18). 

To use GPC for molecular weight determination, we must measure the volume of solvent that passes through the column before a polymer of particular molecular weight is eluted. This quantity is called the retention volume Vj. Figure 9.14 shows schematically the relationship between M and Vj it is an... 

To circumvent this need for calibration as well as to better understand the separation process itself, considerable effort has been directed toward developing the theoretical basis for the separation of molecules in terms of their size. Although partially successful, there are enough complications in the theoretical approach that calibration is still the safest procedure. If a calibration plot such as Fig. 9.14 is available and a detector output indicates a polymer emerging from the column at a particular value of Vj, then the molecular weight of that polymer is readily determined from the calibration, as indicated in Fig. 9.14. 

The basic premise of this method is that the magnitude of the detector output, as measured by hj for a particular fraction, is proportional to the weight of that component in the sample. In this sense the chromatogram itself presents a kind of picture of the molecular weight distribution. The following column entries provide additional quantification of this distribution, however. 

Since K represents the fraction of Vj at which a particular molecular weight fraction emerges from the column, and since In M cc in r we see that this... 

Both preparative and analytical GPC were employed to analyze a standard (NBS 706) polystyrene sample. Fractions were collected from the preparative column, the solvent was evaporated away, and the weight of each polymer fraction was obtained. The molecular weights of each fraction were obtained usmg an analytical gel permeation chromatograph calibrated in terms of both and M. The following data were obtained ... 

Three polystyrene samples of narrow molecular weight distribution were investigatedf for their retention in GPC columns in which the average particle size of the packing was varied. In all instances the peaks were well resolved. The following results were obtained ... 

An example of a size-exclusion chromatogram is given in Figure 7 for both a bench-scale (23.5 mL column) separation and a large-scale (86,000 mL column) mn. The stationary phase is Sepharose CL-6B, a cross-linked agarose with a nominal molecular weight range of 5000-2 x 10 (see Fig. 6) (31). 

Another example is the purification of a P-lactam antibiotic, where process-scale reversed-phase separations began to be used around 1983 when suitable, high pressure process-scale equipment became available. A reversed-phase microparticulate (55—105 p.m particle size) C g siUca column, with a mobile phase of aqueous methanol having 0.1 Af ammonium phosphate at pH 5.3, was able to fractionate out impurities not readily removed by hquid—hquid extraction (37). Optimization of the separation resulted in recovery of product at 93% purity and 95% yield. This type of separation differs markedly from protein purification in feed concentration ( i 50 200 g/L for cefonicid vs 1 to 10 g/L for protein), molecular weight of impurities (<5000 compared to 10,000—100,000 for proteins), and throughputs ( i l-2 mg/(g stationary phasemin) compared to 0.01—0.1 mg/(gmin) for proteins). 

The separation of the polysaccharide components utilizes their different solubUities, polar groups, extents of branching, molecular weights, and molecular flexibUities and may be accompUshed batchwise or with easUy automated column techniques such as column or high performance Uquid chromatography. These procedures have been summarized in several reviews (3,141—143). 

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