Column set

A. Conventional column set consisting of a series of columns with single pore sizes. 

The performances of column set A1 and B are similar, although column set B is more expensive. Using a specially selected column set for oligomer... 

The main criterium for column selection is pore size distribution as it is desirable to have maximum pore volume for separation in the molecular weight range of interest. Having determined the upper molecular weight limit required, a column with a suitable exclusion limit should be selected. In the case of individual pore size columns, it is then a question of selecting other columns with complementary calibration curves to comprise a column set covering the re-... 

These problems can be associated with the fact that various pore size SEC packings, either in a bank of individual pore size columns or in a mixed gel column set, respond differently in this extremely polar solvent. In addition the... 

FIGURE 13.22 Sur ctants on the 6 column set, using a photodiode array (PDA) detector. The Span and Tween 80 can be distinguished from each other very nicely in a mix. THF was the solvent used at 50°C. (A restrictor after the detector minimizes bubbles.)... 

Figures 13.25-13.28 show the ultrahigh resolution separations in chloroform of polystyrene standards, polytetramethylene glycol, urethanes and isocyanates, and epoxy resins, respectively. Multiple column sets of anywhere from two to six columns in series have been used for well over a year with no apparent loss of efficiency. The 500- and 10 -A gels can easily tolerate 15,000 psi or more. In fact, the limiting factor in the number of columns that can be used in series is generally the pump or injector in the FIPLC system. A pump capable of 10,000 psi operation should allow the use of a column bank of 10-12 50-cm columns with a total plate count of 500,000 or more.

Molar Mass Interval. GPC columns are offered for different molar mass intervals for larger intervals it is possible to combine some columns of different pore size types or to combine a few so-called mixed bed or linear columns. Both possibilities have their own special advantages and disadvantages mixed bed columns with a linear separation range of more than four molar mass decades are suitable to quickly get an overall view of a new sample, whereas a column set, carefully selected from different pore size types, often has a much better separation efficiency in a limited mass interval (for details, see Sections in and IV). 

Normally a calibration curve—molar mass against the total retention volume—exists for every GPC column or column combination. As a measure of the separation efficiency of a given column (set) the difference in the retention of two molar masses can be determined from this calibration curve. The same eluent and the same type of calibration standards have to be used for the comparison of different columns or sets. However, this volume difference is not in itself sufficient. In a first approximation the cross section area does not contribute to the separation. Dividing the retention difference by the cross section area normalizes the retention volume for different diameters of columns. The ISO standard method (3) contains such an equation... 

In the classical model of the size exclusion mechanism this difference stands for the effective pore volume of the separating model. Any elution of samples or fractions outside this interval always means a perturbation by a different mechanism. Such conditions have to be avoided. It is not possible to expand this elution difference A significantly for a given column. For this reason, GPC column sets are considerably longer than LG columns for other mechanisms. 

Column set 2 TosoHaas 30 cm X 7.8 mm PW series Detection Refractive index... 

It is difficult to decide what should serve as adequate column quality parameters for describing the performance of a set of GPC columns. The two most common measures are plate count and resolution. While both of these can be useful for monitoring the performance of a column set over time, it is not generally possible to a priori specify the performance needed for a specific analysis. This will depend on the nature of the polymer itself, as well as the other matrix components. 

A greater disparity is found in terms of the resolution requirements that are placed by various organizations on GPC column sets. A common resolution requirement is to measure the resolution R, as defined in Eq. (1) ... 

ASTM D5296-92 requires this R. greater than 1.7 whereas the DIN 55672-1 1995-02 standard requires R to be greater than 2.5. The OECD was uncertain on this point in 1994 and provisionally selected an R of 2.5 but the OECD also recommended this value for further review by the OECD member nations. This resolution criterion in Eq. (1) is a useful one for judging column set quality... 

One final quality parameter that is sometimes specified in test methods is the peak capacity of the column set. In the literature of the OECD, this peak capacity is required to have a value of 6.0 or greater. This peak capacity is defined by Eq. (2) ... 

Some GPC analysts use totally excluded, rather than totally permeated, flow markers to make flow rate corrections. Most of the previously mentioned requirements for totally permeated flow marker selection still are requirements for a totally excluded flow marker. Coelution effects can often be avoided in this approach. It must be pointed out that species eluting at the excluded volume of a column set are not immune to adsorption problems and may even have variability issues arising from viscosity effects of these necessarily higher molecular weight species from the column. 

Traditionally, column efficiency or plate counts in column chromatography were used to quantify how well a column was performing. This does not tell the entire story for GPC, however, because the ability of a column set to separate peaks is dependent on the molecular weight of the molecules one is trying to separate. We, therefore, chose both column efficiency and a parameter that we simply refer to as D a, where Di is the slope of the relationship between the log of the molecular weight of the narrow molecular weight polystyrene standards and the elution volume, and tris simply the band-broadening parameter (4), i.e., the square root of the peak variance. 

A method for determining the molecular weight accuracy of a column set has been described by Yau et al. (5). These relationships are shown as... 

This is a difficult parameter to measure, particularly on one or two evaluation column sets. It was found that most column problems are actually caused by catastrophic instrument failure or poor filtering of the solvent or sample. 

Tables 21.3 and 21.4 show the results of our evaluation on a column set that we felt performed very well. These tables address criteria 1 through 5 described previously. We judged the values listed to be very acceptable for high temperature GPC applications. For room temperature applications, where a smaller particle size column could be used, better values would be expected.

With the first part of the project having been completed, two more challenges remained. We first had to decide how we would ensure that the quality of data from each column set remained high. This meant setting up a quality control procedure for each GPC. The second challenge was determining what to do when we ran out of columns from a particular hatch of gel from the manufacturer. 

The instrumentation of HdC, including a pump, an injector, a column (set), a detector, and a recorder or computer, is very similar to size exclusion chromatography SEC). The essence of this technique is the column. There are two types of HdC columns open microcapillary tubes and a nonporous gel-packed column. This chapter emphasizes column technology and selection and the applications of this technique on the molecular weight analysis of macromolecules. 

With comprehensive GC, we can now choose a rational set of columns that should be able to tune the separation. If we accept that each column has an approximate isovolatility property at the time when solutes are transferred from one column to the other, then separation on the second column will largely arise due to the selective phase interactions. We need only then select a second column that is able to resolve the compound classes of interest, such as a phase that separates aromatic from aliphatic compounds. If it can also separate normal and isoalkanes from cyclic alkanes, then we should be able to achieve second-dimension resolution of all major classes of compounds in petroleum samples. A useful column set is a low polarity 5 % phenyl polysiloxane first column, coupled to a higher phenyl-substituted polysiloxane, such as a 50 % phenyl-type phase. The latter column has the ability to selectively retain aromatic components. 

The ordering of classes of compounds within the separation space was summarized by Ledford et al. (33), who presented an analogy to the separation by using a mixture of objects of varied shapes, colours and sizes. The experimental dimensions could separate objects based on mechanisms which were sensitive to shape, size or colour, and the choice of two of these for the two-dimensional separation was illustrated. Applications showed a variety of petroleum products on different column sets, as well as a perfume sample. 

Experimentation with Controlled-Pore Glass involved the use of several column sets packed with the pore diameters in angstrom inits shown in Table III. 

Percent Recoveries of Polystyrene Latexes Using Column Set II... 

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