Columns Combined

Easy column combinations Pore structure control... 

If two columns with different porosity are used in a column combination, the efficiency of the separation will not change much, but the fractionation range will be increased immensely. In this case, longer chromatography times allow a better separation of broader samples (or samples with high and low molar mass components). 

UniversaP Inject band dilution Column combination... 

HT 2 column with two Styragel HT 6E columns. While such a combination does not provide the highest resolution analysis, it is the best scouting tool for unknown samples. The best column combination can then be chosen for the routine analysis of the polymer. 

The selection of the right gel-permeation chromatography (GPC) column or column combination for a special task is a multistage procedure. First, a decision about the type of column has to be made according to the polymer samples in question ... 

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... 

To reduce or eliminate polymer solute/glass packing interactions the following parameters were optimized a) pH, ionic strength and concentrations of additives such as nonionic surfactants, b) selection of pore sizes in a column combination. 

Figures 6, 7 and 9 show calibration curves using two multi-column combinations and illustrate the degree of "optimization obtained in this system. The mobile phases for Figures 6 and 7 contained 0.025 g polyethylene oxide and ion exclusion and adsorption effects should therefore be largely eliminated. Figure 6 shows that reasonably good resolution can be obtained with a combination of five columns but does exhibit some loss of peak separation at the low cuid high MW ends. In Figure 7 the effect of adding a sixth column of small pore size is illustrated and it is seen that resolution at the low MW end is thereby somewhat improved. This calibration curve is effectively linear with a change of slope at 500,000 MW. It should provide a useful aqueous GPC system for MW and MWD determination of nonionic polyacrylamides.

Figure 6. Molecular weight calibration curve for nonionic polyacrylamides for a 5 column combination (each 4 ft s in, i.d.) with 3000 A, 3000 A, 2000 A, 1000 A, and 729 A CPG-10 (200/400 mesh) packing.

Figure 8. Chromatograms for nonionic polyacrylamide standards and Standard C for the column combination shown in Figure 6.

Figure 10. Chromatograms for nonionic polyacrylamide staruiards for the column combination (mobile phase and flow rate given in Figure 9)...

Principles and Characteristics Multidimensional gas chromatography (MDGC) is widely used, due to the mobile-phase compatibility between the primary and secondary separating systems, which allows relatively simple coupling with less-complicated interfaces. In its simplest form, 2DGC can be carried out in the off-line mode. The most elementary procedure involves manual collection of effluent from a column, followed by reinjection into another column of a different selectivity (e.g. from an apolar to a polar column). Selecting proper GC-column combinations is critical. In on-line mode, the interface in MDGC must provide for the quantitative transfer of the effluent from one column... 

When elution chromatography is used in both dimensions, the valve configurations are similar for the different column combinations. However, when CE is utilized as the second dimension, other types of interfaces not based on valves have been implemented with unique advantages. These and the microfluidic implementation of sampling systems for chip-based two-dimensional separations will be discussed below. 

Many of the possible column combinations that are useful in 2DLC are listed in Chapter 5. Besides the actual types of column stationary phases, for example, anion-exchange chromatography (AEC), size exclusion chromatography (SEC), and RPLC, many other column variables must be determined for the successful operation of a 2DLC instrument. The attributes that comprise the basic 2DLC experiment are listed in Table 6.1. 

Schley, C., Altmeyer, M.O., Swart, R., Muller, R., Huber, C.G. (2006). Proteome analysis of Myxococcus xanthus by off-line two-dimensional chromatographic separation using monolithic poly-(styrene-divinylbenzene) columns combined with ion-trap tandem mass spectrometry. J. Proteome Res. 5, 2760-2768. 

Packed columns are still used extensively, especially in routine analysis. They are essential when sample components have high partition coefficients and/or high concentrations. Capillary columns provide a high number of theoretical plates, hence a very high resolution, but they cannot be used in all applications because there are not many types of chemically bonded capillary columns. Combined use of packed columns of different polarities often provides better separation than with a capillary column. It sometimes happens that a capillary column is used as a supplement in the packed-column gas chromatography. It is best, therefore, to house the capillary and packed columns in the same column oven and use them selectively. In the screening of some types of samples, the packed column is used routinely and the capillary column is used when more detailed information is required. 

Methods have been described for determining chlorinated aliphatic hydrocarbons in soil and chemical waste disposal site samples. The latter method involves a simple hexane extraction and temperature programmed gas chromatographic analysis using electron capture detection and high resolution glass capillary columns. Combined gas chromatography-mass spectrometry was used to confirm the presence of the chlorocarbons in the samples [4],... 

Water Extract serial with methylene chloride at pH >11 (pH 7 for capillary column) and pH 2 analyze extracts separately for packed column, combined for capillary column dry over anhydrous sodium sulfate concentrate inject GC/MS 1-10 pg/L 70-81 Eichelberger et al. 1983... 

Carefully constructed Probe Mixtures based on small molecules and polystyrene standards are used as standardized reference points to better define the functional capabilities of individual columns and column combinations. The result using the method of Probe Mixtures to evaluate columns is better than can be attained from calibration curves alone and is especially useful in this high resolution capability situation. 

Figure 6 shows calibration curves for three other two column combinations, each representing 30,000 to 40,000 plates per set. The 10 A° plus 10 A° curve can be interpreted to show a deficiency in relative pore population in the range equivalent to about 50,000 to 600,000 molecular weight. The other two, properly calibrated, can conceivably be used for determination of molecular weight distributions. However, utility for resolution of specific polymodal mixtures is too difficult to assess from calibration curve alone. How much curvature of a calibration curve translates into utility or non-utility Calibration curves indicating pore size populations all have the same shape for given column combinations whether the plate count level is 5000 plates or 20,000 plates or 80,000 plates. 

The availability of calibration curves for individual columns and various combination banks of columns affords no more than a general insight, based on pare distribution, into the performance of the very high resolving power columns. The use of carefully constructed standard Probe Mixtures will now be discussed to evaluate in more detail the capability of different column combinations. 

The remainder of this paper illustrates that families of reference chromatograms can be developed to determine if a given column combination is the best one to be used with an unknown polymer or polymer mixture, the families of chromatograms being based on the standard Probe Mixtures, one to four column combinations and different plate levels. Extended banks of lower efficiency columns would be required to attain the same degree of resolution of even one of these ULTRASTVRAGEL columns. 

Four banks of columns are used in Figure 13 to determine the optimum three column combination for Mix 5. Each bank has the same level of total plates (approximately 50,000). The best result is with the 10 a° bank. 

Figure 12-18 Catalytic distillation reactor in which catalyst in the distil- t. lation column combines chemical reaction with vapor-liquid equilibrium in the column to achieve conversions higher than obtainahle with a reactor alone.

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