Bimodal columns

Zorbax PSM Bimodal and Trimodal columns are packed with mixed pore-size packing to achieve linear size separations over a broad molecular weight range (Table 3.3). Zorbax PSM Bimodal columns are packed with PSM 60 and PSM 1000 particles, and Trimodal columns contain PSM 60, PSM 300, and PSM 3000 particles (Fig. 3.4). Carefully selecting and mixing different pore-size particles in columns provide much better linearity than coupling columns that are each packed with single pore-size particles. 

The linear MW calibration curve of the bimodal columns, as shown in Figure 1, is the result, of proper matching of the column packing pore size -, and such a matching of pore size is relatively easy to accomplish in practice. li) Th linear MW calibration can be expressed by equation 4 ... 

Sample polystyrene mixture (1.8M + lOOK + 9K) solvent THF, Concentration 0.25% overall, mixture ratio = 1 2 3. 2X Du Pont PSM-Bimodal columns 25 //I sample loop, flow rate =1.5 ml/mm. 

Whereas in the example just described the sample amount was about 50 mg, a similar procedure developed by another group 129) started with 4 g polyethylene copolymer. The sample was applied as a dilute solution in xylene and precipitated by very slow cooling (1.5 K/h) onto the Chromosorb P packing of a 500 x 127 mm column. The first separation was temperature-rising elution fractionation at a flow-rate of 20 ml/min and a Unear temperature increase by 8 K/h. The MMD of the fractions was measured by SEC at 145 °C in o-dichlorobenzene at 0.7 ml/min flow rate. The column set included a pair of bimodal columns 100 A and 1000 A plus a 4000 A column. The apparatus was equipped with an IR detector. The experimental data is computed to show the distribution of short-chain branching and of molar mass simultaneously. 

A set of bimodal columns is useful in this case. These consist of two or more columns with two specific pore sizes which differ by a factor of 10 (e.g. 100 and 1000 A columns). If the pore volume of both gel types is the same, then a calibration graph with optimum linearity is obtained. 

Baker, P.R. Gardner, J.J. Wilkinson, D. Automated high-performance liquid chromatographic method for the determination of nedocromil sodium in human urine using bimodal column switching. J.Chromatogr.B, 1996, 668, 59-65... 

FIGURE 3.4 Calibration plots for Zorbax PSM Bimodal and Trimodal columns. 

Figure 17. Experimental (—) and calculated (+) chromatogram for the bimodal system at 220-nm wavelength using the HDC columns...

C. Linear SEC-[nl Calibration Using Broad Standards. It is to be noted that the viscosity calibration curve in Figure 3 is essentially linear. This linearity is derived from bimodal SEC column sets. 

Equation 7 shows that linear calibration using bimodal colximns can be applied to simplify the SEC-In I calibration procedure as has been done for the SEC-MW calibration. The objective of this [n] calibration then is to determine E and E2 values of the SEC column set for the particular polymer-solvent system of interest. The approach is the same as that for SEC-MW calibration. The similarities between the and the [n] formulations are summarized in Table... 

Novel general expressions were developed for the description of the behaviour of the height equivalent of a theoretical plate in various chromatographic columns such as unpacked (open capillary), packed with spherical nonporous particles and packed with spherical porous adsorbent particles. Particles may have unimodal or bimodal pore size distribution. The expression describing the mass balance in open capillaries is... 

One method that prevents overloading of narrow-bore capillary columns is split injection. The inlets are usually bimodal split/splitless inlets, and either mode can be selected for a given analytical method. In the split mode, the sample is rapidly vaporized in the inlet and a portion is introduced into the column in a narrow band with carrier gas, while the rest of the sample is vented (Dybowski and Kaiser, 2002). The amount introduced can vary for each method and is chosen as a ratio, e.g., 1 100. Easily vaporized compounds may preferentially vent, leading to the introduction of a nonrepresentative sample to the column (Watson, 1999). When the splitless mode is chosen, the entire sample is introduced into the column and the vent is opened after a predetermined period of time, to flush the excess solvent from the injector (Dybowski and Kaiser, 2002). 

Fig. 6.12 Scanning electron microscopy images of a sol-gel derived column material (A) a rigid rod and (B) a magnification of the bimodal pore structure in the resulting monolithic material [28]. Adapted with permission from the American Chemical Society.

The paired /-test is just a special application of the 95 per cent Cl for the mean and the requirement for normally distributed data, described in Chapter 5, applies in this case too. Just be aware that the confidence interval is calculated using the column of individual changes in weight (etc.), so it is these that need to be normally distributed. The subjects placebo and active treated weights could be skewed or bimodal or any other horrible distribution - that would not matter. We just need the changes to be normal. As previously, we do not expect small samples to form perfect classic normal distributions, but if the column of individual changes shows extreme signs of nonnormality, we should not trust a paired /-test (see Chapter 18 for possible solutions). 

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