Fast SEC separations

However, several limitations predicted by chromatographic theory have to be considered. A study of the influence of column dimensions on fast SEC separations has been published in Ref. It has been found difficult to optimize and transfer existing methods and, in many cases, new equipment had to be purchased. 

Surprisingly enongh, the above processes are very fast and separation of macromolecnles on this principle can be considered an equilibrium process. The precise measurements of retention volumes of polymers under conditions of their partial pore permeation in absence of enthalpic interaction did not reveal practically any effect of the eluent flow rate [55]. On the contrary, in a review, Aubert and Tirrell [66] have demonstrated that the SEC exclusion volumes can be flow rate dependent due to both anomalous and physical effects. The former are caused by... 

Figure 17-19. Effect of UV detector time constant on peak shape and noise for fast LC separation of phenones (0.05pg/mL). Time constants, from top to bottom, are 0.5sec, 0.2sec, 0.1 sec, and Osec (no time constant). Peak widths are approximately 1 sec sampling rate is 20 Hz colnmn is 2.1 mm x 50 mm.

These non-SEC modes use high efficiency silica-based columns and binary, ternary and quaternary miscible mixtures of organic solvents and water to achieve fast, selective separations (Figure 9.6). In general, a reverse phase mode of operation is most suitable for lipophilic samples, whilst normal partition or adsorption is used for samples which are lipophobic. 

If we are to maintain the separation (resolution) that has been achieved in the first dimension in the eventual LC x SEC chromatogram, we need to collect a large number of fractions. To maintain a reasonable overall analysis time, this implies that the second-dimension separation should be fast and that the resolution that can be obtained in this second dimension is limited. There have been significant developments toward fast SEC in recent years [9, 28] Moderate-resolution SEC can be performed within one or two minutes. If we want to collect 100 fractions from the first dimension, this implies that typical LC x SEC analysis times are of the order of two to three hours. Indeed, these are the analysis times commonly encountered in practice. 

The comparison among these techniques is tabulated in Table 22.1. In summary, HdC is a separation technique with low selectivity however, the efficiency can be very high. Especially in PCHdC, high analysis speed can be achieved over a wide MW range. ThFFF performs best for the separation of high MW samples. SEC has an intermediate selectivity between FldC and ThFFF. Practicality makes SEC the most suitable method for the common MW range of synthetic polymers. SEC is by far the most commonly used technique for molecular weight distribution determinations. However, HdC is better for the fast analysis purpose. 

For synthetic and naturally occurring polymers, a few well-established techniques have proven useful. The first column pair to try is RPLC, followed by SEC. As SEC has a limited elution range, it can be used as a very fast second-dimension technique with run times on the order of 1-2 min. There are many examples of fast second-dimension SEC columns in the literature (Murphy et al., 1998a van der Horst and Schoenmakers, 2003). If molecules are small and polar and if the number of different solutes is large, RPLC and NPLC can be combined into a very powerful 2DLC separation system (Murphy et al., 1998b) see Chapter 18. 

Although the philosophy of CHAOS is the classical one and very similar to that of other programs which work with a "database" (LHASA, SECS, MARSEIL, etc.), the difference lies in the fact that the "substructures" and the corresponding "disconnection tables" are not in a separate "database", but are an integral part of the program itself. This allows fast access to the necessary information. However, the major novelty of CHAOS is, perhaps, the way in which the "substructures" have been organised for access to them. 

ISEC, which was introduced by Halasz and Martin in 1978 [119], represents a simple and fast method for the determination of the pore volnme, the pore size distribution profile, and the spe-cihc snrface area of porous solids. Generally, ISEC is based on the principle of SEC. SEC, also referred to as gel permeation or gel filtration chromatography, is a noninteractive chromatographic method that separates analytes according to their size by employing a stationary phase that exhibits a well-dehned pore distribution. 

FIGURE 16.13 Schematic representation of separation of a block copolymer poly(A)-block-poly(B) from its parent homopolymers poly(A) and poly(B). The elnent promotes free SEC elntion of all distinct constitnents of mixtnre. The LC LCD procednre with two local barriers is applied. Poly(A) is not adsorptive and it is not retained within colnmn by any component of mobile phase and barrier(s). At least one component of barrier(s) promotes adsorption of both the homopolymer poly(B) and the block copolymer that contains poly(B) blocks, (a) Sitnation in the moment of sample introdnction Barrier 1 has been injected as first. It is more efficient and decelerates elntion of block copolymer. After certain time delay, barrier 2 has been introdnced. It exhibits decreased blocking (adsorption promoting) efficacy. Barrier 2 allows the breakthrongh and the SEC elution of block copolymer but it hinders fast elution of more adsorptive homopolymer poly(B). The time delay 1 between sample and barrier 1 determines retention volume of block copolymer while the time delay 2 between sample and barrier 2 controls retention volume of homopolymer poly(B). (b) Situation after about 20 percent of total elution time. The non retained polymer poly(X) elutes as first. It is followed with the block copolymer, later with the adsorptive homopolymer poly(B), and finally with the non retained low-molar-mass or oligomeric admixture. Notice that the peak position has an opposite sign compared to retention time or retention volume Tr. 

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