Chromatographic systems secondary processes

Application of the partitioning mechanism for the description of the retention process leads to another theoretical consequence applicable to ideal chromatographic systems that is, only one retention mechanism is present and no secondary equilibria effects are observed. Liquid chromatography is a competitive process, where analyte molecules compete with the eluent molecules for the retention on the stationary phase based on that, the standard state of... 

The LSER theory allows the association of the analyte retention behavior with the energetic components of the analyte interactions with the stationary phase. On the other hand, this theory is based on the assumption of the applicability of equation (2-43) to the analyte retention process, and this equation only allows the existence of one single partitioning retention mechanism. Coexistence of several different retention mechanisms, or the presence of secondary equilibria in a chromatographic system, effectively does not allow the applicability of this theory to the description of the analyte retention [59]. 

While dynamic distribution of the analyte between the mobile phase and adsorbent surface is a primary process, there are many secondary processes in the chromatographic system that significantly alter the overall analyte retention and selectivity. Detailed theoretical discussion of the influence of secondary equilibria on the chromatographic retention is also given in Chapter 2. 

In the preceding section, we briefly discussed the separation mechanisms that are exclusively and directly connected with the dimensions of the separated species. Let us designate these mechanisms primary. In the real gel chromatographic systems, however, several other processes are operative, affecting both the retention volumes and the widths of chromatographic zones. These secondary processes can be classified into ... 

The existence of secondary processes brings about another limitation of gel chromatography it is often rather difficult and sometimes even impossible to compare directly the data obtained with different chromatographic systems or under different operational variables. The same limitation is valid for the transfer of experimental results from one type of sample to another — even if an identical column system and operational variables are applied. It is advisable to check the possible influence of secondary processes when separating an unknown sample. 

The efficiency of a real GPC system depends above all on the rate of mass transfer between mobile phase and gel phase, as well as on the extent of secondary processes. Quantitatively, the efficiency can be expressed by the terms like width (w) or deviation (o) of the chromatographic peak, as well as by other terms of the theoretical plate concept. Since the diffusion rate of solute molecules decreases with an increase of their dimensions, one has to expect generally lower efficiency in gel chromatography of macromolecules in comparison with any other mode of liquid chromatographic separation of low molecular substances. 

An unequivocal identification of an unknown compound is unlikely by chromatographic processes alone. Not the least of the reasons for this is the need for comparison to standards thereby assuming reasonable prior assurance of the possible identity of the unknown. It should be noted that in addition to retention time measurements obtained on two or more column systems, if reasonable care has been exercised, quantitative measures of the suspect compound should also correspond, thus providing an additional secondary identification. In other words, whatever the unknown compound may be, it cannot be a mixture of two components on one column and a single component on the second column without... 

In planar chromatography, the fractions are not always transferred to another separation system, but rather a secondary separation is developed, orthogonally on the same chromatographic plate. Therefore, for all substances not completely separated it is possible that baseline separation can be achieved by means of a second separation process with an appropriate mobile (stationary) phase. Figure 8.2 shows that in the second dimension a theoretically unlimited number of secondary columns can be applied. Because of this, the terminology two-dimensional PC is not sufficiently... 

The process of analyte retention in high-performance liquid chromatography (HPLC) involves many different aspects of molecular behavior and interactions in condensed media in a dynamic interfacial system. Molecular diffusion in the eluent flow with complex flow dynamics in a bimodal porous space is only one of many complex processes responsible for broadening of the chromatographic zone. Dynamic transfer of the analyte molecules between mobile phase and adsorbent surface in the presence of secondary equilibria effects is also only part of the processes responsible for the analyte retention on the column. These processes just outline a complex picture that chromatographic theory should be able to describe. 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

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