Phase selection

Fig. 2. Liquid-phase selectivity of UOP Olex adsorbent 0> olefins , paraffins.

Several hundred types of Hquid phases are commercially available. These have been used individually or in combination with other Hquid phases, inorganic salts, acids, or bases. The selection of stationary phases for a particular appHcation is beyond the scope of this article, however, it is one of the most important chromatographic tasks. Stationary phase selection is discussed at length in books, journal articles, and catalogs from vendors. See General References for examples. 

It is also clear from equation (2) that the sample mass can also be extended by increasing the capacity ratio (k ) of the eluted solutes (i.e, by careful phase selection). In this case the maximum load will increase linearly with the value of (k ) but so will... 

Anionic and neutral polymers are usually analyzed successfully on Syn-Chropak GPC columns because they have minimal interaction with the appropriate mobile-phase selection however, cationic polymers adsorb to these columns, often irreversibly. Mobile-phase selection for hydrophilic polymers is similar to that for proteins but the solubilities are of primary importance. Organic solvents can be added to the mobile phase to increase solubility. In polymer analysis, ionic strength and pH can change the shape of the solute from mostly linear to globular therefore, it is very important to use the same conditions during calibration and analysis of unknowns (8). Many mobile phases have been used, but 0.05-0.2 M sodium sulfate or sodium nitrate is common. 

Mobile-phase selection for cationic polymers is similar to that for the other polymers in that ionic strength and pH can change the shape of the solute from linear to globular (9). Mobile phases are often low pH e.g., 0.1% trifluo-roacetic acid, including 0.2 M sodium chloride, has been used successfully for polyvinylpyridines. Sodium nitrate can be substituted for the chloride to avoid corrosive effects. Some salt must be included so that ion exclusion does not occur (3). 

Another advantage of HdC is its generosity in terms of mobile-phase selection. The polymer size and solution properties of a polymer can be studied using HdC, especially OTHdC, in almost any solvent. In SEC, by comparison, the packing material and mobile phase have to be selected to prevent the nonsize exclusion effect. Because the instrumentation of HdC is similar to SEC, and the packing material and columns have become available commercially, this technique will gain in popularity. 

Remarks Moderate selectivity Moderate separation speed Narrow mobile-phase selection Fow selectivity High-speed separation Most generous mobile-phase selection Fow selectivity High-speed separation Generous mobile-phase selection Best for high MW polymers Thermal gradient may be programmed for broad MW separation... 

In common with all multidimensional separations, two-dimensional GC has a requirement that target analytes are subjected to two or more mutually independent separation steps and that the components remain separated until completion of the overall procedure. Essentially, the effluent from a primary column is reanalysed by a second column of differing stationary phase selectivity. Since often enhancing the peak capacity of the analytical system is the main goal of the coupling, it is the relationship between the peak capacities of the individual dimensions that is crucial. Giddings (2) outlined the concepts of peak capacity product and it is this function that results in such powerful two-dimensional GC separations. 

In many respects, the coupling of GC columns is well suited since experimentally there are few limitations and all analytes may be considered miscible. There are, however, a very wide variety of modes in which columns may be utilized in what may be described as a two-dimensional manner. What is common to all processes is that segments or bands of eluent from a first separation are directed into a secondary column of differing stationary phase selectivity. The key differences of the method lie in the mechanisms by which the outflow from the primary column is interfaced to the secondary column or columns. 

The ability of a GC column to theoretically separate a multitude of components is normally defined by the capacity of the column. Component boiling point will be an initial property that determines relative component retention. Superimposed on this primary consideration is then the phase selectivity, which allows solutes of similar boiling point or volatility to be differentiated. In GC X GC, capacity is now defined in terms of the separation space available (11). As shown below, this space is an area determined by (a) the time of the modulation period (defined further below), which corresponds to an elution property on the second column, and (b) the elution time on the first column. In the normal experiment, the fast elution on the second column is conducted almost instantaneously, so will be essentially carried out under isothermal conditions, although the oven is temperature programmed. Thus, compounds will have an approximately constant peak width in the first dimension, but their widths in the second dimension will depend on how long they take to elute on the second column (isothermal conditions mean that later-eluting peaks on 2D are broader). In addition, peaks will have a variance (distribution) in each dimension depending on... 

It is again clear that the two benefits of increased sensitivity and better resolution are both achieved, where these arise from zone compression and phase selectivity, respectively. However, since this mode of analysis is relatively new, it has yet to be tested for a wide range of applications such studies will be required to fully demonstrate its general utility. It is unclear whether this operational mode of selective MDGC constitutes a mode which is consistent with the definition of comprehensive... 

Figure 4.7 The selective or targeted mode of LMCS operation allows selected peaks to be collected sequentially in the cryoti ap, and then pulsed rapidly to the second column. The resulting peaks are naixow and tall provided that the second column phase selectivity and efficiency are adequate, they will also be resolved. The process is repeated as many times as required during the analysis. On this diagram, the lower ti ace response scale will be considerably less sensitive than on the upper ti ace.

Figure 8.20 Combination of bilayer plates and multiple development teclmiques in which total solvent sti ength and mobile phase selectivity are changed simultaneously, in the first direction (a), S- and are varied in n re-chromatograpliic steps, while in the peipendicular, (second) direction (b), and are again varied in m re-clnomatographic steps, to give (c).

Stationary Phases The best general purpose phases are dimethylsiloxanes (DB-1 or equivalent) and 5% phenyl/95% dimethylsiloxane (DB-5 or equivalent). These rather nonpolar phases are less prone to bleed than the more polar phases. The thickness of the stationary phase is an important variable to consider. In general, a thin stationary phase (0.3 /im) is best for high boilers and a thick stationary phase (1.0 /urn) provides better retention for low boilers. (For more detailed information, see Stationary Phase Selection in Appendix 2.)... 

The specific interactions that will produce the necessary retention and selectivitv must dominate in the stationary phase to achieve the separation. It follows that it is important that they are also as exclusive as possible to the stationary phase. It is equally important to ensure that the interactions taking place in the mobile phase differ to as great extent as possible to that in the stationary phase in order to maintain the stationary phase selectivity. 

The theory of the separation of geometric isomers on stationary phases that have a number of sterogenic centers has not been developed to the point where a particular stationary phase together with an appropriate mobile phase can be deduced for the separation of a specific pair of isomers. A number of theories have been put forward to explain the resolution of geometric isomers (some of which have been quite "imaginative" and "colorful") yet a reliable theory to help in phase selection for a hitherto unresolved chiral pair is still lacking. Unfortunately, the analyst is left with only two alternatives. The first is to search the literature for a model separation similar to the problem in hand and start with that phase system or, alternatively, resort to the technique of the early days of LC, namely, find the best phase system by a trial-and-error routine. 

The peak capacity is not pertinent as the separation was developed by a solvent program. The expected efficiency of the column when operated at the optimum velocity would be about 5,500 theoretical plates. This is not a particularly high efficiency and so the separation depended heavily on the phases selected and the gradient employed. The separation was achieved by a complex mixture of ionic and dispersive interactions between the solutes and the stationary phase and ionic, polar and dispersive forces between the solutes and the mobile phase. The initial solvent was a 1% acetic acid and 1 mM tetrabutyl ammonium phosphate buffered to a pH of 2.8. Initially the tetrabutyl ammonium salt would be adsorbed strongly on the reverse phase and thus acted as an adsorbed ion exchanger. During the program, acetonitrile was added to the solvent and initially this increased the dispersive interactions between the solute and the mobile phase. 

In many cases, the solvent systems determined by analytical TLC are directly applicable to PLC with similar results. A proper mobile phase selected for PLC should have a resolution more than 1.5 in the analytical scale. According to theory, PLC resolution, however, decreases with increasing particle size. Improved separa-... 

Mobile phases are of a greater variety than the restricted number of stationary phases. Many solvents and their mixtures are used as a mobile phase. The possibility of slight modification of solvent proportions in a mixmre permits the increase of mobile phase number and, thus, different results in the component separation of the analyzed sample. That is why the optimum mobile phase selection becomes one of the basic operations for the success of the analysis. 

In the mobile phase selection for the separation of compounds on thin silica gel layers, it is necessary to use not only eluotropic series based on the eluting capacity of the solvent but also eluotropic series of compounds established according to their interaction with silica gel. 

The separations of some nonionic tensides having biological activity and consisting of ethyleneoxide oligomer mixtures were performed in many different TEC systems (silica and alumina as the stationary phase and single solvent or binary mixtures as the mobile phase). Selectivity was higher on alumina than on the silica layer. Both... 

The microcircular technique for the mobile phase selection is illustrated in Figure 4.10a. It can be observed that the solvent A3 is the most suitable for nonpolar samples and the solvent B3 is for polar samples. 

For optimum mobile phase selection we have to consider certain chemical characteristics besides the solvent strength [15,16]. From this point of view, the chromatographic solvents can be divided as follows ... 

FIGURE 4.10 Mobile phase selection by microcircular technique, a. Sample of known composition A = nonpolar compound A1 = n-hexane A2 = acetone A3 = n-hexane-acetone, 60-1-40, v/v B = polar compound B1 = methanol B2 = water B3 = methanol-water, 70-1-30, v/v. b. Sample of unknown composition testing with solvents of different Snyder s groups and binary solvent mixture. 

On the basis of Snyder s system for characterization of solvents the PRISMA method for mobile phase optimization has been developed. This system enables the optimization of solvent strength and mobile phase selectivity and also the transfer of the optimized mobile phase to different planar chromatographic techniques, in our case the PLC. 

Emphasis has to be on choice and proper handling of the mobile phase. In Chapter 4 different approaches for mobile phase selection are discussed. General hints for selection are the avoidance of the following ... 

Ion-exchange solid-phase extractions are used for ionic compounds. The pH of the extracts is adjusted to ionize the target analytes so that they are preferentially retained by the stationary bonded phase. Selection of the bonded phase depends on the pK or pA b of the target analytes. Sample cleanup using ion exchange is highly selective and can separate polar ionic compounds that are difficult to extract by the liquid-liquid partition technique. 

---