Stationary phases mixed

In optimization the stationary phase also has to be considered. Various materials should be tested, despite the expenditure necessary. It may even be advantageous to use two different stationary phases at the same time. This can be done with coupled columns, two or more columns with different packings being arranged in series (without column switching). A useful device is the use of mixed stationary phases a column is packed with a mixture of two 

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Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Mixed stationary phases have been widely used in GC due to the unique selectivity that is mostly not achieved with neat stationary phases. Although neat IL-based stationary phases exhibit unique selectivity compared to many nonionic stationary phases, it is not always possible to completely resolve all analytes during the separation, particularly in complex mixtures. 

Each stationary phase is characterised by a calibration curve made with iso-molecular standards of known masses, M polystyrene in THF or polyoxyethylenes, pullulanes, polyethyleneglycols in water (Fig. 7.3 and Table 7.1). The curves representing log(M) as a function of the elution volume have a sigmoidal shape. However, by mixing stationary phases, the manufacturers can provide columns for which the calibration curves are almost linear for a wide range of masses. 

Figure 3.6 illustrates the variation of (a) the specific retention volume (Vff which is proportional to the capacity factor) with the composition of a mixed stationary phase, and (b) the variation of the retention index for benzene with the composition. It is clear from these figures that, whereas straight lines are observed for the variation of the capacity factor with the composition, the retention index varies in a highly non-linear manner. 

Window diagrams were developed by Laub and Purnell for the optimization of the composition of mixed stationary phases for GC (for a review see ref. [501] or ref. (544)). An example of a window diagram is given in figure 5.16. This figure will be explained below. 

Suppose there are two different stationary phases in a column (an example of a mixed stationary phase), phase A holding twice as many molecules at equilibrium as phase B. If A = 0.1, what fraction of all molecules does B hold ... 

Window Method of Purnell. In 1975 Laub and Purnell44 suggested a largely empirical method for finding the optimum mixture of liquids that would give the best separation for a given sample containing at least three analytes. Their only assumption was a common one—that GC retentions as measured by partition coefficients are additive in proportion to the volume fractions of the liquids used to make the mixed stationary phase that is,... 

Dove [74] analysed a complicated mixtuie of aromatic amines after their conversion into TFA derivatives on a mixed stationary phase containing 9.5% of Apiezon L and 3.6% of Carbowax 20M (Fig. 5.5). The derivatization was performed in a 25-ml vial in which the sample of a mixture of amines (0.2 g) and a standard (0.05 g of n-dodecane) were dissolved in 2 ml of tetrahydrofuran and 5 drops of pyridine. The mixture was cooled in an ice-bath and 2 ml of TFA anhydride were added carefully. The contents of the vial were then heated at 50°C for 10 min, cooled and 5 ml of water were added. The mixture was extracted with 8 and 4 ml of dichloromethane. The extract was washed with 5 ml of saturated aqueous NaHC03 and 5 ml of water, dried over anhydrous Na2S04 and a 5-pi portion was chromatographed. 

Cliffe et al. [212] studied the use of TFA-methyl esters of amino acids for quantitative analysis. The yields of the derivatives depend on the reaction conditions esterification was performed with 4 M methanolic HC1 for 90 min at 65°C and acylation with 20% TFA anhydride in dichloromethane at 120°C for 20 min. Injection into the chromatograph was carried out with the aid of a pre-column and the analysis was accomplished on the above-mentioned mixed stationary phase. Fig. 5.16 shows a typical analysis of a known mixture performed by this procedure. Replicate analysis showed a poor reproducibility for Met, Tyr, Arg and Cys (coefficient of variation 20%) and His was not acylated at all. This result is mainly caused by the strong dependence of the yield on the reaction conditions and the instability and high volatility of the derivatives (XE-60 is present in the stationary phase). 

Deshpande, D. D. Patterson, D. Schreiber, H. P. Su, C. S., "Thermodynamic Interactions in Polymer Systems by Gas-Liquid Chromatography. IV. Interactions Between Components in a Mixed Stationary Phase," Macromolecules, 7, 530 (1974). 

MIXED STATIONARY PHASES. REPRODUCED WITH PERMISSION FROM 4. ... 

Several, frequently used mixed stationary phases are given in Table 10.6. 

In such ternary systems comprising a solute and a mixed stationary phase (polymers 2 and 3), the polymer-polymer interaction is derived through the interactions of the solute with the mixed and the pure stationary phases. Note that in ternaiy systems index 3, formerly applied to the carrier gas [Eq. (5)], is now given to a polymer. 

At infinite dilution (0i - 0), the activity of the solute in a mixed stationary phase becomes (74)... 

A new interaction parameter, x (23). d the contact interaction parameter, are thus introduced which can be determined by GC [cf. Eqs. (10) to (12)] from the pure (A"i3, Z12) and mixed stationary phases. The equivalent of the normalized interaction parameter of Eq. (18) is Xij/si where s,- is the molecular surface to volume ratio. 

Table 9. Interaction parameters for mixed stationary phases of -tetracosane (n-Cj4), dioctyl phthalate (DOP) and polyCdimethyl siloxane) (PDMS) 74)... 

Olabisi (79) evaluated the miscibility of polymer pairs by gas chromatr raphy with mixed poly(e-caprolactone)-poly(vinyl chloride) stationa phases. Some interaction parameters for the pure and mixed stationary phases are given in Table 10. 

Table 10. Interaction parameters for mixed stationary phases of poly(vinyl chloride) (PVQ and poly(e-caprolactone) (PCL) at 120 C (79)...

Recently Purnell and collaborators (80) re-examined most of the results published on complexing studies of miscible low molecular weight substances. It was found that the activity coefficient of the solute in the mixed stationary phase could be described by the following relation ... 

Synergistic Effects of Mixed Stationary Phases in Gas Chromatography L. M. Yuan... 

The a-, fi-, or y-cyclodextrins that have been permeth-ylated do not coat well onto the walls of quartz capillaries and must be dissolved in appropriate polysiloxane mixtures for stable films to be produced. In contrast, underivatized cyclodextrins can be coated directly onto the walls of the column with the usual techniques. The thermal stability of a mixed stationary phase can be improved by including some phenyl-polysiloxane in the coating material. Phenylpolysilox-ane also significantly inhibits any oxidation that might... 

The effects of temperature, linear velocity of carrier gas, and relative amounts of the individual phases in the mixed stationary phase were also examined. 

The synergistic effect was only found in mixed stationary phases that have a special selectivity. Those stationary phases were CD, crown ether, liquid crystal-hne, resorcarene, calixarene, AgNOs, and others. Crown ether, CD, cahxarene, and resorcarene possess cyclic moieties with cavity-like structures that are able to form inclusion complexes with metal ions and organic molecules. Liquid crystalhne stationary phases have temperature-dependent ordered structures and the retention is governed by the solute s length-to-breadth ratio. AgNOs retards olefins by the formation of loose adducts. Together with the above special selectivity stationary phases, they have already been the focal point of sup-ramolecular chemistry. 

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