Packed columns mixed stationary phases

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... 

For a given sample and separation mechanism, the separation selectivity of the chromatographic system depends on the nature of both mobile as well as stationary phase. Interactions of solute with mobile and stationary phases may include a mixture of non-polar dispersive and various polar forces. The terms polar and non-polar have been commonly used to describe a property of both the solute, as well as mobile and stationary phases. These terms, however, should not be confused with selectivity. The separation selectivity of a stationary phase can be changed discontinuously by selection of a proper column packing (by its polarity or other parameters), or tuned continuously, synthesizing tailor-made phases or mixing stationary phases differing in polarity. ... 

The increased ability of capillary GC to resolve organic species caused an unanticipated drawback for broad spectrum analysis. The amount of stationary phase on a capillary GC column is much less than on a packed column. This condition increases the likelihood of observing a decrease in chromatographic performance caused by the sample matrix. The altered stationary phase may cause reduced precision of retention times and peak areas. The changes in the chromatographic performance of the stationary phase are measured by the Grob general purpose test mix (9). 

Different chromatographic conditions, sometimes very unusual ones, have been tested for the analysis of TFA-methyl ester derivatives. Hagen and Black [208] applied Carbo-wax 20M as the stationary phase, injected the sample at room temperature, and within 2—3 min increased the temperature to 270°C they detected in the chromatogram even Lys, Arg, Trp and His, thus succeeding where other workers failed, especially as far as His is concerned. The best separation of TFA-methyl esters of amino acids was obtained on a glass column (3.25 m X 2.5 mm I.D.) packed with Diatoport S coated with 2.5% of a mixed silicone phase (XE-60—QF-1—MS-200,46 27 27), both with a temperature programme and under isothermal conditions [211]. 

Fig. 4.6. Dependence of EOF linear velocity on pH of the mobile phase using a CEC column packed with a Mixed Mode stationary phase. Column 25 cm Hypersil Mixed Mode packed capillary. Mobile phase 75Q McCN-.50 mM Na2HP04. Voltage 20 kV. Marker thiourea.

Purnell carried out three experiments to examine the effect of the composition of binary mixtures of stationary phases on solute retention. In the first experiment, the two fractions were mixed, coated on some support, and packed into the column. In the second experiment, each of the two fractions was coated on separate aliquots of support and the coated supports mixed and packed in a column. In the third experiment, each fraction was coated on a support and each support packed into separate columns and the columns joined in series. Purnell demonstrated that all three columns gave exactly the same corrected retention volume for a given solute. 

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