Weakly adsorbing samples, separation

Changes in separation temperature can also be used to deliberately alter sample A values and the course of separation. The separation of very weakly adsorbing sample components has suffered in the past from the small A effect (Section 2-3). Samples such as saturated hydrocarbons and monoolefins are essentially unadsorbed and unseparated under normal conditions, even with the weakest solvents possiblef and the most active adsorbents. However, the simultaneous reduction in temperature can further increase the A values of all sample components to the point where very weakly adsorbing samples can be separated. Clements (14) has given an example of this application for separations of saturated and olefinic lipids t —78°. 

The hydrotreating catalyst (Haldor Topsoe tk-551) consisted of 2.8% Ni, 10.1% Mo and 7.2% P supported on alumina. The 1/16 inch extrudates were crushed and sieved to 40-60 mesh size particles and calcined at 500° C for 16 hours- Catalyst samples were sulfided at 400c 0 in a separate flow reactor with 10% HjS in H for two hours and rapidly transferred to a high pressure, stirred autoclave containing vacuum gas oil (VGO) or a mixture of VGO in mineral oil. The VGO had a corrected boiling range of 265-515° C and a density (15° C) of 0.924 g/cm3, and contained 2.86% S, 740 ppm N, and 200 ppm asphaltenes. Catalysts were coked for 2-3 days at 375 cC in the different concentrations of VGo with added CS, in order to obtain different amounts of coke. Two additional run were made in which the sulfided catalyst was treated with a mixture of n-butylamine or N-phenylcarbazole in hexadecane at 300° C, Coked catalysts were Soxhlet extracted with xylene, followed by an acetone wash, dried at 110° C, and subsequently resulfided in order to assure removal of oil and weakly adsorbed species. Coronene uptakes and diffusivities of... 

As a last resort, the sample can be adsorbed on a small portion of adsorbent by gradually concentrating a slurry of sample solution plus adsorbent. The resulting adsorbent plus adsorbed sample is then placed at the front of the bed and separation is begun. The solvent used in the preparation of the sample-adsorbent mixture must be weak enough so that adsorption is favored over precipitation, otherwise the whole purpose of this technique is set to nought. 

Like silica, alumina may be regarded as a typical polar adsorbent, and sample separation order on alumina and silica is generally similar. The presence of carbon-carbon double bonds in a sample molecule generally increases adsorption energy on alumina more than on silica, and aromatic hydrocarbons which contain different numbers of aromatic carbon atoms are much better separated on alumina than on silica. Active aluminas are markedly sensitive to the differing shapes of various aromatic hydrocarbons and some of their derivatives (weak localization. Section 11-2B), permitting the excellent separation of many aromatic isomers. For these reasons alumina has been used widely to separate mixtures of the aromatic hydrocarbons. The preferential adsorption of acidic samples (according... 

Table 9-1 provides retention volume data for several sample compounds in a model GAC system (benzene plus helium carrier gas) and the corresponding GSC system (helium carrier gas data of Fig. 9-1). For every sample compound, the use of the adsorbing carrier gas decreases its retention volume, and this is fairly typical of GAC separation. Increasing the concentration of benzene in the carrier gas further decreases C/ for all the compounds of Table 9-1, implying that partition effects [case (4)] are relatively unimportant. With the exception of the weakly adsorbing alkanes and those samples with retention volumes smaller than that of benzene (the carrier gas component), the GAC system of Table 9-1 gives two elution bands when a single sample compound is injected into the column. One band corresponds to the injected compound and the other is a benzene band. For these latter compounds it is apparent that adsorption proceeds (at least in part) by case (3) adsorption of the sample... 

The choice of stationary phase and its degree of activity is determined by the nature of the sample. If sample components are adsorbed too strongly, they may be difficult to elute or chemical changes may occur. Weakly polar solutes should be separated on highly active adsorbents otherwise they may elute rapidly with little or no resolution. Strongly polar solutes are better separated on adsorbents of low activity. Silica gel can be prepared with a... 

Apart from the perfluorinated phases, the polarities of the CBPs in table 3.3 are typically intermediate between those of the non-polar alkyl phases and polar adsorbents such as silica. As we saw in figure 3.8, such phases may be operated with a polar eluent in the Reversed Phase mode, or with a non-polar (or weakly polar) eluent in the Normal Phase mode. The elution order of the sample components will be reversed in these two cases. A clear example of this phenomenon has been described by Kirkland [347] for the separation of some urea herbicides using a CBP with aliphatic ether groups. 

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