Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Bonded stationary phases shape selection

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. [Pg.244]

Stationary phases with a high density of bonded alkyl groups can differentiate between two molecules of identical size where one is planar and the other twisted out of plane. This shape selectivity has been described by Sander and Wise [53] for polymeric stationary phases, where in the preparation, water has been added on purpose and trichloro alkyl silanes have been used. The selectivity for the retention of tetrabenzonaphthalene (TEN) and benzo[a]pyrene (BaP) was taken as a measure to differentiate between polymeric and standard RP columns. With standard ( monomeric ) RP columns, the twisted TBN elutes after the planar BaP, which on the other hand is more strongly retarded as TBN on polymeric stationary phases. In these cases the relative retention of TBN/ BaP is smaller than 1, whereas with monomeric phases the value is >1.5. The separation of the standards on three different phases is shown in Figure 2.9. These stationary phases have superior selectivity for the separation of polyaromatic hydrocarbons in environmental analysis. Tanaka et al. [54] introduced the relative retention of triphenylene (planar) and o-terphenyl (twisted), which are more easily available, as tracers for shape selectivity. However, shape selectivity is not restricted to polymeric phases, monomeric ones can also exhibit shape selectivity when a high carbon content is achieved (e.g., with RP30) and silica with a pore diameter >15 nm is used [55]. Also, stationary phases with bonded cholestane moieties can exhibit shape selectivity. [Pg.60]

In comparing the various test procedures, there is always a good agreement found for hydrophobic retention and selectivity as well as for shape selectivity. However, the characterization of silanophilic interaction is still a matter of discussion. In part, the differences are due to the selection of the basic analyte. Therefore, the outcome of every test is different. It has been shown, that the peak asymmetry—used for detection of silanophilic interactions—does not correlate to the pA" value of the basic test solute [64]. A closer look at these data leads to the assumption, that the differences are related to the structure of the basic solute, irrespective of whether a primary, secondary, or a tertiary amine is used. The presence of NH bonds seems to be more important in stationary-phase differentiation than the basicity expressed by the pA value. For comparable test procedures for silanophilic interactions further studies seem to be required. [Pg.73]

Szepesi et al. reported an ion-pair separation of eburnane alkaloids on a chemically bonded cyanopropyl stationary phase. As counter-ion, di-(2-ethyl hexyl)phosphoric acid or (+)-10-camphorsulfonic acid were used in a mobile phase consisting of hexane - chloroform -acetonitrile mixtures (Table 8.8, 8.9). Because of the poor solubility of the latter pairing ion, diethylamine (Table 8.9) was added to the mobile phase. Addition of diethylamine considerably reduced the k1 of the alkaloids, due to suppression of the ionization of the alkaloids. However, due to the strong acidic character of the pairing ion, ion-pairs were still formed under these conditions. The camphorsulfonic acid containing mobile phases were found to be very useful for the separation of optical isomers (Table 8.10, 8.11, Fig.8.8) 6. It was also found that the selectivity of the system could be altered by choosing different medium-polarity solvents (moderator solvents) as dioxane, chloroform or tetrahydrofuran. The polar component of the solvent system affected peak shape. Based on these observations, a method was developed to analyze the optical purity of vincamine and vinpocetine. For the ana-... [Pg.337]

For example, using non covalent bonding, Mossbach et al. have prepared a stationary phase for HPLC in order to resolve ( )-timolol [2). MIP s have fiinctional groups arranged in such a manner that they are complementary in shape and electronic features to the template. Therefore, Wulf et al. have selectively prepared L-Threonine with an enantiomeric excess of 36% by using a polymer which was imprinted with L-DOPA [3). [Pg.517]


See other pages where Bonded stationary phases shape selection is mentioned: [Pg.725]    [Pg.234]    [Pg.823]    [Pg.1145]    [Pg.42]    [Pg.23]    [Pg.62]    [Pg.236]    [Pg.238]    [Pg.242]    [Pg.247]    [Pg.253]    [Pg.253]    [Pg.255]    [Pg.283]    [Pg.285]    [Pg.53]    [Pg.54]    [Pg.69]    [Pg.71]    [Pg.17]    [Pg.368]    [Pg.145]    [Pg.309]    [Pg.115]    [Pg.130]    [Pg.127]    [Pg.70]    [Pg.644]    [Pg.841]    [Pg.100]    [Pg.116]    [Pg.1440]    [Pg.205]    [Pg.24]    [Pg.75]    [Pg.21]    [Pg.113]    [Pg.280]    [Pg.320]    [Pg.346]    [Pg.839]    [Pg.197]    [Pg.80]    [Pg.261]    [Pg.47]   
See also in sourсe #XX -- [ Pg.60 ]




SEARCH



Bond-selectivity

Bonded phase

Bonded phase phases

Bonded stationary phase

Bonds selection

Phase selection

Phase selectivity

Shape selection

Shape selectivity

Stationary phase Bonded phases

Stationary phase selection

Stationary phases selectivity

Stationary selection

© 2024 chempedia.info