Silanols masking

Bij, K.E., Horvath, C., Melander, W.R. and Nahum, A. (1981) Surface silanols in silica-bonded hydrocarbonaceous stationary phases. II. Irregular retention behavior and effect of silanol masking. J. Chromatogr. Z03, 65-84. 

Fig. 7.1. Effect of silanol masking on peptide separation. Chromatographic conditions column, Supelcosil LC-8 (2S0 mm X 4.6 mm I.D.) mobile phase, acetonitrile-SO mM sodium phosphate in water, pH 2.25 (90 10) without (a) or with (b) 5 mM dodecyltri-methylanunonium chloride flow rate, 1.5 ml/min temperature, 25 °C detection, UV at 220 nm. Peptide peaks 1, Boc-Val-Met-Ala-Gly-Val-Ile-Gly-OEt 2,-Boc-Leu-Leu-Ile-Ser(Bzl)-Gly-Oet 3, NHa-Val-Met-Ala-Gly-Val-Ile-Gly-OEt 4, NHj-Leu-Ile-Ser(Bzl)Tyr(Bzl)-Gly-OEt. Reproduced from Bij et al. (1981), with permission.

Diara, H. Fukui, M. Mimaki, T. Poly(4-vinylpyridine) as a reagent with silanol-masking effect for silica and its specific selectivity for PAHs and dinitropyrenes in a reversed phase. Anal. Chim. Acta 2005, 548, 51. 

By inspection of the loading plot obtained (Fig. 5) and the contribution of variables to the selected PC axis (Table 4), we deduce that PCI mainly consists of asymmetry and reduced height, and PC2 of retention. Thus, differences between NAU, ABZ, and DIS are, for example, mainly due to differences in retention. Nevertheless, most of the variance is explained by PCI, meaning that column positions along this axis can be used as ranking criteria of stationary phases in relation to their silanol masking capacity. 

This optimization entails a large reduction in the number of measured chromatographic parameters per stationary phase 252 (7 compounds x 3 injections X 3 columns x 2 mobile phases x 2 chromatographic parameters) instead of 1890 (14 compounds x 3 injections x 5 columns x 3 mobile phases x 3 chromatographic parameters). Furthermore, in the analysis of basic compounds, only the mobile phase at pH 7.0 is effectively necessary to achieve a good evaluation of silanol masking capacity of chromatographic supports. A second mobile phase (at pH 3.0) may be required only when an assessment of the hydrophobic properties of stationary phases is needed. 

In view of the foregoing discussion of the properties of hydrocarbonaceous bonded phases on silica support it is readily appreciated that well-prepared stationary phases presently used in RPC approach the ideal with the exception of their relatively poor stability in contact with aqueous eluents, particularly at high pH, and the fact that surface silanol groups cannot be completely eliminated. The latter may interact with polar solutes, particularly when the dielectric constant of the eluent is rrlHlively low. Neveritieirss, residual surface silanolii can be masked bs alkylamines in the eluent with the result that peak tailing, when it is due to... 

Polymeric polyamines are also strongly adsorbed in the compact region of the electric double layer as a combination of multisite electrostatic and hydrophobic interactions. The adsorption results in masking the silanol groups and the other adsorption active sites on the capillary wall and in altering the EOF, which is lowered and in most cases reversed from cathodic to anodic. One of the most widely employed polyamine coating agents is polybrene (or hexadimetrine bromide), a linear hydrophobic polyquaternary amine polymer of the ionene type [129]. 

To reduce the influence of silanol groups during an isolation, the residual silanols must be masked by using a competing base such as triethylamine or... 

It must be re-emphasized that the use of these apparent rate constants is somewhat tentative. For example, different methoxy groups attached to silicon atoms hydrolyze at different rates the first methoxy groups hydrolyze most slowly. These competing reactions may occur simultaneously, and consequently mask the true rate of silanol production. This is consistent with hydrolysis rate constants of the silane coupling agents obtained from previous authors [16]. 

Cyclodextrins (CDs) are chiral compounds which interact with enantiomers via diastereomeric interactions. The separation is achieved because of the difference in stabilities of the resulting diastereomeric complexes formed between each enantiomer and the CD. In the first CEC experiments incorporating CDs, di-methylpolysiloxane containing chemically bonded permethylated (3- or y-CD (Chirasil-DEX) was chemically bonded to the inner walls of fused silica capillaries [139,140]. Electoosmotic flow is generated in these capillaries in the same manner as in fused silica capillaries. The Chirasil-DEX does not mask all the silanol groups, so while EOF is decreased, it is not entirely diminished by the coating. Since that time, CDs or CD derivatives have been bonded to silica particles which were then packed into capillaries, and the CD has been incorporated into continuous polymer beds known as monoliths. Table 3 shows some different CSPs, enantiomers separated, resolution, and the number of theoretical plates per meter. 

Use of alkylamide phases, in which alkyl chains are attached to the silica surface via an alkylamide group, reduces interactions with free silanols, by an internal masking mechanism. Residual silanols interact by hydrogen bonding with the embedded amide groups and thus become less active toward analytes. The embedded polar amide groups lessen the hydrophobicity of these phases compared to that of C18 bonded phases prepared from the same silica. Improved peak shapes of ionizable compounds were reported and this stationary phase was successfully used under IPC conditions to analyze streptomycin and its dihydrostreptomycin derivative in food... 

Polymeric polyamines are also strongly adsorbed in the compact region of the electric double layer as a combination of multisite electrostatic and hydrophobic interactions. The adsorption results in masking the silanol groups and the other adsorption active sites on... 

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