Endcapping, bonded phases

The relationship of the selectivity to the polarity of the analytes can be understood from the differences in the retention factors of homologous alkanols (Figure 3.12). The polar alkanols are relatively more retained on the non-endcapped bonded phases (LOC-ODS-NE and HIC-ODS-NE) because smaller-size alkanols can reach the unreacted silanol groups on the surface of silica gels. 

The relationship of the selectivity towards rc-electrons can be understood from the differences in the retention factors of polycyclic aromatic hydrocarbons (Figure 3.13). The difference in the retention factors on end-capped and non-endcapped stationary phase materials is less than that of alkylbenzenes. This is due to the water content of the stationary phase. The content may be higher in non-endcapped bonded phases. 

Binders (TbC) 671 Bipolar pulse conductivity detector (LC) 588 Bonded phases (GC) 125 crosslinked 126 estersils 125 nonextractable 126 siloxane 125 Bonded phases (LC) 324 carbon loading 335 cleavage of ligands 336 eluotropic strength (LSC) 382 endcapping 326 hydrophobicity 364 metal impurities 369 models for surface 337 physical characteristics 333, 366... 

V) If the C-18 bonded phase contained un-reacted silanol groups, how would the retention of the solutes be affected by endcapping the stationary phase ... 

FIGURE 6.1 Groups present on the surface of a typical endcapped alkyl bonded phase (C8). 

Imazalil, OPP, and TBZ residues were determined using an HPLC system, consisting of a ternary HPLC pump attached to 250 X 4.6-mm-ID Hichrom RPB 5-/tm column with a 10 X 3.2-mm-ID guard cartridge column. The packing material of both columns was base-deactivated silica, fully endcapped with a bonded phase of Cg/C]8. The mobile phase was methanol/water at a flow rate of 1 ml/min at ambient temperature. Imazalil in the eluate was detected with UV detector at 204 nm a fluorescence detector was used to monitor OPP (excitation 285 nm, emission 350 nm) and TBZ (excitation 296 nm, emission 350 nm) (47). 

Occasionally, one inadvertently may use a mixed-mode mechanism in an SPE method. A good example is the sorption of triazine herbicides onto C-18 bonded phases that are monofunctional and are not endcapped. In this case, the basic compound has the potential for hydrogen bonding to the silanol sites of the silica gel, as well as reversed-phase sorption into the C-18 bonded phase. 

There are three general types of bonded normal-phase sorbents aminopropyl, cyanopropyl, and diol (Fig. 5.3). All three are derivatized as a propyl hydrocarbon, which places them near the surface of the silica. Only the cyanopropyl sorbent is available in either endcapped or nonendcapped forms (Table 5.1). Thus, analytes have the opportunity to interact not only with the bonded phase but also with the underlying silica sorbent. Furthermore, because the hydrocarbon chain is short, only three carbons in length, the... 

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