Column packings endcapping

For strongly cationic compounds, such as aliphatic amines, where silanol effects are most dramatic, the separation can be developed by several good approaches, the first of which is column selection. Choosing a column packed with polymerically bound silica and exhaustive endcapping will deactivate most of the... 

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). 

Fig. 10.27. CEC separation of Cinosulfuron and by-products. Column 335 (250) mm x 100 pm i.d. packed with Synchropak non-endcapped C8, 3 pm, Conditions mobile phase, acetonitrile-20 mtf sodium dihydrogenphosphate, pH 4.0 (60 40 v/v) separation voltage, 15 kV injection, 5 kV, 3s applied pressure, 8 bar on both sides sample, 2.4 mM cinosulfuron dissolved in mobile phase. Reproduced with permission from Rapp et al. [128].

The dominant stationary phase in HPLC is modified silica and, to be more specific, octadecyl silica (ODS). It should be pointed out that there could be great differences between various types of ODS materials or even between different batches of the same material. Carbon load, free silanol content, endcapping, type of silica, and coupling chemistry to the Cig moiety, not to mention the several physical characteristics of the packing material all involve the behavior of an ODS column. However, a rather safe generalization is that, in such material, hydrophobic interactions are a dominant mechanism of separation [3-7]. 

Figure 10.9. Comparative chromatograms of a good and a failed silica-base C18 column due to exposure to high pH mobile phase. Severe peak fronting indicated channeling of the packed bed while peak tailing of the basic analyte (amitriptyline) indicated increase of silanophilic activity due to loss of endcapping or bonded groups. Figure courtesy of Waters Corporation.

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