| Column | Purospher® RP-18 (5 µm) LiChroCART® 125-4 HPLC columns | |
|---|---|---|
| Mobile Phase | Methanol/Water 55/45 (v/v) | |
| Flow rate | 1.0 mL/min | |
| Temperature | ambient | |
| Detection | UV 254 nm | |
| Sample | 1. Thiourea 2. Aniline 3. Phenol 4. p-, m-, o-ethyllaniline 5. N,N-Dimethylaniline 6. Ethylbenzoate 7. Toluene 8. Ethylbenzene |
to basic acidic basic basic neutral neutral neutral |
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Engelhardt Test |
Hydrophobicity and Silanolic Activities of Purospher® Stationary Phases
The Engelhardt test was performed on various Purospher® stationary phases to verify their selectivities. This 8-compound test, developed by Engelhardt et al.*, has become one of the standard procedures for evaluating the hydrophobicity and silanophilic activity of reversed-phase column packings. Toluene and ethylbenzene demonstrate hydrophobic properties, whereas phenol and ethylbenzoate establish neutral polar interactions. The critical behavior towards basic compounds is evaluated by the injection of 5 different anilines. If there is good suppression of the silanolic activity of the stationary phase, then aniline is eluted before phenol with excellent peak shape, and the geometric isomers of ethylaniline co-elute.
* Engelhardt et al., Chromatographia 29, 59, 1990
Purospher® RP-18
Due to the amino endcapping of Purospher® RP-18 with polar interactions to the analytes, co-elution of the geometric isomers of (p-, m-, o-) ethylaniline (4) could not be observed. Both the anilines (2, 5) elute as symmetrical peaks, thus indicating the excellent suitability of Purospher® RP-18 for the separation of strong basic analytes.
Purospher® RP-18 Endcapped
As there are no polar interactions on the surface, Purospher® RP-18 endcapped shows perfect co-elution of p-, m- and o-ethylanilines (4). The anilines (2, 5) both elute with a symmetrical peak shape, demonstrating excellent suitability for the separation of strong basic analytes. The high retention time of ethylbenzene (8) indicates the hydrophobicity of the stationary phase.
Purospher® STAR RP-18 Endcapped
Purospher® STAR RP-18 endcapped shows perfect co-elution of p-, m- and o-ethylanilines (4), indicating no polar interactions due to silanol activity on the surface. Both the anilines (2, 5) elute symmetrically, demonstrating excellent suitability for the separation of strong basic compounds. Purospher® STAR RP-18 endcapped exhibits a similar selectivity as Purospher® RP-18 endcapped, but has an extended pH range.
Purospher® RP-18 HC
As there is no endcapping of the bonded phase, Purospher® RP-18 HC shows very clear polar interactions with the residual silanols on the surface. Aniline and phenol (2,3) elute as one peak, whereas basic compounds demonstrate increased retention but loss in performance. Therefore, this column is suitable for the separation of polar, non-basic analytes, such as explosives.
Purospher® STAR RP-8 Encapped
As there are polar interactions due to the thin layer of hydrophobic coverage with short C8-chains, Purospher® STAR RP-8 endcapped shows no co-elution of p-, m- and o-ethylanilines (4). The anilines (2, 5) elute as symmetrical peaks, demonstrating very good suitability for the separation of strong basic compounds. Both ethylbenzoate (6) and toluene (7) elute as one peak, which is typical for C-8 phases.
Chromatographic Properties of Purospher® Stationary Phases
| Peak symmetry of complexing agents | Polar inter-actions | Steric selectivity | CH2-group selectivity | Silanol group activity | Hydro-phobicity | |
|---|---|---|---|---|---|---|
| Purospher® RP-18 | +++ | ++ | +++ | +++ | ++ | + |
| Purospher® RP-18 endcapped | +++ | - | +++ | +++ | - | +++ |
| Purospher® STAR RP-18 endcapped | +++ | - | +++ | +++ | - | +++ |
| Purospher® STAR RP-8 endcapped | +++ | + | + | ++ | ++ | + |
| Purospher® RP-18 HC | +++ | ++ | +++ | +++ | + | ++ |
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