Instant Notes: Analytical Chemistry

D6 – HPLC: principles and instrumentation 161

Si OH CISi

CH 3

CH 3

R

Si OH

Si O Si

CH 3

CH 3

R

Si OH

(a)

(b)

Si O Si

CH 3

+ CISi(CH 3 ) 3

CH 3

R

Si OH

Si O Si

CH 3

CH 3

CH 3

Si O Si

CH 3

R

CH 3

End capping

Fig. 4. Formation of bonded-phase silicas. (a) Monomeric bonded phases; (b) End-capping

of residual silanols. R = alkyl, aminoalkyl, ion-exchange groups.

Table 3. Stationary phases for HPLC

Stationary phase Sorption mechanism Characteristics

Unmodified silica, SiO 2 Adsorption, normal-phase Polar, retention times variable due to
adsorbed water
Bonded phases
Octadecyl silica, -C 18 H 37 (ODS or C18) Modified partition, reversed- Nonpolar, but unreacted silanol
Octyl silica, -C 8 H 17 phase groups cause polar solutes,
Propyl silica, -C 3 H 7 especially bases, to tail, pH range
limited to 2.5–7.5
All separate a very wide range of
solutes

Aminopropyl, -C 3 H 6 NH 2 Modified partition, normal or Polar, separates carbohydrates
reversed phase pH range limited to 2.5–7.5

Sulphonic acid, -(CH 2 )nSO 3 H Cation-exchange Slow mass transfer broadens peaks,
limited sample capacity, pH range
limited to 2.5–7.5 for silica-based
materials

Quaternary amine, -(CH 2 )nNR 3 OH Anion-exchange

Controlled-porosity silicas (some with Size exclusion Compatible with both organic and
–Si(CH 3 ) 3 groups) aqueous solvents, pH range limited
to 2.5–10

a-, b-, g-cyclodextrin silicas Chiral selectivity based on Expensive, limited life, resolution
adsorptive interactions sensitive to mobile phase
composition
Polymer phases
Cross-linked styrene/divinyl benzene Modified partition, exclusion Nonpolar if unmodified,
co-polymers, unmodified or with or ion-exchange stable over pH range 1–13
ion-exchange groups