Page 165 - Illustrated Pocket Dictionary of Chromatography

166 RESIDUAL SILANOL GROUPS

residual silanol groups Silanol groups that are left on the silica
support surface after the bonding (both primary bonded phase and
endcapping, if done). Free silanol groups have the structure:

Surface Si OH

Residual silanol group
For many silica support materials the normal concentration of silanol
2
groups on the surface is ~8–9mmol/m . Surface silanols can be free,
vicinal, and geminal (see vicinal and geminal entries). The presence
of residual silanol groups is responsible in part for the extreme tailing
that can be generated when amine-containing compounds are sepa-
rated. Mobile-phase modifiers such as trifluoroacetic acid and tri-
ethylamine are added to the mobile phase to help prevent interaction
of the analyte with the residual silanol groups and thereby reduce/
eliminate tailing.

resin An organic polymeric support material used in various sepa-
rations (e.g., IEC, GPC). Resins can be either macro- or microreticu-
lar, which is a function of their manufacture and determines many of
the resins’ physical characteristics. Resins are made from the poly-
merization of various materials such as polystyrene with divinylben-
zene, methacrylate, latexes, dextrans. These materials, either as is or
further modified through the addition of functional groups, are then
used as basic support materials in ion-exchange, size-exclusion, and
other types of separations.

resolution, R s or R A quantitative measure of the distance
between two adjacent peaks. It can be expressed in terms of reten-
tion time difference as a multiple of summed peak widths:
1 t
R s = 2 t - ) (w 1 + w 2 )
( 2
where t 2,w 2 and t 1,w 1 are the retention times, peak, widths for the
second (latest) eluting peak and first eluting peak, respectively. Con-
versely, resolution can be expressed in terms of column parameters:
R s =÷ ( N 4)(a 1 k k +1])
¢ [
¢
- )(
where N is the theoretical plates (column efficiency), a is the sepa-
ration factor (system selectivity), and k¢ is the capacity factor (column
retentivity).