IonPac NS1 Manual
Document No. 034024-06
Page 21 of 35
4.2
Ion Suppression Chromatography
This chromatographic technique takes advantage of the effect of pH on the dissociation constants for the for acidic and basic
organic species. Depending upon the pKa of an organic ionic molecule, the extent of ionization can be controlled by pH.
Because the IonPac NS1 10-µm and NS1-5µm column packings have hydrophobic surfaces, a non-ionic or neutral organic
molecule will have a greater affinity for it. Therefore, retention and selectivity can be enhanced by suppressing ionization at either
end of the pH scale depending upon the acidic and/or basic nature of the molecule. (see figures 8 and 9).
Acidic molecules such as alkyl or aryl carboxylic acids can be protonated at low pH's (<4). Basic molecules can be deprotonated
at high pH's (>9).
O
-
OH
H +
Anionic:
R-C=O
R-C=O
pH < 4
OH -
Cationic:
R-NH
3
+
R-NH
2
+ H
2
0
pH > 9
Ionic
Neutral
Additionally, selectivity can be effected by working within one pH unit of the pKa of a particular organic species such as
substituted benzoic acids, (see Figure 10).
The difference in the pKa's of o-nitrobenzoic acid and benzoic acid was utilized to increase resolution of the compounds.
At the higher pH of 2.52 in the “after” chromatogram compared to pH 2.30 in the “before” chromatogram, the resolution of o-
nitrobenzoic acid and benzoic acid is almost complete. The pKa of o-nitrobenzoic acid is 2.18. As the pH is raised to pH 2.52,
the o-nitrobenzoic acid becomes more ionic and moves to shorter retention times compared to the benzoic acid which has a much
higher pKa of 4.19. The benzoic acid is completely protonated in both eluents and therefore its affinity for the column does not
change as rapidly as the o-nitrobenzoic acid affinity.
