Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 649

Gradient program:

Time %1 %2

0 75 25

5 0 100

Source: Reproduced with permission from Dionex Corporation [24]

excellent selectivity of diprotic organic acids. Due to its higher capacity (approximately
10 times that of the AS5A), the stronger eluant ion p- cyanophenate is necessary to elute
di- and trivalent ions. An example of gradient elution on the HPIC-AS6 using p -
cyanophenate is shown in Fig. 12.18. In this run, the concentration of p -cyanophenate in
the eluant is increased from 2.4mM at the beginning of the run to 35mM at the end.
Carbonate containing eluants are generally not acceptable for gradient elution.
Following suppression, carbonic acid is formed. With a relatively low pK a of 6.2, the
baseline drift is too severe. Although salts of weak acids with pK a s as low as 5 can be
used for isocratic elution, due to their higher background conductivity following
suppression, they are generally not acceptable for gradient elution.

Fig. 12.20 Retention versus ramp slope
Source: Reproduced with permission from Dionex Corporation [24]

The relationship between elution volume and gradient steepness (ie how rapidly the
eluant concentration is being increased) is described by equation (1) and shown