Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 647

fixed concentration of a weak acid is mixed with an increasing concentration of a strong
base such as sodium hydroxide. pH gradients are essentially concentration gradients, as
the purpose of increasing the pH during an anion exchange run is to increase the
concentration of the dissociated form of the weak acid eluant.
Other types of gradients are more difficult to use. For example, a composition gradient
can be performed by changing from a weakly retained eluant ion to a much more strongly
retained ion. There is a wide difference in affinity for the resin between weakly and
strongly retained analytes, such as monovalent acetate and trivalent citrate. To elute the
more strongly retained citrate, ions from the stronger eluant must completely displace the
weakly retained eluant ions from the resin. Depending on the ratio of column ion
exchange capacity to eluant concentration, it may take a long time for the resin to be
converted from the weak eluant to the strong eluant form. (This effect is similar to
solvent demixing in reversed phase high performance liquid chromatography.) Also, to
repeat the run, the resin form must be converted back to the weak eluant. This can only
be done by pumping a high concentration of the weak eluant, thus increasing the
equilibration time between runs. Concentration gradients avoid these problems by always
keeping the form of the resin the same.
With gradient elution, changing the concentration of the eluant results in changes in
background conductivity. By using chemical eluant suppression to reduce the highest
background conductivity to less than 10µS, baseline changes during a run can be limited
to only a few µS. This is accomplished using an appropriate eluant and an Anion Micro-
Membrane Suppressor. In addition, baseline changes of up to 20µS can be compensated
by a technique described later.
There are two types of eluants which can be suppressed to produce background
conductivities of only a few µS. One type is the salt of a weak acid. The weakest acid in
aqueous solution capable of forming a salt is water itself, with the salt being a strong base
such as sodium hydroxide. It is an excellent choice as an eluant for gradient elution as it
is converted in the suppressor to water regardless of its concentration. There are other
salts of weak acids which can be suppressed to produce low background conductivities.
These include borate and many phenates such as p- cyanophenate. In general, the salts of
weak acids with pK a ′ s greater than 7 are acceptable. As pK a decreases, the extent of
dissociation following suppression increases, resulting in increased background
conductivity and increased baseline shift during gradient elution. The other type of eluant
which can be used includes those anions which are anionic at high pH but are converted
to zwitterions or cations following suppression. Amino acids such as glycine are good
choices because they are primarily converted to their cationic form and removed by the
suppressor.
Because sodium hydroxide is converted to water in the suppressor, it is the best choice
for an eluant. As long as the capacity of the suppressor is not exceeded, the eluant
hydroxide concentration has little effect on background conductivity. The suppression
capacity of the AMMS-MPIC Suppressor (P/N 37106) usually exceeds 100mM sodium
hydroxide at a flow rate of 1.0mL min −1 . A gradient run could begin with a few mM
sodium hydroxide and end at 100mM. Examples of gradient elution using sodium
hydroxide are shown in Figs. 12.17 and 12.19. In Fig. 12.19, the HPIC-AS5 column is
used. This column can elute anions with charges ranging from monovalent for chloride to