Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 652

understood, they do combine to produce an acid considerably stronger than either of the
two by themselves. As a neutral polyalcohol, mannitol has little effect on either
separation or detection. Boric acid in the dilute sulphuric acid regenerant is neutral, and
can diffuse through the cation exchange membranes of the MPIC suppressor. The two
then combine in the suppressor, producing a conductive species. By decreasing mannitol
concentration in approximately inverse proportion to the increase in eluant concentration,
the baseline changes can be counteracted. This method is demonstrated in Fig. 12.22, in
which the mannitol was used to balance the baseline in (b), but not in (a).
Another method of baseline compensation is computer baseline subtraction. For this
method, a blank run with no injection is stored in the computer's memory and subtracted
point by point from succeeding runs. This method works well as long as the baseline
profile is reproducible.
A problem encountered in gradient elution is the presence of extraneous peaks in the
chromatogram caused by trace impurities in the eluant. In anion exchange, the most
common impurities are chloride, sulphate and carbonate. In the early part of a gradient
run, while the eluant is weak, these ions are concentrated at the front of the separator. As
the eluant strength increases, they are eventually eluted, and appear as interfering peaks
at the expected retention times. Carbonate can be minimised by using properly deionised
water and low carbonate sodium hydroxide. The effect of contaminants can be minimised
by placing a small column, called the anion trap column, in front of the injection valve.
The anion trap column is filled with high capacity, low efficiency anion exchange resin.
It successfully prevents trace anionic contaminants from reaching the separator column
during the early part of the run. Later in the run, the eluant strength may be high enough
to elute the contaminants. The low efficiency of the resin spreads out the contaminant
peaks so that they do not interfere with the chromatography. A consequence of the use of
an anion trap column is that if a composition gradient is used, the time needed to convert
the form of the resin in the anion trap column must be considered.
An alternate approach is to use a higher efficiency trap column which is regenerated at
the end of each gradient run with a high concentration eluant, removing any accumulated
ionic contaminants. The HPIC-AG8 guard column can perform as a trap column because
of its high affinity for carbonate, the primary contaminant in sodium hydroxide eluants.
An example of the use of an AG8 as a trap column is the separation of anions shown in
Fig. 12.23. For this chromatogram, carbonate and other contaminants on the trap column
were removed with a strong borate eluant. The sample is then injected before the sodium
hydroxide eluant has removed all of the borate from the trap column. The small amount
of borate left forms a complex with quinate, increasing its retention from co-elution with
acetate to elution between glycolate and formate.
12.2.5 Miscellaneous
Motomizu et al. [25] compared the efficiencies of derivatives of various
benzenecarboxylic acids as eluting agents in the ion chromatographic determination of
anions and cations.
Various workers have discussed the applications of ion chromatography to the analysis
of mixtures of anions and cations in peatland waters [26] and non saline waters [27-34].