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and varying baseline will result. In this case the AMMS III or the ASRS-ULTRA will need to be serviced as outlined
in the AMMS III manual (see Document No. 031727, Product Manual for the Anion MicroMembrane Suppressor III,
the AMMS III or Document No. 031367, the “Product Manual for the Anion Self-Regenerating Suppressor-ULTRA,
the ASRS-ULTRA.”)
Carbonate Peak
When using a carbonate eluent, carbonate in the sample is not normally evident in the chromatogram. However, when
using other eluents (e.g., borate) a carbonate peak will be observed. The elution time of this peak will depend upon
the pH of the eluent. The higher the pH, the higher the carbonate/bicarbonate ratio. The divalent carbonate form tends
to elute later then the bicarbonate form. The converse is true as the pH is lowered. The borate eluent described in this
manual is an equimolar solution of sodium tetraborate and boric acid. The boric acid is added to lower the tetraborate
eluent pH. This places the carbonate peak between chloride and nitrite (see Figure 5, Anion Separation using 22 mM
Borate Eluent) when using a 22 mM borate eluent formulation. When using a more dilute eluent the carbonate is
retained longer since it is divalent. This explains why the carbonate peak elutes after the nitrite peak when using the
10 mM borate eluent formulation. Since the carbonate peak is reduced by almost two orders of magnitude during
passage through the AMMS III or the ASRS-ULTRA, this method is not useful for carbonate quantification.
Eluent Impurities
Use an Anion Trap Column (ATC-3, P/N 059660) to retard divalent anion contaminants (e.g., sulfate) in dilute eluents
(e.g., 10 mM borate) so that they do not concentrate on the analytical column when the dilute eluent is being run through
the column and then elute as sharp bands when the following strong eluent (e.g., 50 mM borate) is pumped through
the column. Dilute eluent contaminants eluting as sharp bands in the strong eluent can interfere with the quantification
of sample divalent anions (see Figure 6, Anion Separation using 10 mM Borate Eluent with Column Purge after
Nitrate). If the tetraborate salts or deionized water are contaminated (e.g., with sulfate) this will prolong the
reequilibration process from the strong eluent back to the dilute eluent.
6.2.3 A Contaminated Guard or Analytical Column
Remove the IonPac AG9-SC Guard and AS9-SC Analytical Columns from the system. If the background conductivity decreases,
the column(s) is (are) the cause of the high background conductivity. Clean or replace the AG9-SC at the first sign of column
performance degradation (compared to the original test chromatogram) to eliminate downtime. Clean the column(s) as instructed
in “Column Care.”
6.2.4 A Contaminated Anion Trap Column, ATC-3
When doing gradient analysis, has the Anion Trap Column, the ATC-3 (4-mm) been installed correctly? If it has not, install one
as directed in Section 3.5, Installing the Anion Trap Column, and watch the background conductivity. If the background
conductivity is now low, this means that the ATC-3 is trapping contaminants from the eluent. The eluents probably have too many
impurities (see items 1 - 3 above).
If the ATC-3 is already installed, remove it. Is the background conductivity still high? If the background conductivity decreases,
the ATC-3 is the source of the high background conductivity.
A. Disconnect the ATC-3 (4-mm) from the injection valve and direct the outlet to waste.
B. Flush the ATC-3 with 200 mL of 70 mM Na
2
B
4
O
7
at 2.0 mL/min on a 4-mm system.
C. Equilibrate the ATC-3 with the strongest eluent used during the gradient run. Use a flow rate of 2.0 mL/min on a 4-
mm system.
D. If the problem persists, replace the ATC-3.
