snowysummer,
The negative excursion you observe at the beginning of the chromatograms is a direct consequence of the fact that the background you are working from when using the standard eluent system is due to the presence of 4.5 mM carbonic acid (the byproduct of the suppression reaction). When you inject a sample containing no carbonate, there is a diminished background signal corresponding to the system void volume due to the fact that the sample contained little or no carbonate. If you inject eluent, you will see no disturbance is located at the system void volume. If you inject a double concentration solution of eluent you will see a peak at the system void volume location. It might seem confusing as to why you observe this disturbance at the void volume considering the fact that the retention time for carbonate is significantly different than the time for this baseline disturbance to manifest but this is a direct consequence of the fact that stoichiometry must be maintained for anions to relative to cations as the sample passes through the column. Consequently, there is a disturbance corresponding at the system void volume with an ionic strength exactly the same as the ionic strength of the sample that was injected. If you studied the situation further with radiotracers you can show that all analytes, including carbonate present in the sample are eluted normally as one would exact in an ion exchange process. The carbonate present in the sample in such a case actually elutes at the retention time one would normally expect for carbonate. The disturbance at the system void volume involves carbonate present in the eluent not carbonate present in the sample. The situation is actually a bit more complex than this due to the fact that you are using a buffered system but the above explains the basic cause of the negative excursion.
