Chromatography Forum

Hi all,

I'm having problem of getting several small negative peaks (at 5.3mins, 6.4mins and 7.3mins-this negative peak is larger than the other), followed by a large peak (7.6mins) in my blank, which is just ultrapure water.

But when I injected my eluent, the baseline is stable and constant.

What does it signify? Any idea? I have got no clue of it.

The system I'm using is suppressed Metrohm Compact IC pro with conductivity detector. It's for my anion analysis, like fluoride, chloride, bromide, nitrite, phosphate etc.

Thanks.

The negative peaks wheninjecting ultrapure water indicate ions in the eluent.
You will find the retention times identical to e.g. chloride, nitrate, and sulfate.
This means that you add these ions with the eluent components (including the water used for eluent preparation).
It is clear that you see a flat baseline when injecting eluent as you inject the same blank concntrations.

Be aware that your results are influenced by this. If you find e.g. a negative chloride peak corresponding to 1 ppb and you inject a sample of 2 ppb the peak you get will correspond to 1 ppb only!

The negative peaks wheninjecting ultrapure water indicate ions in the eluent.
You will find the retention times identical to e.g. chloride, nitrate, and sulfate.
This means that you add these ions with the eluent components (including the water used for eluent preparation).
It is clear that you see a flat baseline when injecting eluent as you inject the same blank concntrations.

Be aware that your results are influenced by this. If you find e.g. a negative chloride peak corresponding to 1 ppb and you inject a sample of 2 ppb the peak you get will correspond to 1 ppb only!

I see. Thanks for the clarification.

And yes, those retention time are identical with my standards.

I've got a few questions regarding it:
1. If that's the case as you mentioned, does that indicated that there are ions contamination in my ultrapure water system? Or maybe somewhere in the IC system?

2. I'm thinking of using eluent as my blank, and also using the eluent to do dilution on my standards. Is it be okay?
If the eluent itself showed flat baseline, probably it won't influence much on my results, right?
(Because if my ultrapure water is the main cause of contamination, it'll interrupting my result by showing negative peak in the blank; but not for the case of eluent).

Thanks again.

The source of ion contamination may be the water. If this is the case, make sure that your purification system is serviced regularely. Ultrapure Water is ultrapure as long as it is within the purification system. Therefore for eluent preparation use it fresh from the system, do not store it in between.
Another source might be the chemicals used for eluent preparation. Use just highest grade chemicals and make sure that nobody else it using the same bottle.

to 2.: this is OK also multipoint calibration (NOT forced though origin) takes care.

I see. Thanks for the help and information!