Hello,
I´m working on the Ion chromatography in anion mode detecting Lithium salts in product. In time I´m observing that the peak height of analyte is continuously increasing even though the sample % concentration is still the same. Can anyone help me and explain why is this happening?
Example of peak heights vs. concentration of one Li salt at different measurements in one week.
23 - 12.00 %
24.5 - 12.00 %
25 - 12.00 %
26.3 - 12.00 %
It used to be the same height for the same concentrations but past few days we are observing this phenomena.
Thank you for your help.
1) Are you sure that you analyse exactly the same sample? No time effect? What is the nature of your sample, 'lithium salts in product' does not contain clear information.
2) How do you inject? Using an injection valve"? Is the valve correct functioning and well serviced?
3) How is the retention time and peak area. If the peak area is more ore less the same, the deterioration might be based on fouling or poor calibration as William suggested. Personally I strongly favor measuring peak areas in stead of heights.
Other questions: Do you use fresh solvents / mobile phase or do you recirculate them? Do you use a gradient?
May be your method need little improvement It might be possible that not all of your sample comes down in a fisrst run so you accumulate the compound over several runs.
You could try to clean system after each run by running a blank after the first injection to see if you actually get a blank spectrum or not. .
Jos Wielders thank you for your comments. To clearify the information. The product is electrolyte, containing organic solvents and additives such as mentioned Li salts, composition must be strictly the same at all samples. We are the QA laboratory. I can´t be specific about the composition. Anyway, we are using autosampler and injection valve. Retention time is ok, reproducible. As we are a QA laboratory we have on everything specific procedures that must be followed and it says to measure peak heights not areas, unfortunatelly, which I personally also prefer and agree with you. We use fresh mobile phase, which is changed every day.
It is better to handle the problem by measuring the signal-to-noise (S/N) ratio...If the height of your baseline conductivity is also increasing it is expected to see your analyte signal height will be increased. In that case, considering the mobile phase composition, freshness, and used salts (batch, brand, etc.) are appropriate and reproducible, then you may think on the suppression module (if applicable). Just record the starting conductivity and noise height of your baseline.
Do you obtain the same heights when you make the consecutive injections using the same sample on the same day?(for instance, 5 replicates as intra-day repeatability)
OK I understand that you cannot be specific about the sample. But is it in a water, / organic solvent mix that is fully miscible with the mobile phase you are using.
Is the mobile phase buffered? Lots of questions, I am only searching for possible sources of variance.
Is you Li peak free of other neighbouring peaks? Smooth baseline?
If the peak height measurement is mandatory, probably the area is recorded as well. If your peak area remains more or less constant, than you might have a clue .
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