Chromatography Forum

I have started to get reduced recoveries for my calibration verification standard which is run every 10 samples to check the curve. The reduced recoveries post at 70% and return to 100% randomly. I have run a batch of 60 consecutive samples with known values and i noticed random drops in recoveries. There are no drops in pressure or visible leaks.

Does anyone have any suggestions on possible cause? Thanks

Ion chromatography:
Dionex ICS 2000
Dionex AS-AP Autosampler
Principal analytes: F, Cl, SO4
CRD 200 4mm
AERS 500 4mm Suppressor
Eluent: EGC III KOH @ 32mM
Flow rate: 1.00 mL/min
Supressor: 80mA
Pressure: ~2010 PSI

I've never used the AS-AP but it is possible that it is incompletely filling the injection loop. If the total background is consistent, pressure is consistent and separation is consistent, I'd look to make sure the autosampler is performing consistently.

Is it a syringe based system? I know occasionally our syringe based autosampler had air bubble issues.

Out of curiosity, which column are you using?

Thanks for the response.

The inconsistency of recoveries aka increase variation in reproducibility may be itself a symptom of autosampler inconsistency. AS-AP autosampler uses a syringe which is primed several times before any runs commence. Reagent grade nanopure water (18 mega ohm resistance) is sonicated for atleast 1 hour before being used to prime the syringe. There are other degas measures installed to eliminate the problem of air bubbles.

How did you resolve the problem with your autosampler air bubble issues?

For your curiosity, I use an AS18 Column

With the AS3500, I think we rinsed the syringe with a mixture of degassed DI and methanol. Most of our systems now use the old style AS40 which are not syringe driven.

When is the last time your injection valve was serviced? You may have a partial blockage in the valve or sample loop that is preventing consistent injection volumes?