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ALEXYS system with poor reproducibility

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Applies to

ALEXYS system, AS 100 autosampler

Summary

Poor reproducibility in peak height and peak area. Typically %RSD improves with concentration, however sometimes the %RSD is much worse than expected.

Cause

Possible causes:

  1. Autosampler
    • most often: air in sampling tubing or syringe
    • leakage of injection loop, valve, syringe, needle or tubing
    • not enough sample in vial
    • air bubble on bottom of micro vial
    • contamination between injections
  2. Data acquisition
    • baseline not correctly drawn
    • unstable baseline, large noise, small peaks
    • interfering peaks, shouldering peaks
  3. Pump
    • leakage at connectors or seals
  4. Detector, HPLC
    • contamination of working electrode
    • reference electrode contaminated, chloride concentration not constant
    • poor column performance, contamination of column
    • not optimised detector settings
    • carbohydrate analysis: carbonate in mobile phase increases ionic strength, constant shift in retention times

Remedy

  1. Autosampler
    • remove air from tubing and syringe
    • apply maintenance to autosampler
    • needle wash ‘after each injection’ (in stead of ‘after each vial’)
  2. Data acquisition
    • optimise acquisition parameters
  3. Pump
    • apply maintenance, replace seals in case of leakage
  4. Detector, HPLC
    • apply maintenance to flow cell
    • add constant chloride concentration to REF or mobile phase (ISAAC)
    • replace column (apply cleaning procedure first)
    • optimise detector settings
    • carbohydrate analysis: proper degassing and application of He to prevent carbonate from dissolving in mobile phase.
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Next My bio-inert HPLC system for carbohydrates show a rapid decrease in peak heights
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