Sample  Uptake  Notes

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What is the use of measuring sample flow   when the sample is being pumped ? :

  • Firstly, pumps vary in accuracy :  consider the peristaltic pump for example -
      peristaltic pump tubes deliver liquid sample in a way that can change with time,
    especially if tubes are strongly compressed.
      - a typical result of flow measurement is shown below - only in region 2 is the flow reasonably constant.
      - thus constant pump rotational speed is not always a guarantee of constant sample flow.

  • Secondly, the sensitivity of most analytical instruments varies with sample flow in a non-linear way -

  • In region A, even small pump irregularities will show up markedly in the analytical signal ...

  • In region B, signal fluctuations from the pump are less noticeable, and ...

  • In the 'saturation region' C, pump fluctuations will have the least effect,
      - but sample consumption will be at a maximum.

    If sample uptake is measured by weight or volume differences over periods of minutes,
    these fluctuations won't appear in long-term flow measurements.
    The analytical signal could still be unstable however.

    By making measurements in 'real-time' with a meter such as the  XENA ,
    short-term flow changes will be recorded ... and noticed.
    For quality assurance, place a  XENA  permanently between sample and instrument.

    ** An added benefit of these meters  is that unwanted air bubbles will be noticed : these bubbles,
    even though they are often very small,  are a common cause of instability.

    Contact EPOND for the real-time flow meter to suit your application : ICP-OES, ICP-MS, AAS or other technique

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