Carbon deposits on a quartz cell

What is the reason for the appearance of black carbon deposits on a quartz cell in an air-acetylene flame? The ratio of air/acetylene gas mixture is 13.5 / 2 and flow rate 10 dm3/min. Agilent 240fs with VGA 77.

  • Hello Paul, it looks to me that there is too much acetylene in the gas mixture coming to the burner. This is causing the carbon build-up.

    I'd suggest you first remove the absorption cell from the holder and clean this by soaking in dilute nitric acid for at least 30 minutes. Rinse thoroughly with distilled water and allow it to air dry. Then you can refit this to the holder.

    Make sure the cell and the holder is removed from the burner. Then ignite the flame and adjust (reduce) the acetylene flow until you have a blue flame with a thin lighter blue section close to the burner slot. Then leave the gas flows at that setting. Turn the flame off.

    Then refit the cell and holder. You can then reignite the burner. With the adjusted acetylene flow, I don't think you will have any further carbon build-up.

    Hope that helps you,
    Cheers Eric

  • Thank you for you cooperation. The cell and its holder were cleaned by method that recommended. Acetylene flow rate were set at the lower level for Agilent 240FS - 1.5 dm3/min. The carbon build up less, but its present still. What kind of flow rate level is minimal for my system? 

  • Hello Paul,

    I would expect that working with flows set to these values would ensure no carbon build-up:
    air 13.5 L/min
    Acetylene 1.5 L/min.

    As I understand, you had set those flows - but are still seeing carbon build-up. What does the flame look like?
    Is it blue in color - or is there any yellow tinges in the flame?
    Is there any chance you can include a photo of the flame?

    Do you see any difference in the appearance of the flame as you change the set flow rate for acetylene?
    For example, if you reduce the acetylene flow to 1.0 L/min and then increase to say 2.0 L/min

    I'm wondering if there is an issue with the flow control for the acetylene on your instrument - and this troubleshooting is intended to confirm if this is likely,

    Cheers Eric

  • This is my conditions of flow rates.

    At this conditions flame looks like that:

    If I set up acetylene flow 1 dm3/min - flame does not appear. 1.5 dm3/min is lower lever for this set up.

    This is how cell looking after 1 min of working:

    If I increase acetylene flow rate up to 2 dm3/min flame beginning looks like that (its color is yellow)

    Just frustrated. Don"t even know what to do.

  • Hi Paul,

    Thanks for attaching the photos. This does confirm there is excessive acetylene flow - so there is a fault with the gasbox in your instrument.
    I would recommend you request a service call to get this checked.

    In the interim, you should be able to complete your analysis by with the acetylene flow set to the minimum flow of 1.5 L/min.
    While there is still some carbon build-up on the cell, this is on the outside of the cell and should not impact on the light throughput through the centre of the cell.

    Hope this helps you,

    Cheers Eric

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