Ghost peaks

Has anyone seen ghost peaks appearing prior to "real" peak by LC-MS/MS ? I've even seen some ghost peaks while the flow coming from the UPLC was diverted to the waste !

Answers

  • Greetings Fred,

    In my experience with ACQUITY, the only, consistent, "ghost" peaks I have encountered have been air peaks. If you are employing the Partial Loop "pressure assist" (PA) mode of injection, the trailing air gap is injected along with the sample. This peak is usually observed slightly after the void (solvent front) and prior to any peaks of interest. The Full Loop and Partial Loop with Needle Overfill (PLNO) injection modes do not co-inject either of the air gaps. This air gap peak should be present in blank injections as well as standards and samples. If you are using the PA injection mode, you can test this hypothesis several ways. You can try one of the other injection modes and determine if the ghost peak persists or not. You can reduce the trailing (post) air gap and see if the ghost peak scales with the reduction. Finally, if you determine that this is an air peak and find that it interferes with your peak detection, you can transfer to the PLNO injection mode and reduce the Needle Flush Volume down to almost nothing (I believe the minimum is 3 uL). This should keep the air gaps out of the sample loop and not use an excessive amount of sample.

    If you are not using the PA injection mode, then this is all (perhaps) interesting, but irrelevant. In that case, please provide more information and I will try to help in discovering the source.

    Hope this is helpful,

    pcb

  • Here is an excellent article for anyone battling with "ghost peaks". It has a section dedicated to "air peaks".

    Williams, Stuart. “Ghost peaks in reversed-phase gradient HPLC: a review and update”. Journal of Chromatorgraphy A, 2004, 1052, 1-11.

    Regards,

    Shen

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