Why does the solvent front often have a sinusoidal shape?

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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I am wondering why the solvent front in HPLC often appears as an upward peak followed by a downward peak, or vice-versa. I imagine this has something to do with the different refractive index of the diluent and the mobile phase. When the diluent hits the detector (the solvent front) the different refractive index of the diluent causes a change in absorbance. However, I don't see why the solvent front signal is TWO peaks, one up and one down.

Thank you for any information on this!
The true solvent front does not have a sinusoidal shape. Perhaps you are looking at the pressure pulse made by the injector, which shows up as an artifact at the T zero time? That small "pulse" does indeed have a sinusoidal shape as it it caused by the making and breaking of pressure (the pressure change) which results from the injector valve switching between, in and out-of the fluid stream of mobile phase. Because this artifact is the result of a pressure disturbance or change in the system pressure (and flow), it will be observed at low sensitivity on the baseline (via UV wavelength or RI, or really most any detector).

If observed, any actual 'solvent front' peak shape for an unretained compound will not be sinusoidal, but look like a regular peak, at tzero, often with some tailing. If the solvent front shows overloading, then it may appear as a truncated, off-scale peak with tailing, but not sinusoidal.

If you want to learn more about HPLC principles and fundamentals, please consider reading one of the classic texts on the topic such as, "Introduction to Modern Liquid Chromatography".

    More information on "T zero" and Column Void Volume: "Determination of HPLC Column Void Volume / Dead Volume, Dead Time (T zero)"; https://hplctips.blogspot.com/2011/05/d ... -time.html
    Hi Multi, I'm relatively new to chromatography and have also noticed this sinusoidal shape on my injections. When you say that both the sinusoidal "peak" and solvent front "peak", if there is any, both come out at t0, how can I separate them/identify which of these phenomena I'm looking at. I know, for example, that the negative area from the sinusoidal peak correlates to the pressure and flow change upon injection, but where should I look for the solvent front peak? Directly before the negative peak usually?

    Anyway thanks for reading, this was my first comment.
    3 posts Page 1 of 1

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