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In reverse-phase HPLC, Thiourea, Uracil, Nitromethane or $\ce{KNO3}$ are used as dead time markers because of their polarity and therefore weak interactions with the stationary phase. However, I could not find any substances typically used as dead time markers in normal-phase-applications. These would have to have small, non-polar molecules and a high boiling point (I know that's a little contradictory since nonpolar molecules only form weak intermolecular bonds).

Thanks in advance.

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Interesting and complex question for normal phase LC! Couple of years ago, it was addressed in quite detail. Most of the practitioners have accepted 1,3,5-Tri-tert-butylbenzene for normal phase LC because it is extremely non-polar. It works with most normal phase mobile phases. Boiling point has nothing to do with NPLC.

However, if you really want true "void" volume of a column, pycnometry is the way to go. It is quite simple.

Determination of void volume in normal phase liquid chromatography in the Journal of Chromatography A See paper here

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