# Normal phase dead time marker

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).