During a lecture, my professor said that sublimation at atmospheric pressure (1 atm), as a purification/separation technique, can be performed when the sublimating substance has a triple point falling at pressures above 100 mmHg. Why is this assumption made? Shouldn't sublimation at atmospheric pressure only occur when the atmospheric pressure (1 atm) falls below the triple point? In fact, only under this condition can be found a temperature at which the vapor pressure of the solid equals atmospheric pressure.
I don't understand why we consider 100 mmHg as the limit. Furthermore, it has been said that the sublimation point of vanillin, at atmospheric pressure, falls at 48°C, so at these pressure and temperature values we actually fall on the sublimation curve. Right?