Is pressure important in microwave digestion?

Pressure is only important for microwave digestion because it allows for higher temperatures to be achieved once you seal the vessel.  (PV=nRT).  It is temperature that allows for the more complete digestion to occur as compared to atmospheric techniques such as hot plates or hot blocks.  In theory you would like all of the pressure generated in the vessel to be the vapor pressure of the acid.    However, with digestion comes decomposition gases such as CO₂ and NO_x.  These gases must be vented off in order to allow for higher temperatures needed for complete digestion.  CEM’s vent and seal vessels allow for the safe off gassing of excessive pressure without losing any volatile analytes.   So while pressure does not play a role in actually digesting the samples, it is a factor you must take into account when performing microwave digestion.