Communication. Liquid nitrogen cooling in microwave digestion

In an attempt to minimize the delay in opening Teflon pressure vessels following microwave acid digestion, and thus significantly reduce sample preparation time, various approaches to vessel cooling have been investigated. These include the feasibility of carrying out digestions with the pressure vessels immersed in liquid nitrogen and the use of liquid nitrogen as a pre- and post-digestion coolant. Liquid nitrogen cooling in the microwave unit was found to decrease digestion rates considerably, although the prevention of rapid and uncontrollable increases in pressure during digestion of organic material could be useful in some cases. Liquid nitrogen cooling subsequent to or between heating cycles was found to be very effective, especially where several heating, cooling and venting cycles were required. As well as enabling the vessels to be cooled rapidly and opened, it stopped any continuing increase in pressure which could otherwise cause the hot vessels to vent with potential loss of sample. Pre-digestion cooling also helped to delay the onset of rapid increases in pressure.