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 sam ple preparation time, various approaches to vessel cooling have been i nvestigated. These include the feasibility of carrying out digestions with the pressure vessels immersed in liquid nitrogen and the use of l iquid nitrogen as a pre- and post-digestion coolant. Liquid nitrogen c ooling in the microwave unit was found to decrease digestion rates con siderably, although the prevention of rapid and uncontrollable increas es 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 heatin g, cooling and venting cycles were required. As well as enabling the v essels to be cooled rapidly and opened, it stopped any continuing incr ease in pressure which could otherwise cause the hot vessels to vent w ith potential loss of sample. Pre-digestion cooling also helped to del ay the onset of rapid increases in pressure.