Microwave Enzyme Inactivation System: Electronic Control to Reduce Dose Variability

The use of microwave energy to inactivate enzymes by rapid heating permits measurement of heat stable metabolites with minimal postmortem artifact. Reproducible results, however, require that the energy applied (dose) and the exposure geometry be reproducible. The commercial systems that we have examined are limited in this regard. A method to achieve a reproducible dose by electronic control of the applied power and exposure time is described. The electronics are compatible with any 220 V, signal phase commercial microwave power source. Without electronic control, exposure time varies by 50 ms, due to random contacter closure time. Output power varies by 15%, due to tube aging and variations in line voltage, magnetron temperature, and load impedance during exposure. With the addition of electronic circuitry, the exposure time can be controlled within 3 ms, and the power within 2%. Timing is done by counting energy bursts from the triac controlled magnetron. Leveling is accomplished using a feedback loop to vary the magnetron magnet current to keep the output power constant. The use of these electronic controls makes microwave irradiation systems more reliable as a biochemical research tool.