An optimized differential heat conduction solution microcalorimeter for thermal kinetic measurements.

Heat conduction calorimeters are widely used in the biological sciences, but baseline instability, low resolution, electrical noise and motion artifacts have limited their utility. Two main sources of noise, baseline fluctuation or drift and a motion artifact, were traced to amplifier drift, a small (0.015 degrees C) gradient within the constant temperature cylinder, and the method of installing the thermopiles. The addition of heaters to the top and bottom of the cylinder reduced the gradient to approximately 0.003 degrees C and greatly reduced the slow component of the motion artifact. The drift error was reduced by proper mounting of the amplifier and its external components and the enclosure of the calorimeter in a temperature-controlled box. An R-C model of the heat flow in the calorimeter was developed which was employed to discover several means of increasing sensitivity without increasing the rise-time of the calorimeter. Analysis, also based on the model, showed that variations in the air gap between the cell and cell holder can be a major source of error when the calorimeter is used to investigate the kinetics of a chemical reaction. This analysis also showed that the time for the heat to flow through the solution in the cell can be the dominant factor in determining the rise-time of the instrument. The heat conduction calorimeter described here has improved characteristics: a baseline stability of 200 nJ x s-1 (peak-to-peak) over a 48 h period; a resolution of 200 nJ x s-1; a sensitivity of 6.504 +/- 0.045 J x V-1 x s-1 referred to the sensor output; and a rise-time of 122 s for the 10-90% response.