Thermal characterization of a bio sample using a heat flux sensor-based multipurpose AC microcalorimeter

Microcalorimetry is used to measure the thermodynamic state of a bio sample in several microliters of a liquid medium. To determine the heat from a single unit, the total heat from the sample and the number of units are needed at the same time. A scaling law increases the effect of evaporation in terms of the volume decrease and latent heat during the heat flux measurement. In this paper, to characterize those multivariable parameters, the AC calorimetric technique is applied to a heat flux-based microcalorimeter. An oscillation current is applied to the heater and the temperature responses are analyzed in the frequency domain. The oscillation heat from the heater propagates through the sample and reaches the sensor as DC and AC signals in the frequency domain. The DC signal represents the heat flow from the sample while the AC signal indicates the change in heat capacity of the sample. An FFT analysis was performed to extract these values. The experimental results show that, not only the latent heat of water, but also the growth of Escherichia coli bacteria in an LB medium can be measured with very high accuracy. Consequently, the AC calorimeter can be used to distinguish latent heat and the change in heat capacity. Moreover, the growth of E. coli can be characterized in terms of metabolism and concentration increment.