Multi-frequency sawtooth modulation of a power-compensation differential scanning calorimeter

In this paper experiments with temperature-modulated differential scanning calorimetry (TMDSC) are described in which the modulation consists of multiple, superimposed frequencies. The harmonics of the Fourier series of the measured heat-flow rate and temperature are deconvoluted to extract data pertaining to the different frequencies. In order to derive 1st, 3rd, 5th, and 7th harmonics of similar amplitudes, a complex, but simple-to-program sawtooth is generated. In TMDSC with sawtooth modulation with a period of less than about 150 s, a correction by extrapolation to zero frequency is necessary. It is shown that the extrapolation of the heat capacity calculated from the amplitudes of the first and higher harmonics of the Fourier-transforms of heat-flow rate and the sample temperature permits this evaluation of the heat capacity from a single experiment. The method is tested on a power-compensation type TMDSC with temperature control close to the calorimeter heater. The time constant for the response of the calorimeters is less than 30 s (τ < 5 s rad^-1). Instabilities of the calorimeter response are removed by smoothing.