Heat capacity measurements using temperature-modulated heat flux DSC with close control of the heater temperature

The determination of heat capacity data with sawtooth-type, temperature-modulated differential scanning calorimetry is analyzed using the Mettler-Toledo 820 ADSC(TM) temperature-modulated differential scanning calorimeter (TMDSC). Heat capacities were calculated via the amplitudes of the first and higher harmonics of the Fourier series of the heat flow and heating rates. At modulation periods lower than about 150 s, the heat capacity deviates increasingly to smaller values and requires a calibration as function of frequency. An earlier derived correction function which was applied to the sample temperature-controlled power compensation calorimeter enables an empirical correction down to modulation periods of about 20 s. The correction function is determined by analysis of the higher harmonics of the Fourier transform from a single measurement of sufficient long modulation period. The correction function reveals that the time constant of the instrument is about 5 s rad(-1) when a standard aluminum pan is used. The influence of pan type and sample mass on the time constant is determined, the correction for the asymmetry of the system is described, and the effect of smoothing of the modulated heat flow rate data is discussed.