Temperature modulated DSC (TMDSC) - Applications and limits of phase information, c(p) determination and effect separation

The ADSC (Alternating DSC [1]) technique superimposes upon the conventional constant heating rate a periodically varying modulation [2-8]. The modulat ion creates high instantaneous heating rates which increases sensitivity. T he low underlying constant heating rate is used to get better resolution. W ith ADSC it is possible to separate overlapping thermal effects without los s of sensitivity and to determine heat capacities under quasi-isothermal co nditions. It has been reported that there are also some limitations for the use of the modulation techniques, i.e. that the accuracy of c(p) determina tion is reduced at higher modulation frequencies due also to thermal diffus ivity within the sample itself [9, 10]. In this contribution, the limitations given by the measuring system itself will be discussed. A key value is the limit frequency of the sensor arrange ment. In the Mettler Toledo DSC821 degrees this frequency is approximately 1/3 Hz. From these findings the following recommendations amongst others ca n be given: for light mass crucibles, 30 s periods are reasonable with ampl itudes not exceeding the heating/cooling rates possible. A blank and a cali bration measurement will eliminate cell asymmetry and will enhance the accu racy of c(p) measurements even at higher modulation frequencies.