Micro flow calorimetry
Micro flow calorimeters are calorimeters with measuring arrangements in which the sample is passed over the thermal measuring device. They measure heat quantities , heat outputs or heat capacities and are used in particular to determine process heat and specific thermal parameters. While the sample is usually fixed in the device with classic calorimeters, the change in temperature over time is not used as a measured variable in flow-through calorimeters, but rather the local temperature difference is measured , similar to scanning calorimetry (e.g. dynamic differential calorimetry ) . In terms of the measuring principle, micro-flow calorimetry is therefore less suitable as a method for determining absolute heat quantities than for measuring heat output. A quick setting of stationary thermal conditions is advantageous for this. This calls for small heat capacities for the measuring system. This is why flow calorimetry is particularly suitable for the smallest sample quantities and miniaturized measuring arrangements. Chip calorimeters, in which the thermal transducers are designed using thin-film technology and therefore have particularly small thermal masses, can be used in particular in micro-flow calorimetry. They are characterized by high sensitivity and low time constants.
Thermopiles as transducers in micro flow calorimetry
Thermoresistive or thermoelectric sensors are preferably used as sensors. Small temperature differences can be read out particularly sensitively by arranging pairs of thermocouples in rows , so-called “thermopiles”, as they are also used in IR radiation sensors . Measuring resistors, thermopiles and resistance heaters for calibration can be implemented using thin-film technology, which means that the corresponding metal or semiconductor layers are only a few 100 nm to a few micrometers thick. Therefore, these measuring elements have low masses and therefore low heat capacities. The proportion of these materials in the thermal conduction in the component can also be reduced as a result. The thin-film technology also offers the possibility of including insulation and protective layers of small thickness, so that the parasitic heat capacities of the electrical functional elements can be kept low overall.
application
Micro flow calorimeters can be used to determine the heats of reaction or - in the case of known heats of reaction after appropriate calibration - to determine concentrations and reaction rates , i. H. can also be used to measure kinetic quantities. In addition to chemical analysis , the measurement of substrate turnover in enzymatic reactions and the determination of enzyme activities are important fields of application. By using microfluid segment technology , flow calorimetry can also be carried out on test series with a high sample sequence or used as a monitoring method.
Individual evidence
- ↑ JM Koehler et al., Thermochimica Acta 310 (1998), 25-35
- ↑ S. Adamovsky et al., Thermochim. Acta 415: 1-7 (2004)
- ↑ JM Koehler et al., Microchimica Acta 120 (1995), 309-319
- ↑ J. Lerchner et al., Thermochim. Acta 445: 144-150 (2006)
- ↑ W. Lee et al., PNAS 106: 15225-15230 (2009)
- ↑ J. Lerchner et al., J. Therm. Anal. Calorimetry 127 (2017), 1307-1317
- ↑ A. Wolf et al .: Thermochimica Acta 603. 2015. 172-183