Instrumentation isothermal microcalorimetry

Wadso (11) states that one of the main applications for isothermal microcalorimetry is the investigation of noncovalent binding processes by means of titration techniques. This use of the instrument employs a cell with a liquid (pure liquid, solution, or suspension), which is stirred. It is then possible to inject small quantities of another liquid and to measure the heat flow from the events that take place. One event will be the dilution of the injected material into the fluid in the cell thus, it is necessary to correct for these dilution effects. Other responses will be due to any interaction between the injected material and the solute or suspended matter in the cell. If it is assumed that the titrated material all interacts with the solute and suspended material, then there will be almost zero free concentration of the titrated sample. This will allow calculations of the enthalpy for the binding process and the stoichiometry. 

Overall, it should be noted that isothermal microcalorimetry is really a bank of related techniques. The fact that almost any system can be studied, in one or more ways, with this technique makes it a valuable tool. However, the fact that concurrent processes and artifacts are also measured with ease makes mistakes probable. Consequently, it is important to remember that it is the easiest thing in the world to use this instrument to obtain data, but it can be very hard to define the exact process to which the data relate, and to provide appropriate interpretation. 

Isothermal microcalorimetry offers ultimate instrument sensitivity in the nanowatt region, which is 1000-fold greater than traditional DSC. This allows for the detection of reactions at lower, more relevant temperatures, thereby improving the likelihood of valid extrapolations. See Section 8.9.2 for more details of isothermal microcalorimetry. 

In the discussion that follows it will be shown that isothermal microcalorimeters can be used to detect many different types of processes. The combination of high sensitivity with the ability to detect almost any process makes the instrument a valuable tool. The potential problem, however, is the lack of specificity in many experiments. Consequently, it is quite possible to measure a process that is not the one that was the intended subject of the study. Even in the best experiments, it is extremely likely that one will measure more than just the one process that is the subject of the study. This Catch-22 aspect of microcalorimetry must always be at the forefront of the researcher s mind, i.e., the ability to measure all things is an advantage, but the disadvantage is that it becomes hard to prove that the measured response was in fact due to the process that was the intended subject of the study. 

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