How to correctly calibrate a Differential Scanning Calorimeter
Why are calibrations of thermal analyzers necessary?
Thermal analytical measurements are often used in quality assurance, so the measurement results must be precise and reproducible. This means the measured temperatures must be correct. Precise calibration and adjustment of the analysis equipment is therefore important.
Calibration determines the deviation of a reference substance from the published data. If the deviation is not within the desired tolerance range, the device must be adjusted.
A new calibration is required:
- with new devices
- according to specified time intervals
- if the device was exposed to external interference (e.g. vibrations)
- or when routine measurements yield dubious results.
Calibrating a DSC
As a rule, phase transitions of reference substances with known transition temperatures and transition enthalpies are used to calibrate and adjust DSC measuring cells.
Deviations can occur in the temperature, heating rate, and heat flow parameters on the sensor, so these need to be calibrated; see also ASTM E 967-97 and ASTM E 968-99. If the deviations are within the tolerance range, the measurement of the samples can be started. If this is not the case, adjustments will need to be made.
The crucible material (thermal conductivity, mass, geometry, size of the holes in the lid), the furnace atmosphere, the flow rate of the purging gas, and the DSC sensor itself can influence the parameters. Due to the countless resulting combinations of measurements, a separate adjustment is required for each combination.
The following sequence of adjustments has proven useful: first tau lag, then temperature, and finally sensor adjustment.
Tau lag adjustment
Tau lag is a time constant that describes the behavior of the furnace. When adjusted, the dynamic behavior of the measuring cell is corrected. It also ensures that the apparent influence of the heating rate has no effect on the measurement result.
If the onset temperature of indium is determined at different, increasing heating rates without dew point adjustment, the onset melting temperature shifts to higher temperatures, which is physically incorrect. At least two substances with different onset temperatures within the temperature range of interest are useful for an adjustment.
Temperature adjustment ensures that the onset temperatures correspond exactly to the published values. An adjustment with at least two substances with different onset temperatures is also recommended here.
The sensor adjustment is used to correct the signal value of the heat flow from sensor to crucible and crucible to sample. Here, too, we recommend adjusting with at least two references to obtain a high level of accuracy within the temperature range of interest.
A measurement process therefore requires the following steps: calibration, adjustment, checking the adjustment through repeated calibration, and only then sample measurement.
Certified reference substances for calibration
Reference substances should be easy to use, easily accessible, and stable.
Their melting point or enthalpy of fusion should be within the measurement range of interest, since extrapolations lead to inaccurate results.
Pure metals (>99.99% purity) such as gallium, indium, and tin, but also organic substances such as adamantane, benzoic acid, and naphthalene are used as calibration references.
Certified reference substances for temperature and heat calibration of DSCs are offered by LGC (UK), NIST (USA) and PTB (Germany). These are traceable reference materials. Such substances meet all the requirements of relevant quality management standards such as ISO 9000 and DIN EN ISO/IEC 17025.
In addition, some liquid crystals are available as certified secondary reference substances for the cooling mode. For purity determinations of slightly contaminated substances, LGC also offers reference substances with certified impurities.