The property measured in derivative thermogravimetric analysis (DTG) is :
Rate of change in weight
In ordinary thermogravimetric analysis (TG) the sample's mass is recorded as the temperature is raised, so the plot is mass against temperature and each decomposition step appears as a downward step in the curve.
Derivative thermogravimetry (DTG) plots the first derivative of that curve, \(\frac{dW}{dT}\) (or \(\frac{dW}{dt}\)), against temperature. The quantity measured is therefore the rate of change in weight, not the weight itself.
The practical value of taking the derivative is resolution. A step in the TG curve becomes a peak in the DTG curve, and two decomposition steps that overlap into one sloping shoulder on the TG trace resolve into two distinguishable peaks on the DTG trace. The peak maximum gives the temperature of fastest mass loss, and the peak area is proportional to the total mass lost in that step.
Change in weight is what plain TG measures, so it describes the parent technique rather than its derivative form.
Heat evolved is measured by differential scanning calorimetry (DSC) or differential thermal analysis (DTA), which follow enthalpy changes rather than mass.
Change in temperature is the independent variable that is scanned in all of these methods, not the measured response.
Hence DTG measures the rate of change in weight.
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