热重曲线以质量m为纵坐标，温度T或时间t为横坐标，即m～T（或t）曲线。将热重曲线中的质量（m）对时间（t）进行一次微商，得到dm/dt～T（或t）曲线，称为微商热重曲线（Derivative thermogravimetry，DTG），表示质量随时间的变化率（失重速率）与温度（或时间）的关系。相应地，以微商热重曲线表示结果的热重法称为微商热重法或导数热重法。目前，新型的热天平都有质量微商单元，可直接记录和显示微商热重曲线。
微商热重曲线与热重曲线的对应关系是：微商曲线上的峰顶点（d2m/dt2=0，失重速率最大）与热重曲线上的拐点相对应。微商热重曲线上的峰数与热重曲线上的台阶数相等，微商热重曲线峰面积则与失重量成正比。
图4-13为钙、锶、钡3种元素水合草酸盐的微商热重曲线与热重曲线图。
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图4-13 钙、锶、钡3种元素水合草酸盐的微商热重曲线与热重曲线图
热重分析基本原理的英文描述如下：
Thermogravimetry（TG）or thermogravimetric analysis（TGA）provides a quantitative measurement of any weight changes associated with thermally induced transitions.For example，TG can record directly the loss in weight as a function of temperature or time（when operating under isothermal conditions）for transitions that involve dehydration or decomposition.Thermogravimetric curves are characteristic of a given compound or material due to the unique sequence of physical transitions and chemical reactions that occur over definite temperature ranges.
The rates of these thermally induced processes are often a function of the molecular structure.Changes in weight result from the physical and chemical bonds forming and breaking at elevated temperatures.These processes may evolve volatile products or form reaction products that result in a change in weight of the sample.TG data are useful in characterizing materials as well as in investigating the thermodynamics and kinetics of the reactions and transitions that result from the application of heat to these materials.The usual temperature range for TG is from ambient to 1200℃ in either inert or reactive atmospheres.
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