Instant Notes: Analytical Chemistry

melted when the temperatures of the liquid indium and alumina again rise at

the same rate.

Two alternative strategies can now be adopted. If the temperatures of sample

S and reference R are measured and the temperature difference recorded (the

differential thermal analysis or DTAstrategy),

DT =TS−TR

a downward peak (i.e. a minimum) is recorded. Under carefully controlled

instrumental conditions, this may be related to the enthalpy change for the

thermal event:

DHK

f

i

DTdtKA

where Ais the area of the temperature-time peak from initial (i) to final (f)

point.

This leads to quantitative or heat-flux differential scanning calorimetry

(heat-flux DSC). The negative sign is required since the enthalpy change on

melting is positive, but DTfor melting is negative.

The second strategy is to control the amount of heat supplied to sample and

reference so that their temperatures stay as nearly the same as possible. Using

separate heaters for sample and reference allows measurement of the difference

in power DPto be measured. With proper control and calibration, this will give

the enthalpy change of the peak directly:

DH =DPdt

This is known as power-compensated DSC. Essentially, it has been demon-

strated that both strategies produce equivalent results with similar accuracy.

Instrumentation The schematic diagram of the apparatus for DSC/DTA is shown in Figure 1. The
temperature, both for the sample and the reference and also the furnace is
measured by thermocouples, or resistance sensors. Higher sensitivity and
greater stability are obtained if multiple sensors of inert material are used.

312 Section G – Thermal methods

Fig. 1. Schematic diagram of DTA or DSC apparatus (Drepresents either the temperature

or power difference).

Atmosphere

control

Sensor

amplifier

Furnace

Sample

Sensor

Programmer

or computer

Cooling

control

Data recorder

or computer

SR

∆