TPD (Temperature Programmed Desorption) often refers to the quantification of desorbed gas most commonly using a mass-spectrometer to measure gases evolved from a sample while ramping the temperature of the sample at a constant rate. Alternatively, infrared spectroscopy may be used when MS is problematic, i.e. if there are complex mixtures of gases to be studied. The method is generally referred to as TDS (Thermal Desorption Spectroscopy). The experimental difficulty in this method is the proper calibration of the mass-spectrometer to provide quantitative measurements of evolved gas.

The quantities of gases evolved may be derived experimentally from the measured area under the MS response vs. time curve by comparison to a calibrated volumetric dose of the gases delivered to the system under the identical conditions used for the TDS experiment. Alternatively, calibration of hydrogen may be achieved by decomposing a metal hydride sample with a very well known hydrogen content or by carefully weighing a hydride sample before and after desorption. In addition to the calibration, good response times are needed in coupled DSC–MS systems. That means that the time lag between the signal detected by the DSC and that detected by the MS should be small. It is also desirable that the shape of the MS signal accurately reflects decomposition reaction of the sample. Both characteristics, time lag, and shape of the MS signal, are taken into account by the response time function of the system which is mainly affected by the flow and type of carrier gas and the geometry of the experimental system.

A simple TPD alternative to study the gas–solid interaction of hydrides can be studied using the using a thermo-volumetric analyzer (TVA), based on a modified Sievert’s type apparatus. A TVA system uses a calibrated high pressure reactor vessel and calibrated reservoir vessel. The temperature of the reactor vessel is usually controlled a PID temperature controller that can ramp the temperature of the sample at a constant rate. As the sample desorbs the hydrogen pressures inside the vessels are recorded using high precision pressure transducers. These values are then converted into capacity via the volumetric method. As with all volumetric and gravimetric devices, it is not possible to be sure that the evolved gas pressure or mass loss is purely due to hydrogen without performing some form of gas analysis. It is also important to note that TPD methods are by design dynamic not equilibrium measurements. Thus, great care should be taken in evaluating stabilities and capacities to individual hydride phases.

An example of TPD measurements can be seen in the thermal decomposition of Zr and Ti doped sodium aluminum hydride system by Zidan et al. One such measurement is shown in below figure.

Thermal-programmed desorption (28°C/min) of hydrogen from various doped samples of NaAlH4 after three cycles of dehydriding / rehydriding