Thermogravimetry and Gas Analysis, Part 2: TGA-MS

TGA experiments in combination with a suitable evolved gas analysis (EGA) technique not only provide quantitative information about the change in mass of a sample but also qualitative information about the gaseous reaction or decomposition products that are evolved. In this series of articles, we will discuss the possibilities that METTLER TOLEDO offer.

In Part 1 of this series, we presented an overview of the advantages of coupling a thermobalance to a gas analyzer for the qualitative analysis of the reaction or decomposition products [1]. In this part, we show in more detail how TGA-MS experiments are performed and evaluated.

Introduction

TGA-MS is used as an analysis method to detect small gaseous molecules such as H₂O, HCl or CO₂ evolved from samples that are not too complex. A typical example is the detection of residual solvents in pharmaceutical products. In this analysis method, the evolved gases from the TGA experiment are transferred online through a capillary tube into the MS. The temperature at which substances are evolved can therefore be determined exactly.

Normally, short lenghts of heated capillary tubing (about 0.5 to 1 m long) are used to transfer the gases from the TGA into the MS. The capillaries are solely for gas transfer and have no separation function. This is different to GC/MS where suitable GC capillary columns are used to separate the evolved substances. For TGA-MS experiments, METTLER TOLEDO recommends coupling the TGA to a Pfeiffer Vacuum ThermoStar Analysis system [2].

TGA-MS Setup

The schematic diagram in Figure 1 shows the setup of a TGA-MS coupling. The MS capillary embedded in a heated transfer line (normally at 200 °C) is inserted into the TGA furnace by means of a special TGA-MS interface. The interface consists of a glass body, a support for the capillary, and a second gas outlet through which the excess gas emitted from the TGA can be vented to the surroundings. The position of MS capillary with respect to the TGA furnace is very important.

Conclusions

TGA-MS is an excellent technique. It enables you to identify gases evolved in a TGA analysis even at very low concentrations (ppm level). This is particularly the case when we are dealing with small molecules and mixtures that are not too complex.

_{Thermogravimetry and Gas Analysis, Part 2: TGA-MS | Thermal Analysis Application No. UC 461 | Application published in METTLER TOLEDO Thermal Analysis UserCom 46}