TGA-FTIR and TGA-MS Measurements

Introduction

In thermogravimetry (TGA), the weight of the sample is measured continuously as a function of temperature and time. During the measurement, volatile products are evolved resulting in weight loss steps in the TGA curve. Information on the chemical structure and functional groups of the evolved gases can be obtained by coupling the TGA to an evolved gas analyzer (EGA). This is normally either an FTIR spectrometer, which measures the IR absorption of molecules, or a mass spectrometer in which the gas molecules are ionized and the positively charged fragment ions separated according to their mass-to-charge ratio. The purpose of this article is to make the user aware of some important practical points and to suggest several tests that should be performed before and after the measurement.

Temperature of the Transfer Line

The transfer line is maintained at about 200 °C for all measurements so that evolved vapors do not condense during their passage to the analyzer. It prevents carry-over of fractions and possible blockage of the transfer line. Blockage test If the transfer line is blocked, the EGA no longer reacts to thermal effects taking place in the thermobalance. Blockage of the transfer line can occur when substances such as rubber decompose. In the MS, an unusually high vacuum (less than 10-6 mbar) can also indicate that the transfer line is blocked. The transfer line can be checked using a volatile solvent. About 5 mg acetone are sealed hermetically in a 40 µl aluminum crucible. The crucible is inserted in the TGA and heated to 200 °C at 10 K/min. The pressure in the crucible increases until it finally bursts. Acetone vapor is suddenly developed and should be detected in the EGA: wavenumber range 1725-1640 cm-1 (FTIR) or m/z 15, 43, 58 (MS).

Choice of Carrier Gas

FTIR: the carrier gas should not exhibit any absorption bands. Nitrogen, oxygen, synthetic air and argon are therefore suitable. MS: the choice of the carrier gas depends on the mass of the ion you want to detect. The following pairs interfere: m/z 28 (N₂ and CO), m/z 36 (Ar and HCl), m/z 44 (CO₂ and NO₂).

Leak-Tightness of FTIR

The volatile substances evolved from the sample in the TGA can only reach the gas cell in the FTIR if the TGA-FTIR system is leak-tight. For this reason, the entire system should be tested to make sure that it is gas-tight. This can be done by connecting a piece of flexible rubber tubing to the FTIR outlet and dipping the other end into a beaker of water. The height of the water column that still produces gas bubbles should be at least 10 cm. If no bubbles appear, press the furnace to the right against the O-ring. If necessary, replace the O-ring. Check the flange on the FTIR interface.

FTIR Sensitivity

For the best possible FTIR sensitivity use a relatively low carrier gas flow rate, e.g. 20 ml/min. Another parameter is the heating rate: the higher the heating rate the better the sensitivity. Normally heating rates of 5 K/min to 20 K/min are used. Be careful, however, because high heating rates lead to effects overlapping each other! A weight loss of at least 0.5 mg in the TGA curve is sufficient to ensure good IR absorption. This should be taken into account in the sample weight used. A weight loss that is too large could lead to undesirable aerosol formation, the droplets of which could then be deposited as a film in the gas cell. FTIR background spectrum The background spectrum recorded prior to the actual measurement is automatically subtracted in the following measurements. Even so, the residual carbon dioxide and the moisture in the FTIR spectrometer should be reduced as much as possible by purging with dry air or nitrogen free of CO₂.