Using Mass Spectrometry to Yield Extra Information about Thermal Analysis

By AZoM

Table of Contents

TA-MS Overview
QIC Series Mass Spectrometers
The TA- MS Interface
QIC Response
TA- MS Examples
     Calcium Oxalate Decomposition
     Decomposition of an Organo- Sulphur Compound
     Dehydration of Lactose Monohydrate
Conclusion
About Hiden Analytical

TA-MS Overview

Evolved gas analysis methods can be utilized to offer essential information about decomposition or desorption processes when combined with thermal analysis techniques such as DSC, TGA, and DTA mass spectrometry (MS) and offers several benefits for evolved gas analysis. Along with being highly sensitive, modern mass spectrometers can offer simultaneous, unequivocal and rapid detection of several gaseous species within the available mass range. MS hence compliments TA techniques with speciation as a function of time and/or temperature.

The key requirements of the TA-MS Interface are:

• Minimum dead volume
• Controllably heated sample inlet – no cold spots
• Inert materials
• High performance gas handling for operation with low molecular weight gas components (H₂, He) and for flow matching with the TGA

Figure 1. Example of a Hiden TGA-MS Interface

QIC Series Mass Spectrometers

The QIC Series mass spectrometers are small, bench top gas analysis systems as shown in Figure 2, which feature Hiden’s heated Quartz Inert Capillary (QIC) inlet with controlled by-pass for quick and continuous sampling in the process pressure range 100 mbar to 2 bar absolute. The innovative QIC inlet design enables a range of custom adaptors, flow and temperature matched to be interfaced with specific TA/TGA instruments.

Figure 2. HPR-20 QIC Mass Spectrometer

In order to ensure easy usage, the QIC Series systems are designed for automatic start/stop from an external trigger signal (from the TGA) allowing synchronization of weight, temperature and MS data. Additionally, two 0-10V analogue inputs read weight and temperature data directly into the MS.

The TA- MS Interface

Thermal Analysis equipment functions at near atmospheric pressure whereas mass spectrometers operate at high vacuum, some nine decades lower. The TA-MS interface is crucial for precise measurement of both evolved gases and vapors. Hiden Analytical has worked closely with major TA instrument manufacturers and end users to produce a comprehensive range of TA-MS interface adapters for the QIC Series. These adapters are interchangeable to suit different instruments and can be specified at point of sale with new systems or as simple retro-fits to the existing apparatus. Figure 1 shows a typical Hiden interface design.

QIC Response

The integration of low dead volume sampling with fully heated transfer line, inert material construction and superior gas handling/inlet dynamics mean that the QIC Series offer unrivalled precision and response speed to both vapors and gases.

Figure 3 shows typical response curves for the QIC inlet when compared to traditional capillary inlets. Response times of less than 150 ms are possible with the QIC inlet.

Figure 3. QIC Inlet Response

TA- MS Examples

Calcium Oxalate Decomposition

Coupled TG-MS analysis data reveals i) water desorption, ii) partial oxalate decomposition with CO / CO₂ evolution and iii) full decomposition of oxalate with CO₂ evolution. There is also a perfect correlation between mass loss and MS signals.

Figure 4. Calcium Oxalate Decomposition

Decomposition of an Organo- Sulphur Compound

Figure 5 below shows there is only one major weight loss feature shown by the TGA instrument. However, the MS data shows that this weight loss is because of the coincident loss of several species including H₂S, formaldehyde, carbonyl sulphide, isopropyl alcohol and CS₂. This example demonstrates the advantages of attaching the HPR-20 mass spectrometer to a TGA system.

Figure 5. TG-MS Plot of Organosulphur Compound Decomposition

Dehydration of Lactose Monohydrate

Lactose is used broadly in the pharmaceutical industry as a filler or binder in tablet manufacture. Therefore, the adsorption of water during prolonged storage is of interest.

Figure 6 shows a TG-MS plot of the dehydration of freshly prepared lactose monohydrate. Here one distinct weight loss can be seen. The mass spectrometer trace shows that this is due to the loss of water probably from the dehydration of lactose.

Figure 6. TG-MS Plot of Lactose Monohydrate Dehydration

Figure 7 shows the TG-MS plot of a sample of lactose monohydrate that was exposed to the atmosphere for 24 hrs. Here, it can clearly be seen that there is an additional loss of water detected in the mass spectrometer trace at lower temperature, approximately 70 °C. This is due to desorption of water adsorbed on the lactose during exposure to the atmosphere.

Figure 7. TG-MS Plot of Lactose Monohydrate

Conclusion

The examples show that coupling a mass spectrometer like the Hiden QIC to TA equipment can offer invaluable information about the identity of desorbed species or decomposition products formed during a thermal analysis experiment. This information is complimentary to that obtained by the TGA to give a complete understanding of the desorption / decomposition process.

About Hiden Analytical

Hiden Analytical is a leading manufacturer of quadrupole mass spectrometers for both research and for process engineering. Their products Our products address a diverse range of applications including:

• Precision gas analysis
• Plasma diagnostics by direct measurement of plasma ions and ion energies
• SIMS probes for UHV surface science
• Catalysis performance quantification
• Thermo-gravimetric studies

These analytical instruments are designed to work over a pressure range extending from 30 bar processes down to UHV/XHV.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

For more information on this source, please visit Hiden Analytical.