Evolved Gas Analysis for Precise TGA Determinations
On Demand Webinar

Evolved Gas Analysis – Precise TGA Determinations

On Demand Webinar

Evolved gas analysis complements TGA by characterizing decomposition products and volatiles

Thermogravimetric Analysis (TGA) provides quantitative information on the change in mass of a sample as a function of time or temperature. But TGA cannot identify or characterize the gaseous products evolved during a measurement.
The combination of a TGA with a mass spectrometer (MS) or a Fourier transform infrared spectrometer (FTIR) allows the nature of the gaseous reaction products formed in the TGA to be investigated online.

In this Webinar, we will discuss the advantages of TGA-MS and TGA-FTIR systems and present some interesting applications.

33:01 min
English , 日本語

The Webinar covers the following topics:

  • Principles of Evolved Gas Analysis
  • TGA/DSC 1
  • Measurement possibilities
  • Why use Evolved Gas Analysis?
  • Industries and applications
  • Applications

Evolved gas analysis is a method used to study the nature of volatile products released by a substance as it is heated. This can be done using many different types of techniques and equipment.

The TGA-EGA system
The focus is on evolved gas analysis as it is commonly performed in thermal analysis, namely by coupling a thermogravimetric analyzer to a mass spectrometer or to a Fourier transform infrared spectrometer to create a TGA-EGA system.

The thermogravimetric analyzer records the loss-of-mass of the sample while the evolved gas analyzer simultaneously analyzes and so provides information about the gaseous products evolved. The products result from processes such as evaporation, desorption, decomposition, and chemical reactions.

Industries and applications
Evolved gas analysis has numerous potential applications for the analysis of volatiles, additives and decomposition products. It can be used in practically all industries ranging from the automotive field to chemicals, plastics, and pharmaceuticals. This webinar presents several different application examples that demonstrate the analytical power and versatility of evolved gas analysis.

Evolved Gas Analysis

Evolved Gas Analysis

Ladies and Gentlemen

Welcome to the METTLER TOLEDO webinar on “Evolved Gas Analysis”, or EGA for short.

 

In the broader definition, evolved gas analysis has to do with the investigation of the nature of volatile products released by a substance as it is heated. This can be done using many different types of techniques and equipment.

 

More specifically, this webinar describes evolved gas analysis as it is commonly performed in thermal analysis, namely by coupling a thermogravimetric analyzer to a mass spectrometer or to a Fourier transform infrared spectrometer to create a TGA-EGA system.

 

Contents

First, I would like to explain the principles involved in evolved gas analysis and show how this capability can easily be added to an existing METTLER TOLEDO TGA/DSC 1 instrument in your laboratory.

I also want to briefly explain the techniques used and mention their benefits and limitations.

Finally, I will present several examples to illustrate the different application possibilities of TGA-EGA techniques.

Principles of TGA-EGA

As I mentioned before, the most commonly used TGA-EGA instrument combinations are TGA-mass spectrometry and TGA-Fourier transform infrared spectroscopy. Instrument combinations like this are often referred to as hyphenated techniques.

 

The thermogravimetric analyzer records the loss-of-mass of the sample while the evolved gas analyzer simultaneously analyzes and so provides information about the gaseous products evolved. The products result from processes such as evaporation, desorption, decomposition, and chemical reactions.

 

The example in the figure on the right shows the measurement curves obtained when result of heating a sample of fluorinated insulation material was heated in the TGA furnace of a TGA-FTIR system.

The TGA curve displays a single, large, weight-loss step that begins at about four hundred degrees Celsius (400 °C) and ends at about five hundred and fifty degrees (550 °C). During this step, the polymeric insulation material decomposes and releases volatile products.

Unfortunately, the weight-loss curve alone does not provide any information about the nature of the gaseous decomposition products evolved. This is the task of evolved gas analysis. In the diagram on the right, we can see that hydrogen fluoride is produced because the FTIR spectrum measured at about five hundred degrees (500 °C) corresponds closely to a reference spectrum of hydrogen fluoride gas.

Principles of TGA-EGA

TGA is a quantitative technique that measures the loss of mass of the substance as it is heated. If the sample is a pure substance of defined molecular weight, some information about decomposition products can be obtained from stoichiometric considerations but in general TGA is not an identification technique.

The combination of a TGA with an MS or FTIR instrument allows gaseous products evolved during a TGA experiment to be characterized or even identified.

Mass spectrometric analysis detects the presence and intensity of specific fragment ions and correlates this information with molecular structure.

In infrared spectroscopy, the infrared absorption bands are assigned to particular functional groups of a molecule.

These two techniques provide largely qualitative information. It is not always be possible to unambiguously identify substances because the masses of fragment ions and the wavenumbers of infrared absorption bands are not always unique to a specific molecule.

Principles of TGA-EGA

The schematic diagram shows the general setup of a TGA-EGA system.

The TGA and EGA instruments are connected to a computer. The computer controls the TGA and records the weight-loss curve of the sample as a function of temperature or time.

Starting from the left, the purge gas flows over the sample in the furnace area of the TGA and sweeps the gaseous decomposition products into the MS or FTIR instrument via a transfer line. This is heated to prevent condensation occurring during transfer.

The spectral data recorded by the MS or FTIR spectrometer is stored in the same computer as a function of time simultaneously to the recording the TGA weight-loss curve. This means that the MS or FTIR spectra can be selected and displayed for any point-in-time on the TGA weight-loss curve.

TGA/DSC 1 – EGA Interface Options

Several different interfaces are available for connecting external analytical equipment to the METTLER TOLEDO TGA/DSC 1.

 

The Standard Interface is shown on the left of the slide. This interface consists of a normal gas outlet, connected to the end of the furnace. Tubing can be connected to this gas outlet to vent the gas to a fume hood or to the atmosphere. Alternatively, a collection tube or a cold trap can be connected to this outlet to trap any volatile decomposition products purged out of the TGA furnace.

The pictures in the middle and on the right show the FTIR and MS interfaces used for coupling the TGA to a Fourier transfer infrared instrument or a mass spectrometer.

Measurement Possibilities

We have seen from the previous slides that three different interfaces are available at the outlet of the TGA furnace - for a mass spectrometer, for a Fourier transform infrared spectrometer, and for collection tubes or cold traps.

 

Mass spectrometry measures the masses of fragment ions according to their mass-to-charge ratio. This information allows the structure of a molecule to be deduced.

 

Infrared spectroscopy is a non-destructive technique. Infrared spectra record the wavelengths at which groups of atoms or functional groups in a molecule absorb infrared light. Infrared spectra are specific for different molecules and allow substances to be identified or the type class of compound to be classified.

 

Alternatively, volatile components can be trapped in collection tubes or in cold traps for different time-intervals during the measurement. These devices can then transferred to a stand-alone analyzer such as a gas chromatograph or high-pressure liquid chromatograph and analyzed off-line at the end of the TGA analysis.

This type of off-line analysis is less elegant and more time-consuming than on-line analysis. The time resolution of events that occur is poorer because fractions are collected over longer time intervals. The number of collection tubes or cold traps that can be used in any one experiment is of course limited. Sensitivity and the possibility of using other analytical separation or identification techniques however are potential advantages.

 

The remainder of this seminar will deal specifically with evolved gas analysis using on-line mass spectrometry and infrared spectroscopy.

Measurement Possibilities – Mass Spectrometry

The schematic diagram shows the operating principle of a quadrupole mass spectrometry of the type used for TGA-MS measurements.

When the purge gas with the volatile products from the TGA reaches the mass spectrometer through the transfer line, only about 1 percent is allowed to enter the instrument MS, the rest is removed by a pump. This is necessary because the mass spectrometer MS operates at high vacuum and the vacuum would otherwise collapse. However, the small amount of sample is not a problem because a mass spectrometer the MS is extremely sensitive.

 

The molecules that arrive in the ionization chamber are bombarded by an energetic electron beam. This causes the molecules to fragment to smaller, positively charged fragment ions, most of which are singly charged.

The ions are then accelerated into a chamber where they are separated according to their mass-to-charge ratio through a combination of electrostatic and electromagnetic fields. The mass spectrometer detector system measures either the entire mass spectrum or continuously monitors the intensity of a number of characteristic fragment ions. The ions formed are directly related to the structure of the molecule.

Interpretation of the masses of the fragment ions and their intensities allows compounds to be identified.

Alternatively the measured data can be compared with reference data in mass spectral libraries of known compounds.

Measurement Possibilities – Mass Spectrometry

This slide summarizes the advantages and limitations of mass spectrometry.

 

In TGA-EGA applications, mass spectrometry is primarily a qualitative technique used for the identification of gaseous molecules. Semi-quantitative measurements are also possible with suitable calibration.

The technique is extremely sensitive. This means that either only very small quantities of sample are needed or that very low concentrations of evolved gases can be detected.

The measurement of the mass spectrum is extremely fast so that ions of all masses within the mass range of the detector are recorded almost instantaneously. This means that many different fragment ions can be simultaneously monitored as a function of time. This allows overlapping mass-loss effects in the TGA to be interpreted. The decomposition products are measured almost immediately as soon as they are purged from the TGA furnace. The timescale of the MS data corresponds exactly to that of the weight-loss curve.

 

Since the molecules are analyzed as different molecular ion fragments, a certain amount of experience in interpretation as well as knowledge about the sample is required to elucidate the original structure nature of the sample.

Furthermore, some fragment ions have the same mass-to-charge ratio and cannot be distinguished, for example both carbon monoxide and nitrogen have the same molecular mass of 28.

Finally, decomposition products that are not volatile are not transferred into the spectrometer and are not detected.

Measurement Possibilities – FTIR

The second identification technique used for TGA-EGA is Fourier transform infrared spectroscopy. In a TGA-FTIR combination, the entire effluent from the TGA passes through a heated capillary into the sample cell of the FTIR spectrometer. Spectra are recorded at a rapid rate, typically one per second, so the time delay between sample decomposition in the TGA and the measurement of the corresponding spectrum is more or less negligible.

 

FTIR spectroscopy is based on the absorption of electromagnetic radiation by molecules in the mid-infrared spectral region from four thousand to four hundred (4000 to 400) wavenumbers (cm-1). The energy in this spectral range is low compared with that used in mass spectrometry MS so that no ionization or fragmentation occurs.

 

The slide shows the different modes of vibration of a group of atoms. The modes comprise the energetically strongersymmetrical and antisymmetrical stretching-vibrations, and the weaker,scissoring, rocking, wagging and twisting vibrations.

The molecules absorb infrared energy at frequencies that depend on the structure of the molecule. The absorption of energy causes the molecule or certain parts of the molecule (the so-called functional groups) to vibrate at the same frequencies at which they absorb the energy. The absorption frequencies are unique for a particular molecule and can therefore be used to characterize or identify a substance or the class of substance through interpretation or the use of spectral libraries.

 

Measurement Possibilities – FTIR

The slide summarizes the advantages and limitations of FTIR.

 

As in mass spectrometry, measurements are performed very rapidly so that real-time-spectra of the decomposition products are obtained. Spectral libraries are available for comparing the measured spectra with the reference spectra of known molecules.

 

FTIR is not as sensitive as mass spectrometry so that larger sample quantities are required for some measurements.

Infrared spectroscopy identifies functional groups rather than fragments of molecules as in MS.

 

One limitation of FTIR is that symmetrical diatomic molecules such as oxygen and nitrogen are not infrared-active and are not detected. For practical reasons, this is sometimes an advantage. Larger molecules will however always have one or more vibrational modes that will absorb energy and hence be detected.

Different molecules with the same functional group, for example a carbonyl group, cannot be distinguished via that particular functional group, but might be differentiated through other functional groups they possess.

Finally, as with MS, any decomposition products that are not volatile cannot be measured.

Why use Evolved Gas Analysis?

The slide lists the main application areas of evolved gas analysis.

The most important application is the characterization and identification of volatile impurities and decomposition products liberated when a material is heated. This also applies to the release of substances trapped in the crystal lattice or bound as solvates within a material.

If different isotopes are present in a material, their ratio can be determined by mass spectrometry.

Products formed in polycondensation and crosslinking reactions can also be measured. Harmful gases produced in decomposition reactions can be detected and the information used in safety studies.

 

Industries and Applications

Evolved gas analysis has numerous potential applications and can be used in practically all industries. The slide summarizes the main industries and applications.

The industries range from the automotive field to chemicals, plastics, and pharmaceuticals. The applications have to do with volatiles, additives and decomposition products.

 

I would now like to present several different application examples that demonstrate the analytical power and versatility of evolved gas analysis.

Application 1                 TGA-MS: Calcium oxalate monohydrate

The first application illustrates the use of TGA-MS to investigate the decomposition of calcium oxalate monohydrate.

The TGA curve shows that the substance decomposes in three distinct steps.

The decomposition was investigated by monitoring the emm over zee (m/z) 18, 28 and 44 ions that correspond to water, carbon monoxide and carbon dioxide.

The results confirm that the first step corresponds to the loss of water of crystallization, the second step to the release of carbon monoxide from the anhydrous calcium oxalate, and the third step to the liberation of carbon dioxide from the calcium carbonate formed in the second reaction step.

Stoichiometrically, one mole of calcium oxalate monohydrate produces one mole each of water, carbon monoxide and carbon dioxide. The emm over zee (m/z) 44 curve however shows that a small amount of carbon dioxide is also formed in the second weight-loss step, in contrast to the theoretical reaction scheme.

This effect is due to the disproportionation reaction of two molecules of carbon monoxide to one molecule each of carbon dioxide and carbon. This additional information can only be obtained by simultaneous measurement of the mass spectrum, and is not evident from the weight-loss curve alone.

 

Application 2                                         TGA-MS: Residual solvents

This example summarizes the results obtained from the TGA-MS analysis of an active pharmaceutical ingredient, or API.

In production, APIs are often recrystallized from different solvents in order to obtain the desired crystalline form. Afterwards, it is important to check that no unwanted solvent residues are left in the API.

 

The TGA curve shows several weight loss steps in the temperature range up to 350 degrees Celsius. The initial weight loss below 60 degrees is probably due to the gradual evaporation of adsorbed moisture or solvents while the final step above 250 degrees is no doubt due to decomposition of the product.

The two intermediate weight loss steps cannot be assigned from the TGA data alone. The MS intensity curves of the fragment ions 31 (for methanol) and 43 (for acetone) were measured simultaneously with the TGA curve and correspond to the weight loss steps in the temperature range 70 to 190 and 190 to 225 degrees.

 

The temperatures at which methanol and acetone are released during heating are much higher than the normal evaporation temperatures of these two solvents. This indicates that the solvents were bound inside the crystal structure of the API. The methanol is released at lower temperatures and over a wider temperature range, whereas the acetone is released at higher temperatures and in a rather narrow temperature range. This could mean that the acetone is more firmly bound in the substance, possibly as a solvate, whereas the methanol is trapped but not bound in the crystal lattice.

 

Application 3                                         TGA-MS: Amino resin

The purpose of the experiment shown here was to investigate the curing reaction of an amino resin and to analyze the main decomposition products.

Ideally, the condensation reaction of an amino resin forms water during the crosslinking reaction. Depending on the conditions, undesired side reactions with the formation of other reaction products such as methanol and formaldehyde can also occur. The nature of the volatile products released in the reaction indicates whether or not the reaction is proceeding favorably.

 

The TGA curve shows a gradual loss of weight in the range 40 to 220 degrees Celsius (°C) due to the curing or polycondensation reaction of the amino resin. The maximum rate of weight-loss is at about 115 degrees as shown by the DTG curve. The marked change in the TGA and DTG curves at about 220 degrees indicates that decomposition of the product begins at this temperature.

 

MS fragment ion curves were recorded for several different mass-to-charge ratios. The evaporation of residual moisture and water with ion masses of 17 and 18 formed during the polycondensation reaction can be detected from about 50 degrees onward. The ion intensity curves of methanol, with a mass-to-charge ratio of 31, and formaldehyde with mass 30 are flat up to about 220 degrees. This proves that these compounds are not formed below 220 degrees.

 

Above 220 degrees, the intensities of the ions with masses of 17, 30, and 31 show a marked increase, indicating that the sample has begun to degrade. The intensity of the ion with mass 18 however decreases noticeably. Since water is characterized by ions of mass 17 and 18, the intensity ratio of these two masses would normally be expected to be constant. This is obviously not the case indicating that there must be another reason for the increase of the emm over zee (m/z) 17 curve, for example another ion with the same nominal mass.

 

Measurements with FTIR were performed to clarify this point, but are not shown here. A direct comparison of the FTIR spectrum measured at 240 degrees with database spectra indicated the presence of methanol, carbon dioxide and ammonia. The presence of ammonia, with a mass-to-charge ratio of 17 explains why this curve increases.

 

Application 4                                         TGA-FTIR: Polyvinyl chloride

The next application illustrates the use of TGA-FTIR.

The aim of the experiment was to monitor the degradation and identify the nature of the decomposition products evolved during the pyrolysis of PVC up to 600 degrees Celsius.

The TGA weight-loss curve, derivative weight-loss curve and an infrared chemigram show two distinct weight loss steps at about 310 degrees and 465 degrees. In this case, the infrared chemigram was a total intensity curve of the infrared absorption between 3090 and 3075 wavenumbers and is shown by the dotted green black line.

 

The inset diagram on the left shows the FTIR spectrum recorded at 310 degrees. The sharp absorption bands between 3100 and 2600 wavenumbers correspond to the spectrum of hydrogen chloride gas. This shows that the first weight-loss step is caused by cleavage of hydrogen chloride gas from the main polymer chain of the PVC.

 

The inset diagram on the right shows an FTIR spectrum measured at about 465 degrees. The spectrum in this spectral range is typical for benzene. The benzene is formed through cyclization of the carbon chain, which is made possible through the double bonds formed previously during the elimination of hydrogen chloride.

 

Careful examination of the FTIR spectrum at 310 degrees indicates that a small amount of benzene was released in addition to hydrogen chloride. This is visible from the underlying benzene pattern in the spectrum. The presence of benzene could very likely be confirmed by performing additional MS measurements. This should be possible because mass spectrometry MS is much more sensitive than gas-phase infrared spectroscopy (IR).

Application 5                                                     TGA-FTIR: BHET

In this application, TGA-FTIR was used to investigate the thermal degradation of bis-hydroxyl-ethyl-terephthalate or BHET. This substance is used in the production of PET bottles. The monomer is sometimes also formed during the recycling of PET bottles during the depolymerization process.

 

The TGA and DTG curves in the upper part of the diagram show two distinct weight-loss steps. FTIR spectra were recorded simultaneously throughout the TGA measurement to identify the components released from the substance. Four different FTIR chemigrams or functional group profiles are plotted in the lower half of the diagram. The absorption frequencies are shown next to the chemigram and correspond to the characteristic CH (speakersay “see-aich”) stretching bands of alkanes, the CO (speakersay “see-oh”) absorption bands of alcohols, and the carbonyl bands of carboxylic acids and esters.

 

The chemigrams or functional group profiles show that the first weight-loss step involves a decomposition product that contains alkane and alcohol functional groups. The FTIR spectrum measured at 300 degrees Celsius shows an excellent match with a reference library spectrum of ethylene glycol.

 

Direct comparison of the spectrum recorded at 450 degrees with database spectra did not however identify any one particular decomposition product. The functional group profiles show that the decomposition product contains an alcohol CO (speaker say “see-oh”) bond, and carboxylic acid and ester functional groups. This information, together with some knowledge of the material, leads one to assume that the second weight-loss step is due to the elimination of hydroxy-formic-acid-ester created through the cleavage of the side chains of the aromatic group.

Summary: Evolved Gas Analysis

TGA-EGA opens up many important new application possibilities. It is an excellent technique for characterizing decomposition products and volatile components as they are produced when a material is heated. This slide summarizes the most important features and benefits.

 

The measurement data obtained by combining a TGA with a mass spectrometer or an infrared spectrometer provides much more information about products and processes and the effect of temperature.

Measurements require only very small amounts of sample. TGA-MS in particular can detect very low concentrations of substances.

 

The METTLER TOLEDO concept allows an existing TGA instrument to be combined with a mass spectrometer or FTIR instrument at a later date in order to meet new requirements.The TGA is directly connected to the MS or FTIR. The software is triggered by the TGA instrument and the data is integrated into the STARe software.

For More Information on Evolved Gas Analysis

Finally, I would like to draw your attention to further information about evolved gas analysis that you can download from the Internet.

METTLER TOLEDO publishes articles on thermal analysis and applications from different fields twice a year in UserCom, the well-known METTLER TOLEDO technical customer magazine. Back issues can be downloaded as PDF files from the Internet as shown at the bottom of the slide. Individual applications can be searched for on the METTLER TOLEDO homepage.

For More Information on Evolved Gas Analysis

In addition, you can download information about application handbooks, webinars or of a more general nature from the Internet addresses given on this slide.

Thank You

This concludes my presentation on Evolved Gas Analysis. Thank you very much for your interest and attention.

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