Thermal Analysis of Organic Compounds
On Demand Webinar

Thermal Analysis of Organic Compounds

On Demand Webinar

"Thermal Analysis of Organic Compounds" presents techniques for characterizing organic materials

Thermal Analysis of Organic Compounds
Thermal Analysis of Organic Compounds

The field of organic chemistry focuses on the synthesis, purification and qualification of organic compounds. Thermal analysis can be used to study, predict and check the many processes in the production of organic compounds.

In this webinar, we will show how thermal analysis is used to analyze organics and will present some typical examples of samples measured by DSC, Flash DSC, TOA and TGA.

44:25 min
English

The Webinar covers the following topics:

  • Introduction
  • Basic properties of organic compounds.
  • Typical questions
  • Thermal analysis
  • Industries and applications
  • Instrumentation and applications
    - Differential Scanning Calorimetry (DSC) and Flash DSC
    - Thermogravimetry (TGA)
    - Thermo-optical Analysis (TOA)
  • Summary

In the webinar titled "Thermal Analysis of Organic Compounds", we describe a number of techniques and methods that can be used to characterize their properties and effects under variable conditions.

Organic chemistry and applications to industry

Organic chemistry is the study of the structure and properties of carbon compounds. The field of organic chemistry covers the synthesis, isolation, purification and reactions of organic compounds and materials. Analyses have to be performed during synthesis and work-up processes to make sure that the desired compounds have been produced and to verify their concentration. Many of the methods used are based on thermal analysis techniques such as DSC, TGA and thermo-optical analysis.

During synthesis and work-up, the focus is mainly on reaction kinetics and safety aspects, but content and purity might also be determined at many of the stages in the process.

When the final product has been isolated, properties such as purity, stability, polymorphism, melting, reactivity, and so on will also be checked.

 

Thermal analysis of organic compounds

The most important properties and effects that can be analyzed by DSC are melting, crystallization and polymorphism, enthalpies of melting, crystallization, and thermal stability.

TGA is normally used to study compositional analysis, thermal stability and decomposition, evaporation and desorption behavior. It can also be used to identify gaseous products released during a TGA experiment if it is combined with evolved gas analysis techniques such as MS, FTIR or GC/MS.

TOA is the method of choice for measuring visible changes such as changes in morphology, melting and crystallization.

Thermal Analysis of Organic Compounds

Slide 0: Thermal Analysis of Organic Compounds

 

Ladies and Gentlemen,

Welcome to the METTLER TOLEDO webinar on the Thermal Analysis of Organic Compounds.

 

Small and medium-size organic molecules of natural or synthetic origin are used as raw materials, additives, active ingredients or processing aids in many industries.

Organic polymers such as polyethylene and polyesters will not be discussed here because they have already been covered in separate webinars on thermoplastics, elastomers and thermosets.

 

Thermal analysis is an excellent method for characterizing organic compounds because many of their properties are temperature dependent. In this webinar, I want to describe a number of interesting application examples that demonstrate this.

 

Slide 1: Contents

This slide lists the main topics I plan to cover.

 

I would first like to discuss the most important thermal properties of organic compounds and describe the thermal analysis techniques that can be used to measure them.

The techniques include:

Differential Scanning Calorimetry, or DSC,

Flash Differential Scanning Calorimetry, or FDSC,

Thermo-Optical Analysis, or TOA, and finally

Thermogravimetric Analysis, or TGA.

 

I will then present a number of examples that illustrate how these techniques can be used to investigate the properties and behavior of organic compounds.

 

Finally, I want to summarize the different thermal analysis techniques and their application fields, and list a number of useful references for further information and reading.

 

Slide 2: Introduction

Organic chemistry is the study of the structure and properties of carbon compounds. In an organic molecule, the carbon atoms are covalently linked to other carbon atoms and to atoms of elements such as hydrogen, oxygen, nitrogen, phosphorous or sulfur.

 

The simplest organic compounds contain molecules composed of carbon and hydrogen atoms. For example, methane contains just one carbon atom covalently bonded to four hydrogen atoms.

Organic substances are often classified according to their functional groups or their particular ring structure. The slide shows several examples of functional groups that occur in organic compounds, such as ketones, alcohols, amines, or esters.

 

The field of organic chemistry covers the synthesis, isolation, purification and reactions of organic compounds and materials. Analyses have to be performed during synthesis and work-up processes to make sure that the desired compounds have been produced and to verify their concentration. Many of the methods used are based on thermal analysis techniques such as DSC, TGA and thermo-optical analysis.

 

Slide 3: Applications Related to Organic Chemistry

This slide summarizes some of the different applications of thermal analysis for the analysis of raw materials, during synthesis and work-up, and for finished products.

 

Nowadays, raw materials are usually obtained from the supplier with an analysis certificate and are used as delivered. The supplier will already have performed the required quality control steps, such as checking the melting point, purity and composition. Sometimes, additional steps such as recrystallization or drying may be performed to obtain the proper starting product for the synthesis.

 

During synthesis and work-up, the focus is mainly on reaction kinetics and safety aspects, but content and purity might also be determined at many of the stages in the process.

 

When the final product has been isolated, properties such as purity, stability, polymorphism, melting, reactivity, and so on will also be checked.

 

Slide 4: Thermal Analysis

So what exactly is Thermal Analysis?

 

The ICTAC definition is:

“A group of techniques in which a physical property of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature program”.

The schematic diagram on the right shows a simple linear temperature program in which the temperature of a sample is increased at a constant heating rate.

 

The lower half of the slide illustrates typical events and processes that occur when a sample is heated. These include initial melting where the sample changes from the crystalline to the liquid state, followed by oxidation if the sample is exposed to air or oxygen, and finally decomposition.

Thermal analysis techniques are widely used in quality control and in research and development to measure properties such as heat capacity, mass as a function of temperature, and chemical stability to name just a few.

 

Slide 5: Thermal Analysis Techniques

The slide shows four important thermal analysis techniques that can be employed to characterize organic compounds, namely:

 

Differential Scanning Calorimetry, or DSC.

Conventional DSC is the most widely used thermal analysis technique. It allows you to determine the energy absorbed or released by a sample as it is heated, cooled or held at constant temperature.

The picture shows a DSC sensor with a crucible containing a sample, colored red, and a reference crucible.

Flash Differential Scanning calorimetry, or FDSC can measure DSC effects at ultra-fast heating and cooling rates - rates that cannot be achieved using conventional DSC instruments.

This enables ust to investigate fast reorganization processes in metastable materials and polymorphs. 

The picture shows the sensor on which a sample can be measured at heating rates of up to 2.4 million Kelvin per minute and cooling rates of up to 240 thousand Kelvin per minute.

 

Thermogravimetric Analysis, or TGA. The technique measures the mass of a sample as a function of temperature in a defined atmosphere.

The picture shows the unique METTLER TOLEDO ultra-micro balance with its automatic internal ring weights which are used to calibrate the balance.

 

And finally, Thermo-optical Analysis, or TOA. This technique enables us to visibly monitor material properties, such as the structure of crystals, as a function of temperature or time. The picture shows the hot-stage accessory. This can be placed underneath a microscope to observe the sample while it is subjected to a defined temperature program.

 

I will explain these techniques in more detail in the following slides and describe some application examples.

 

Slide 6: DSC and Flash DSC

Let’s begin with DSC.

This technique allows us to measure the amount of heat absorbed or released by a sample as it is heated or cooled.

 

DSC instruments are available in various versions and differ in their temperature range, the type of sensor used, and their maximum heating and cooling rates.

 

The METTLER TOLEDO DSC operates from minus one hundred and fifty degrees Celsius to plus seven hundred degrees at heating rates of up to three hundred kelvin per minute. Samples are measured in small crucibles made of aluminum, alumina or other materials, using sample amounts of two to twenty milligrams.

 

In comparison, the METTLER TOLEDO Flash DSC 1 measures samples at much higher heating and cooling rates. To do this, it uses very small samples of about one hundred nanograms and no sample crucibles - the sample is in direct contact with the sensor. The ultra-fast heating and cooling rates allow industrial processing conditions to be simulated in which materials undergo extremely rapid cooling.

 

The schematic curve on the left shows a DSC measurement curve with a number of typical effects that can be observed depending on the sample studied. Exothermic effects point in the upward direction, endothermic effects downward.

 

The effects are numbered next to the curve and explained in the table. They are:

 

One, the initial deflection or start-up transient of the DSC;

Two, the baseline where no thermal effects occur;

Three, a glass transition with enthalpy relaxation;

Four, cold crystallization;

Five, melting of the crystalline fraction; and finally

Six, oxidative exothermic decomposition.

 

Special instruments enable samples to be measured at higher or lower gas pressures (high-pressure DSC), or with optical accessories like DSC-Microscopy or UV-DSC.

 

Slide 7: Differential Scanning Calorimetry (DSC)

The table summarizes the main applications of DSC for organic compounds. These include purity analysis, the construction of phase diagrams, the determination of specific heat capacity, the measurement of chemical reactions and catalyst behavior, the determination of thermal stability, and the study of melting and crystallization behavior.

 

The picture on the right shows a view of an open DSC furnace with sample and reference crucibles.

 

I will now present several DSC applications including one involving Flash DSC.

 

Slide 8: Application 1: DSC                                                                                          Melting curves

 

The melting point can be used to identify crystalline or semi-crystalline substances. The melting points of organic molecules vary widely depending on their chemical structure. In general, small organic molecules with no significant intermolecular interactions have a lower melting point than larger molecules with strong interactions caused by intra-molecular or inter-molecular hydrogen bonding. Molecules with high melting temperatures often decompose before they melt.

The slide shows the melting curves of several organic compounds.

The onset of melting is a characteristic value for a substance. The crystals of four of the compounds are stable and exhibit single well-defined melting peaks.

In contrast, the heating curve of chlorpropamide indicates that metastable crystals reorganize before they melt at the heating rate used for the measurement.

 

Slide 9: Application 2a: DSC                                                                               Purity determination

The degree of purity of substance is another factor that influences both the melting curve and the melting point. The presence of an impurity in an otherwise pure substance lowers the melting point of the substance.

The effect can be used to determine the degree of purity of a substance. This value is an important criterion in the quality control of pure substances and can easily be determined in a single DSC measurement. The program for the determination of purity by DSC is based on the van’t Hoff equation, which describes the melting point depression of eutectic systems.

 

The slide displays DSC curves of three samples of dimethyl terephthalate. The top curve, in black shows the melting of pure dimethyl terephthalate with a melting point of 140.25 degrees Celsius.

The other two samples contained different amounts of salicylic acid, which had been added as an impurity. The middle and bottom curves show that the melting peaks become wider and flatter, and shift to lower temperatures. 

Dimethyl terephthalate and salicylic acid form a so-called eutectic mixture. The eutectic behaves like a pure substance and melts at a well-defined temperature, in this example at about 111 degrees. The eutectic melting peak increases in size with increasing amounts of impurity.

The example shows that, besides determining a numerical value for the degree of purity, the relative purity of different samples of a substance can be quickly estimated by comparing the individual melting curves with those of suitable standards.

 

Slide 10: Application 2b: DSC                                                          Construction of phase diagrams

We can use DSC melting curves to study the behavior of eutectic systems and to construct a binary phase diagram. To do this, we measure the DSC melting curves of a range of mixtures of different compositions.

In the example shown, we used mixtures of benzoic acid, BA, and dimethyl terephthalate, DMT. The upper black curve displays the melting curve of pure DMT and the lower black curve the melting curve of pure benzoic acid. From top to bottom, the other curves show the melting curves of six different mixtures containing 4 to 89 percent benzoic acid.

The eutectic mixture melts at about 97degrees Celsius, the so-called eutectic point. The upper five curves with benzoic acid contents of 4 to 62% show that the melting peak of the excess DMT shifts to lower temperatures and becomes smaller as the percentage of the benzoic acid impurity increases, until the peak is finally no longer apparent, as can be seen in the 62% BA curve.  This occurs at concentrations around the eutectic composition.

At higher benzoic acid concentrations, a peak corresponding to the melting of the benzoic acid becomes visible at a higher temperature. We can see this in the curve of 89% benzoic acid - the DMT now acts as an impurity that lowers the melting point of the benzoic acid.

The melting points measured for the eutectic mixture and the excess amounts of the two substances can then be used to construct the binary phase diagram shown in the next slide.

 

Slide 11: Application 2b: DSC                                                                                     Phase diagram

The binary phase diagram shows the mole fraction of benzoic acid on the x-axis and the melting points of the individual sample mixtures on the y-axis. The melting point of the eutectic mixture remains constant and is plotted as a horizontal line at about 97 degrees Celsius. This line is referred to as the solidus line. The sample is completely crystalline below this temperature.

The melting point depression of the DMT is shown by the liquidus line, which is drawn through the individual measurement points. The line shows how the melting point decreases with increasing benzoic acid content. Above the liquidus line, the sample is completely liquid. At a benzoic acid content of 72%, only the melting of the eutectic mixture is observed. This corresponds to the eutectic composition. Between the liquidus and solidus lines, part of the sample is molten and the remainder is still crystalline.

As the benzoic acid content increases, the melting point of the benzoic acid shifts to higher temperatures and continues up to 100% benzoic acid where the melting temperature of pure benzoic acid is measured. Conversely, as the mole fraction of benzoic acid approaches 0% the temperature corresponds to the melting point of DMT.

 

The example shows that we were able to construct a binary phase diagram for this system from just eight DSC measurements.

 

Slide 12: Application 3: DSC                                                                              Safety investigations

Another important application area is the identification and assessment of possible hazards and risks in chemical processes. This is vital for developing and controlling chemical reactions on the laboratory scale and in an industrial environment. Loss of control over a reaction can lead to a so-called thermal runaway, and in the worst case, to an explosion with considerable damage and personal injury.

One approach to safety is to investigate the reaction kinetics of the individual substances used in the reaction.

The slide shows how the thermal stability of ethyl acrylate was investigated using the METTLER-TOLEDO model-free-kinetics software. Ethyl acrylate is stabilized because it would otherwise immediately polymerize. It is well-known that polymerization reactions show a tendency to cause thermal runaways.

The three dynamic DSC measurements (above left) performed at different heating rates show that the production of heat suddenly increases after a certain temperature increase or time. This indicates that the reaction behavior is complex. The assumption is that, at this point, the stabilizer has been consumed and that polymerization suddenly occurs with great intensity. The reaction enthalpy of about 720 joules per gram is sufficiently large to decompose the polymerization product if the reaction container is not immediately cooled. Since the reaction takes place so rapidly, the system behaves practically adiabatically.

 

The kinetics software calculates the apparent activation energy as a function of conversion from the three DSC curves. This activation energy curve is shown in the bottom left diagram. The activation energy was then used to predict conversion curves for ethyl acrylate at isothermal temperatures of 110, 120 and 130 degrees Celsius as shown in the diagram on the right. We see, for example, that at 110 degrees it takes 5 hours until the stabilizer has been consumed, after which the reaction suddenly begins.

The accuracy of this prediction must of course be checked. We did this by performing an isothermal DSC measurement at 110 degrees. The dashed green line shows that a conversion of 60 percent is reached after 337 minutes - the prediction was 304 minutes. A risk assessment must now be performed to decide whether this difference is acceptable or not.

 

Slide 13: Application 4: DSC                                                                                Chemical reactions

DSC measurements can also be used to study other types of chemical reactions.

In this experiment, we dissolved dibenzoyl peroxide in dibutyl phthalate and heated samples from room temperature to 175 degrees Celsius at heating rates of 2, 5 and 10 kelvin per minute. The resulting curves are shown in the right part of the diagram. As expected, the exothermic reaction peak shifts to higher temperatures at higher heating rates.

The DSC curves were then evaluated using a kinetics program. The inset diagram on the left of the slide shows predictions for conversion at isothermal temperatures of 115, 130 and 145 degrees. The information is plotted as a function of time and is also shown numerically in the table. For example, at 130 degrees, we see that it takes about 1.3 minutes to reach a conversion of 50%.

 

Slide 14: Application 5: FDSC                                                                  Melting and decomposition

It is often difficult to measure the true melting point of some organic compounds because their melting and decomposition processes overlap. The measured value is not correct because decomposition products act as impurities and lower the melting point.

 

However, as we saw in the previous slide, chemical reactions are heating-rate-dependent, in contrast to melting points. This means that it should be possible to separate melting and decomposition by using higher heating rates to shift the decomposition reaction to a higher temperature. Unfortunately, the maximum heating rates provided by conventional DSC instruments are not high enough. The ultra-fast heating rates of the Flash DSC 1, however, make this type of analysis possible.

 

The example shows measurement curves an organic substance obtained by conventional DSC and by Flash DSC.

The DSC curve in the lower part of the diagram was measured at 150 Kelvin per minute. This heating rate is relatively high but is much too low to separate melting from decomposition.

The upper part of the slide shows the first and second heating runs of the same substance measured by Flash DSC. The first heating run, shown in black, was recorded at a heating rate of three hundred thousand kelvin per minute or 5000 kelvin per second. The second heating run, shown in red, was recorded at 50 kelvin per second, that is, a heating rate 100 times lower than that used in the first heating run. 

 

The first Flash DSC heating run clearly shows an isolated melting peak with an onset temperature of 345.5 degrees. At this heating rate, decomposition has shifted to such a high temperature that it doesn’t occur in the measured temperature range.

In contrast, the second heating run at 50 kelvin per second shows the exothermic decomposition reaction. At this heating rate, decomposition occurs at a much lower temperature – in fact, roughly 30 degrees below the onset of melting.

 

Slide 15: Thermo-Optical Analysis (TOA)

Let’s now turn to thermo-optical analysis or TOA for short.

Thermo-optical techniques allow us to observe a sample as it is heated or cooled. Some thermo-optical systems can even simultaneously measure DSC heat flow.

 

The table lists the most important analytical applications of TOA in organic chemistry. They include melting, polymorphism and crystallization behavior as well as glass transitions and thermochromism.

 

The picture shows a hot-stage measuring cell for visual sample observation. A microscope and a video camera connected to the measuring cell allow us to capture video images at different temperatures.

 

Slide 16: Application 1: TOA                                                                                   Phase transitions

Most analyses with hot-stage microscopy are performed dynamically – that is, the sample is heated at a constant rate and the changes that occur in the sample are observed as a function of temperature.

 

The slide shows the different phase transitions exhibited by cholesteryl benzoate, a typical example of a liquid crystal.

Liquid crystals are non-isotropic liquids after melting and experience further phase transitions above the melting point before they finally change to isotropic liquids.

 

The DSC curve shows the phase transitions as a function of temperature. The main melting process is observed at about 140 degrees Celsius; the liquid crystalline phase exists from melting up to about 170 degrees. Above this temperature, the substance is an isotropic liquid. During the DSC measurement, the substance was observed through a microscope equipped with polarizer filters. Images were captured at relevant temperatures. 

 

In the first image, the crystals appear as large flakes. These undergo a solid-solid transition at about 110 degrees to another crystalline form, which diffracts the polarized light differently. At 140 degrees, the crystals melt and transform to a cholesteric liquid, which yields a structureless gray image. This phase finally undergoes a liquid-liquid transition to form an isotropic fluid that no longer transmits polarized light; the image now appears black.

 

Slide 17: Application 2: TOA                                                                       Isothermal crystallization

Crystal morphology influences the solubility and melting of a substance. The size, number and structure of crystals formed during crystallization depend on the cooling rate, and, under isothermal conditions, on the crystallization temperature. Hot-stage microscopy can be used to investigate the morphology of crystals.

 

The images on the right show different crystalline forms of chlorpropamide that were obtained after cooling the substance from the melt to isothermal crystallization temperatures of eighty, ninety, and one hundred degrees Celsius at ten kelvin per minute.

 

At high temperatures, the nucleation rate is low and crystal growth rate is high, so that a small number of large crystals are formed. The lower the crystallization temperature, the higher the nucleation rate. The effect of this can be seen in the increasing number of smaller crystals formed at lower temperatures.

 

Slide 18: Application 3: TOA                                                                                       Polymorphism

Another way to study crystal morphology is to heat a metastable compound and observe the different solid-solid transitions that the substance undergoes.

 

This is illustrated using the polymorphism of sulfapyridine as an example. The sample was first quench-cooled from the melt. This fast cooling process produces an amorphous solid with no crystal structure. On heating, increased molecular mobility after the glass transition allows the substance to reorganize to energetically more favorable, crystalline configurations.

Several of these transitions are displayed in the DSC heating curve shown in the slide. The slide also shows images that were simultaneously captured at different temperatures during the DSC run.

 

At the beginning of the DSC curve, the sulfapyridine exhibits a glass transition at about 55 degrees Celsius. The material is then liquid but far below its stable melting temperature of 190 degrees. It therefore crystallizes soon afterward at about 95 degrees in a process known as cold-crystallization. This produces an exothermic peak in the DSC curve.

The first image captured shortly after cold-crystallization shows that the sulfapyridine has crystallized to a multitude of different forms.

The less stable crystals reorganize at the exothermic, monotropic, solid-solid transition at 120 degrees to form crystals that are more stable. The second image shows the newly formed crystals; these are mainly visible on the right hand side of the image. The image also shows that the material is composed of different crystals even after the solid-solid transition.

The next effect observed in the DSC curve on further heating is the melting of certain crystals. This melting process occurs at about 180 degrees and is immediately followed by another exothermic crystallization process.

The third image displays crystals that are more stable, but there is also a black area due to the presence of an isotropic liquid. The sample now consists of stable crystals, and the melt from the unstable crystals. The stable crystals finally melt at 190 degrees.

 

Slide 19: TGA/DSC

I now want to discuss thermogravimetric analysis, or TGA.

In this technique, the mass of a sample is continuously measured as it is heated or cooled in a defined atmosphere. We simply put a few milligrams of the sample into a crucible, weigh the sample, heat it and continuously record the weight change.

 

The schematic curve on the left shows a typical TGA measurement curve, in this case of an organic polymer. Initially, before the heating ramp begins, the TGA measures the mass of the sample. At the end of the measurement, only inorganic residues and ash remain behind. The steps due to loss of mass provide valuable information about the composition of materials and the stability of substances.

 

The steps are numbered next to the curve and explained in the table. They are:

One, heating begins and volatile components vaporize;

Two, pyrolysis of organic substances;

Three, at some point, the atmosphere is switched from nitrogen to oxygen to obtain oxidative conditions;

Four, soot or any carbon black residues burn;

Five, inorganic material that was present in the starting material is left behind as a residue.

 

Slide 20: Thermogravimetric Analysis (TGA)

TGA is used to investigate processes such as vaporization or decomposition. Evolved gases can be analyzed online using hyphenated techniques such as TGA-MS, TGA-FTIR or TGA-GC/MS.

 

The table on the left summarizes the main analytical applications of TGA for organic compounds. The technique provides information about the composition of compounds, thermal stability, and sorption/desorption behavior.

 

The picture on the right is a view of an open TGA furnace showing a sample holder with two positions for the sample and reference crucibles. The standard crucibles are made of aluminum oxide in order to withstand high temperatures.

 

Slide 21: Application 1: TGA/DSC                                                                            Thermal stability

An important application of TGA in the field of organic chemistry is to obtain information about the stability of substances. Stability is normally measured as a function of temperature.

The diagram shows the TGA/DSC measurement of calcium gluconate monohydrate. This compound exhibits intumescence. This means that it swells and forms a low-density foam when heated. The foam acts as flame protection barrier. Such compounds are used in flame- or fire-retardant coatings.

 

The upper curve in the diagram displays the mass loss curve of a sample of calcium gluconate monohydrate that was heated from room temperature to its intumescence temperature.

The curve indicates that the sample loses mass in three separate processes. The first mass loss of just 0.3 percent occurs between room temperature and 70 degrees Celsius. Mass losses in this temperature range are normally due to the evaporation of moisture.

The second mass loss results from the loss of water of crystallization and amounts to about 3% of the total mass. This agrees well with the value calculated from the enthalpy of vaporization determined from the simultaneously measured DSC curve shown in the lower left part of the diagram. Water of crystallization however makes up 4% of the total mass of calcium gluconate monohydrate. The result could mean that not all the water was released before the intumescence reaction began, or that not all the molecules were in fact hydrated.

In the third mass loss step, intumescence begins and calcium oxide is formed. The intumescence process is accompanied by profuse foaming and a large increase in the volume of the sample. This is observed in the TGA curve at about 175 degrees - the foam makes contact with the TGA furnace and causes a sudden apparent increase in the TGA measurement signal.

 

Slide 22: Application 2: TGA/DSC                                                                               Hygroscopicity

The water content of a substance is an important quality factor. Hygroscopic behavior determines storage conditions. Hygroscopic materials should be stored in desiccators or hermetically sealed containers. The stability of such materials in dry or humid atmospheres can be investigated using a TGA-Sorption System. This consists of a TGA connected to a humidity generator by means of a special interface. Measurements can be performed with real-time mass and humidity information.

 

The slide shows the results of measurements performed on amiloride hydrochloride dihydrate. The sample was placed in the TGA furnace and pre-dried at 125 degrees Celsius. This removed the water of crystallization as can be seen from the mass loss shown by the black TGA curve during the first 30 minutes of the experiment. After this initial drying step, the relative humidity was increased in steps from 5 to 95% as shown by the red curve. With increasing relative humidity, the amiloride hydrochloride took up more and more water until the substance regained its original dihydrate form at a relative humidity of about 50%. Further increase of the relative humidity above 50% resulted in the uptake of a small amount of water through adsorption on the surface of the substance.

After this, the relative humidity was decreased in steps. The adsorbed water was gradually released but not the water of crystallization - this more-strongly-bound water can only be eliminated by heating.

 

Slide 23: Application 3: TGA-EGA                                                                          Volatile impurities

Volatile substances released in a TGA experiment can be identified by combining the TGA online with other techniques such as mass spectrometry, infrared spectroscopy, or a combination of gas-chromatography and mass-spectrometry. The instruments are coupled directly to the furnace outlet of the TGA using special interfaces.

 

The slide shows TGA, DTG and DSC curves simultaneously measured using a TGA/DSC instrument. The sample was a substance that had previously been recrystallized from a solvent. The TGA curve shows several overlapping mass losses that can be attributed to moisture, volatiles, and the release of solvents from solvates during melting and decomposition.

The TGA was connected to an FTIR spectrometer in order to identify these substances and, in particular, to identify the substance responsible for the mass loss step at about 88 degrees Celsius.

The infrared spectrum corresponding to a furnace temperature of 88 degrees is shown in blue in the lower right corner of the slide. The best fit from the infrared spectral library was diglyme, the dimethyl ether of diethylene glycol.

Diglyme is commonly used as a solvent for organic reactions. The mass loss was therefore due to the release of this substance.

 

Slide 24: Summary 1

The table lists the main effects that can be used to characterize organic compounds and the thermal analysis techniques recommended for their analysis. A red square denotes the most suitable technique; a blue circle indicates that the technique can also be used, but possibly with some limitations.

 

The most important properties and effects that can be analyzed by DSC are melting, crystallization and polymorphism, enthalpies of melting, crystallization, and thermal stability.

TGA is normally used to study compositional analysis, thermal stability and decomposition, evaporation and desorption behavior. It can also be used to identify gaseous products released during a TGA experiment if it is combined with evolved gas analysis techniques such as MS, FTIR or GC/MS.

TOA is the method of choice for measuring visible changes such as changes in morphology, melting and crystallization.

 

Slide 25: Summary 2

This slide summarizes the temperature ranges of the METTLER TOLEDO DSC, TGA and TOA instruments.

 

In general, DSC experiments are performed at temperatures between minus one hundred and fifty degrees Celsius and plus seven hundred degrees. If special equipment or accessories are used, the temperature ranges, and heating and cooling rates may be different.

 

TOA experiments can be performed between minus sixty degrees and plus three hundred and seventy five degrees. This depends on the particular system used. Several modes of operation are available depending on the information required.

 

TGA measurements normally begin at room temperature. The maximum possible temperature is about sixteen hundred degrees. Various combinations of different furnace sizes and sensors are available. If needed, the TGA can also be operated at pressures below 1 bar. Furthermore, it can be combined with a moisture generator or gas analysis systems.

 

Slide 26: For More Information on Organic Analyses

Finally, I would like to draw your attention to further information about the use of thermal analysis in the field of organic chemistry 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 from www.mt.com/ta-usercoms.

 

A large collection of applications can be found in the “Thermal Analysis in Practice” and other specific handbooks.

 

Slide 27: Further information on Thermal Analysis

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

 

Slide 28: Thank You

 

This concludes my presentation on the thermal analysis of organic compounds. Thank you very much for your interest and attention.

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