ICAN 5122: Other Analytical Methods Final

What does DSC stand for?

Differential Scanning Calorimetry

What does “Differential'“ mean in DSC?

The difference in the amount of heat applied (or removed) to increase (or decrease) the temperature of a sample material and a reference material are measured

What does “Scanning” mean in DSC

The differential is measured across a temperature range at a specific rate (controlled temperature program) where we scan for these differences

Calorimetry

The term used to describe the science of counting the heat of chemical reactions or physical changes

True or False: Calorimetry can be used to measure both endothermic and exothermic heat flows.

True

Overall, what does DSC provide?

Accurate thermodynamic data and information regarding reactivity and/or phase transformation

In DSC, endothermic heat will flow into the sample as a result of either

Glass transition (Tg), melting, evaporation, etc.

In DSC, exothermic heat will flow out of the sample as a result of either

Crystallation, curing, oxidation, etc.

When a physical transformation occurs, thermodynamics tells us that heat will flow in or out of a sample. In the DSC technique, we need to compare to a reference and when a physical transformation occurs, more (or less) heat will need to flow to the sample in comparison to the reference. This will manifest itself as an upwards peak (exothermic), or as a downwards peak (endothermic). Why?

DSC peaks go up or down because the instrument must adjust the heat flow to keep the sample and reference at the same temperature. DSC is not plotting the heat of the sample. It’s plotting the power difference needed to counteract the sample’s behavior.

Will an exothermic transformation cause the peak to go upwards or downwards in DSC?

Upwards

Will an endothermic transformation cause the peak to go upwards or downwards in DSC?

Downwards

Endo or Exo? Water freezing

Exo

Endo or exo? Ice melting

Endo

Endo or exo? Photosynthesis

Endo

Endo or exo? Cooking an egg

Endo

Endo or exo? Thermite

Exo

Glass transition temperature (T_g)

The range of temperature over which there is an onset of molecular mobility departing from a glassy state and into a more rubbery or viscous state

Why is glass transition temperature a very important property when dealing with amorphous material?

Above this temperature, a material may crystallize, which may or may not be desirable

What is the reverse phenomenon of glass transition temperature called?

Vitrification

DSC Heat Flow Equation Explanation

Expressed using sample specific heat capacity, heating rate and heat flow as a function of time and temperature

Heat capacity

Amount of heat (calories, kilocalories, or joules) needed to raise the temperature of 1 gram of a substance by 1 degree Celsius

Heat flow

Transfer of heat as a function of time and temperature (comparing a sample to a reference)

What are the two main types of instrument desings for DSC?

Heat Flow DSC and Heat Flux DSC

What are some less common types of DSC instruments?

High-pressure DSC and ultra-violet DSC

What is fast scan DSC?

Conventional DSC with fast heating rates > 100 degrees Celsius/min

What is modulated temperature DSC?

Conventional DSC with a twist with heating, cooling and isothermal cycles

What does Heat Flow DSC measure?

The heat that enters and leaves the sample

How does heat flow DSC work?

Calorimeter uses a feedback loop to keep temperature within the material constant, measuring power to do so against another material, usually a reference material

What is the reference material for DLS usually made of?

A known standard (eg. alumina) or an empty pan (very common way)

What does heat flow DSC provide an accurate measure of?

Enthalpy and heat capacity

How does heat flux DSC work?

Instrument measures temperature changes between sample and a reference material or an empty pan. Due to differences in heat capacity, the reference heats faster than the sample. The temperature of the sample does not change during melting but the reference temperature remains unaffected and continues exhibiting a linear increase. When the melting is complete, the sample temperature also begins to increase again.

Strengths of heat flux DSC, relative to heat flow DSC?

Stable baselines, higher sensitivity but slower heating and cooling

Benefits of using aluminum as crucible for DSC

Inexpensive, low temperature

Benefits of using copper as crucible for DSC

Used as catalyst (testing polymers)

Benefits of using gold as crucible for DSC

Higher temperature, expensive

Benefit of using alumina (Al₂O₃) as crucible for DSC

Very high temperature

Benefits of using sapphire as crucible for DSC

Crystalline alumina, more chemically resistant than amorphous alumina

Sample preparation steps for DSC

Need to know the weight of the sample (usually a few mg), need it to be flat and uniform (less is more), keeping sample as thin as possible to minimize sample gradients, covering as much of the pan as possible

The sample pan in DSC is usually crimped before analysis. It is crucial that it not be deformed during analysis. Why?

A DSC pan must not deform during analysis because deformation alters thermal contact, changes heat‑transfer resistance, and can break the seal of the pan. This disrupts the symmetry between the sample and reference pans, leading to inaccurate heat‑flow measurements, false peaks, mass loss, and shifted transition temperatures. Maintaining an intact, properly crimped pan ensures that the DSC measures only the sample’s thermal events, not artifacts caused by pan deformation.

Why would you select an open pan for DSC?

You want to know about atmospheric interactions with the sample

Why would you select a crimped (hermetic sealed) pan for DSC?

Improves the thermal contact between the sample, pan and disc, reducing thermal gradients in the sample, minimizing spillage

Why would you select an inverted crimped pan for DSC?

To analyze samples which are larger in size and cannot be compressed, allowing for use of more material if needed

Why would you select a crimped or inverted crimped with pinhole?

Provides release of solvent (minimize impact of solvent vapor on measured melting, enthalpy, glass transition, etc.)

What purge gas would you typically use for DSC?

Nitrogen

He as a purge gas for DSC is useful for

Removal of volatiles

Ar is preferred as an inert purge gas for DSC when examining samples that could. .

React with nitrogen

Parameters you need to optimize in DSC

Heating rate, starting and end temperature, cycling, cooling rate, isothermal hold

How are the x and y axes in the DSC thermogram usually presented as?

Temperature and heat flow, respectively

In DSC, the y axis, or the heat flow refers to

The heat added in order for the thermal transitions to occur

DSC applications

Pharmaceuticals, polymers, oxidation testing, ceramics, resins, etc., QA testing, safety screening, metals, protein analysis, nucleic acids

DSC can be used to identify contaminants, or in conjunction with other techniques. In this scenario, you noticed that one sample of your materials that you synthesized has particulates on the bottom of the container. How would you resolve it?

Isolate particles, analyze them separately using DSC and complementary techniques, adjust synthesis accordingly

What is a hydrate?

A solid adduct containing both the parent compound (e.g. the anhydrous compound) and water

Gamma-ray spectroscopy methods are useful for

Isotope identification, environmental radioactivity measurements, nuclear medicine, geological and materials analysis

Properties of gamma rays

High-energy photons emitted from the nucleus with no mass, no charge, and very high penetrating power

How are gamma rays different from X-rays?

X-rays usually come from electronic processes, and Gamma rays come from nuclear processes

Half-life (radiation)

The time required for half of the radioactive nuclei in a sample to decay

Activity (radiation)

The number of decay events per second

A sample with higher activity produces ___ (more/less) radiation events in a given time

More

In gamma ray spectroscopy methods, the measured signal depends on 3 factors:

How much radioactive material is present, how quickly it decays, how long the measurement is collected

Why do Gamma rays need special detectors?

Gamma rays are highly penetrating and interact less readily than UV or IR light; therefore, efficient detection requires dense, high-Z materials so that interactions occur inside the detector. Shielding is also important because background counts can obscure weak signals.

A useful gamma detector must achieve these 2 things:

Stop the photon and report how much energy was deposited

Photoelectric effect

Photon gives all of its energy to one bound electron → full-energy deposition is possible

Compton scattering

Photon transfers only part of its energy, then leaves with lower energy → continuum

Pair production in Gamma-ray spectroscopy

Above 1.022 MeV, a photon can create an electron-positron pair near a nucleus

These three processes determine whether the detector on a Gamma-ray spectrometer sees a sharp peak, a continuum, or escape features.

Photoelectric effect, Compton scattering, pair production

Basic Layout of a Gamma-Ray spectrometer

Source/sample → detector → preamplifier + multichannel analyzer → computer/spectrum

How a spectrum is processed in a Gamma-ray spectrometer

Source → detector interaction → electrical pulse → processed spectrum

What are the two common detectors in Gamma-ray spectrometers?

NaI(TI) scintillator, HPGe semiconductor

NaI(TI) scintillator

Gamma interaction produces flashes of light, photomultiplier converts light to an electrical pulse, efficient and relatively simple, lower energy resolution

HPGe semiconductor Properties in Gamma-Ray Spectroscopy

Interaction creates electron-hole pairs, collected charge is proportional to energy, excellent energy resolution, usually requires cooling, sharper peaks

NaI(TI) Scintillator Properties in Gamma-Ray Spectroscopy

Gamma interaction produces flashes of light, photomultiplier converts light to an electrical pulse, efficient and relatively simple, lower energy resolution, broader peaks

How would you select a detector for Gamma-ray spectroscopic methods?

Choose NaI(TI) when simplicity and efficiency matter; choose HPGe when peak separation matters most.

The photopeak (full-energy peak) in a Gamma-ray spectrum represents the point when the detector. .

Receives the full gamma-ray energy

The Compton continuum in a Gamma-ray spectrum represents

Partial energy deposition from Compton scattering

In a Gamma-ray spectrum, peak position helps ______ the isotope

Identify

In a Gamma-ray spectrum, peak area helps ______ the isotope

Quantify

A Gamma-ray spectrum contains

Useful peaks and interaction-related background features

What Gamma-ray spectrocopy measures

Emitted gamma-ray energies and intensities

Gamma-ray spectroscopy is best for

Isotope identification and quantification

Limitation of Gamma-ray spectroscopy (motivates Mossbauer spectroscopy)

Resolution is finite and ordinary gamma absorption is not perfectly resonant

Mossbauer spectroscopy turns gamma-ray absorption into. .

A high-resolution probe of the local environment around a nucleus

Information Mossbauer spectroscopy provides

Local electronic environment, oxidation state, site symmetry, magnetic ordering

Mossbauer spectroscopy is a nuclear spectroscopy technique based on

Resonant absorption and emission of gamma rays

The phenomenon observed in Mossbauer spectroscopy when

Recoil is effectively eliminated for nuclei embedded in a solid

Mossbauer spectroscopy is especially useful for iron-containing materials because

Fe-57 is a practical Mossbauer isotope

Why is Mossbauer spectroscopy not just “another gamma technique”?

It probes tiny hyperfine energy shifts

Why ordinary Gamma absorption is difficult

A free nucleus must recoil when it emits or absorbs a gamma ray because momentum is conserved, resulting in some energy being diverted into recoil, so the emitted photon energy is slightly different from the exact nuclear transition energy. That mismatch spoils perfect resonance between emission and absorption.

The main obstacle to simple resonant gamma-ray absorption in free atoms

Recoil

If emission and absorption energies do not match exactly in Gamma absorption, resonant absorption becomes

very weak

Thermogravimetric analysis (TGA)

Technique that allows you to measure the mass of a substance (or change in mass) as a function of temperature usually under a controlled heating program

What is TGA used to study?

The thermal stability of a sample, rates of reactions, reaction processes, etc.

Can TGA be used to study the onset of molecular mobility in an amorphous material (glass transition)?

No. Tg involves a change in heat capacity and molecular mobility, but no mass change, so TGA cannot detect it. You’d use DSC instead.

Can you use TGA to study a reaction where X → Y + gas?

Yes. Gas leaves the sample, so you see a mass loss at the temperature where the reaction occurs.

Can you use TGA to study dehydration?

Yes. Loss of water gives a clear mass loss step.

Can TGA be used to study thermally induced crystallization?

No (in general). Crystallization is a rearrangement of the solid with no change in mass, so TGA won’t see it. DSC would.

Can TGA be used to study a reaction where X + gas → Y?

Yes. If the gas is incorporated (e.g., oxidation), you see a mass gain as temperature increases.

Six major components that make up the anatomy of a TGA instrument

The balance, the sample platform, the furnace, gas flow controllers, electronics and computer

What does FT-IR measure?

Absorption of IR light by molecular vibrations.

What makes a vibration IR-active?

It must involve a change in dipole moment.