Thermal Methods: DSC, DTA, TGA principles and applications MCQs With Answer

Introduction: Thermal Methods: DSC, DTA, TGA principles and applications MCQs With Answer is designed for M.Pharm students to strengthen understanding of key thermal analysis techniques used in pharmaceutical research and quality control. This collection emphasizes the physical principles, instrumentation differences, data interpretation and practical applications of Differential Scanning Calorimetry (DSC), Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA). Questions cover thermodynamic concepts (glass transition, melting, crystallization), kinetic analysis of decomposition, sample preparation, atmosphere effects, and hyphenated techniques (TG-FTIR/TG-MS). Practically oriented items will help students prepare for exams and lab applications by linking theory to real-world pharmaceutical problems such as polymorphism, stability and formulation compatibility.

Q1. What is the fundamental physical quantity directly measured in a Differential Scanning Calorimeter (DSC)?

• Mass change of the sample as temperature increases
• Temperature difference between sample and reference without heat flow measurement
• Heat flow difference between the sample and reference as a function of temperature
• Infrared absorption changes during heating

Correct Answer: Heat flow difference between the sample and reference as a function of temperature

Q2. Which statement best distinguishes power-compensated DSC from heat-flux DSC?

• Power-compensated DSC measures mass loss while heat-flux DSC measures heat capacity
• Power-compensated DSC adjusts electrical power to keep sample and reference at the same temperature; heat-flux DSC measures temperature difference across a known sensor
• Heat-flux DSC uses differential thermocouples while power-compensated DSC measures infrared emission
• They are identical; the names are interchangeable

Correct Answer: Power-compensated DSC adjusts electrical power to keep sample and reference at the same temperature; heat-flux DSC measures temperature difference across a known sensor

Q3. In DSC thermograms, the glass transition (Tg) is typically observed as:

• A sharp exothermic peak corresponding to crystallization
• A step change in baseline heat flow corresponding to change in heat capacity
• A mass loss event correlated with volatile evolution
• A large endothermic peak corresponding to melting

Correct Answer: A step change in baseline heat flow corresponding to change in heat capacity

Q4. Which parameter is most directly obtained from Thermogravimetric Analysis (TGA)?

• Heat capacity (Cp) of a drug substance
• Weight change of a sample as a function of temperature or time
• Enthalpy of fusion for crystalline drugs
• Activation energy from modulated heat flow

Correct Answer: Weight change of a sample as a function of temperature or time

Q5. Differential Thermal Analysis (DTA) differs from DSC primarily because DTA measures:

• The absolute heat flow into the sample
• The temperature difference between sample and reference without direct heat-flow calibration
• The mass spectrometric signal of evolved gases
• The sample’s refractive index change during heating

Correct Answer: The temperature difference between sample and reference without direct heat-flow calibration

Q6. Which of the following is a crucial advantage of modulated DSC (MDSC) over conventional DSC for pharmaceutical samples?

• MDSC measures mass loss directly during decomposition
• MDSC separates reversing (heat capacity-related) and non-reversing (kinetic) heat flow components, improving detection of overlapping events
• MDSC eliminates the need for calibration standards
• MDSC provides molecular weight information of degradants

Correct Answer: MDSC separates reversing (heat capacity-related) and non-reversing (kinetic) heat flow components, improving detection of overlapping events

Q7. During TGA analysis of a drug-excipient mixture, an early weight loss at low temperature most likely indicates:

• Polymer crosslinking
• Loss of adsorbed or bound water (moisture)
• Primary thermal decomposition of the API
• Increase in crystallinity

Correct Answer: Loss of adsorbed or bound water (moisture)

Q8. Which kinetic method uses peak temperature shift with different heating rates to estimate activation energy from DSC or DTA data?

• Kissinger method
• Arrhenius direct integration without heating-rate variation
• Van ‘t Hoff analysis
• HPLC peak area method

Correct Answer: Kissinger method

Q9. Which atmosphere would you choose in TGA to evaluate oxidative stability of a pharmaceutical compound?

• Inert gas such as nitrogen or helium
• Vacuum only
• Oxidative atmosphere such as air or oxygen
• Argon with hydrogen spike

Correct Answer: Oxidative atmosphere such as air or oxygen

Q10. A DSC endothermic peak corresponding to melting will shift to higher temperature when:

• The heating rate is decreased substantially
• A small amount of impurity is added that acts as a melting point depressant
• The heating rate is increased, leading to thermal lag and peak shift to higher temperatures
• The sample is measured in vacuum rather than inert gas

Correct Answer: The heating rate is increased, leading to thermal lag and peak shift to higher temperatures

Q11. Which of the following best explains the use of derivative thermogravimetry (DTG)?

• DTG filters out baseline noise from DSC curves
• DTG enhances resolution by plotting the rate of weight change (dW/dt or dW/dT) to identify overlapping decomposition steps
• DTG measures heat capacity as a derivative signal
• DTG provides direct information about heat of combustion

Correct Answer: DTG enhances resolution by plotting the rate of weight change (dW/dt or dW/dT) to identify overlapping decomposition steps

Q12. When determining melting enthalpy (ΔHf) by DSC for a crystalline pharmaceutical, which step is essential for accurate integration?

• Using a very large sample mass to maximize signal
• Baseline subtraction and correct integration limits around the melting peak
• Measuring in humid air to mimic storage conditions
• Avoiding instrument calibration

Correct Answer: Baseline subtraction and correct integration limits around the melting peak

Q13. Which instrument combination allows identification of decomposition products evolved during TGA?

• DSC coupled with FTIR
• TGA coupled with FTIR or mass spectrometer (TG-FTIR or TG-MS)
• DTA alone with no coupling
• Polarized light microscopy coupled to DSC

Correct Answer: TGA coupled with FTIR or mass spectrometer (TG-FTIR or TG-MS)

Q14. In pharmaceutical compatibility studies using DSC, the disappearance or shift of an API melting peak in a mixture most likely indicates:

• Instrument drift only
• Possible interaction such as eutectic formation, solid-state reaction or amorphization
• Complete thermal stability of both components
• Exclusive hydration without any solid-state change

Correct Answer: Possible interaction such as eutectic formation, solid-state reaction or amorphization

Q15. Which factor does NOT significantly influence the results of a TGA experiment?

• Sample pan material and mass
• Heating rate and gas flow rate
• Thermocouple calibration of a DSC not used in TGA
• Atmosphere composition (inert vs oxidative)

Correct Answer: Thermocouple calibration of a DSC not used in TGA

Q16. A multi-step weight loss in TGA of a polymeric excipient usually suggests:

• A single-step clean melting process
• Sequential events such as moisture loss, side-chain cleavage and backbone degradation
• Pure recrystallization without decomposition
• Instrumental baseline artifacts only

Correct Answer: Sequential events such as moisture loss, side-chain cleavage and backbone degradation

Q17. How is heat capacity (Cp) of a sample commonly measured by DSC?

• By integrating a melting peak only
• By using a modulated or isothermal DSC procedure and comparing heat flow to a calibrated reference material
• By measuring mass loss at multiple temperatures
• By visual inspection of the thermogram

Correct Answer: By using a modulated or isothermal DSC procedure and comparing heat flow to a calibrated reference material

Q18. Which experimental strategy improves resolution between closely spaced thermal events in DSC?

• Increasing sample mass significantly to amplify signals
• Reducing heating rate and using smaller sample mass or modulation (MDSC)
• Using moisture-saturated atmosphere for all measurements
• Removing baseline correction from analysis

Correct Answer: Reducing heating rate and using smaller sample mass or modulation (MDSC)

Q19. When performing non-isothermal kinetic analysis from TGA data, why is it important to run experiments at several different heating rates?

• To intentionally create inconsistent results
• To obtain activation energy and kinetic parameters by methods like Kissinger and Ozawa-Flynn-Wall that require multiple heating rates
• Because single heating-rate data are always free of noise
• To produce more mass loss peaks for easier interpretation

Correct Answer: To obtain activation energy and kinetic parameters by methods like Kissinger and Ozawa-Flynn-Wall that require multiple heating rates

Q20. In a DSC experiment, an exothermic event observed above the melting point of an API during heating most likely indicates:

• Simple endothermic melting only
• Crystallization on heating, oxidative degradation, or an exothermic solid-state reaction such as recrystallization or decomposition
• Exclusive water loss
• Measurement in an inert atmosphere always eliminates exotherms

Correct Answer: Crystallization on heating, oxidative degradation, or an exothermic solid-state reaction such as recrystallization or decomposition

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