Field ionization MCQs With Answer

Introduction: Field ionization MCQs With Answer is a focused quiz set designed for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). Field ionization (FI) is a soft, high‑field ionization technique widely used in mass spectrometry to generate molecular ions with minimal fragmentation, particularly for volatile and thermally stable organic molecules. This blog provides concise, concept‑driven multiple‑choice questions that cover FI principles, mechanisms (tunneling), emitter design, operating parameters (field strength, tip radius, temperature), instrumentation coupling, and application limits. The questions reinforce understanding of how electric fields create molecular ions, practical considerations for analysis, and differences between FI and related techniques like field desorption and electron impact ionization.

Q1. What best describes field ionization (FI) as used in mass spectrometry?

• Ionization by collisions with high energy electrons
• Ionization by strong electric field causing electron tunneling
• Chemical ionization involving reagent gases
• Ionization by laser ablation of the sample

Correct Answer: Ionization by strong electric field causing electron tunneling

Q2. What is the primary microscopic mechanism responsible for ion formation in FI?

• Direct collision between gas molecules and the emitter
• Electron tunneling from the molecule to the emitter tip
• Thermal evaporation followed by electron impact
• Photoionization by stray photons

Correct Answer: Electron tunneling from the molecule to the emitter tip

Q3. Typical electric field strengths required for field ionization are on the order of:

• 10^3–10^5 V/m
• 10^6–10^7 V/m
• 10^8–10^10 V/m
• 10^12–10^14 V/m

Correct Answer: 10^8–10^10 V/m

Q4. Field ionization is most suitable for which class of analytes?

• Large nonvolatile polymers and salts
• Highly polar ionic compounds in solution
• Volatile, thermally stable organic molecules
• Inorganic metal clusters requiring laser desorption

Correct Answer: Volatile, thermally stable organic molecules

Q5. Which of the following materials is commonly used to make FI emitter tips?

• Silicon wafer
• Woven nylon fiber
• Tungsten or other refractory metal needle
• Glass capillary

Correct Answer: Tungsten or other refractory metal needle

Q6. The active region for field ionization typically lies how far from the emitter tip?

• Several micrometers away (1–10 µm)
• Hundreds of nanometers (100–500 nm)
• Sub‑nanometer to a few nanometers (≈0.5–2 nm)
• More than 10 micrometers

Correct Answer: Sub‑nanometer to a few nanometers (≈0.5–2 nm)

Q7. Which theoretical relationship is commonly used to describe current from a field emitter as a function of field?

• Arrhenius equation
• Michaelis–Menten equation
• Fowler–Nordheim equation
• Beer’s law

Correct Answer: Fowler–Nordheim equation

Q8. Ionization efficiency in FI correlates most strongly with which molecular property?

• Boiling point
• Ionization potential (IP) of the molecule
• Molecular weight only
• Optical absorbance at UV wavelengths

Correct Answer: Ionization potential (IP) of the molecule

Q9. Which background/probe gas is most commonly used in field ion microscopy and related FI experiments?

• Nitrogen (N2)
• Argon (Ar)
• Helium (He)
• Oxygen (O2)

Correct Answer: Helium (He)

Q10. Compared with electron impact ionization, field ionization typically produces:

• Much more extensive fragmentation
• Predominantly molecular ions with minimal fragmentation
• Only negative ions
• No usable ions for mass analysis

Correct Answer: Predominantly molecular ions with minimal fragmentation

Q11. A primary analytical application of FI in mass spectrometry is:

• Ion imaging of biological tissues
• Determination of molecular weight of volatile organics
• Ionization of inorganic salts in solution
• Analysis of large nonvolatile proteins without any sample preparation

Correct Answer: Determination of molecular weight of volatile organics

Q12. Which combination of emitter properties most strongly lowers the voltage required for FI?

• Large tip radius and high work function
• Small tip radius and low work function
• Rough surface and insulating coating
• High temperature and large surface area

Correct Answer: Small tip radius and low work function

Q13. The reason FI commonly yields radical cations (M+•) is because:

• The molecule gains an electron from the emitter
• An electron is removed by tunneling, leaving a radical cation
• Photons produce excited cationic fragments
• Molecules undergo protonation on the emitter

Correct Answer: An electron is removed by tunneling, leaving a radical cation

Q14. Which experimental action will most increase ionization probability in FI?

• Decrease electric field strength at the tip
• Increase distance between emitter and counter‑electrode
• Increase the local electric field strength at the tip
• Lower the vacuum level (increase pressure)

Correct Answer: Increase the local electric field strength at the tip

Q15. Compared to electron impact (EI) ionization, FI is preferred when the analyst wants to:

• Obtain extensive fragment ion patterns for structural elucidation
• Generate abundant molecular radical cations with minimal fragmentation
• Ionize inorganic salts dissolved in water
• Measure ultraviolet absorbance simultaneously

Correct Answer: Generate abundant molecular radical cations with minimal fragmentation

Q16. Field desorption (FD) differs from field ionization (FI) primarily because FD:

• Requires gas‑phase analytes only
• Is used predominantly for nonvolatile or thermally labile compounds deposited on the emitter
• Always produces only negative ions
• Uses photon absorption to create ions

Correct Answer: Is used predominantly for nonvolatile or thermally labile compounds deposited on the emitter

Q17. FI sources have historically been coupled to which type of mass analyzer for molecular ion detection?

• Magnetic sector mass spectrometer
• Capillary electrophoresis detector only
• Infrared spectrometer
• Electrochemical detector

Correct Answer: Magnetic sector mass spectrometer

Q18. Increasing the emitter temperature in FI experiments most directly affects which process?

• Directly lowers the ionization potential of analytes
• Enhances desorption rate and can promote field desorption contributions
• Eliminates the need for high electric fields
• Converts all ions to negative ions

Correct Answer: Enhances desorption rate and can promote field desorption contributions

Q19. A major limitation of field ionization for routine pharmaceutical analysis is:

• Its extensive fragmentation for all organic compounds
• Unsuitability for large, nonvolatile, or highly polar/ionic molecules
• Incompatibility with vacuum systems
• Generation of only multiply charged ions

Correct Answer: Unsuitability for large, nonvolatile, or highly polar/ionic molecules

Q20. Which physical parameter is most commonly used to quantify how “sharp” an emitter tip is for FI?

• Emitter temperature
• Radius of curvature at the tip
• Emitter electrical resistance
• Bulk density of the emitter material

Correct Answer: Radius of curvature at the tip

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