Field ionization MCQs With Answer

Field ionization MCQs With Answer

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Introduction

Field ionization (FI) is a soft ionization technique in mass spectrometry that creates molecular ions using extremely high electric fields at the tips of sharp emitter needles. For M. Pharm students studying Modern Pharmaceutical Analytical Techniques, FI offers a valuable way to obtain intact molecular ion peaks for volatile and thermally stable analytes with minimal fragmentation—crucial for accurate molecular weight determination and structural elucidation. This blog presents carefully designed Field Ionization MCQs with answers, focusing on principles, instrumentation, operating conditions, analyte suitability, and pharmaceutical applications. Each question is crafted to reinforce conceptual clarity, highlight differences from techniques like EI, CI, FD, ESI, and MALDI, and provide exam-ready practice aligned with advanced analytical curricula.

Q1. What is the fundamental principle behind field ionization in mass spectrometry?

  • Electron tunneling from a neutral molecule to a high-field emitter, producing M+•
  • Proton transfer from a reagent gas to form [M+H]+
  • Photon-induced ionization via UV absorption
  • Thermal emission of electrons from the sample at high temperature

Correct Answer: Electron tunneling from a neutral molecule to a high-field emitter, producing M+•

Q2. Which best describes the typical emitter used for field ionization sources?

  • Gold-coated MALDI plate with matrix crystals
  • Activated carbon micro-needles grown on a tungsten or rhenium wire
  • Stainless steel capillary used for electrospray
  • Porous silicon wafer etched by HF

Correct Answer: Activated carbon micro-needles grown on a tungsten or rhenium wire

Q3. What is the approximate electric field strength required near the emitter tip for efficient field ionization?

  • 10–100 V/cm
  • 103–104 V/cm
  • 105–106 V/cm
  • 107–108 V/cm

Correct Answer: 107–108 V/cm

Q4. Field ionization is best suited for which class of analytes in pharmaceutical analysis?

  • Small, volatile, thermally stable or moderately stable organic molecules
  • High-mass, non-volatile peptides and proteins
  • Inorganic salts and metal clusters in aqueous media
  • Strongly ionic, highly polar polymers

Correct Answer: Small, volatile, thermally stable or moderately stable organic molecules

Q5. Which pressure range is typically required for field ionization sources?

  • Atmospheric pressure (≈100 bar)
  • Medium vacuum (10-3–10-4 torr)
  • High to ultra-high vacuum (≈10-7–10-9 torr)
  • Pressurized helium (1–2 bar)

Correct Answer: High to ultra-high vacuum (≈10-7–10-9 torr)

Q6. What is the predominant ion type observed in field ionization mass spectra?

  • Even-electron [M+H]+ ions
  • Odd-electron molecular radical cations M+• with minimal fragmentation
  • Multiply charged ions like [M+nH]n+
  • Negative ions formed by electron capture

Correct Answer: Odd-electron molecular radical cations M+• with minimal fragmentation

Q7. Compared to electron ionization (EI), field ionization typically provides:

  • Greater fragmentation and lower molecular ion abundance
  • Similar fragmentation but higher signal-to-noise
  • Much less fragmentation and higher molecular ion abundance
  • No molecular ion, only adducts

Correct Answer: Much less fragmentation and higher molecular ion abundance

Q8. Which statement best differentiates field ionization (FI) from field desorption (FD)?

  • FI deposits analyte on the emitter; FD uses vapor-phase introduction
  • FD requires a laser; FI does not
  • FI uses gas-phase analyte near the emitter; FD deposits analyte directly onto the emitter surface
  • FD operates at atmospheric pressure; FI in high vacuum

Correct Answer: FI uses gas-phase analyte near the emitter; FD deposits analyte directly onto the emitter surface

Q9. In pharmaceutical applications, a key advantage of FI-MS is:

  • Quantitative analysis of peptides via multiple charging
  • Obtaining intact molecular ions for volatile drugs and steroids
  • High salt tolerance in aqueous samples
  • Direct imaging of drug distribution in tissue

Correct Answer: Obtaining intact molecular ions for volatile drugs and steroids

Q10. Which is a common sample introduction method for field ionization?

  • Direct infusion through a nano-ESI emitter
  • Thermally vaporizing the analyte into the source near the emitter tip
  • Pulsed UV laser ablation of the sample
  • Desorption via matrix-assisted co-crystallization

Correct Answer: Thermally vaporizing the analyte into the source near the emitter tip

Q11. Which limitation most constrains the use of FI for pharmaceutical compounds?

  • Inability to detect halogen-containing molecules
  • Requirement that analytes be sufficiently volatile and not decompose upon vaporization
  • Excessive adduct formation with alkali metals
  • Mandatory use of reagent gases that react with samples

Correct Answer: Requirement that analytes be sufficiently volatile and not decompose upon vaporization

Q12. What is the approximate internal energy imparted to ions by FI compared to EI?

  • FI imparts around 0–1 eV; EI imparts ~70 eV
  • FI imparts ~70 eV; EI imparts 0–1 eV
  • Both impart ~30 eV
  • Both impart negligible energy

Correct Answer: FI imparts around 0–1 eV; EI imparts ~70 eV

Q13. The immediate ionization zone in FI is located:

  • Several millimeters away from the emitter tip
  • Within a few nanometers to tens of nanometers from the emitter tip
  • At the ion detector surface
  • Inside the mass analyzer flight tube

Correct Answer: Within a few nanometers to tens of nanometers from the emitter tip

Q14. Which mass analyzers are historically and practically compatible with FI sources?

  • Double-focusing sector and TOF analyzers
  • Ion mobility spectrometers only
  • Orbitrap only
  • Quadrupole ion trap only

Correct Answer: Double-focusing sector and TOF analyzers

Q15. Which statement about reagent gases applies to field ionization?

  • Requires methane or ammonia as a reagent gas
  • Uses nitrogen as a collision gas to generate ions
  • Does not require any reagent gas for ion formation
  • Uses helium as a photon mediator

Correct Answer: Does not require any reagent gas for ion formation

Q16. Which observation is typical in FI mass spectra of pharmaceuticals?

  • Dominant [M+Na]+ adduct peaks
  • Predominant molecular radical cation with minimal fragment ions
  • Complete absence of molecular ion
  • Highly multiply charged envelopes

Correct Answer: Predominant molecular radical cation with minimal fragment ions

Q17. What maintenance or operational challenge is common with FI emitters?

  • Salt build-up from aqueous sprays
  • Emitter burnout or degradation of carbon tips under high fields and heat
  • Matrix crystal heterogeneity
  • Need for frequent UV lamp replacement

Correct Answer: Emitter burnout or degradation of carbon tips under high fields and heat

Q18. Which comparison between FI and CI (chemical ionization) is correct?

  • FI forms mainly [M+H]+ via reagent gas; CI forms M+• via tunneling
  • FI often yields M+• with little fragmentation; CI typically forms [M+H]+ using a reagent gas
  • Both require reagent gases for soft ionization
  • Both primarily operate at atmospheric pressure

Correct Answer: FI often yields M+• with little fragmentation; CI typically forms [M+H]+ using a reagent gas

Q19. In GC–MS coupling, using an FI source is advantageous because:

  • It produces strong molecular ions for volatile analytes, aiding identification
  • It allows direct analysis of non-volatile biomolecules
  • It eliminates the need for vacuum systems
  • It ensures extensive fragmentation for structural elucidation

Correct Answer: It produces strong molecular ions for volatile analytes, aiding identification

Q20. Which statement regarding charge states in FI is most accurate?

  • FI commonly produces doubly and triply charged ions
  • FI predominantly yields singly charged radical cations
  • FI exclusively forms negative ions
  • FI produces only protonated molecules

Correct Answer: FI predominantly yields singly charged radical cations

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