FAB and MALDI ionization MCQs With Answer

FAB and MALDI ionization MCQs With Answer

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Welcome, M. Pharm students! This quiz focuses on two cornerstone soft-ionization techniques in mass spectrometry—FAB (Fast Atom Bombardment) and MALDI (Matrix-Assisted Laser Desorption/Ionization). Both have transformed the analysis of thermolabile and high-mass biomolecules by enabling gentle desorption and ionization with minimal fragmentation. The following MCQs span principles, instrumentation, matrices, analyte suitability, ion types, sample preparation, adduct formation, mass range, and performance considerations. You will test your understanding of laser types, beam energies, adduct chemistry, charge states, delayed extraction, and the impact of salts and matrices on spectra. Use this set to sharpen conceptual clarity and practical insight for exams and real-world method development in modern pharmaceutical analytical techniques.

Q1. What best describes the ionization mechanism in Fast Atom Bombardment (FAB)?

  • Sputter desorption/ionization by fast neutral atoms impacting a viscous matrix containing analyte
  • Thermal desorption by resistive heating under vacuum
  • Photodissociation by UV laser in a crystalline matrix
  • Electron impact ionization at 70 eV

Correct Answer: Sputter desorption/ionization by fast neutral atoms impacting a viscous matrix containing analyte

Q2. The primary species used to bombard the sample in FAB are:

  • Fast neutrals generated by charge-exchange from an ion beam (e.g., Xe, Ar)
  • Electrosprayed charged droplets
  • UV photons at 337 nm
  • Alpha particles from a radioactive source

Correct Answer: Fast neutrals generated by charge-exchange from an ion beam (e.g., Xe, Ar)

Q3. Which set lists common matrices for FAB?

  • Glycerol, 3-nitrobenzyl alcohol (3-NBA), thioglycerol
  • Toluene, hexane, benzene
  • Sinapinic acid and CHCA only
  • Water and methanol only

Correct Answer: Glycerol, 3-nitrobenzyl alcohol (3-NBA), thioglycerol

Q4. In positive-ion FAB, the most common ions observed are:

  • Predominantly [M+H]+ and alkali metal adducts such as [M+Na]+
  • Multiply charged ions with z > 10
  • Radical cations [M]+• exclusively
  • Doubly deprotonated [M−2H]2−

Correct Answer: Predominantly [M+H]+ and alkali metal adducts such as [M+Na]+

Q5. The fundamental principle of MALDI is best described as:

  • Laser energy absorbed by matrix crystals leads to desorption and ionization of embedded analyte molecules
  • Ionization by collisions with atmospheric nitrogen at ambient pressure
  • Emission of secondary electrons under electron bombardment
  • Direct thermal evaporation of neutrals only

Correct Answer: Laser energy absorbed by matrix crystals leads to desorption and ionization of embedded analyte molecules

Q6. Common MALDI matrices for peptides and proteins include:

  • α-Cyano-4-hydroxycinnamic acid (CHCA), sinapinic acid (SA), 2,5-dihydroxybenzoic acid (DHB)
  • Acetone, acetonitrile, water
  • Glycerol and 3-NBA exclusively
  • Sodium chloride and ammonium sulfate

Correct Answer: α-Cyano-4-hydroxycinnamic acid (CHCA), sinapinic acid (SA), 2,5-dihydroxybenzoic acid (DHB)

Q7. Which lasers are most commonly used in MALDI?

  • Nitrogen laser at 337 nm and frequency-tripled Nd:YAG at 355 nm
  • CO₂ laser at 10.6 μm exclusively
  • HeNe laser at 632.8 nm
  • Argon-ion laser at 488 nm continuous wave only

Correct Answer: Nitrogen laser at 337 nm and frequency-tripled Nd:YAG at 355 nm

Q8. Why is MALDI frequently coupled with time-of-flight (TOF) mass analyzers?

  • Pulsed ion production matches TOF extraction and allows very high mass range
  • Requires continuous ion beam for quadrupole operation
  • Produces exclusively negative ions suited for sector instruments
  • Needs high pressure interface only TOF provides

Correct Answer: Pulsed ion production matches TOF extraction and allows very high mass range

Q9. Which statement best contrasts the mass range capabilities of MALDI and FAB?

  • MALDI readily measures intact proteins (>100 kDa), whereas FAB is generally limited to small peptides and polar molecules up to a few kDa
  • Both techniques routinely measure >1 MDa without fragmentation
  • FAB outperforms MALDI for intact megadalton protein complexes
  • Neither technique is suitable for peptides

Correct Answer: MALDI readily measures intact proteins (>100 kDa), whereas FAB is generally limited to small peptides and polar molecules up to a few kDa

Q10. Which best describes the sample preparation difference between FAB and MALDI?

  • FAB uses a viscous liquid matrix spot; MALDI requires co-crystallization of matrix and analyte
  • Both require electrospray of analyte into the source
  • MALDI uses liquid matrices exclusively; FAB uses solid crystals
  • FAB requires derivatization with chromophores; MALDI does not

Correct Answer: FAB uses a viscous liquid matrix spot; MALDI requires co-crystallization of matrix and analyte

Q11. Regarding charge states, which statement is accurate for MALDI and FAB?

  • MALDI and FAB typically produce mostly singly charged ions
  • Both produce highly multiply charged ions similar to ESI
  • FAB produces only radical cations
  • MALDI produces only doubly charged ions

Correct Answer: MALDI and FAB typically produce mostly singly charged ions

Q12. A common analytical challenge in MALDI small-molecule analysis is:

  • Intense matrix-related peaks below m/z 500 can interfere with analyte detection
  • Lack of vacuum prevents desorption
  • Excessive multiple charging complicates spectra
  • Laser cannot ionize aromatic compounds

Correct Answer: Intense matrix-related peaks below m/z 500 can interfere with analyte detection

Q13. FAB is often categorized as which related technique?

  • A form of liquid secondary ion mass spectrometry (LSIMS)
  • Atmospheric pressure chemical ionization (APCI)
  • Electron ionization (EI)
  • Field desorption (FD)

Correct Answer: A form of liquid secondary ion mass spectrometry (LSIMS)

Q14. What are the key functions of the matrix in MALDI?

  • Absorbs laser energy, assists desorption, and mediates proton transfer to form ions
  • Provides a conductive path for electron emission
  • Eliminates the need for vacuum
  • Separates ions by time-of-flight

Correct Answer: Absorbs laser energy, assists desorption, and mediates proton transfer to form ions

Q15. In FAB, the matrix primarily:

  • Acts as a viscous medium to dissolve analyte, dissipate energy, and facilitate ion–molecule reactions during sputtering
  • Provides seed crystals for co-crystallization
  • Generates electrons for 70 eV ionization
  • Traps ions for FTICR detection

Correct Answer: Acts as a viscous medium to dissolve analyte, dissipate energy, and facilitate ion–molecule reactions during sputtering

Q16. The purpose of delayed extraction in MALDI is to:

  • Mass resolution and mass accuracy by compensating initial ion velocity spread
  • Ion transmission at atmospheric pressure only
  • Laser fluence stability
  • Crystal homogeneity on the target

Correct Answer: Mass resolution and mass accuracy by compensating initial ion velocity spread

Q17. Negative-ion mode in FAB and MALDI is particularly useful for:

  • Useful for acidic analytes (e.g., sulfated oligosaccharides), in both FAB and MALDI
  • Not feasible in either technique
  • Only used for noble gases
  • Produces only radical anions

Correct Answer: Useful for acidic analytes (e.g., sulfated oligosaccharides), in both FAB and MALDI

Q18. Which statement best reflects the effect of salts on ionization?

  • MALDI is relatively tolerant to moderate salts; FAB and ESI are more prone to ion suppression
  • Both MALDI and FAB are unaffected by salts
  • FAB is more salt tolerant than MALDI
  • Salts increase ionization efficiency in MALDI

Correct Answer: MALDI is relatively tolerant to moderate salts; FAB and ESI are more prone to ion suppression

Q19. The primary beam energy typically used for FAB is:

  • 2–10 keV neutral atoms generated from Xe or Ar
  • 70 eV electrons
  • 100 keV protons
  • 1–5 eV thermal neutrals

Correct Answer: 2–10 keV neutral atoms generated from Xe or Ar

Q20. In MALDI positive-ion mode, which adducts are commonly observed?

  • [M+H]+, [M+Na]+, and [M+K]+
  • [M]•+ exclusively
  • [M−2H]2− only
  • [M+Cl]− in positive mode

Correct Answer: [M+H]+, [M+Na]+, and [M+K]+

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