Ionization techniques – MALDI and FAB MCQs With Answer

Ionization techniques – MALDI and FAB MCQs With Answer

Understanding ionization techniques like matrix-assisted laser desorption/ionization (MALDI) and fast atom bombardment (FAB) is essential for B. Pharm students working with mass spectrometry of drugs, peptides, polymers and biomolecules. This introduction explains principles, sample preparation, matrices, ion formation (proton transfer, cationization), instrumentation (MALDI-TOF), common matrices (DHB, CHCA, sinapinic acid, glycerol), advantages, limitations (matrix background, suppression) and practical tips for improving sensitivity and mass accuracy. Emphasis is on comparing soft ionization behaviors, mass range suitability, and applications in pharmaceutical analysis. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the core difference between MALDI and FAB ionization?

• MALDI uses a laser and crystalline matrix, FAB uses a beam of high-energy atoms and a liquid matrix
• MALDI uses electron impact, FAB uses chemical ionization
• MALDI requires gas-phase sample introduction only, FAB uses direct infusion
• MALDI produces multiply charged ions, FAB produces only negative ions

Correct Answer: MALDI uses a laser and crystalline matrix, FAB uses a beam of high-energy atoms and a liquid matrix

Q2. Which matrix is commonly used for peptide analysis in MALDI?

• 2,5-Dihydroxybenzoic acid (DHB)
• Glycerol
• Perchloric acid
• Silica gel

Correct Answer: 2,5-Dihydroxybenzoic acid (DHB)

Q3. What primary role does the MALDI matrix perform?

• Absorb laser energy, assist desorption and facilitate proton/cation transfer to analyte
• Ionize atoms by electron impact
• Convert analyte into volatile derivatives
• Act as a chromatographic stationary phase

Correct Answer: Absorb laser energy, assist desorption and facilitate proton/cation transfer to analyte

Q4. Which of the following is a typical primary beam for FAB?

• Xenon or argon fast atoms
• UV laser at 337 nm
• Electron beam at 70 eV
• Proton beam at 1 MeV

Correct Answer: Xenon or argon fast atoms

Q5. For FAB, what is a typical liquid matrix used to dissolve the analyte?

• Glycerol
• CHCA (α-cyano-4-hydroxycinnamic acid)
• DHB (2,5-dihydroxybenzoic acid)
• Acetonitrile alone

Correct Answer: Glycerol

Q6. Which ion charge state is most commonly observed in MALDI spectra for peptides and proteins?

• Predominantly singly charged ions
• Highly multiply charged (10+)
• Negatively charged only
• Only radical cations

Correct Answer: Predominantly singly charged ions

Q7. Why is MALDI considered a “soft” ionization technique?

• It causes minimal fragmentation, preserving intact molecular ions
• It fragments molecules extensively to produce structural information
• It only ionizes inorganic salts
• It uses high temperatures to vaporize samples

Correct Answer: It causes minimal fragmentation, preserving intact molecular ions

Q8. Which laser wavelength is commonly used in MALDI instruments?

• UV lasers such as 337 nm (nitrogen) or 355 nm (Nd:YAG)
• Visible 600 nm diode lasers
• Infrared 1064 nm only
• Gamma rays

Correct Answer: UV lasers such as 337 nm (nitrogen) or 355 nm (Nd:YAG)

Q9. Which type of analyte is traditionally best analyzed by FAB?

• Small to medium polar, thermally labile molecules (e.g., steroids, lipids)
• Very large intact proteins (>200 kDa)
• Nonpolar gases
• Inorganic metal clusters only

Correct Answer: Small to medium polar, thermally labile molecules (e.g., steroids, lipids)

Q10. What is a common limitation of MALDI for small molecule (<500 Da) analysis?

• Matrix-related background peaks often obscure low mass signals
• MALDI cannot ionize organic compounds
• MALDI always fragments small molecules completely
• MALDI produces only negative ions for small molecules

Correct Answer: Matrix-related background peaks often obscure low mass signals

Q11. In MALDI, what effect does a high matrix-to-analyte ratio typically have?

• Improves crystallization and signal but may increase matrix background
• Always eliminates all background peaks
• Makes the analyte non-ionizable
• Converts singly charged ions to multiply charged ions

Correct Answer: Improves crystallization and signal but may increase matrix background

Q12. Which of the following describes the main ionization mechanism in MALDI?

• Desorption followed by proton transfer or cation attachment from matrix to analyte
• Direct electron removal by collision with fast atoms
• Chemical derivatization to volatile esters
• Thermal evaporation and electron impact ionization

Correct Answer: Desorption followed by proton transfer or cation attachment from matrix to analyte

Q13. Which compound is commonly used as a matrix for intact proteins and high-mass peptides?

• Sinapinic acid (SA)
• Formaldehyde
• Tetramethylsilane
• Guanidine hydrochloride

Correct Answer: Sinapinic acid (SA)

Q14. How does adding a salt (e.g., NaCl) to a MALDI sample typically affect the spectrum?

• Promotes formation of sodium adducts [M+Na]+ leading to shifted m/z peaks
• Prevents any ion formation
• Converts all peaks to negative ions
• Causes vaporization of the matrix only

Correct Answer: Promotes formation of sodium adducts [M+Na]+ leading to shifted m/z peaks

Q15. In a MALDI-TOF instrument, what physical quantity is measured to determine m/z?

• Time-of-flight of ions from source to detector
• Absorbance of the matrix crystal
• Electrical conductivity of the sample
• Temperature change after laser pulse

Correct Answer: Time-of-flight of ions from source to detector

Q16. Which matrix is particularly favored for oligosaccharides and glycopeptides?

• 2,5-DHB (Dihydroxybenzoic acid)
• Glycerol
• n-Hexane
• Acetone

Correct Answer: 2,5-DHB (Dihydroxybenzoic acid)

Q17. Which statement about FAB spectra is correct?

• FAB often gives molecular ions with some in-source fragmentation and matrix-related peaks
• FAB produces only singly charged ions with no background
• FAB always gives better resolution than MALDI-TOF
• FAB cannot analyze polar molecules

Correct Answer: FAB often gives molecular ions with some in-source fragmentation and matrix-related peaks

Q18. Which parameter is important to optimize in MALDI sample preparation for best spectral quality?

• Matrix selection, matrix-to-analyte ratio and crystallization uniformity
• Use of pure water only without organic solvent
• Heating the sample to dryness at 300 °C
• Adding strong oxidizing agents

Correct Answer: Matrix selection, matrix-to-analyte ratio and crystallization uniformity

Q19. What is a common reason to choose FAB over MALDI in older pharmacological analyses?

• Better sensitivity for certain small polar, nonvolatile compounds using liquid matrix
• FAB provides higher mass range for intact proteins
• FAB requires no vacuum system making it simpler
• FAB gives exclusively multiply charged ions for large peptides

Correct Answer: Better sensitivity for certain small polar, nonvolatile compounds using liquid matrix

Q20. How can matrix-related suppression of small drug signals in MALDI be minimized?

• Use alternative matrices, chemical derivatization, or employ MALDI-TOF/TOF with delayed extraction
• Always heat the sample to 500 °C
• Only analyze in negative mode regardless of chemistry
• Add large amounts of salt to drown matrix peaks

Correct Answer: Use alternative matrices, chemical derivatization, or employ MALDI-TOF/TOF with delayed extraction

Q21. In MALDI, what is delayed extraction used to improve?

• Mass resolution and mass accuracy by temporal focusing of ions
• Conversion of ions to neutral molecules
• Increased laser power only
• Selective ion precipitation

Correct Answer: Mass resolution and mass accuracy by temporal focusing of ions

Q22. Which ion type is frequently observed for analytes in positive-mode MALDI?

• Protonated molecules [M+H]+ and alkali adducts [M+Na]+
• Only radical cations M+•
• Only deprotonated molecules [M-H]-
• Only dimeric neutral species

Correct Answer: Protonated molecules [M+H]+ and alkali adducts [M+Na]+

Q23. Which practice improves reproducibility of MALDI imaging on tissue sections?

• Uniform matrix application by automated spraying or sublimation
• Hand spotting large droplets of matrix in one corner
• Drying tissue at 200 °C briefly
• Using no matrix and relying on laser alone

Correct Answer: Uniform matrix application by automated spraying or sublimation

Q24. What energy range are the fast atoms in FAB typically generated at?

• KeV range (e.g., 4–10 keV)
• Milli-electron volt (meV) range
• Gigavolt (GV) range
• Zero energy atoms at room temperature

Correct Answer: KeV range (e.g., 4–10 keV)

Q25. In pharmaceutical peptide analysis, why is MALDI often preferred over FAB?

• Higher sensitivity for larger peptides/proteins and simpler spectra due to mainly singly charged ions
• MALDI always yields complete sequence information without fragmentation
• MALDI does not require any sample preparation
• MALDI can only detect inorganic impurities

Correct Answer: Higher sensitivity for larger peptides/proteins and simpler spectra due to mainly singly charged ions

Q26. Which matrix would you choose for small drug molecules where CHCA causes interference?

• Try alternative matrices like DHB or use matrix-free methods such as SALDI
• Use concentrated hydrochloric acid instead
• Never change the matrix; dilute the sample instead
• Add silica gel to the sample spot

Correct Answer: Try alternative matrices like DHB or use matrix-free methods such as SALDI

Q27. What is a practical tip to reduce salt adduct formation in MALDI?

• Desalt the sample (e.g., ZipTip C18) or use matrix additives to favor protonation
• Add extra sodium chloride to the sample
• Apply matrix in 100% water only
• Use a hotter laser to burn off salts

Correct Answer: Desalt the sample (e.g., ZipTip C18) or use matrix additives to favor protonation

Q28. Which factor most directly affects MALDI spot homogeneity and peak variability?

• Crystallization pattern of matrix-analyte co-crystals
• The color of the MALDI plate
• Using distilled water as the only solvent
• Ambient room humidity alone, without impacting crystallization

Correct Answer: Crystallization pattern of matrix-analyte co-crystals

Q29. Which analytic improvement can MALDI-TOF/TOF provide compared with MALDI-TOF?

• Tandem MS fragmentation for sequence information and structural characterization
• Lower mass range only
• Removal of need for matrix entirely
• Converting positive ions to negative ions automatically

Correct Answer: Tandem MS fragmentation for sequence information and structural characterization

Q30. For accurate mass calibration in MALDI experiments, what is recommended?

• Use internal calibrants spotted with the sample for highest mass accuracy
• Never calibrate the instrument because TOF is inherently accurate
• Only calibrate using room air peaks
• Use colored ink as an external calibrant

Correct Answer: Use internal calibrants spotted with the sample for highest mass accuracy

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com