Metastable ions MCQs With Answer

Metastable ions MCQs With Answer

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Metastable ions MCQs With Answer

Metastable ions, central to advanced mass spectrometric interpretation, are ions that fragment after acceleration but before detection, providing crucial structural insights without deliberate collisions. For M. Pharm students studying Modern Pharmaceutical Analytical Techniques, understanding metastable ion behavior, formation regions, peak characteristics, and detection strategies is essential for reliable data interpretation in EI, CI, FAB, MALDI, ESI, and sector-based instruments. This quiz emphasizes the physics of metastable dissociation (unimolecular decay), equations governing metastable peak positions (e.g., m* = m2²/m1 in magnetic sector instruments), and practical aspects such as linked scans, post-source decay (PSD) in TOF, and the impact of kinetic energy release on peak shape. Use these MCQs to consolidate theory and enhance problem-solving in structural elucidation.

Q1. In mass spectrometry, a metastable ion is best described as:

  • An ion that fragments inside the ion source before acceleration
  • An ion that fragments after acceleration but before detection in a field-free region
  • An ion that does not fragment at any stage of analysis
  • An ion formed only via collision-induced dissociation in a collision cell

Correct Answer: An ion that fragments after acceleration but before detection in a field-free region

Q2. In a single-focusing magnetic sector instrument (scanning B at constant accelerating voltage), the metastable peak m/z position (m*) for a transition m1 → m2 is given by:

  • m* = m1 × m2
  • m* = (m1 + m2)/2
  • m* = (m2² / m1)
  • m* = (m1² / m2)

Correct Answer: m* = (m2² / m1)

Q3. Metastable peaks typically appear as:

  • Sharp, high-resolution peaks at integer m/z values
  • Broad, low-intensity peaks at non-integer m/z values
  • Intense peaks shifted to lower kinetic energy without broadening
  • Isotopically resolved doublets

Correct Answer: Broad, low-intensity peaks at non-integer m/z values

Q4. Which statement about the lifetime of metastable ions is most accurate?

  • They typically dissociate within femtoseconds
  • They typically dissociate on the microsecond to millisecond timescale
  • They are indefinitely stable in vacuum
  • Their lifetime is independent of internal energy

Correct Answer: They typically dissociate on the microsecond to millisecond timescale

Q5. The primary mechanism of metastable ion formation is:

  • Unimolecular dissociation due to excess internal energy
  • Electron capture dissociation
  • Photodissociation by laser pulses
  • Plasma ionization of neutrals

Correct Answer: Unimolecular dissociation due to excess internal energy

Q6. A precursor ion of m/z 150 fragments metastably to a product ion of m/z 120 in a magnetic sector (B-scan). Where will the metastable peak appear?

  • m* = 96
  • m* = 120
  • m* = 135
  • m* = 80

Correct Answer: m* = 96

Q7. Why are metastable peaks often not observed in simple quadrupole mass spectra?

  • Quadrupoles only detect neutral fragments
  • Metastable ions formed inside the RF/DC filter lose stability and are not transmitted
  • Quadrupoles always operate at pressures that suppress fragmentation
  • Quadrupoles convert all ions to the same m/z

Correct Answer: Metastable ions formed inside the RF/DC filter lose stability and are not transmitted

Q8. In TOF instruments, metastable fragmentation after the source is commonly referred to as:

  • Post-source decay (PSD)
  • Electron impact (EI) decay
  • Thermal desorption
  • Chemical ionization (CI)

Correct Answer: Post-source decay (PSD)

Q9. The appearance of metastable peaks is enhanced when:

  • Field-free path length is long and vacuum is high
  • Pressure is high and path length is short
  • Acceleration voltage is zero
  • Only negative ions are analyzed

Correct Answer: Field-free path length is long and vacuum is high

Q10. In double-focusing BE sector instruments, a B/E linked scan is primarily used to:

  • Measure ionization cross-sections
  • Record daughter ions arising from metastable transitions of a selected precursor
  • Calibrate flight time in TOF analyzers
  • Increase detector gain

Correct Answer: Record daughter ions arising from metastable transitions of a selected precursor

Q11. Metastable ion peaks are broadened mainly because:

  • Detector saturation artifacts
  • Distribution of kinetic energy release during fragmentation
  • Magnet hysteresis in the sector field
  • Isobaric interferences from solvents

Correct Answer: Distribution of kinetic energy release during fragmentation

Q12. Which ionization method typically produces fewer metastable ions due to lower internal energy imparted?

  • Electron impact (EI)
  • Electrospray ionization (ESI)
  • Field ionization (FI)
  • Penning ionization

Correct Answer: Electrospray ionization (ESI)

Q13. For a metastable transition A+ (m1) → B+ (m2) + neutral, which statement is most correct in a field-free region?

  • B+ retains approximately the same velocity as A+
  • B+ retains approximately the same kinetic energy as A+
  • B+ has greater charge than A+
  • The neutral always carries away negligible momentum

Correct Answer: B+ retains approximately the same velocity as A+

Q14. Which analytical advantage do metastable ions provide in pharmaceutical mass spectrometry?

  • They enable isotopic abundance quantitation
  • They reveal unimolecular fragmentation pathways useful for structural elucidation
  • They eliminate the need for tandem MS
  • They increase absolute sensitivity of detection

Correct Answer: They reveal unimolecular fragmentation pathways useful for structural elucidation

Q15. A precursor at m/z 100 gives a metastable daughter at m/z 80. In a magnetic sector B-scan, what is the expected position of the metastable peak?

  • m* = 80
  • m* = 90
  • m* = 64
  • m* = 125

Correct Answer: m* = 64

Q16. Which statement correctly compares metastable fragmentation with collision-induced dissociation (CID)?

  • Both require a collision gas at high pressure
  • Metastable fragmentation is unimolecular; CID is collisionally activated and can be controlled via collision energy
  • CID produces only neutral losses; metastable produces only bond cleavages
  • Metastable requires lasers; CID does not

Correct Answer: Metastable fragmentation is unimolecular; CID is collisionally activated and can be controlled via collision energy

Q17. In TOF-PSD experiments, why do fragment ions often appear at longer flight times than expected from their m/z?

  • They are formed later and inherit the precursor’s velocity rather than its kinetic energy
  • They collide with the detector grid
  • They carry multiple charges unexpectedly
  • They undergo electron emission en route

Correct Answer: They are formed later and inherit the precursor’s velocity rather than its kinetic energy

Q18. Which condition tends to suppress observation of metastable peaks in sector instruments?

  • Very high source temperature
  • Short field-free region between sectors
  • High detector gain
  • Using a refractory metal filament

Correct Answer: Short field-free region between sectors

Q19. The intensity of metastable peaks generally correlates with:

  • First-order rate constant for unimolecular decay and residence time in the field-free region
  • Second-order collision rates with background gas
  • Detector dark current
  • Magnet sweep speed only

Correct Answer: First-order rate constant for unimolecular decay and residence time in the field-free region

Q20. In an electrostatic (E) sector analyzer operated for kinetic energy analysis, kinetic energy release (KER) during metastable decay primarily causes:

  • Narrowing of peaks
  • Shift of peaks to exactly integer m/z
  • Energy dispersion leading to asymmetric broadening
  • Complete loss of all product ions

Correct Answer: Energy dispersion leading to asymmetric broadening

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