Chapter: Mass Spectrometry

Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio (m/z) of ions. It is used to determine the elemental composition of a sample or molecule, to elucidate the chemical structures of molecules, and to quantify known or unknown compounds in a sample. It operates by converting molecules into ions, separating them based on their m/z ratio, and then detecting them.

Fundamental Principles of Mass Spectrometry

All mass spectrometers operate on the same basic principle, involving five key stages:

1. Sample Introduction: The sample, which can be solid, liquid, or gas, is introduced into the mass spectrometer.
2. Ionization: Molecules in the sample are converted into gas-phase ions. This is a crucial step as only charged particles can be manipulated by electric and magnetic fields.
3. Mass Analysis (Mass Analyzer): The ions are separated according to their mass-to-charge ratio (m/z). This is the core of the mass spectrometer.
4. Detection: The separated ions are detected, and their abundance is measured. This generates an electrical signal that is converted into a mass spectrum.
5. Data Processing: The raw data is processed to generate a mass spectrum, which is a plot of relative ion abundance versus m/z.

1. Sample Introduction Systems

The method of sample introduction depends on the physical state and volatility of the sample.

• Batch Inlet Systems: For volatile liquid or gaseous samples. The sample is vaporized in a heated reservoir and then slowly leaks into the ionization source.
• Direct Probe Inlet: For solid or low-volatility liquid samples. The sample is placed in a small crucible on the tip of a probe, which is then inserted directly into the ionization source. Heating the probe vaporizes the sample.
• Chromatographic Coupling: Most commonly, MS is coupled with separation techniques:
  • Gas Chromatography-Mass Spectrometry (GC-MS): The effluent from a GC column (gaseous analytes) is directly introduced into the MS ionization source. Excellent for volatile and semi-volatile compounds.
  • Liquid Chromatography-Mass Spectrometry (LC-MS): The effluent from an LC column (liquid analytes, often non-volatile) is introduced via an interface that removes the solvent and ionizes the analytes. Essential for polar, non-volatile, and thermally labile compounds.
  • Capillary Electrophoresis-Mass Spectrometry (CE-MS): Similar to LC-MS, but coupled with CE for highly efficient separations.

2. Ionization Sources

The choice of ionization method depends on the sample’s properties (volatility, thermal stability, polarity) and the desired information. Ionization methods are broadly categorized into “hard” and “soft” sources.

• Hard Ionization Sources: Impart significant energy to the molecules, causing extensive fragmentation. Useful for structural elucidation.
  • Electron Ionization (EI) / Electron Impact (EI): (For volatile, thermally stable compounds, typically GC-MS).
    • Mechanism: Sample molecules are bombarded with high-energy electrons (typically 70 eV). This removes an electron from the molecule, forming a radical cation (molecular ion, M+∙).
    • Fragmentation: The excess energy causes the molecular ion to fragment into smaller daughter ions.
    • Spectrum: Produces characteristic fragmentation patterns, making it excellent for library searching and structural identification. The molecular ion peak might be small or absent for unstable molecules.
    • Advantages: High reproducibility, well-established libraries, good for volatile compounds.
    • Disadvantages: Not suitable for non-volatile or thermally labile compounds, strong fragmentation can obscure molecular ion.
• Soft Ionization Sources: Impart less energy, resulting in less fragmentation, often preserving the molecular ion (or protonated/deprotonated molecule). Ideal for determining molecular weight.
  • Chemical Ionization (CI): (For volatile to semi-volatile compounds).
    • Mechanism: A reagent gas (e.g., methane, isobutane, ammonia) is first ionized by electron impact. These primary ions then react with sample molecules via ion-molecule reactions (e.g., proton transfer), leading to protonated molecules [M+H]+ or other adducts.
    • Fragmentation: Less energetic than EI, resulting in simpler spectra with abundant molecular ion species and fewer fragments.
    • Advantages: Clear molecular ion or pseudomolecular ion peak, useful for molecular weight determination, can be used for compounds not easily ionized by EI.
    • Disadvantages: Requires reagent gas, may still cause some fragmentation.
  • Electrospray Ionization (ESI): (For polar, non-volatile, thermally labile compounds, typically LC-MS).
    • Mechanism: The sample solution is sprayed through a fine capillary at high voltage, creating charged droplets. Solvent evaporates from these droplets, increasing charge density, leading to ion emission (desorption of pre-formed ions or charge residue formation).
    • Ions Formed: Primarily protonated molecules [M+H]+ (positive mode) or deprotonated molecules [M−H]− (negative mode), and multiply charged ions for large molecules (e.g., proteins).
    • Advantages: Excellent for large, polar, and thermally labile biomolecules (proteins, peptides, oligonucleotides), produces multiply charged ions making high molecular weight compounds accessible to lower m/z range mass analyzers, compatible with LC.
    • Disadvantages: Sensitive to salts and buffers, can be easily suppressed by matrix effects.
  • Matrix-Assisted Laser Desorption/Ionization (MALDI): (For large biomolecules, polymers, often coupled with TOF MS).
    • Mechanism: The analyte is mixed with a large excess of a UV-absorbing matrix compound, which co-crystallizes with the analyte. A laser pulse irradiates the mixture, causing the matrix to rapidly vaporize and transfer charge to the analyte molecules.
    • Ions Formed: Primarily singly charged molecular ions (e.g., [M+H]+) for large molecules.
    • Advantages: Excellent for very large molecules (e.g., proteins up to several hundred kDa), produces predominantly singly charged ions (simplifies spectra), high throughput.
    • Disadvantages: Requires matrix, matrix peaks can interfere at low m/z, less quantitative than ESI.
  • Atmospheric Pressure Chemical Ionization (APCI): (For less polar, semi-volatile compounds, typically LC-MS).
    • Mechanism: Sample is nebulized and vaporized into a corona discharge at atmospheric pressure. Solvent ions are produced and then react with analyte molecules via chemical ionization processes.
    • Advantages: Good for relatively non-polar compounds, less susceptible to salt effects than ESI, compatible with LC.
    • Disadvantages: Requires vaporization, not suitable for very large or highly polar biomolecules.

3. Mass Analyzers

Mass analyzers separate ions based on their m/z ratio. Key performance characteristics include:

• Mass Range: The range of m/z values that can be measured.
• Resolution: The ability to distinguish between ions with very similar m/z values. Higher resolution means better separation of closely spaced peaks. Defined as R=m/Δm, where Δm is the peak width at half maximum height.
• Accuracy: How close the measured m/z value is to the true m/z value. Often expressed in parts per million (ppm).
• Sensitivity: The lowest concentration of analyte that can be detected.

Common types of mass analyzers:

• Quadrupole (Q):
  • Mechanism: Consists of four parallel rods. Ions travel through an oscillating electric field (DC and RF voltages). Only ions with a specific m/z ratio can pass through the quadrupole at a given voltage setting.
  • Advantages: Compact, relatively inexpensive, robust, high scan speed, good for GC-MS.
  • Disadvantages: Lower resolution and accuracy compared to other analyzers.
• Time-of-Flight (TOF):
  • Mechanism: Ions are accelerated to a uniform kinetic energy and then drift through a field-free flight tube. Lighter ions travel faster and reach the detector first, while heavier ions arrive later.
  • Advantages: Very high mass range, very fast acquisition rates (full spectrum in microseconds), high resolution (especially with reflectron), can analyze multiple ions simultaneously. Excellent for MALDI.
  • Disadvantages: Requires precise timing electronics, resolution can be affected by initial kinetic energy distribution.
• Magnetic Sector (B) / Double Focusing (EB or BE):
  • Mechanism: Ions are deflected by a magnetic field according to their momentum. Ions then pass through an electrostatic analyzer (E) which separates them by kinetic energy, achieving higher resolution.
  • Advantages: Very high resolution and accuracy, excellent for accurate mass measurements and elemental composition determination.
  • Disadvantages: Large, expensive, slow scan speed, low mass range compared to TOF.
• Ion Trap (IT) / Quadrupole Ion Trap (QIT):
  • Mechanism: Ions are trapped in a 3D quadrupole electric field formed by a ring electrode and two end-cap electrodes. Ions are then destabilized and ejected to the detector in order of increasing m/z.
  • Advantages: Compact, good sensitivity, allows for multiple stages of MS (MSⁿ) for fragmentation studies.
  • Disadvantages: Lower mass capacity, space charge effects can reduce performance, limited resolution and accuracy compared to FT-ICR or Orbitrap.
• Fourier Transform Ion Cyclotron Resonance (FT-ICR):
  • Mechanism: Ions are trapped in a strong magnetic field and excited by RF pulses. The frequency at which they orbit (cyclotron frequency) is inversely proportional to their m/z. This frequency is measured and converted to a mass spectrum via Fourier Transform.
  • Advantages: Highest resolution, highest mass accuracy, can perform MSⁿ.
  • Disadvantages: Very expensive, requires superconducting magnet, large footprint, ultra-high vacuum.
• Orbitrap:
  • Mechanism: Ions are trapped in an electrostatic field between a central electrode and an outer electrode. They orbit the central electrode while oscillating axially. The axial oscillation frequency is inversely proportional to the square root of m/z.
  • Advantages: Very high resolution and mass accuracy (comparable to FT-ICR, but without superconducting magnet), good sensitivity, relatively compact.
  • Disadvantages: Expensive, not as good for MSⁿ as ion traps or FT-ICR (though hybrid instruments exist).

4. Detectors

Detectors convert the ion current into an electrical signal.

• Electron Multiplier: Most common. Ions strike a dynode surface, releasing secondary electrons, which are then amplified in a cascade fashion.
• Faraday Cup: Ions strike a metal cup, and the current produced is measured. Less sensitive than electron multipliers but provides absolute current readings.
• Microchannel Plate (MCP): Used in TOF-MS for fast, spatially resolved detection. Similar to electron multipliers but in a flat plate format.

5. Data and Mass Spectra

A mass spectrum is a plot of ion signal intensity (relative abundance) versus mass-to-charge ratio (m/z).

• Molecular Ion Peak (M$^{\boldsymbol{+\bullet}}$): Represents the intact molecule that has lost one electron. Its m/z value gives the molecular weight of the compound. Its presence and intensity are crucial for molecular weight determination.
• Base Peak: The most intense peak in the spectrum, assigned a relative abundance of 100%. All other peaks are scaled relative to it.
• Fragment Ions: Peaks resulting from the dissociation of the molecular ion or other precursor ions. Fragmentation patterns are characteristic of molecular structure and can be used for identification.
• Isotope Peaks: Small peaks appearing at m/z values higher than the molecular ion or fragment ions due to the presence of heavier isotopes (e.g., $^{13}$C, $^{37}$Cl, $^{81}$Br). The pattern and intensity ratios of these peaks can provide information about the elemental composition (e.g., presence of Cl, Br).
  • A+1 peak: Due to $^{13}$C, $^{15}$N, etc.
  • A+2 peak: Due to $^{18}$O, two $^{13}$C, or presence of Cl/Br. For compounds with a single chlorine atom, an M+2 peak approximately one-third the intensity of the M peak is observed. For a single bromine atom, an M+2 peak approximately equal in intensity to the M peak is observed.
• Adduct Ions: (Common in soft ionization, especially ESI). Ions formed by the non-covalent association of the molecule with other species (e.g., [M+Na]+,[M+K]+,[M+NH4​]+).
• Multicharged Ions: (Common in ESI for large molecules). Ions carrying more than one charge (e.g., [M+2H]2+,[M+3H]3+). The observed m/z for a doubly charged ion would be (M+2)/2.

Tandem Mass Spectrometry (MS/MS or MSⁿ)

MS/MS involves multiple stages of mass analysis, typically used for structural elucidation of complex molecules.

• Principle: A precursor (parent) ion is selected in the first mass analyzer, then fragmented (e.g., by collision-induced dissociation – CID), and the resulting product (daughter) ions are analyzed in a second mass analyzer.
• Applications:
  • Structural Elucidation: Provides detailed fragmentation pathways, allowing identification of functional groups and connectivity within a molecule.
  • Peptide/Protein Sequencing: By fragmenting peptides and analyzing the product ions, amino acid sequences can be determined.
  • Quantification: Enhanced selectivity for quantifying target compounds in complex mixtures by monitoring specific precursor-product ion transitions (Selected Reaction Monitoring – SRM or Multiple Reaction Monitoring – MRM).
  • Impurity Identification: Identification of impurities in pharmaceutical products or environmental samples.

Applications of Mass Spectrometry

Mass spectrometry is a versatile tool used in a wide array of fields:

• Chemistry: Structure elucidation of organic and inorganic compounds, reaction monitoring, purity assessment.
• Biochemistry and Proteomics: Identification and quantification of proteins, peptides, post-translational modifications, protein sequencing, protein-protein interactions.
• Metabolomics and Lipidomics: Comprehensive analysis of metabolites and lipids in biological systems.
• Pharmacology and Drug Discovery: Drug metabolism studies, pharmacokinetic analysis, drug screening, impurity profiling.
• Environmental Analysis: Detection and quantification of pollutants (e.g., pesticides, PCBs) in water, soil, and air.
• Forensics and Toxicology: Drug testing, analysis of explosives, identification of unknown substances.
• Clinical Diagnostics: Biomarker discovery, newborn screening, therapeutic drug monitoring.
• Food and Beverage Analysis: Authenticity testing, contaminant detection, flavor profiling.
• Geochronology and Isotope Ratio Mass Spectrometry (IRMS): Dating geological samples, tracing elemental origins, studying biogeochemical cycles.
• Space Exploration: Analysis of extraterrestrial materials and atmospheric composition.

Multiple Choice Questions (MCQs)

Here are 30 multiple-choice questions with answers and explanations, covering the concepts discussed in Mass Spectrometry.

1. What fundamental property does a mass spectrometer measure? A) Molecular weight B) Mass-to-charge ratio (m/z) C) Charge of an ion D) Ionization energyAnswer: B Explanation: Mass spectrometers separate and detect ions based on their mass-to-charge ratio (m/z), not directly their molecular weight or charge independently.
2. Which of the following is NOT one of the five basic components of a mass spectrometer? A) Ionization Source B) Mass Analyzer C) Chromatographic Column D) DetectorAnswer: C Explanation: While chromatographic columns (GC or LC) are often coupled with mass spectrometry for sample introduction, they are not intrinsic components of the mass spectrometer itself. The five basic components are sample introduction, ionization, mass analysis, detection, and data processing.
3. Which ionization technique is classified as a “hard” ionization method, causing extensive fragmentation? A) Electrospray Ionization (ESI) B) Chemical Ionization (CI) C) Electron Ionization (EI) D) Matrix-Assisted Laser Desorption/Ionization (MALDI)Answer: C Explanation: Electron Ionization (EI) uses high-energy electrons (typically 70 eV) to ionize molecules, imparting significant excess energy that leads to characteristic fragmentation patterns.
4. A mass spectrum is typically plotted as: A) Time vs. m/z B) Relative abundance vs. time C) Relative abundance vs. m/z D) Absolute abundance vs. molecular weightAnswer: C Explanation: A mass spectrum displays the relative intensity (abundance) of detected ions against their corresponding mass-to-charge ratio (m/z).
5. Which ionization source is best suited for analyzing large, polar, and thermally labile biomolecules, often producing multiply charged ions? A) Electron Ionization (EI) B) Chemical Ionization (CI) C) Electrospray Ionization (ESI) D) Atmospheric Pressure Chemical Ionization (APCI)Answer: C Explanation: Electrospray Ionization (ESI) is renowned for its ability to ionize large, non-volatile, and thermally sensitive molecules, frequently producing multiply charged ions which is beneficial for protein analysis.
6. The most intense peak in a mass spectrum is known as the: A) Molecular ion peak B) Isotope peak C) Base peak D) Adduct ion peakAnswer: C Explanation: The base peak is by definition the most abundant ion in the spectrum, and its intensity is set to 100% relative abundance.
7. If a compound contains a single chlorine atom, what is the approximate intensity ratio of the M+2 peak to the M peak due to the natural abundance of isotopes? A) 1:1 B) 1:2 C) 1:3 D) 1:4Answer: C Explanation: Chlorine has two common isotopes, $^{35}$Cl and $^{37}$Cl, in roughly a 3:1 ratio. Therefore, a compound with one chlorine atom will show an M+2 peak that is approximately one-third the intensity of the molecular ion (M) peak.
8. Which mass analyzer separates ions based on their flight time through a field-free tube? A) Quadrupole B) Ion Trap C) Time-of-Flight (TOF) D) Magnetic SectorAnswer: C Explanation: In Time-of-Flight (TOF) mass analyzers, ions with the same kinetic energy but different m/z values travel at different speeds, with lighter ions arriving at the detector faster.
9. What does “resolution” in mass spectrometry refer to? A) The ability to detect very low concentrations of analytes. B) The speed at which a spectrum can be acquired. C) The ability to distinguish between ions with very similar m/z values. D) The maximum mass that can be measured.Answer: C Explanation: Resolution defines the ability of a mass spectrometer to separate two ions that have very close mass-to-charge ratios. It is typically defined as m/Δm.
10. Which ionization technique typically results in predominant singly charged ions, making it ideal for very large molecules like proteins above 100 kDa? A) Electrospray Ionization (ESI) B) Atmospheric Pressure Chemical Ionization (APCI) C) Matrix-Assisted Laser Desorption/Ionization (MALDI) D) Electron Ionization (EI)Answer: C Explanation: MALDI is particularly well-suited for very large biomolecules because it typically produces singly charged ions, simplifying the interpretation of spectra for high molecular weight compounds.
11. Tandem Mass Spectrometry (MS/MS or MSⁿ) is primarily used for: A) Improving sample introduction speed. B) Structural elucidation and quantification through fragmentation. C) Increasing the vacuum level in the mass spectrometer. D) Only detecting molecular ion peaks.Answer: B Explanation: MS/MS involves selecting a precursor ion, fragmenting it, and then analyzing the product ions, which provides detailed structural information and enhances specificity for quantification.
12. Which term refers to the intact molecule that has lost one electron, and its m/z value gives the molecular weight? A) Fragment ion B) Adduct ion C) Molecular ion (M$^{\boldsymbol{+\bullet}}$) D) Isotope peakAnswer: C Explanation: The molecular ion (M$^{\boldsymbol{+\bullet}}$) is the radical cation formed by the removal of one electron from the neutral molecule, and its m/z corresponds to the molecular weight.
13. Which type of mass analyzer traps ions in a 3D quadrupole electric field and allows for MSⁿ experiments? A) Time-of-Flight (TOF) B) Magnetic Sector C) Quadrupole Ion Trap (QIT) D) OrbitrapAnswer: C Explanation: Ion traps, particularly Quadrupole Ion Traps (QITs), are capable of performing multiple stages of mass spectrometry (MSⁿ) by isolating and fragmenting ions sequentially within the trap.
14. What is the common detector in most mass spectrometers, which amplifies ion signals through a cascade of secondary electron emissions? A) Faraday Cup B) Photodiode Array C) Electron Multiplier D) Flame Ionization DetectorAnswer: C Explanation: Electron multipliers are the most common and sensitive detectors, providing high amplification of the ion signal.
15. Which coupling technique is primarily used for volatile and semi-volatile compounds? A) LC-MS B) GC-MS C) CE-MS D) MALDI-MSAnswer: B Explanation: Gas Chromatography-Mass Spectrometry (GC-MS) is ideal for analyzing volatile and semi-volatile compounds because GC separates them in the gas phase before introduction to the MS.
16. What phenomenon gives rise to M+1, M+2, etc., peaks in a mass spectrum? A) Adduct formation B) Fragmentation C) Isotopic abundance D) Multiple chargingAnswer: C Explanation: These peaks are due to the natural abundance of heavier isotopes of elements like carbon ($^{13}C),nitrogen(^{15}N),oxygen(^{18}O),chlorine(^{37}Cl),andbromine(^{81}$Br).
17. Which mass analyzer offers the highest resolution and mass accuracy, utilizing a strong superconducting magnetic field? A) Quadrupole B) Time-of-Flight (TOF) C) Fourier Transform Ion Cyclotron Resonance (FT-ICR) D) OrbitrapAnswer: C Explanation: FT-ICR mass spectrometers are known for providing the highest resolution and mass accuracy due to the precise measurement of ion cyclotron frequencies in a powerful magnetic field.
18. Which ionization technique involves co-crystallizing the analyte with a UV-absorbing matrix? A) Electrospray Ionization (ESI) B) Chemical Ionization (CI) C) Atmospheric Pressure Chemical Ionization (APCI) D) Matrix-Assisted Laser Desorption/Ionization (MALDI)Answer: D Explanation: MALDI requires the analyte to be embedded within a matrix material that absorbs laser energy, facilitating the desorption and ionization of the analyte.
19. What is a disadvantage of Electron Ionization (EI)? A) It produces too few fragment ions. B) It is not suitable for non-volatile or thermally labile compounds. C) It produces too many molecular ions. D) It is very expensive.Answer: B Explanation: EI requires the sample to be in the gas phase and stable at relatively high temperatures, making it unsuitable for non-volatile or thermally sensitive compounds.
20. If a large protein has a molecular weight of 15000 Da and is observed as a triply charged ion in ESI-MS, what would be its approximate m/z ratio? A) 15000 B) 7500 C) 5000 D) 3750Answer: C Explanation: For a triply charged ion, m/z=(M+3H)/3≈M/z=15000/3=5000. (Assuming M >> H).
21. Which application of mass spectrometry involves the comprehensive analysis of all metabolites in a biological system? A) Proteomics B) Genomics C) Metabolomics D) TranscriptomicsAnswer: C Explanation: Metabolomics is the large-scale study of metabolites within cells, biofluids, tissues or organisms, which is typically performed using mass spectrometry.
22. What is the primary purpose of the vacuum system in a mass spectrometer? A) To cool the detector. B) To prevent ion-molecule reactions and ensure a long mean free path for ions. C) To facilitate sample introduction. D) To increase the ionization efficiency.Answer: B Explanation: A high vacuum is necessary to prevent collisions between ions and neutral gas molecules, which would scatter ions and reduce sensitivity, and to ensure that ions can travel unimpeded to the detector.
23. In Chemical Ionization (CI), what type of ions are typically formed for the analyte? A) Radical cations (M$^{\boldsymbol{+\bullet}}$) B) Fragment ions only C) Protonated molecules [M+H]+ or other adducts D) Singly charged molecular ions onlyAnswer: C Explanation: CI involves ion-molecule reactions, most commonly proton transfer, leading to the formation of protonated molecules ([M+H]+) or other adduct ions, with less fragmentation than EI.
24. Which mass analyzer works by simultaneously trapping ions and measuring their axial oscillation frequencies, resulting in high resolution and accuracy without a superconducting magnet? A) Magnetic Sector B) Time-of-Flight (TOF) C) Ion Trap (IT) D) OrbitrapAnswer: D Explanation: The Orbitrap is unique in its design, trapping ions in an electrostatic field and measuring their characteristic axial oscillation frequencies, providing high performance comparable to FT-ICR.
25. What is an “adduct ion” in mass spectrometry? A) An ion formed by the loss of a proton from the molecule. B) An ion formed by the non-covalent association of the molecule with another species (e.g., Na⁺, K⁺). C) A fragment ion. D) A multiply charged ion.Answer: B Explanation: Adduct ions are commonly observed in soft ionization techniques like ESI, where the analyte molecule associates with small ions present in the solvent (e.g., sodium, potassium, ammonium).
26. What information can isotope peaks (e.g., M+2) provide about an unknown compound? A) Its exact molecular weight. B) The presence of certain elements like chlorine or bromine. C) Its fragmentation pathway. D) Its ionization efficiency.Answer: B Explanation: The characteristic patterns and intensity ratios of isotope peaks (especially M+2) can strongly indicate the presence of elements like chlorine (M:M+2 ≈ 3:1) or bromine (M:M+2 ≈ 1:1).
27. Why is LC-MS preferred over GC-MS for analyzing highly polar and non-volatile compounds? A) LC-MS provides more fragmentation. B) GC-MS requires derivatization for such compounds, while LC-MS interfaces are better suited for direct liquid introduction. C) LC-MS is faster. D) LC-MS is less expensive.Answer: B Explanation: Highly polar and non-volatile compounds are often thermally labile and cannot be easily volatilized for GC. LC-MS interfaces (like ESI, APCI) directly handle liquid effluents, making them suitable for these types of compounds without extensive derivatization.
28. Which term describes the measurement of how close the measured m/z value is to the true m/z value? A) Resolution B) Sensitivity C) Accuracy D) Mass rangeAnswer: C Explanation: Accuracy in mass spectrometry refers to the precision of the mass measurement itself, often expressed in parts per million (ppm).
29. What is the typical energy of electrons used in Electron Ionization (EI)? A) 5 eV B) 20 eV C) 70 eV D) 150 eVAnswer: C Explanation: 70 eV is the standard electron energy used in EI because it typically provides highly reproducible and characteristic fragmentation patterns that are well-documented in spectral libraries.
30. Which application of mass spectrometry is crucial for determining the amino acid sequence of proteins? A) Elemental analysis B) Isotope ratio mass spectrometry (IRMS) C) Peptide/Protein sequencing via MS/MS D) Forensic toxicologyAnswer: C Explanation: Tandem mass spectrometry (MS/MS) is widely used in proteomics for peptide sequencing, where peptides are fragmented, and the m/z values of the resulting fragments allow for the deduction of the amino acid sequence.