What Is Mass Spectrometry?
Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio (m/z) of ions to determine molecular masses, molecular formulas, and structural features of compounds. It is arguably the most powerful and versatile tool in modern analytical chemistry — capable of detecting single molecules, sequencing proteins, dating ancient artifacts, and detecting drugs in blood at nanogram-per-milliliter concentrations.
The fundamental process in mass spectrometry involves three steps: 1. Ionization: Convert neutral molecules into gas-phase ions 2. Mass analysis: Separate ions by their m/z ratio 3. Detection: Count ions at each m/z value
The output is a mass spectrum — a plot of relative ion abundance vs. m/z.
Ionization Methods
The choice of ionization method depends on the type of compound and the information needed.
Electron Ionization (EI)
Electron ionization is the classic "hard" ionization technique for volatile organic compounds. A beam of high-energy electrons (70 eV) bombards vaporized sample molecules, ejecting an electron to produce a molecular ion (M⁺•):
M + e⁻ → M⁺• + 2e⁻
At 70 eV, there is excess energy to break bonds, producing fragment ions. This fragmentation pattern is characteristic and reproducible — EI spectra are stored in searchable libraries (NIST, Wiley) containing over 350,000 compounds. EI is the ionization method used in GC-MS.
The molecular ion peak (m/z = molecular weight) is often visible and confirms the molecular mass. However, for large or thermally labile molecules, the molecular ion may not survive — EI is too harsh.
Electrospray Ionization (ESI)
ESI is the dominant technique for large, polar, and thermally labile molecules such as proteins, peptides, nucleic acids, and drugs. A liquid sample is sprayed through a fine needle at high voltage. Solvent evaporation leaves multiply protonated (or deprotonated) ions:
M + nH⁺ → [M + nH]^n+
ESI produces multiply charged ions, allowing proteins with molecular masses of hundreds of thousands of daltons to be analyzed by instruments with m/z ranges of only a few thousand. The observed m/z peaks correspond to [M + nH]^n+, and the true molecular mass is calculated from two adjacent charge state peaks.
Matrix-Assisted Laser Desorption/Ionization (MALDI)
MALDI embeds the analyte in a UV-absorbing matrix. A laser pulse vaporizes the matrix, carrying analyte molecules into the gas phase as singly charged ions. MALDI produces primarily singly charged ions (M + H⁺ or M + Na⁺) and is ideal for protein and polymer molecular weight determination.
Chemical Ionization (CI)
CI uses a reagent gas (methane, ammonia, isobutane) to gently ionize the analyte via ion-molecule reactions. CI is a "softer" technique than EI — it produces abundant molecular ions with less fragmentation, useful when EI destroys the molecular ion.
Mass Analyzers
Quadrupole
Four parallel metal rods with oscillating electric fields filter ions so only those with a specific m/z pass through at any given moment. Quadrupoles are robust, inexpensive, and fast — standard in GC-MS and LC-MS instruments.
Time of Flight (TOF)
Ions are accelerated to the same kinetic energy, then allowed to drift down a tube. Lighter ions travel faster and arrive at the detector sooner:
KE = ½mv² = qV → v = √(2qV/m) → t ∝ √(m/z)
TOF analyzers have essentially unlimited mass range and very high scan speeds — essential for MALDI analysis of proteins and for fast chromatographic peak detection.
Orbitrap
Ions orbit around a central electrode while oscillating along its axis. The oscillation frequency is inversely proportional to √(m/z), allowing ultra-high resolution (> 100,000 resolving power). Orbitraps can distinguish ions differing by a few millionths of a dalton — enabling exact molecular formula determination from the accurate mass alone.
Ion Trap and FT-ICR
These store ions and perform multiple rounds of fragmentation (MS/MS or MSⁿ), enabling structural sequencing of complex molecules.
Interpreting a Mass Spectrum
Key features of a mass spectrum:
- Molecular ion (M⁺•): The highest m/z peak (for EI) corresponding to the intact molecule. In ESI, the equivalent is the [M+H]⁺ or multiply charged peaks.
- Base peak: The most abundant ion (set to 100% relative abundance).
- Isotope pattern: Natural isotope abundances create characteristic patterns:
- M+1 peak: ¹³C contribution (~1.1% per carbon)
- M+2 peak: Halogen signature — Cl gives M : M+2 of ~3:1; Br gives ~1:1.
- Fragment ions: Characteristic losses reveal structure. Loss of 15 Da = CH₃; loss of 18 = H₂O; loss of 28 = CO or C₂H₄; loss of 31 = OCH₃.
Tandem Mass Spectrometry (MS/MS)
In MS/MS, a precursor ion is selected, fragmented (typically by collision-induced dissociation, CID), and the product ions analyzed. This provides detailed structural information and is the basis of: - Proteomics: Peptide sequencing from fragmentation of [M+nH]^n+ ions - Metabolomics: Identifying metabolites in biological samples - Clinical toxicology: Quantifying drugs and metabolites in urine/blood with high selectivity (triple quadrupole instruments)
Real-World Applications
Proteomics and drug discovery: MS identifies proteins by "peptide mass fingerprinting" — digesting with trypsin, measuring peptide masses, and searching databases. This underpins modern biomarker discovery and protein interaction studies.
Environmental analysis: GC-MS detects persistent organic pollutants (POPs), pesticides, and VOCs in water, soil, and air at sub-parts-per-trillion levels.
Doping control: WADA-accredited sports anti-doping labs use LC-MS/MS to detect performance-enhancing drugs and their metabolites in athlete urine and blood.
Forensic science: MS confirms illicit substances, identifies accelerants in arson, and profiles explosives residues.
Radiocarbon dating: Accelerator mass spectrometry (AMS) counts individual ¹⁴C atoms (m/z = 14) in ancient materials, determining age from the ¹⁴C/¹²C ratio up to ~50,000 years with milligram-scale samples.
Clinical diagnostics: MS is transforming newborn screening — neonatal blood spots are analyzed by MS/MS to detect 50+ inherited metabolic disorders from a single dried blood spot within hours of birth.
Mass spectrometry's combination of sensitivity, specificity, and structural information makes it the gold standard analytical technique across science and medicine.