Weighing Molecules One at a Time
Mass spectrometry (MS) measures the mass-to-charge ratio (m/z) of ions derived from a sample. By converting neutral molecules into gas-phase ions and separating them according to their masses, the technique provides the molecular weight and, through fragmentation analysis, detailed structural information. Mass spectrometry is among the most sensitive analytical methods available, capable of detecting compounds at femtomole (10^-15 mol) or even attomole (10^-18 mol) levels.
The power of modern mass spectrometry lies in its versatility. It can analyze small organic molecules, proteins with masses exceeding 100,000 daltons, synthetic polymers, metabolites in blood, pesticide residues in food, and pollutants in water. Coupled with chromatographic separation techniques, it has become the gold standard for identifying and quantifying complex mixtures.
Ionization Methods
The first step in mass spectrometry is ionization -- converting neutral molecules into charged species that can be manipulated by electric and magnetic fields. Different ionization methods suit different types of analytes.
Electron ionization (EI) bombards gas-phase molecules with a beam of 70 eV electrons, knocking out an electron to produce a radical cation (M+). EI is a "hard" ionization method that imparts considerable excess energy, causing extensive fragmentation. The resulting fragmentation pattern is highly reproducible and can be matched against databases such as the NIST Mass Spectral Library, which contains spectra for over 350,000 compounds. EI is primarily used with gas chromatography (GC-MS).
Electrospray ionization (ESI) sprays a liquid solution through a charged capillary at atmospheric pressure, producing a fine mist of charged droplets. As the solvent evaporates, multiply charged ions are released into the gas phase. ESI is a "soft" method that typically produces intact molecular ions ([M+H]+ or [M-H]-) with little or no fragmentation. It is ideal for large, polar, and thermally labile molecules including proteins, peptides, and nucleic acids. ESI is the standard ionization method for liquid chromatography-mass spectrometry (LC-MS).
Matrix-assisted laser desorption/ionization (MALDI) embeds the analyte in a crystalline matrix that absorbs ultraviolet laser light. The laser pulse ablates the matrix and analyte together, producing primarily singly charged ions. MALDI is widely used in proteomics (identifying proteins from enzymatic digests), polymer analysis, and tissue imaging.
Mass Analyzers
Once ions are formed, they must be separated by mass-to-charge ratio. Several types of mass analyzers are in common use:
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Quadrupole: Four parallel rods with oscillating RF and DC voltages act as a mass filter, transmitting ions of a selected m/z while deflecting others. Quadrupoles are compact, robust, and widely used in routine LC-MS and GC-MS.
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Time-of-flight (TOF): Ions are accelerated through a fixed potential and allowed to drift through a field-free region. Lighter ions travel faster and arrive at the detector first. TOF analyzers offer very high mass range (theoretically unlimited) and fast scan speeds, making them ideal for MALDI and high-throughput analyses.
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Orbitrap: Ions orbit around a central spindle electrode, and their axial oscillation frequency is measured. The Fourier transform of the signal yields mass spectra with resolving power exceeding 500,000, enabling the determination of molecular formulas by exact mass measurement.
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Ion trap: Ions are confined in a three-dimensional electric field and sequentially ejected by mass. Ion traps can perform multiple stages of fragmentation (MS^n), providing deep structural information.
Fragmentation Patterns and Molecular Formulas
In EI mass spectrometry, the fragmentation pattern reveals structural features. Common fragmentations include loss of water (18 Da) from alcohols, loss of CO (28 Da) from aldehydes and ketones, and cleavage adjacent to heteroatoms or branch points. The McLafferty rearrangement, a gamma-hydrogen transfer followed by beta-cleavage, is diagnostic for carbonyl compounds with a gamma-hydrogen.
High-resolution mass spectrometry (HRMS) can determine molecular formulas by measuring exact masses. Carbon-12 has a mass of exactly 12.00000 Da, hydrogen-1 is 1.00783 Da, nitrogen-14 is 14.00307 Da, and oxygen-16 is 15.99491 Da. A compound with molecular formula C8H10N4O2 (caffeine) has a calculated exact mass of 194.0804 Da, distinguishable from other formulas with the same nominal mass of 194.
Tandem Mass Spectrometry
Tandem mass spectrometry (MS/MS) involves two stages of mass analysis separated by a fragmentation step. A precursor ion is selected in the first analyzer, fragmented by collision with an inert gas (collision-induced dissociation, CID), and the product ions are analyzed in the second analyzer. MS/MS provides structural specificity and selectivity, making it essential for proteomics, metabolomics, pharmaceutical analysis, and clinical diagnostics. Multiple reaction monitoring (MRM) on triple quadrupole instruments achieves extraordinary sensitivity and selectivity for targeted quantitation.