Weighing Molecules
Mass spectrometry achieves what no balance can — measuring the mass of individual molecules and atoms with precision exceeding six decimal places. By converting molecules into charged ions and separating them in electric or magnetic fields, mass spectrometers produce spectra that encode molecular weight, elemental composition, and structural information. It is arguably the most versatile analytical technique in modern science.
Ion Optics and Separation
In a magnetic sector instrument, ions accelerated through a potential V enter a uniform magnetic field B perpendicular to their velocity. The Lorentz force bends their paths into circles whose radii depend on m/z. Lighter ions curve more tightly; heavier ions swing wider. This mass-dependent deflection separates ions for sequential detection. Time-of-flight instruments instead measure how long ions take to traverse a field-free drift tube — heavier ions arrive later.
Fragmentation as Fingerprint
When molecules are ionized by electron impact (70 eV), excess energy breaks chemical bonds in reproducible patterns. A molecule of mass 100 might fragment into ions at m/z 85, 71, 57, 43 — each fragment revealing a structural subunit. The simulation generates schematic fragmentation patterns showing how molecular ion peaks, base peaks, and fragment ions compose a characteristic mass spectrum.
From Proteomics to Forensics
Mass spectrometry identifies proteins in biological research (proteomics), detects drugs and metabolites in clinical testing, characterizes environmental pollutants, authenticates food products, and analyzes crime scene evidence. Coupled with chromatography (GC-MS, LC-MS), it separates complex mixtures before mass analysis, achieving detection limits below parts per trillion.