Mass Spectrometry for Visual Learners

Learn the three main spectroscopy techniques (infrared, NMR, and mass spectrometry) used by chemists to identify unknown molecules. Understand how mass spectrometry determines molecular mass, isotope abundances, and molecular structure through a basic four-step process: ionization, fragmentation, acceleration, and detection.

Use electron ionization to create a beam of high-energy electrons from a heated filament. Pass the sample through this electron beam, which knocks electrons off molecules to create radical cations, leading to molecular fragmentation into various radical and cationic fragments.

Perform chemical ionization by using gaseous cations to protonate sample molecules (forming MH+ ions without radical formation), or use electrospray ionization by dissolving the sample and spraying it through a charged needle. These soft techniques preserve the molecular ion with minimal fragmentation, allowing analysis of large biomolecules.

Use charged electrodes to accelerate the cations toward the detector while deflecting neutral radicals away. Apply an electric or magnetic field that deflects cations to varying extents based on their mass-to-charge ratio, allowing only ions with the correct m/z ratio at a given field strength to reach the detector.

Accelerate all cations to the same kinetic energy, then allow them to drift through an ion drift chamber (flight tube). Lighter cations travel faster and reach the detector sooner than heavier cations, allowing mass determination based on flight time.

Use kinetic energy (KE = ½mv²) and velocity equations (v = distance/time) to solve time-of-flight problems. Convert mass numbers to SI units using Avogadro's constant, then calculate the time required for ions to traverse the flight tube and determine unknown mass values.