How can a mass spectrometer distinguish molecules using charged particles?

A chemical sample can be distinguished by sending its charged particles through an electric field and comparing their motion. A mass spectrometer first gives some atoms or molecules charge, then guides them into an analyzer. Their behavior depends on mass divided by charge, providing information that a balance weighing the whole sample would not reveal.

In a simple time-of-flight analyzer, ions start under comparable conditions and are accelerated through the same voltage difference. Electrical work gives each ion kinetic energy equal to its charge times that voltage. Ions with the same charge therefore receive approximately the same added energy, even when their masses differ.

Kinetic energy is ½mv². With equal energy, a less massive ion must move faster. Along the same field-free flight distance it arrives sooner. More generally, flight time depends on the square root of mass-to-charge ratio under these ideal conditions. Four times the mass at unchanged charge gives twice the flight time, rather than four times.

The detector turns arrivals into signals, and calibration connects flight times to mass-to-charge values. A spectrum displays peaks at those values. Isotopes can give related peaks, while ionization or later fragmentation can create pieces of molecules. The pattern can therefore carry more information than one peak alone.

One matching mass need not uniquely identify a compound: different compounds can share masses, ions can carry multiple charges, and instrument conditions affect the observed species. Reference measurements, fragmentation patterns and chromatography provide further constraints. The ideal equal-charge flight comparison and invented spectrum illustrate that reasoning without a calibrated scale or measured sample.