 Mass spectrometer

A mass spectrometer is an instrument that produces or receives ions, separates them according to mass-to-charge ratio, and records their signals. Its major functions are supplied by an ion source, a mass analyzer and a mass spectrometer detector. The result is a mass spectrum; interpreting a peak also requires knowing which ionic chemical species produced it.

The 1955 instrument described by W. C. Wiley and I. H. McLaren provides a concrete apparatus example. An ion source and accelerating fields launch ions into an evacuated drift region toward a collector. Arrival times separate mass-to-charge ratios; the paper analyses how initial positions and velocities broaden the recorded peaks, and uses a two-field ion-gun geometry to improve focusing.^[ref] This is one analyzer design, not a definition requiring every mass spectrometer to measure flight time.

For an ideal time-of-flight mass spectrometer, neglect initial speed and acceleration duration. Across an electric potential difference $U$, an ion of positive electric charge $q$ gains $qU=m{v}^2/2$. A field-free drift of length $L$ then givesIf one peak arrives at $40\mu \mathrm{s}$ and another at $44\mu \mathrm{s}$, identical acceleration and drift conditions imply a mass-to-charge ratio ratio $(44/40{)}^2=1.21$. It is a mass ratio only if the charge states are also equal. Peak position, width and intensity answer different questions: ionic identity, resolving ability and response-weighted abundance.