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.<Time-of-Flight Mass Spectrometer with Improved Resolution by W. C. Wiley and I. H. McLaren>[Page 1150, general description; subsequent resolution analysis and two-field ion gun]{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 $qU=mv^2/2$. A field-free drift of length $L$ then gives
$$
t=L\sqrt{\frac{m}{2qU}}.
$$
If one peak arrives at $40\,\mathrm{\mu s}$ and another at $44\,\mathrm{\mu s}$, 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.


 Mass spectrometer (source code)