mass spectrum
/ mass SPEK-trum /
Imagine sorting a big bag of coins by value and stacking each kind into its own column — pennies here, dimes there — so that one glance shows you which coins you have and how many of each. A mass spectrum is that kind of sorted picture, but for ions: it shows which masses are present and how abundant each one is.
Formally, a mass spectrum is a plot of ion abundance (vertical axis) against mass-to-charge ratio (horizontal axis). Each vertical line, or peak, marks a particular m/z value, and its height shows how many ions of that m/z reached the detector. The tallest peak is the base peak; the peak from the whole molecule is the molecular ion.
It matters because the spectrum is the readable output of the entire experiment — its pattern of peaks is what you compare against libraries or interpret by hand to identify a compound. The caveat is that a spectrum reflects not just what is in the sample but also how it was ionized and analyzed, so the same compound can give different-looking spectra under different conditions.
An analyst overlays the mass spectrum of an unknown solvent on a reference spectrum from a library; the peaks line up at the same m/z values with matching heights, confirming the unknown is acetone.
A mass spectrum charts ion abundance against mass-to-charge ratio.
Do not confuse a mass spectrum (abundance versus m/z) with a chromatogram (signal versus time). In a coupled technique like GC-MS, you first separate compounds in time, then record a mass spectrum for each.