accurate mass
/ AK-yer-it mass /
Imagine weighing a letter on a postage scale that reads only whole grams — it says '5 grams', and you must guess what is inside. Now switch to a jeweler's balance reading to a thousandth of a gram: suddenly the tiny extra weight of a paperclip versus a coin is unmistakable. Accurate mass is mass spectrometry weighing ions to that fine, decimal level.
Accurate mass is a mass-to-charge measurement made precisely enough to capture an ion's mass to several decimal places, close to its true theoretical value. Because each element's atoms have characteristic non-integer exact masses, a molecule's accurate mass is effectively a coded version of its elemental formula. The closeness of measurement to truth is often reported as mass error in parts per million.
It matters because an accurate mass can narrow a vast list of candidate compounds down to a handful of possible molecular formulas, often resolving ambiguities that whole-number masses cannot. The honest caveat is that accurate mass alone rarely proves a structure — several formulas may fit within the error window, so it is usually combined with isotope pattern and fragmentation evidence.
A high-resolution instrument measures an ion at m/z 180.0634; that value matches the formula for glucose to within two parts per million, ruling out other formulas that round to the same nominal mass of 180.
Accurate mass measured to several decimals points to a molecular formula.
Do not confuse accurate mass (a precise measurement, with error) with exact mass (the calculated theoretical value from a formula). Analysts compare the measured accurate mass against the calculated exact mass and report how far apart they are, in parts per million.