Exact Mass Practice (HRMS)
Every HRMS value in a paper connects three things: the neutral molecule M, the adduct it flew as, and the m/z the instrument recorded. These problems are real published measurements, and you are given two of the three: either name the adduct from a known formula, or decide which molecular formula the measurement supports for a known adduct (each candidate shows its neutral exact mass, so the work is handling the adduct and then judging the milli-Daltons). Set the toggle to Either and the problem picks a direction for you.
Common Questions
How does an exact mass identify a molecular formula?
Only ¹²C sits at an exact integer mass; every other isotope carries a small surplus or deficit (H is 1.00783, N is 14.00307, O is 15.99491). Different formulas with the same nominal mass therefore differ in their fourth decimal: C₁₈H₂₄N₂O and C₁₉H₂₈N₂ both sit at nominal 284, but their exact masses differ by 36 mDa. A measurement good to a few mDa (which is what real instruments deliver) picks one formula out of the crowd.
Why is there no M⁺• peak in an electrospray mass spectrum?
Electrospray ionization doesn't knock electrons out of molecules the way electron ionization does: it transfers ions to them. The molecule leaves the source carrying a proton ([M+H]⁺), a sodium ion ([M+Na]⁺), sometimes potassium or ammonium, or missing a proton in negative mode ([M−H]⁻). So the peak you see is never the molecule's own mass; it is always offset by the adduct, and reading HRMS means knowing those offsets.
What do "calcd" and "found" mean in a paper's HRMS report?
"Calcd" is the theoretical m/z of the proposed ion (formula plus adduct, e.g. C₁₂H₂₄O₃Na for [M+Na]⁺); "found" is what the instrument measured. Agreement within about 5 mDa (often much better: the RealOChem bank of 8,935 published pairs has a median gap of 0.5 mDa) is the standard evidence that the synthesized compound has the claimed formula.
Where do sodium and potassium adducts come from?
Na⁺ and K⁺ are everywhere in real samples (glassware, solvents, buffers), and oxygen-rich molecules coordinate them well. [M+Na]⁺ sits 21.98 mass units above [M+H]⁺ and won't fragment the same way, so recognizing it matters. In the 8,935 published HRMS reports behind this page, sodium adducts are the second most common ion after [M+H]⁺, while potassium and ammonium together are under 1%: rare, but real enough that a working chemist has to recognize them.