Representative example of a molecular family level annotation from spectral library searching that matches to hexenoylcarnitine. The MS/MS spectrum contains several diagnostic fragments and neutral losses that make it possible to assign it to the acylcarnitines molecular family, as indicated on the molecular structures . However, routine spectral library matching cannot distinguish between the 14 potential stereo-and regioisomers, resulting in a level 3 annotation. This highlights the need for new strategies to communicate the results from spectral library searching, as narrowing down to the molecular family, even when the exact molecular identity is unknown, can often already be valuable for biological interpretation. Top is the experimental observed MS/MS spectrum, with a precursor m/z deviation of 11.6 ppm compared to the calculated m/z of the protonated ions.