Determining an organic compound's structure from its mass spectrum is challenging. If, as in the case of soft ionization - chemical ionization (CI), atmospheric pressure chemical ionization (APCI), field ionization (FI), electrospray (ES), atmospheric pressure photoionization (APPI), or fast atom bombardment (FAB) - the mass spectrum only provides the mass of the intact molecule, the challenge can be even greater. At best, an elemental composition can be determined if the measured mass is sufficiently accurate; however, many of these elemental compositions represent large numbers of compounds. There are 127 compounds in the NIST08 Mass Spectral Database that have the elemental composition C10H20O. Without an accurate mass measurement, the challenge is even more overwhelming. The elemental composition C10H20O has a nominal mass of 156 Da. The NIST08 Mass Spectral Database has 684 compounds with this nominal mass. To get past this point of an elemental composition, fragmentation of the ion..