A problem with EI mass spectrometry is that, for fragile molecules, the energy of the bom-barding electron can be sufficient to cause it to fragment completely, losing all trace of the molecular ion. Some useful information can be gained from fragmentation patterns, but in general it is more useful to aim to weigh the molecule all in one piece. This can be achieved using any of a number of other techniques, of which the most common are chemical ionization (CI) and electrospray (ES). Chemical ionization is achieved by mixing a gas such as ammonia with the substrate in the spectrometer. Bombardment of NH₃ with electrons leads to formation of some NH₄ + by proton transfer, and reaction of this ion with the substrate makes a charged complex, which can be accelerated by the electric fi eld. The masses observed by chemical ionization spectroscopy carried out in this way are usually M + 1 or M + 18 (the mass of NH₄+) relative to the mass of the substrate. With electrospray mass spectroscopy, an aerosol of the substrate is ionized, and ionization in the presence of sodium ions means that masses of M + 1 and M + 23 are often seen, or, if the ionization forms anions, M – 1. This is the electrospray mass spectrum of heptan-2-one. Notice how a single molecular ion is clearly visible this time, but that it has a mass of 137, which is 23 more than the mass of 114 (in other words, this is the mass of M + Na⁺).