Desorption Ionization Mass Spectrometry

Common to all experiments in mass spectrometry is the creation of gas-phase ions. Electron impact (EI) ionization is a familiar method for creating ions from volatile gas-phase molecules (M). In EI ionization, fast-moving electrons remove an electron from the neutral molecule to create the odd-electron molecular ion M`Õ. Another common ionization method is chemical ionization (CI). Here, an ion-molecule reaction between the sample molecule and a reagent ion such as or results 5 4 in proton transfer to M to form [M ] H]`. In each of these methods, the sample molecule is originally in the gas phase. The volatilization process must create these gas-phase sample molecules without sample decomposition or reorganization. For non-volatile sample molecules, other ionization methods must be used and have been developed over the past 20 years. These methods include fast atom bombardment (FAB), (static) secondary ion mass spectrometry (SIMS), liquid secondary ion mass spectrometry (LSIMS), laser desorption and electrospray ionization. The diversity of approaches deÐes tidy organization, but most methods can be classiÐed into desorption ionization (DI) or nebulization ionization methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyzer. In EI and CI, the processes of volatilization and ionization are distinct and separable ; in DI, they are intimately associated. Table 1 compares some aspects of various ionization methods used in mass spectrometry, placing DI in perspective with other methods available to the analyst. In nebulization ioniza-