Special feature: Tutorial . Desorption ionization mass spectrometry

CH 5 ` NH 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 decomposi- tion 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 spec- trometry (SIMS), liquid secondary ion mass spectrom- etry (LSIMS), laser desorption and electrospray ionization. The diversity of approaches de-es tidy organization, but most methods can be classi-ed 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 sub- sequent 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- ¤ EditorÏs Note: This is the -rst of several "TutorialÏ articles which will appear this year in the Special Features section of the journal. These tutorials will describe fundamental aspects and applications of mass spectrometry with the general reader, not the authorÏs peer group, in mind. The aim will be to cover some speci-c areas of mass spectrometry in a manner in which a teacher might present the subject in a graduate level course. Although these articles are normally invited, comments and suggestions from readers are welcome. Please address these to the Special Features Coordinator; Graham Cooks, Dept of Chemistry, Purdue University, W. Lafayette, IN, USA, 47907. Further, as a special o†er to readers, copies of the -gures of this "TutorialÏ article, as color slides, are available free of charge on request from the editor-in-chiefÏs office. Supplies are limited and slides will be sent out in the order requests are received. tion, an aerosol spray is used at some point to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectro- meter. These ionization methods include thermospray and electrospray ionization. Since this paper is focused on DI processes, nebulization methods will not be dis- cussed further. This paper will -rst consider general aspects of DI hardware (the mechanics), followed by a brief descrip- tion of sample preparation and ionization mechanisms. Issues of interest include energy input and energy trans- fer, in addition to ionization, association and disso- ciation reactions. Although detailed elements of the mechanisms of ionization may still be debated, DI source conditions and sample attributes are easily manipulated so that high-quality data can generally be obtained. DI methods tend to produce even-electron ions such as protonated molecules (M ) H)` or cationized molecules such as (M ) Na)`; these stable ions undergo only a minimum amount of fragmenta- tion. The development of DI methods ampli-ed the impact of mass spectrometry many fold. The inherent restrictions in EI an CI that limited applications to volatile sample molecules constituted severe limitation especially in the -eld of biological chemistry. DI was applied immediately with great enthusiasm to a wide range of analytical problems, and the -eld of mass spec- trometry has Ñourished as a result. In their infancy, DI methods were relegated to the periphery of mass spectrometric research, pursued and developed by those for whom the inability to deal with non-volatile samples in the ion source constituted an analytical challenge. Progress was dependent on the solution of problems related to sample handling and ion transport through substantial pressure and potential gradients. Non- volatile samples, regardless of their molecular mass, were difficult samples to handle. Figure 1 shows the positive-ion plasma desorption mass spectrum recorded in the mid-1970s for tetrodotoxin and chiriquitoxin.2 The mass of the tetrodotoxin is only 320, but the non- volatility rendered the molecule resistant to EI and CI methods of analysis prevalent at the time. The impact of the new capabilities of plasma desorption (PD) for the determination of molecular mass for these samples was enormous. Such leading work for PD and other desorp- tion ionization methods foretold of extensive applica- tions to problems in biochemical and natural products analysis, since sample volatility was no longer required.