Two disparate paths can lead to increased performance in mass spectrometric analysis. Improvements in fundamental parameters of instrument performance can be achieved through optimized interface design, improvements in ion source and analyzer efficiency, and enhanced detector performance. Increased performance also can be achieved through a better sampling protocol, or through better chromatographic performance. Alternatively, because we are clever chemists, we might choose to alter the chemical and physical characteristics of the sample itself, however collected, to achieve a more complete transport through the sample purification/selection/chromatographic process, and to achieve better ionization through enhanced sensitivity or selectivity. This second path involves a change in either the physical or chemical form of the sample. A change in the chemical form of the sample is sample derivatization, which is the focus of this column.
Sample derivatization is a general term used for a chemical transformation designed to improve analytical capabilities, and it is a mainstay of analytical chemistry and instrumental analysis. The chemical structure of the sample can be modified to a form consistent with a better outcome for separation from other mixture components, or into a form that is more easily introduced to the measuring instrument, or into a chemical form that provides an enhanced response in terms of either improved selectivity or sensitivity. Derivatization reactions are chemical reactions. As such, there are many variables that control reaction completeness, speed, and specificity. These include temperature, solvents, catalysts, and supports. Each of these variables can be optimized within a specific application. Years of experience with widely used derivatization reactions have culminated in automated miniaturized kits available from a variety of commercial vendors. Websites of such vendors are populated with tables that describe such derivatization kits and application to certain functional groups, and provide examples of applications in chromatography and spectrometry. More complete information is found in monographs and handbooks (1,2). Mass spectrometry (MS) journals routinely contain reports of new derivatization reagents, and new uses for older reagents. Logically, much of the applications work is reported in the appropriate journals, and while derivatization may be an integral part of the procedure, it may not be highlighted as a search or indexing keyword.
Derivatization reactions used in MS often overlap with those used in other venues of analytical chemistry. However, there are some derivatization reactions specifically designed for MS, such as those that enhance ionization or introduce a specific mass shift to the sample ions that becomes evident in the mass spectrum. The general goals of derivatization in MS are outlined in Table I. Most of these goals should be self-explanatory, and they are, of course, not mutually exclusive. For instance, derivatization to create a new compound as a surrogate for the original almost always changes the physical and chemical properties of the compound (thereby changing the volatility and the thermal stability), almost always changes the mass, and usually affects the limits of detection that can be achieved. Constant demands for better selectivity and sensitivity in MS analysis, the use of a wide variety of different ionization methods, and the expanding areas of application of MS combine to ensure a steady stream of literature reports of new derivatization reagents or modifications of past procedures (search "novel derivatization" + "mass spectrometry" on the web). Reviews of new and more established reactions are especially valuable. Halket and Zaikin have published reviews of MS derivatization (3,4), and their series of reviews published in European Mass Spectrometry (Table II) is an excellent complement. Other recent reviews (5–10, for example) deal with specific ionization methods of focus on specific areas of application.
Table I: General goals of derivatization in mass spectrometry
Given the length of comprehensive reviews, clearly this column must focus on a limited aspect of MS derivatization. Many of the most established derivatization reactions were developed for use in gas chromatography–mass spectrometry (GC–MS) and electron ionization. Specifically, sample compounds that were not sufficiently volatile or stable to pass through the GC system were converted to more stable forms. As new ionization methods were developed, a shift in focus was apparent. For example, negative ion analysis was not widespread for analytical applications until chemical ionization sources were put in place. As the parallels between electron-capture detectors (used in GC) and negative ion MS became clear, derivatization reactions that modified the sample with an electrophoric (such as a pentafluorobenzyl derivative) group quickly appeared. When the first of the desorption ionization methods (fast atom bombardment [FAB] and liquid secondary ion mass spectrometry [LSIMS]) appeared in the late 1980s, and mass spectrometry–mass spectrometry (MS-MS) was accepted for analytical purposes, different forms of derivatization appeared. The observation (11) that precharged compounds usually exhibited a higher ionization efficiency in desorption ionization methods such as FAB soon generalized to electrospray ionization (ESI) and matrix assisted laser desorption ionization (MALDI) MS, although covariant factors relevant to sample transport in solid and liquid matrices soon became apparent. The ability of a localized charge to direct fragmentation both in the mass spectrum and in the MS-MS spectrum was also broadly recognized. Given this new direction in derivatizaton, the literature and the handbooks were consulted for chemical reactions that offered functional-group selectivity in the creation of a charged derivative. Some well-known name reactions showed an immediate applicability in MS analyses, and often, derivatization reactions were rediscovered by research groups working in widely divergent areas.
Table II: Series of review articles by Zaikin and Halket on derivatization in mass spectrometry