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Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometer As a New Tool for Organic Chemists Jan St NEW TOOLS IN SYNTHESIS 249 Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometer as a New Tool for Organic Chemists Jan St. Pyrek University of Kentucky Mass Spectrometry Facility, ASTeCC Bldg, Lexington, KY 40506-0286, USA. E-mail: [email protected] Received 16 December 1998 This article is dedicated to Professor Zofia Kasprzyk for her 81 birthday 1 Introduction Abstract: Currently, mass spectral determination of molecular weights and elemental compositions of synthetic products is a com- mon practice. It is aided by truly impressive developments in meth- Mass spectrometry (MS) is the technique that renders ods and instrumentation enabling to characterize great variety of mass-to-charge ratios (m/z) and relative intensities of ions compounds including even those of a relatively high molecular formed (with exceptions), separated, and detected in the weight. Confidence in such measurements, however, depends upon high vacuum. The remarkable analytical potentials of MS selecting a suitable ionization method, unambiguous identification have been immediately recognized by its inventor J. J. of a molecular-type ion, and the adequate precision of mass deter- Thomson and, in years immediately following the First minations. This review addresses major aspects of the ion formation and analysis with special attention given to the ion-cyclotron reso- World War, the detection and precise characterization of nance. This powerful technique, different in its principle from most isotopes of almost all elements have been achieved em- other methods of ion analysis, offers a superb combination of ion ploying the very first mass spectrographs and spectrome- manipulation and measurement parameters and full compatibility ters.1 With some delay, however, these seminal with an array of ionization methods. discoveries have been followed by a systematic effort to Key words: mass spectrometry, ultra-high resolution mass spec- analyze organic compounds. Ever since, thanks to a trometry, ion-cyclotron-resonance, soft ionization, stable isotopes, steady progress in techniques and instrumentation, MS molecular weight determination. continues to serve in a variety of inorganic and organic ap- plications. Moreover, by gaining truly astonishing capa- 1. Introduction bilities in terms of ionization, mass range, sensitivity, and 2. Ion analysis 2.1. Types of mass analyzers accuracy, MS methods cover classes of compounds that 2.2. Compatibility of ion sources with mass analyzers originally have been considered as entirely unsuitable for 2.3. Fourier Transform-Ion Cyclotron Resonance-Mass such an analysis.2 Spectrometer Still, as exemplified by the recent synthesis of two diter- 2.3.1. Overview 3 2.3.2. Trapping ions in the ICR-cell penoids, synthetic work can be carried out with the com- 2.3.3. Detection of the trapped ions plete omission of MS. As a rule, however, MS data are 2.3.4. Getting ions into the ICR Cell used to confirm MW and to replace elemental analyses of 2.4. Tandem mass spectrometry the final reaction products. Only seldom, synthetic publi- 2.4.1. Tandem mass spectrometry with magnetic-sectors and cations include detailed spectral interpretations and use quadrupoles more advanced MS techniques. This low visibility of MS 2.4.2. Tandem-MS with FT-ICR-MS contrasts with the structure elucidation of natural and met- 3. Ion formation 3.1. Overview abolic products and, more recently, with the characteriza- 3.2. Electron impact ionization tion and sequencing of linear biopolymers. Still, an early 3.3. Chemical ionization implementation of MS analysis in the synthetic practice 3.4. Fast atom bombardment may greatly facilitate structural assignments well before 3.5. Matrix assisted laser desorption/ionization time and effort is invested in separation and purification 3.6. Cationization of products. MS analysis, especially when combined with 3.7. Electrospray ionization the on-line chromatography, provides an advantage in 4. Mass spectrometry and molecular weight determination 4.1. Stable isotopes and molecular ions confirming identity and purity of substrates, in finding if 4.2. Isotopic depletion and substitution the expected compounds are formed, in identifying inter- 4.3. High or ultra-high resolution? mediates and by-products and, finally, in guiding through- 4.4. Mass measurement accuracy out their isolation and purification. Such an extensive use 4.5. Can molecular weight be unequivocally determined by of MS methods, however, requires an access to instrumen- mass spectrometry? tation supported by the comprehension of novel and tradi- tional methods with the full awareness of their inherent limitations. This review, in an attempt to assess the relevance of MS in establishing MW, surveys selected methods of ion for- Synlett 1999, No. 2, 249–266 ISSN 0936-5214 © Thieme Stuttgart · New York 250 J. St. Pyrek NEW TOOLS IN SYNTHESIS mation and analysis. It gives special attention to Fourier of ion sources and in some configurations must work with Transform-Ion Cyclotron Resonance-Mass Spectrometry interfaces handling a considerable flow of solvents. More- (FT-ICR-MS), an advanced method providing an unprec- over, for techniques such as ESI (Section 3.7) mass ana- edented ultra-high mass resolution, high sensitivity, and lyzers of an extended mass range (2,000-4,000 u) and versatile MS/MS, but still only seldom used in a routine resolution depicting multiply charged ions are highly de- characterization of organic compounds. Easier access to sirable. Although magnetic-sector instruments usually FT-ICR-MS instruments, by offering gains analogous to furnish all such necessary parameters, quadrupoles and those of now routine use of the high field NMR, should ion traps4 prevail as detectors in chromatographic applica- help in traditional and combinatorial synthesis aimed at tions. Newer solutions to LC and GC interfacing come targets of ever increasing MW and architectural complex- from the recent developments in time-of-flight (TOF) in- ity. struments offering very high rate of spectral repetitions and good resolution. Still, registering spectra at medium and high resolution, rather than a unit resolution, provides 2 Ion Analysis richer structural information and considerably improves specificity of the detection in analytical applications. While a simple, on-line MS detection characterizes com- 2.1 Types of mass analyzers pounds by their chromatographic mobility and spectra, Selection of an ion analyzer (Table 1) results from spe- tandem-MS (MS/MS) measurements (Section 2.4) pro- cific analytical tasks and compatibility with the mode of vide another dimension that delineates specific parent- sample introduction and ionization. For every mass spec- daughter (or according to PC nomenclature, “precursor- trometer, configuration of its three principal parts, ion product”) ion relations. This information considerably source, ion analyzer, and ion detector, determines mass augments selectivity in the detection of selected targets range, scanning speed, mass resolution, mass accura- whereas for novel compounds may help in their structure cy, sensitivity, and the dynamic range of ion intensity elucidation. measurements. As exemplified by interfacing MS detec- Recent developments are aimed primarily at the emerging tion with chromatographic methods, some of these para- applications of soft ionization methods pertinent to medi- meters can be compromised, especially in instrument ded- um and high MW biological compounds. There is partic- icated to a single task. Thus, for gas chromatography- ular attention paid to the TOF and ion trap instruments, mass spectrometry (GC-MS), mass analyzers capable of mostly because of their great simplicity, and to FT-ICR, fast scanning to only about 1,000 u and resolving power mostly because of its superb parameters. Yet, high resolu- separating only the adjacent ions (unit resolution) may be tion magnetic-sector instruments still suit well all major quite adequate. Such instruments may operate only with ionization methods including those that are most frequent- EI (Section 3.2) but require good sensitivity and broad dy- ly used in the organic MS and, in terms of resolution and namic range defining ions that differ in their intensity by dynamic range, provide benchmark spectra. Even older 2-3 orders, in components that may differ by another 2-3 sector instruments upgraded with new data systems and orders. For a comparison, LC-MS utilizes greater variety new ion sources may still offer very good performance. Biographical Sketch Jan Pyrek, born in 1942 in Then, still in the same Insti- Currently, as a faculty in the Jaslo - a subcarpatian town, tute and while visiting labo- College of Pharmacy, he di- was educated in Krakow and ratories of Professors G. rects the MS-Facility at the Warsaw. He participated in Jommi (University of Mi- University of Kentucky. He Polish Chemistry Olympi- lan) and E. Wenkert (Rice has authored over 90 publi- ads as early as 1957. He University), he continued to cations concerned mostly studied and worked in the work on natural products. In with structure, metabolism, Biochemistry Department of 1979, he joined the faculty and synthesis of various nat- Warsaw University (1960- of the University of Texas at ural products. Apart from 7) under the direction of Houston, initiating new in- science, he has a long inter- Prof. Zofia Kasprzyk. Sub- terests in steroids, bile acids, est in art and, while living in sequently, he obtained his and biomedical application rural Kentucky with his wife Ph.D. degree
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