Tandem mass spectrometry in the study of fatty acids, bile acids, and steroids

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I. Introduction 82 II. Collision‐Energy Effects and Charge‐Remote Fragmentation 83 III. Fatty Acids and Related Lipids 85 A. Saturated and Unsaturated Fatty Acids 85 B. Hydroxy‐Polyunsaturated Fatty Acids and Eicosanoids 89 C. Derivatization of Fatty Acids for FAB and ES Analysis 90 D. Fatty Alcohols and Aldehydes 94 IV. Complex Lipids 96 A. Phospholipids 97 B. Triacylglycerols 106 V. Bile Acids and Conjugated Steroids 112 A. Bile Acids 112 B. Steroid Conjugates: Sulfates and Glucuronides 115 C. Mechanism of Fragmentation of Bile Acids, Steroid Sulfates, and Steroid Glucuronides 126 1. Side‐Chain Fragmentation 127 2. Ring Fragmentation 127 3. Sulfate Fragmentation 128 4. Bile Acid Fragmentation 128 VI. Neutral Steroids 128 A. Estradiol Metabolites and Synthetic Derivatives 128 B. Oxosteroids and Their Derivatives 131 1. 3‐Oxo‐Δ4‐Steroid 3‐Oximes 132 2. 20‐Oxosteroid and 17‐Oxosteroid Oximes 133 3. Methyl Oximes 134 4. Underivatized Oxosteroids 134 5. Steroid Esters 138 6. Girard T Hydrazone 139 VII. Novel Derivatives of Neutral Steroids 140 VIII. Conclusions 142 Acknowledgments 143 Appendix 143 A. Abbreviations and Relevant References 143 B. Use of the Prime 144 C. Some Steroid Structures 145 References 145 Over the last 50 years, the mass spectrometry of lipids has evolved to become one of the most mature techniques in biomolecule analysis. Many volatile and non‐polar lipids are directly amenable to analysis by gas‐chromatography‐mass spectrometry (GC‐MS), a technique that combines the unsurpassed separation properties of gas‐chromatography with the sensitivity and selectivity of electron ionization mass spectrometry. Less volatile and/or thermally labile lipids can be analyzed by GC‐MS, following appropriate sample derivatization. However, many complex lipids are not readily analyzed by GC‐MS, and it is these molecules that are the subject of the current review. Since the early 1970s, there have been three outstanding developments in mass spectrometry that are particularly appropriate in lipid analysis; i.e., the introduction of (i) fast atom bombardment (FAB); (ii) electrospray (ES); and (iii) tandem mass spectrometry (MS/MS). The FAB and ES ionization techniques will be discussed in relation to MS/MS, and examples of their application in biochemical studies will be presented. The review will concentrate on the analysis of fatty acids, bile acids, steroid conjugates, and neutral steroids. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 22:81–152, 2003; Published online in Wiley Interscience ( www.interscience.wiley.com). DOI 10.1002/mas.10046

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