Fast atom bombardment combined with tandem mass spectrometry for determination of bile salts and their conjugates

High-energy collision-induced dissociation by MALDI TOF/TOF causes charge-remote fragmentation of steroid sulfates

A method for structural elucidation of biomolecules dating to the 1980s utilized high-energy collisions (~10 keV, laboratory frame) that induced charge-remote fragmentations (CRF), a class of fragmentations particularly informative for lipids, steroids, surfactants, and peptides. Unfortunately, the capability for high-energy activation has largely disappeared with the demise of magnetic sector instruments. With the latest designs of tandem time-of-flight mass spectrometers (TOF/TOF), however, this capability is now being restored to coincide with the renewed interest in metabolites and lipids, including steroid-sulfates and other steroid metabolites. For these metabolites, structure determinations are required at concentration levels below that appropriate for NMR. To meet this need, we explored CRF with TOF/TOF mass spectrometry for two groups of steroid sulfates, 3-sulfates and 21-sulfates. We demonstrated that the current generation of MALDI TOF/TOF instruments can generate charge-remote fragmentations for these materials. The resulting collision-induced dissociation (CID) spectra are useful for positional isomer differentiation and very often allow the complete structure determination of the steroid. We also propose a new nomenclature that directly indicates the cleavage sites on the steroid ring with carbon numbers.

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)

This document contains recommendations for terminology in mass spectrometry. Development of standard terms dates back to 1974 when the IUPAC Commission on Analytical Nomenclature issued recommendations on mass spectrometry terms and definitions. In 1978, the IUPAC Commission on Molecular Structure and Spectroscopy updated and extended the recommendations and made further recommendations regarding symbols, acronyms, and abbreviations. The IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy’s Subcommittee on Mass Spectroscopy revised the recommended terms in 1991 and appended terms relating to vacuum technology. Some additional terms related to tandem mass spectrometry were added in 1993 and accelerator mass spectrometry in 1994. Owing to the rapid expansion of the field in the intervening years, particularly in mass spectrometry of biomolecules, a further revision of the recommendations has become necessary. This document contains a comprehensive revision of mass spectrometry terminology that represents the current consensus of the mass spectrometry community.

Delineating mechanisms of dissociation for isomeric heparin disaccharides using isotope labeling and ion trap tandem mass spectrometry

Heparin and heparan sulfate (HS) glycosaminoglycans have been identified as important players in many physiological as well as pathophysiological settings. A better understanding of the biosynthesis and structure of these molecules is critical for further elucidation of their biological function. We have demonstrated the successful use of negative electrospray ionization tandem mass spectrometry in the differentiation of all twelve standard heparin-building blocks, including the potentially important N-unsubstituted disaccharides. Collision induced dissociation of each of the isomeric disaccharides provided unique product ion spectra, useful for identification and quantification of the relative amounts of each isomer present. In the research presented herein, isotopic labeling studies using (18)O and (2)H were used to determine the origins of each of the neutral losses observed in the product ion spectra, and mechanisms of dissociation consistent with the observed data were postulated. The general mechanisms postulated were for the generation of B, Y, and Z ions formed from glycosidic cleavages, as well as A and X ions formed from cross-ring cleavages. The eight isomeric heparin disaccharides all underwent cross-ring cleavage to form (0,2)X(1) and (0,2)A(2) ions, and further experiments suggest that the mechanisms of formation of these ions are through a charge-remote process. The tandem mass spectrometry data presented herein also provide a foundation for further developments towards a practical analysis tool for the structural elucidation of larger, biologically important heparin/HS oligosaccharides by using mass spectrometry.

High-energy collision-induced dissociation of oxosteroids derivatised to Girard hydrazones

Neutral oxosteroids have been derivatised with Girard T and P hydrazine reagents to give the corresponding Girard hydrazone quaternary ammonium salts. Both Girard T (GT) and Girard P (GP) hydrazones of oxosteroids give very intense [M](+) ion signals in electrospray (ES) mass spectra and fragment within the ES interface and collision cell to give characteristic fragment ions. GT and GP derivatives give informative high-energy collision-induced dissociation spectra, from which the structure of the precursor oxosteroid can be determined. Both charge-remote and charge-mediated mechanisms are responsible for the formation of the fragment ions at high collision-energy.

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

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.

Clinical applications of tandem mass spectrometry: ten years of diagnosis and screening for inherited metabolic diseases

This paper reviews the clinical applications of tandem mass spectrometry (MS-MS) in diagnosis and screening for inherited metabolic diseases in the last 10 years. The broad-spectrum of diseases covered, specificity, ease of sample preparation, and high throughput provided by the MS-MS technology has led to the development of multi-disorder newborn screening programs in many countries for amino acid disorders, organic acidemias, and fatty acid oxidation defects. Issues related to sample acquisition, sample preparation, quantification of metabolites, and validation are discussed. Our current experience with the technique in screening is presented. The application of MS-MS in selective screening has revolutionized the field and made a major impact on the detection of certain disease classes such as the fatty acid oxidation defects. New specific and rapid MS-MS and LC-MS-MS methods for highly polar small molecules are supplementing or replacing some of the classical GC-MS methods for a multitude of metabolites and disorders. New exciting applications are appearing in fields of prenatal, postnatal, and even postmortem diagnosis. Examples for pitfalls in the technique are also presented.

Electrospray and tandem mass spectrometry in biochemistry

Over the last 20 years, biological MS has changed out of all recognition. This is primarily due to the development in the 1980s of 'soft ionization' methods that permit the ionization and vaporization of large, polar, and thermally labile biomolecules. These developments in ionization mode have driven the design and manufacture of smaller and cheaper mass analysers, making the mass spectrometer a routine instrument in the biochemistry laboratory today. In the present review the revolutionary 'soft ionization' methods will be discussed with particular reference to electrospray. The mass analysis of ions will be described, and the concept of tandem MS introduced. Where appropriate, examples of the application of MS in biochemistry will be provided. Although the present review will concentrate on the MS of peptides/proteins and lipids, all classes of biomolecules can be analysed, and much excellent work has been done in the fields of carbohydrate and nucleic acid biochemistry.

Applications and mechanisms of charge-remote fragmentation

Studies on the applications, energetics, and mechanisms of charge-remote fragmentations are reviewed, with emphasis given to those articles published after 1992. Independent of the charge status, charge-remote fragmentations are analogous to gas-phase thermolysis. Under collisional activation and with a fixed charge, ions containing long-chain or poly-ring structures undergo charge-remote fragmentations, generating productions that are structurally informative. Interpretation of the production spectra enables one to elucidate molecular structures. Although charge-remote fragmentations have been successfully used in the structural determination of fatty acids, phospholipids, glycolipids, triacylglycerols, steroids, peptides, ceramides, and other systems, the energetics and mechanisms of these reactions are still debated because none of the existing mechanisms can explain all the experimental data.

Electrospray and atmospheric pressure chemical ionization tandem mass spectrometric behavior of eight anabolic steroid glucuronides

Mass spectrometric and tandem mass spectrometric behavior of eight anabolic steroid glucuronides were examined using electrospray (ESI) and atmospheric pressure chemical ionization (APCI) in negative and positive ion mode. The objective was to elucidate the most suitable ionization method to produce intense structure specific product ions and to examine the possibilities of distinguishing between isomeric steroid glucuronides. The analytes were glucuronide conjugates of testosterone (TG), epitestosterone (ETG), nandrolone (NG), androsterone (AG), 5alpha-estran-3alpha-ol-17-one (5alpha-NG), 5beta-estran-3alpha-ol-17-one (5beta-NG), 17alpha-methyl-5alpha-androstane-3alpha,17beta-diol (5alpha-MTG), and 17alpha-methyl-5beta-androstane-3alpha,17beta-diol (5beta-MTG), the last four being new compounds synthesized with enzyme-assisted method in our laboratory. High proton affinity of the 4-ene-3-one system in the steroid structure favored the formation of protonated molecule [M + H]+ in positive ion mode mass spectrometry (MS), whereas the steroid glucuronides with lower proton affinities were detected mainly as ammonium adducts [M + NH4]+. The only ion produced in negative ion mode mass spectrometry was a very intense and stable deprotonated molecule [M - H]- . Positive ion ESI and APCI MS/MS spectra showed abundant and structure specific product ions [M + H - Glu]+, [M + H - Glu - H2O]+, and [M + H - Glu - 2H2O]+ of protonated molecules and corresponding ions of the ammonium adduct ions. The ratio of the relative abundances of these ions and the stability of the precursor ion provided distinction of 5alpha-NG and 5beta-NG isomers and TG and ETG isomers. Corresponding diagnostic ions were only minor peaks in negative ion MS/MS spectra. It was shown that positive ion ESI MS/MS is the most promising method for further development of LC-MS methods for anabolic steroid glucuronides.

Analysis of bile acids and bile alcohols in urine by capillary column liquid chromatography-mass spectrometry using fast atom bombardment or electrospray ionization and collision-induced dissociation

Solid-phase extraction and group separation by anion exchange chromatography were combined with capillary column liquid chromatography-mass spectrometry (LC/MS) to permit a thorough characterization of bile acids and intact conjugates of bile alcohols in human urine. Groups of compounds were separated according to acid strength and were analysed on a capillary column, 0.25 x 500 mm, packed with 5 microns particles of Chromasil C18, and connected via a fused silica capillary to the continuous-flow fast atom bombardment (CF-FAB) or electrospray (ES) sources of an AutoSpec-TOFFPD hybrid mass spectrometer. Acetonitrile:water mixtures containing 30 mM ammonium acetate pH 7.2 were used as mobile phases, with 5% glycerol added for FAB Ionisation. Bile acids were analysed directly or after derivatization of carboxyl groups with 4-aminobenzenesulphonic acid. Negative-ion spectra (m/z 1000 or 800 to 300 or 100) were recorded using the point detector or, in the case of ES ionization, the focal plane array detector (FPD). Deprotonated molecules of bile acids containing a sulphonic acid group were detected with a spectral signal to noise ratio of 5:1 when about 90 fmol were injected onto the column of the LC/CF-FAB system. The corresponding peak in the reconstructed ion chromatogram gave a signal-to-noise ratio of about 25:1. The sensitivity could be increased 20-50 times by using ES ionization and the FPD. Bile acids without a sulphonic acid group gave about 70% of the signal of sulphonic acids using ES ionization. The capillary column LC/MS systems were evaluated by analyses of urine from an infant with cholestatic liver disease. More than 150 different bile acids and bile alcohol conjugates were detected, some of which were partially characterized using collision induced dissociation (CID) of the deprotonated molecules and B/E linked scans. A number of compounds were detected for the first time, e.g. di-, tri-, and tetra-hydroxycholestanoic acids conjugated with N-acetylhexosamine and cholestenediol, cholestenetriol and cholestanetriol doubly conjugated with sulphuric acid and glucuronic acid. The relative merits of ES and FAB ionization are discussed.

Fast-atom bombardment mass spectrometry and low energy collision-induced tandem mass spectrometry of tauroconjugated bile acid anions

Fragmentation of negative ions produced by fast-atom bombardment (FAB) from 14 tauroconjugated bile acids and some of their deuterated analogs has been studied by mass spectrometry and by collision-induced dissociation (CED) tandem mass spectrometry at low energy.Low energy collision-induced dissociation of the deprotonated molecules [M - H](-) of these tauroconjugated bile acids leads to both charge-driven and charge-remote fragmentations (CRF). The former yields neutral loss from the side chain with charge migration during the fragmentation process. These fragments dominate the CID spectra, but are absent from the FAB spectra. Their relative abundances are dependent on the number and the positions of the hydroxyl groups in the steroid nucleus and thus permit distinction among some positional isomers.The CRF fragments correspond to cleavages in the side chain up to fragmentations across the steroid rings with charge retention on the sulfonate group. These CRF fragments, which also are useful for structural identification, are less intense in CID than in FAB spectra. It appears that these charge-remote fragments are favored by unsaturation in the steroid rings, either as keto groups or as endocyclic double bonds. Tandem mass spectrometry combined with the use of deuterated analogs demonstrates that the structures of the survivor pseudomolecular ions and of the CRF fragments are not rearranged.

Ion spray liquid chromatographic/mass spectrometric characterization of bile acids

The ion spray mass spectra of selected bile acid standards are presented. Full-scan positive ion spectra obtained in solutions of acetonitrile-water-trifluoroacetic acid and methanol-water-trifluoroacetic acid show the presence of intense [M + S + H]+ and [M + 2S + H]+ ions (where S is either methanol or acetonitrile, depending on the solvent system) in addition to [M + H]+ ions. The relative abundance of the protonated and solvent adduct ions are reproducible and unique for several of the bile acids, and provide a means of identifying isomeric bile salts. Adduct ions are most abundant for free acids and weaker for glycine and taurine conjugates. On-line ion spray liquid chromatography/mass spectrometry (LC/MS) using a 3 cm x 4.6 mm cartridge column containing 3 microns particle size C18 packing was utilized for the separation and identification of components of monkey bile. Eight common bile acid conjugates were separated in less than 8 min utilizing a water-acetonitrile gradient with 0.1% trifluoroacetic acid as the mobile-phase modifier. Full-scan data were acquired from 1 microliter of untreated bile injected on-column. Identification of component bile acids from the equivalent of 1 nl injected on-column was obtained with the use of a 1 mm diameter microbore column. Limits of detection for bile acid conjugates using a microbore column and selected ion monitoring (SIM) range from 40 to 100 fmol.

The use of negative ion thermospray liquid chromatography/tandem mass spectrometry for the determination of bile acids and their glycine conjugates

A study was carried out on the negative ion thermospray liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry of a group of bile acids and their glycine conjugates. The liquid chromatographic/tandem mass spectrometric experiments were performed using low-energy collisions on a triple-quadrupole mass spectrometer. It was found that relatively high collision energies and collision gas pressures were required to produce fragmentation and that some unusual fragments were produced.

Application of fast atom bombardment with tandem mass spectrometry and liquid chromatography/mass spectrometry to the analysis of acylcarnitines in human urine, blood, and tissue

Using a precursor-ion scan function on a triple quadrupole mass spectrometer, acylcarnitines were detected in the target matrices at or below concentrations of 1 nmol per gram by fast atom bombardment mass spectrometry. Acylcarnitine profiles from patients with known metabolic disorders were consistent with previously acquired data. Putative acylcarnitine signals were confirmed in one case by administration of stable isotope-labeled carnitine, which equilibrated rapidly with the endogenous pool. The addition of a continuous flow system enabled rapid sequential analysis without operator intervention, indicating the potential for automation of the analytical procedure. Incorporation of a micro-LC column enabled on-line liquid chromatographic/mass spectrometric analysis of selected patient samples. Large-scale screening and quantitative analysis of urine or blood for diagnostic acylcarnitines are now practicable.

Determination of pyrrolizidine alkaloid metabolites from mouse liver microsomes using tandem mass spectrometry and gas chromatography/mass spectrometry

The rapid and sensitive identification and quantification of important pyrrolizidine alkaloid metabolites using tandem mass spectrometry (MS/MS) and gas chromatography/mass spectrometry (GC/MS) is described. Identifications of N-oxide and hydrolytic metabolites of the pyrrolizidine alkaloids senecionine and monocrotaline in extracts of mouse hepatic microsomal incubations were accomplished by comparing collisionally activated decomposition/mass-analyzed ion kinetic energy spectra of specific ions from microsomal extracts with spectra obtained from synthetic standards of suspected metabolites. Trace amounts of the toxic metabolite dihydropyrrolizine (DHP) were observed by GC/MS of trimethylsilyl (TMS) derivatives, but the amounts present in hepatic microsomal extracts were below the MS/MS limit of detection. Quantitative determinations of senecionine N-oxide were performed by fast atom bombardment MS/MS. Suppression of N-oxide ionization by other substances in the extracts was judged to be minimal. The TMS derivatives of the metabolites senecic acid, monocrotalic acid and DHP were quantified using capillary GC/MS. Results from the study demonstrate that the relative contributions of the three major pathways of pyrrolizidine alkaloid metabolism (N-oxidation, hydrolysis and oxidation to pyrrolic compounds) can be assessed using a single analytical instrument and minimal sample preparation.

Fast atom bombardment and tandem mass spectrometry for structure determination: remote site fragmentation of steroid conjugates and bile salts

A series of 35 steroid conjugates (sulfates and glucuronides) and bile salts were investigated by using fast atom bombardment and tandem mass spectrometry. Collisional activation of the [M - H]- anions of sulfate conjugates and bile salts predominantly yields fragment ions arising by reactions occurring remote from the charge site. These reactions are sometimes sensitive to differences in stereochemistry at positions remote from the charge site and are useful for positional isomer differentiation. On the other hand, collisional activation of the [M - H]- anions of the glucouronide conjugates leads primarily to charge-driven fragementations.

A mass spectrometric approach to the identification of conjugated drug metabolites

For the identification of intact underivatized drug conjugates, the mass spectrometric technique of choice is fast atom bombardment (FAB); the combined use of both positive and negative ion FAB usually provides information on the molecular mass and nature of conjugates under study, while the number of exchangeable hydrogen atoms can be determined using trideuterated glycerol as the FAB-matrix. Electron impact and desorption chemical ionization spectra can be used to study the aglycone part of the conjugated metabolites. With this approach metabolites conjugated with glucuronic acid, sulphuric acid and amino acids have been identified. The identification was supported by analysis of reference compounds, prepared by chemical synthesis. The examples given are selected from current metabolism studies on drug candidates in development within Organon's research.

FAB MS/MS for phosphatidylinositol, -glycerol, -ethanolamine and other complex phospholipids

Fast atom bombardment (FAB) of phosphatidylinositol, phosphatidylethanolamine, cardiolipin, phosphatidic acid and phosphatidylglycerol produces a limited number of very informative negative ions. Especially significant is the formation of (M-H)-ions and ions that correspond to the carboxylate portions of these molecules. FAB desorption in combination with collisional activation allows for characterization of fragmentation and determination of structural features. Collisional activation of the carboxylate anion from complex lipids is especially informative. Structural characterization of the fatty acids can be achieved as the released saturated carboxylate anions undergo highly specific charge remote fragmentations that are entirely consistent with the chemistry of carboxylate anions desorbed from free fatty acids. This permits both identification of the modification and assignment of its location on the acid chain. FAB-desorbed alkyl acetyl glycerophosphocholines (platelet-activating factor) do not produce (M-H)-ions. However, significant high mass ions are formed, and these can be collisionally activated for structural characterization.