Fragmentation of diketopiperazines from Aspergillus fumigatus by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

Diketopiperazines (DKPs) corresponding to cyclic dipeptides have been reported to exhibit antimicrobial, antitumor, antimutagenic and antiviral properties. These compounds are commonly isolated from microorganisms and sponges and from a variety of tissues and body fluids. In this work, we used electrospray ionization tandem mass spectrometry (ESI-MS/MS) to investigate the fragmentation of a series of DKPs previously isolated from Aspergillus fumigatus, which exhibit the same structural core. Loss of CO directly from the protonated molecule was found to be a fragmentation process common to all the compounds analyzed. However, our results revealed a series of ions that are diagnostic for the substituents at C(4) and C(9). In order to rationalize the differences in the fragmentation pathways of substituted and nonsubstituted DKPs, the relative Gibbs energies (DeltaG) of the product ions and intermediate ions were estimated using the B3LYP/6-31 + + G(d,p) model. The data reported here can be used for the structural elucidation of DKPs from low sample amounts, as an alternative to NMR.     

Trace detection of inorganic oxidants using desorption electrospray ionization (DESI) mass spectrometry

Desorption electrospray ionization (DESI), an established ambient ionization method in mass spectrometry (MS) for the analysis of organic compounds, is applied here to trace detection of inorganic salts, including inorganic oxidants. In-situ surface analysis of targeted compounds, including nitrogen-, halogen- and sulfur-salts, down to sub-nanogram levels, was performed using DESI-MS. Successful experiments were carried out in both the negative and the positive ion modes; simple anions and cations as well as small cluster ions were observed. Various surfaces are examined and surface porosity effects were briefly explored. Absolute detection limits on porous polytetrafluoroethylene (PTFE) of 120 pg (surface concentration 0.07 ng mm⁻²) and 50 pg (surface concentration 0.03 ng mm⁻²), were achieved for sodium chlorate and sodium perchlorate, respectively. The compounds of interest were examined in the presence of a hydrocarbon mixture to assess matrix effects: only a two- or three-fold decrease in the target ion intensity was observed. Commercial fireworks were analyzed to determine perchlorate salts in complex mixtures. This work demonstrates the potential applicability of ambient ionization mass spectrometry to forensic investigations involving improvised explosives.      

Forensic analysis of inks by imaging desorption electrospray ionization (DESI) mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is employed in the forensic analysis of documents. Blue ballpoint pen inks applied to ordinary writing paper are examined under ambient conditions without any prior sample preparation. When coupled to an automated moving stage, two-dimensional molecular images are generated. Proof-of-principle experiments include characterization of a simulated forged number and examination of older written records. This application of DESI has advantages over extractive techniques in terms of speed and sample preservation. The effects of the desorbing solvent composition, in this case a mixture of methanol and water, and of flow rate, are evaluated. Results suggest that the solubility of the analyte (dyes Basic Blue 7, Basic Violet 3 and Solvent Blue 26) plays an important role in desorption from the paper surface.     

Coupling of liquid chromatography with mass spectrometry by desorption electrospray ionization (DESI)

A liquid chromatography/mass spectrometry (LC/MS) method using desorption electrospray ionization (DESI) as a versatile interface has been established, which allows a wide range of elution flow rates, online derivatization via reactive DESI and further combination with electrochemistry.     

Identification and fragmentation of hydrolyzed aluminum species by electrospray ionization tandem mass spectrometry

Earlier characterization of some hydrolysis products of AlCl₃·6H₂O was confirmed by electrospray ionization tandem mass spectrometry with increasing collision energy of projectile ions. At lower collision energies, the aqua ligands were stripped off. At higher energies, two hydroxo groups formed a bridging oxo group with loss of one water molecule. Aluminum complexes could also capture aqua ligands in the collision chamber so long as the parent ion did not fragment, and the fragment ion spectra broadened toward higher m/z values. The chloro ligands were eliminated as hydrochloric acid. The aluminum cores remained highly intact. Copyright © 2004 John Wiley & Sons, Ltd.     

Structural determination of picomole amounts of phospholipids via electrospray ionization tandem mass spectrometry

The remarkable sensitivity of electrospray ionization was exploited to achieve great increases in the sensitivity of tandem mass spectrometric analyses of phospholipids derived from both synthetic and biologic sources. Herein, we demonstrate that (1) product-ion spectra after electrospray ionization can be obtained easily by utilizing ≤ 5 pmol of phospholipid with a mass-selected window of less than 2 mass units, (2) the low energy inherent in the electrospray ionization method facilitates analysis of labile molecular ions that are not easily detected with the relatively high energy employed during fast-atom bombardment desorption, and (3) collision-induced dissociation of precursor ions generated from electrospray ionization often resulted in novel product-ion patterns. Collectively, these results underscore the utility of electrospray ionization tandem mass spectroscopy for the structural determination of diminutive amounts of phospholipids.     

Effects of an arginine-selective tagging procedure on the fragmentation behavior of peptides studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

The presence of arginine as the naturally occurring amino acid with the highest gas-phase basicity strongly influences the fragmentation behavior of peptides undergoing collision-induced dissociation. Using a derivatization procedure recently developed in our group, based on a reversible reaction of the guanidino group with 2,3-butanedione and an arylboronic acid, we examined how this label affects the fragmentation patterns of labeled versus unlabeled peptides in MS/MS experiments. As part of this fundamental study, two groups of model peptides (angiotensins and bradykinins) as well as tryptic peptides were labeled according to our protocol and subjected to collision-induced dissociation (CID) in both a triple quadrupole and a quadrupole ion trap instrument. It was found that for angiotensins containing an AspArg sequence, C-terminal cleavage at Asp that occurs for native peptides was completely inhibited in Arg-labeled peptides. For bradykinins and peptides obtained from tryptic digests of standard proteins, some sample peptides were little affected by the tagging of arginine residues. Others, in contrast, exhibited an almost total loss of nonspecific backbone cleavage and their fragment ion spectra were dominated by loss of the arginine tag. These and other experimental results are discussed in view of the nature of the arginine tag and the concept of proton mobility.     

Desorption electrospray ionization mass spectrometry of intact bacteria

Desorption electrospray ionization (DESI) mass spectrometry (MS) was used to differentiate seven bacteria species on the basis of their measured DESI-mass spectral profile. Both gram-positive and gram-negative bacteria were tested and included Escherichia coli, Staphyloccocus aureus, Enterococcus sp., Bordetella bronchiseptica, Bacillus thuringiensis, Bacillus subtilis and Salmonella typhimurium. Distinct DESI-mass spectra, in the mass range of 50-500 u, were obtained from whole bacteria in either positive or negative ion modes in less than 2 mins analysis time. Positive ion DESI-mass spectral fingerprints were compared using principal components analysis (PCA) to investigate reproducibility for the intraday and the day-to-day measurements and the method selectivity to differentiate the bacteria studied. Detailed study of variances in the assay revealed that a large contribution to the DESI-mass spectral fingerprint variation was the growth media preparation procedure. Specifically, experiments conducted with the growth media prepared using the same batch yielded highly reproducible DESI-mass spectra, both in intraday and in day-to-day analyses (i.e. one batch of growth media used over a 3-day period versus a new batch every day over the same 3-day period). Conclusions are drawn from our findings in terms of strategies for rapid biodetection with DESI-MS.     

Ionization of anabolic steroids by adduct formation in liquid chromatography electrospray mass spectrometry

The ionization of 46 anabolic steroids has been studied. The absence of basic or acidic moieties in most of these analytes makes their direct ionization as [M + H]+ by atmospheric pressure interfaces difficult. The formation of adducts with different components of the mobile phase has been found to be an efficient way to ionize anabolic steroids by electrospray. Different mobile phases using methanol (MeOH) or acetonitrile as organic solvent and HCOOH, Na+ or NH4+ as additives have been tested to favor the adduct formation. A direct correlation between the chemical structure of the anabolic steroid and the possibility to ionize it in a particular chromatographic condition has been found. According to their ionization, anabolic steroids can be divided into seven different groups depending on both the nature and the relative position of their functional groups. The formation of different adducts such as [M + Na + MeOH]+ or [M + H + CH3 CN - H2O]+ is required in order to ionize some of these groups and the optimal mobile phase composition for each group of anabolic steroids is proposed. Despite the ionization limitations due to their chemical structure, most of tested anabolic steroids could be ionized using the adduct formation approach.     

Characteristic fragmentation behavior of some glucuronide-type triterpenoid saponins using electrospray ionization tandem mass spectrometry

The fragmentation behavior of some glucuronide-type triterpenoid saponins from Symplocos chinensis, and their analogues escin Ia and Ib, were investigated by positive ion electrospray ionization tandem mass spectrometry using a quadrupole linear ion trap mass spectrometer. The fragmentation patterns of these saponins significantly changed in accordance with structural variations in the glucuronyl residue of the oligosaccharide chain. It was found that the carboxyl group and hydroxyl group at the C-3' position of the glucuronyl residue were the key sites for determining the fragmentation behavior of these compounds. When the carboxyl group was esterified, only the C(2alpha) ion, and no B(2alpha) ion, and cationized aglycone were observed. When the hydroxyl group at C-3' was acylated, the inherent cross-ring cleavage was hindered. However, glycosidic cleavages always occurred, regardless of the crucial structural variations. The results of the present studies can benefit the determination of trace triterpenoid saponins of this type in crude plant extracts, and also provide background information to aid the structural investigations of similar glycoconjugates.     

Metal ion complexes in the structural analysis of phospholipids by electrospray ionization tandem mass spectrometry

The effects of metal cationization on collisionally activated dissociation (CAD) of phospholipids were investigated by electrospray ionization with quadrupole ion trap tandem mass spectrometry. The metal ions include Li(+), Na(+), K(+), Sr(2+), Ba(2+), and the first transition series. CAD of the transition metal ion-bound lipid complexes gave significant yields of product ions that identify the positions of the two fatty acyl substituents on the glycerophospholipid backbone. The cobalt(II) ion, which has a single naturally occurring isotope, was expected to be a better cationization reagent as it produces simpler mass spectra than other transition metal ions. CAD of the cobalt(II) ion complexes of glycerophosphoethanolamines, glycerophosphoglycerols and glycerophosphoserines yielded product ions that revealed information regarding both the lipid classes and the regiospecific positions of the two fatty acyl substituents.     

A shotgun tandem mass spectrometric analysis of phospholipids with normal-phase and/or reverse-phase liquid chromatography/electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry is used in lipidomics studies. The present research established a top-down liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) shotgun analysis method for phospholipids (PLs) using a normal-phase column or a C30 reverse-phase column with the data-dependent MS/MS scanning mode. A normal-phase column can separate most of the major different classes of PLs. By using LC/ESI-MS/MS with a normal-phase column, approximately 50 molecular species were identified in a PL mixture from rat liver. When the reverse-phase column was used, the PLs could be separated depending on their hydrophobicity, essentially the length of their fatty acyl chains and the number of unsaturated bonds in them. The LC/ESI-MS/MS method using a C30 reverse-phase column was applied to phosphatidylcholine (PC) and phosphatidylethanolamine (PE) mixtures as test samples. Molecular species with the same molecular mass but with different pairs of fatty acyl chains were separately identified. As a result, about 60 PC and 50 PE species were identified. PLs from rat liver were subjected to LC/ESI-MS/MS using the C30 reverse-phase column and about 110 molecular species were identified. Off-line two-dimensional LC/ESI-MS/MS with the normal-phase and C30 reverse-phase columns allowed more accurate identification of molecular species by using one-dimensional C30 reverse-phase LC/ESI-MS/MS analysis of the collected fractions.     

Derivatization reagents in liquid chromatography/electrospray ionization tandem mass spectrometry

Liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) is one of the most prominent analytical techniques owing to its inherent selectivity and sensitivity. In LC/ESI-MS/MS, chemical derivatization is often used to enhance the detection sensitivity. Derivatization improves the chromatographic separation, and enhances the mass spectrometric ionization efficiency and MS/MS detectability. In this review, an overview of the derivatization reagents which have been applied to LC/ESI-MS/MS is presented, focusing on the applications to low molecular weight compounds.     

Desorption electrospray ionization mass spectrometric analysis of organophosphorus chemical warfare agents using ion mobility and tandem mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI‐MS) has been applied to the direct analysis of sample media for target chemicals, including chemical warfare agents (CWA), without the need for additional sample handling. During the present study, solid‐phase microextraction (SPME) fibers were used to sample the headspace above five organophosphorus CWA, O‐isopropyl methylphosphonofluoridate (sarin, GB), O‐pinacolyl methylphosphonofluoridate (soman, GD), O‐ethyl N,N‐dimethyl phosphoramidocyanidate (tabun, GA), O‐cyclohexyl methylphosphonofluoridate (cyclohexyl sarin, GF) and O‐ethyl S‐2‐diisopropylaminoethyl methyl phosphonothiolate (VX) spiked into glass headspace sampling vials. Following sampling, the SPME fibers were introduced directly into a modified ESI source, enabling rapid and safe DESI of the toxic compounds. A SYNAPT HDMS? instrument was used to acquire time‐aligned parallel (TAP) fragmentation data, which provided both ion mobility and MSⁿ (n = 2 or 3) data useful for the confirmation of CWA. Unique ion mobility profiles were acquired for each compound and characteristic product ions of the ion mobility separated ions were produced in the Triwave? transfer collision region. Up to six full scanning MSⁿ spectra, containing the [M + H]⁺ ion and up to seven diagnostic product ions, were acquired for each CWA during SPME fiber analysis. A rapid screening approach, based on the developed methodology, was applied to several typical forensic media, including Dacron sampling swabs spiked with 5 µg of CWA. Background interference was minimal and the spiked CWA were readily identified within one minute on the basis of the acquired ion mobility and mass spectrometric data. Copyright © 2010 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.     

On-probe pyrolysis desorption electrospray ionization (DESI) mass spectrometry for the analysis of non-volatile pyrolysis products

An on-probe pyrolyzer has been constructed and interfaced with desorption electrospray ionization (DESI) mass spectrometry (MS) for the rapid analysis of non-volatile pyrolysis products. The detection and analysis of non-volatile pyrolysis products of peptides, proteins and the synthetic polymer poly(ethylene glycol) were demonstrated with this instrument. The on-probe pyrolyzer can be operated off-line or on-line with the DESI source and was interfaced with a tandem MS (MS/MS) instrument, which allowed for structure characterization of the non-volatile pyrolytic products. Advantages of this system are its simplicity and speed of analysis since the pyrolysis is performed in situ on the DESI source probe and hence, it avoids extraction steps and/or the use of matrices (e.g., as in MALDI–MS analyses).     

Development of capabilities for imaging mass spectrometry under ambient conditions with desorption electrospray ionization (DESI)

Aspects of the development of mass spectrometry over the past three decades are briefly reviewed and growth points in the subject are identified. Molecular imaging by mass spectrometry is one such growth area. The development of a capability for 2D chemical imaging of surfaces is described, based on the combination of a desorption electrospray ionization (DESI) ion source with an automated surface stage capable of x, y translational motion. The lateral resolution of this new system is found to be less than 200 microns, using a test ink pattern. Chemical imaging of surfaces is demonstrated using model examples of organic and biological systems: (i) imaging of a 2D pattern written in different colored inks on photographic paper and (ii) imaging of thin coronal sections of rat brain tissue fixed onto a glass microscope slide. In both cases, full mass spectra are recorded as a function of x,y-position on the surface. In the chemical imaging example, the distributions of the two different inks on the paper surface were mapped by tracking the abundance of the intact organic cation which characterizes each particular ink dye. In the tissue imaging example, distributions of specific lipids in coronal sections of rat brain tissue were followed from the abundance distributions in 2D space of the deprotonated lipid molecules recorded in the negative ion mass spectra. These latter distributions reveal distinct anatomical features of the rat brain. The results of these studies demonstrate the feasibility of performing surface imaging studies using DESI and show that at this stage of its development it has a lateral spatial resolution of a few hundred microns.     

Differentiation of diiodothyronines using electrospray ionization tandem mass spectrometry

Diiodothyronines 3,5-diiodothyronine (3,5-T2), 3',5'-diiodothyronine (3',5'-T2), and 3,3'-diiodothyronine (3,3'-T2) are important metabolites of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3; reverse T3). In this paper, a novel and rapid method for identifying and quantifying 3,5-T2, 3',5'-T2 and 3,3'-T2 has been introduced using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Fragmentation patterns were proposed on the basis of our data obtained by ESI-MS/MS. MS2 spectra in either negative ionization mode or positive ionization mode can be used to differentiate 3,5-T2, 3',5'-T2 and 3,3'-T2. On the basis of the relative abundance of fragment ions in MS2 spectra under the positive ionization mode, quantification of the 3,5-T2, 3',5'-T2 and 3,3'-T2 isomers in mixtures is also achieved without prior separation.