Dissociation Tandem Mass Spectrometry Research Articles

Direct Experimental Characterization of Glycosyl Cations by Infrared Ion Spectroscopy.

Glycosyl cations are crucial intermediates formed during enzymatic and chemical glycosylation. The intrinsic high reactivity and short lifetime of these reaction intermediates make them very challenging to characterize using spectroscopic techniques. Herein, we report the use of collision induced dissociation tandem mass spectrometry to generate glycosyl cations in the gas phase followed by infrared ion spectroscopy using the FELIX infrared free electron laser. The experimentally observed IR spectra were compared to DFT calculated spectra enabling the detailed structural elucidation of elusive glycosyl oxocarbenium and dioxolenium ions.

Characterization of Isomeric Glycans by Reversed Phase Liquid Chromatography-Electronic Excitation Dissociation Tandem Mass Spectrometry.

The occurrence of numerous structural isomers in glycans from biological sources presents a severe challenge for structural glycomics. The subtle differences among isomeric structures demand analytical methods that can provide structural details while working efficiently with on-line glycan separation methods. Although liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a powerful tool for mixture analysis, the commonly utilized collision-induced dissociation (CID) method often does not generate a sufficient number of fragments at the MS2 level for comprehensive structural characterization. Here, we studied the electronic excitation dissociation (EED) behaviors of metal-adducted, permethylated glycans, and identified key spectral features that could facilitate both topology and linkage determinations. We developed an EED-based, nanoscale, reversed phase (RP)LC-MS/MS platform, and demonstrated its ability to achieve complete structural elucidation of up to five structural isomers in a single LC-MS/MS analysis. Graphical Abstract.

Selection of Collision Energies in Proteomics Mass Spectrometry Experiments for Best Peptide Identification: Study of Mascot Score Energy Dependence Reveals Double Optimum.

Collision energy is a key parameter determining the information content of beam-type collision induced dissociation tandem mass spectrometry (MS/MS) spectra, and its optimal choice largely affects successful peptide and protein identification in MS-based proteomics. For an MS/MS spectrum, quality of peptide match based on sequence database search, often characterized in terms of a single score, is a complex function of spectrum characteristics, and its collision energy dependence has remained largely unexplored. We carried out electrospray ionization-quadrupole-time of flight (ESI-Q-TOF)-MS/MS measurements on 2807 peptides from tryptic digests of HeLa and E. coli at 21 different collision energies. Agglomerative clustering of the resulting Mascot score versus energy curves revealed that only few of them display a single, well-defined maximum; rather, they feature either a broad plateau or two clear peaks. Nonlinear least-squares fitting of one or two Gaussian functions allowed the characteristic energies to be determined. We found that the double peaks and the plateaus in Mascot score can be associated with the different energy dependence of b- and y-type fragment ion intensities. We determined that the energies for optimum Mascot scores follow separate linear trends for the unimodal and bimodal cases with rather large residual variance even after differences in proton mobility are taken into account. This leaves room for experiment optimization and points to the possible influence of further factors beyond m/ z.

Methyl group transfer upon gas phase decomposition of protonated methyl benzoate and similar compounds.

Gas phase decompositions of protonated methyl benzoate and its conjugates have been studied by using electrospray ionization-collision induced dissociation-tandem mass spectrometry. Loss of CO2 molecule, thus transfer of methyl group, has been observed. In order to better understand this process, the theoretical calculations have been performed. For methyl benzoate conjugates, it has been found that position of substituent affects the loss of CO2 molecule, not the electron donor/withdrawing properties of the substituent. Therefore, electrospray ionization-mass spectrometry in positive ion mode may be useful for differentiation of isomers of methyl benzoate conjugates.

Roles of Macrophage Exosomes in Immune Response to Calcium Oxalate Monohydrate Crystals.

In kidney stone disease, macrophages secrete various mediators via classical secretory pathway and cause renal interstitial inflammation. However, whether their extracellular vesicles, particularly exosomes, are involved in kidney stone pathogenesis remained unknown. This study investigated alterations in exosomal proteome of U937-derived macrophages (by phorbol-12-myristate-13-acetate activation) after exposure to calcium oxalate monohydrate (COM) crystals for 16-h using 2-DE-based proteomics approach. Six significantly altered proteins in COM-treated exosomes were successfully identified by nanoscale liquid chromatography–electrospray ionization–electron transfer dissociation tandem mass spectrometry as proteins involved mainly in immune processes, including T-cell activation and homeostasis, Fcγ receptor-mediated phagocytosis, interferon-γ (IFN-γ) regulation, and cell migration/movement. The decreased heat shock protein 90-beta (HSP90β) and increased vimentin were confirmed by Western blotting. ELISA showed that the COM-treated macrophages produced greater level of interleukin-1β (IL-1β), one of the markers for inflammasome activation. Functional studies demonstrated that COM-treated exosomes enhanced monocyte and T-cell migration, monocyte activation and macrophage phagocytic activity, but on the other hand, reduced T-cell activation. In addition, COM-treated exosomes enhanced production of proinflammatory cytokine IL-8 by monocytes that could be restored to its basal level by small-interfering RNA targeting on vimentin (si-Vimentin). Moreover, si-Vimentin could also abolish effects of COM-treated exosomes on monocyte and T-cell migration as well as macrophage phagocytic activity. These findings provided some implications to the immune response during kidney stone pathogenesis via exosomal pathway of macrophages after exposure to COM crystals.

Gas-phase relative stabilities of Tl+-crown ether complexes and Rb+-crown ether complexes.

The gas-phase stabilities of Tl+-crown ether complexes and Rb+-crown ether complexes were studied using the electrospray ionization-collision-induced dissociation-tandem mass spectrometry. Tl+ and Rb+ have identical ionic radii, thus a comparison of the properties of the crown ether complexes with these two cations seems to be justified. The selected crown ethers were 12C4 (it has a cavity smaller than the cation radius), 18C6 (it has a cavity of size similar to the cation radius), 24C8 (it has a cavity greater than the cation radius) and their conjugates. It has been found that the crown ether complexes of stoichiometry 1:1 with Tl+ are more or equally stable in the gas phase than the crown ether complexes with Rb+. However, 2:1 complexes with Tl+ are less stable than the complexes with Rb+.

Structural investigation by tandem mass spectrometry analysis of a heterogeneous mixture of Lipid An isolated from the lipopolysaccharide of Aeromonas hydrophila SJ-55Ra.

We report herein the electrospray ionization mass spectrometry (ESI-MS) negative ion mode and low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) analysis of a mixture of lipid An isolated from the lipopolysaccharide (LPS) of a rough-resistant wild strain of the Gram-negative bacteria Aeromonas hydrophila grown in the presence of phages (SJ-55Ra). This investigation indicates that the presence of a mixture of lipid A acylated disaccharides, whose molecular structures were not relatively conserved, resulted from the incomplete LPS biosynthesis caused by the phage treatment. The heterogeneous lipid An mixture from the LPS-SJ55Ra was obtained following growth of the Gram-negative bacteria Aeromonas hydrophila (SJ-55R) in the presence of phages and isolation by the aqueous phenol method. Following hydrolysis and purification of the lipopolysaccharide, ESI-MS and low-energy CID-MS/MS analyses were performed on a triple-quadrupole (QqQ) and a Fourier transform ion cyclotron resonance (FTICR) instrument. ESI-MS analysis suggested that this lipid An mixture contained eight molecular disaccharide anions and three monosaccharide anions. This series of lipid An was asymmetrically substituted with ((R)-14:0(3-OH)) fatty acids located at O-3 and N-2 and with branched fatty acids: (Cl4:0(3-(R)-O-C14:0)) and (C12:0(3-(R)-O-(14:0)) at the O-3' and N-2' positions. Tandem mass spectrometric analyses allowed the exact determination of the fatty acid acylation locations on the D-GlcpN disaccharide. The MS/MS results established that it was possible to selectively cleave C-O, C-N, and C-C bonds, together with glycosidic C-O and cross-ring cleavages, affording excellent structural analysis of lipid A biomolecules.

Influence of the metals and ligands in dinuclear complexes on phosphopeptide sequencing by electron-transfer dissociation tandem mass spectrometry.

Phosphorylation is one of the most important protein modifications, and electron-transfer dissociation tandem mass spectrometry (ETD-MS/MS) is a potentially useful method for the sequencing of phosphopeptides, including determination of the phosphorylation site. Notably, ETD-MS/MS typically provides useful information when the precursor contains more than three positive charges. It is not yet used as an analysis method for large-scale phosphopeptide production due to difficulties occurring in the production of acidic phosphopeptides having more than three positive charges. To increase the charge state of phosphopeptides, we used dinuclear metal complexes, which selectively bind to the phosphate group in phosphopeptides with the addition of positive charge(s). Dinuclear copper, zinc, and gallium complexes were tested and it was found that the type of metal present in the complex strongly affected the affinity of the phosphorylated compounds and their ETD fragmentation. The dinuclear copper complex interacted weakly with the phosphate groups and ETD-induced peptide fragmentation was largely suppressed by the presence of Cu2+, which worked as an electron trap. The dinuclear gallium complex was strongly bound to a phosphate group. However, the ligand binding to gallium acted as an electron trap and the presence of dinuclear gallium complex in the precursor for ETD-MS/MS hampered the sequencing of the phosphopeptides, as in the case of dinuclear copper complexes. In contrast, dinuclear zinc complexes efficiently bind to phosphopeptides with an increase in the charge state, facilitating phosphopeptide sequencing by ETD-MS/MS. The fragmentation of the ligand and peptide backbone in the dinuclear zinc-phosphopeptide complex were competitively induced by ETD. These processes are influenced by the ligand structure and so the detailed ETD fragmentation pathways were investigated using density functional theory calculations.

Homolytic Scission as the Main Pathway in the Liquid Chromatography/ Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry of Mesotrione and its Photoproducts

The indirect photochemical degradation of mesotrione induced by humic substances was studied by excitation with a solar light simulator. The disappearance rate constant increased by increasing the concentration of molecular oxygen. Three main byproducts were observed. They all arise from the dissociation of the non- aromatic cycle. Collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of the deprotonated molecule was carried out and the effect of the collision energy as well as the elemental compositions of the products ions was used to propose the chemical structures. The presence in the chemical structure of the SO2-Me moiety permitted several types of homolytic scissions leading to the formation of radical anions.

Four unreported types of glycans containing mannose-6-phosphate are heterogeneously attached at three sites (including newly found Asn 233) to recombinant human acid alpha-glucosidase that is the only approved treatment for Pompe disease

Myozyme is a recombinant human acid alpha-glucosidase (rhGAA) that is currently the only drug approved for treating Pompe disease, and its low efficacy means that a high dose is required. Mannose-6-phosphate (M6P) glycosylation on rhGAA is a key factor influencing lysosomal enzyme targeting and the efficacy of enzyme replacement therapy (ERT); however, its complex structure and relatively small quantity still remain to be characterized. This study investigated M6P glycosylation on rhGAA using liquid chromatography (LC)–electrospray ionization (ESI)–high-energy collisional dissociation (HCD) tandem mass spectrometry (MS/MS). The glycans released from rhGAA were labeled with procainamide to improve mass ionization efficiency and the sensitivity of MS/MS. The relative quantities (%) of 78 glycans were obtained, and 1.0% of them were glycans containing M6P (M6P glycans). These were categorized according to their structure into 4 types: 3 newly found ones, comprising high-mannose-type M6P glycans capped with N-acetylglucosamine (GlcNAc) (2 variants, 17.5%), hybrid-type M6P glycans (2 variants, 11.2%), and hybrid-type M6P glycans capped with GlcNAc (3 variants, 6.9%), as well as high-mannose-type M6P glycans (3 variants, 64.4%). HCD-MS/MS spectra identified six distinctive M6P-derived oxonium ions. The glycopeptides obtained from protease-digested rhGAA were analyzed using nano-LC-ESI-HCD-MS/MS, and the extracted-ion chromatograms of M6P-derived oxonium ions confirmed three M6P glycosylation sites comprising Asn 140, Asn 233 (newly found), and Asn 470 attached heterogeneously to nine M6P glycans (two types), eight M6P glycans (four types), and seven M6P glycans (two types), respectively. This is the first study of rhGAA to differentiate M6P glycans and identify their attachment sites, despite rhGAA already being an approved drug for Pompe disease.

PH-Dependent Singlet O2 Oxidation Kinetics of Guanine and 9-Methylguanine: An Online Mass Spectrometry and Spectroscopy Study Combined with Theoretical Exploration.

We report a kinetic and mechanistic study on the title reactions, in which 1O2 was generated by the reaction of H2O2 with Cl2 and bubbled into an aqueous solution of guanine and 9-methylguanine (9MG) at different pH values. Oxidation kinetics and product branching ratios were measured using online electrospray ionization mass spectrometry coupled with absorption and emission spectrophotometry, and product structures were determined by collision-induced dissociation (CID) tandem mass spectrometry. Experiments revealed strong pH dependence of the reactions. The oxidation of guanine is noticeable only in basic solution, while the oxidation of 9MG is weak in acidic solution, increases in neutral solution, and becomes intensive in basic solution. 5-Guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) were detected as the major oxidation products of guanine and 9MG, and Sp became dominant in basic solution. A reaction intermediate was captured in mass spectra, and assigned to gem-diol on the basis of CID measurements. This intermediate served as the precursor for the formation of Gh. After taking into account solution compositions at each pH, first-order oxidation rate constants were extracted for individual species: that is, 3.2-3.6 × 107 M-1 s-1 for deprotonated guanine, and 1.2 × 106 and 4.6-4.9 × 107 M-1 s-1 for neutral and deprotonated 9MG, respectively. Guided by approximately spin-projected density-functional-theory-calculated reaction potential energy surfaces, the kinetics for the initial 1O2 addition to guanine and 9MG was evaluated using transition state theory (TST). The comparison between TST modeling and experiment confirms that 1O2 addition is rate-limiting for oxidation, which forms endoperoxide and peroxide intermediates as determined in previous measurements of the same systems in the gas phase.

The structural features of the sulfated heteropolysaccharide (ST-1) from Sargassum thunbergii and its neuroprotective activities

Polysaccharide (ST) was prepared from Sargassum thunbergii using hot water. Two fractions (ST-1 and ST-2) were prepared using anion exchange chromatography. One desulfated polysaccharide (ST-1-DS) was also prepared. Electrospray ionization mass spectrometry (ESI–MS) performed on ST-1-DS showed that the desulfated polysaccharides contained methyl glycosides of mono-sulfated and di-sulfated galacto-fucooligosaccharides. This result suggested that ST-1 might contain sulfated galactofucan, which consists of a backbone of alternating (Gal)n and (Fuc)n and sulfated randomly on Gal and mainly on C-2 in Fuc. In addition, ST-1 was degraded in 1M sulfuric acid. The solution was centrifuged, and the supernatant was concentrated and precipitated in ethanol to obtain the precipitate (ST-1-P). ST-1-P was then separated using gel chromatography and anion exchange chromatography to obtain the oligomers. ESI–MS spectra of oligomers indicated that ST-1 mostly contained sulfated glucuronomannan and fucoglucuronan. ESI–MS with collision-induced dissociation tandem mass spectrometry (ESI-CID-MS/MS) suggested that glucuronomannan contained alternating 2-linked Man and 4-linked GlcA, while fucoglucuronan contained 4-linked glucuronan with branched Fuc at C-3. Finally, the neuroprotective activities of ST, ST-1, ST-2 and MIX (a mixture of ST-1 and ST-2) were determined. ST showed the most neuroprotective activity, which indicated that ST might be a good candidate for curing neurodegenerative diseases.

Does low-energy collision-induced dissociation of lithiated and sodiated carbohydrates always occur at anomeric carbon of the reducing end?

Collision-induced dissociation (CID) tandem mass spectrometry is one of the major methods in the structural determination of carbohydrates. Previous experimental studies and theoretical investigation of lithiated and sodiated underivatized carbohydrates seem to indicate that dehydration reactions and cross-ring dissociation of low-energy CID mainly occur at the anomeric carbon of the reducing end. However, these studies only investigated a few carbohydrates. ESI-MS/MS spectra of [M+Li]+ and [M+Na]+ ions of several 18 O1-labeled monosaccharides and disaccharides at O1 of the reducing end were studied using a linear ion trap mass spectrometer. Dissociations from the losses of both labeled and unlabeled neutral fragments were observed. The branching ratios of dissociations from the losses of unlabeled neutrals for dehydration reactions are larger than that for cross-ring dissociation; lithiated carbohydrates are larger than sodiated carbohydrates, and 1-4 linkages of disaccharides are larger than the other linkages. For some lithiated carbohydrates, dehydration reactions from the losses of unlabeled neutrals have larger branching ratios than that from the losses of labeled neutrals. The fragments from the losses of unlabeled neutrals investigated using MS3 showed that the losses of unlabeled H2 O mainly occur at the reducing monomer for sodiated carbohydrates, but the losses of unlabeled C2 H4 O2 for lithiated carbohydrates can occur at both reducing and nonreducing monomers. The ratio of B1 and Y1 ions to C1 and Z1 ions of disaccharides is related to the cis or trans configuration of the O1 and O2 atoms in the nonreducing monomer. The results are explained by the properties of transition states of dissociation channels. Our data shows that dehydration reactions and cross ring dissociation do not always occur at the anomeric carbon atom of the reducing monomer.

Curvulin and spirostaphylotrichins R and U from extracts produced by two endophytic Bipolaris sp. associated to aquatic macrophytes with antileishmanial activity

In the present study, biological activity and chemical composition of two crude extracts of endophytic fungal strains of Bipolaris genera isolated from two species of aquatic macrophytes: Eichhornia azurea (Kunth) and Eichhornia crassipes (Mart.) were investigated. The nuclear magnetic resonance and mass spectrometry data provided the identification of three main compounds: curvulin (1), spirostaphylotrichin R (2) and U (3). The fragmentation mechanism of the precursor ions towards collision induced dissociation (CID) tandem mass spectrometry experiment (MS/MS) is also proposed. Furthermore, biological screening of the crude extracts displayed antileishmanial activity with IC50 values ranging from 70–84.2 μg.mL−1.

Formation of organometallic species from complexes of N-phenylaza-crown ether conjugates with lead cations in CID-MS/MS conditions

Gas phase decomposition of the ions [M+PbNO3]+ (M stands for fluoro-substituted N-nitrophenylaza-crown ether conjugate), has been studied by the electrospray ionization-collision induced dissociation-tandem mass spectrometry (ESI-CID-MS/MS). The conjugates containing hydrogen atom at ortho position to the crown moiety, form abundant fragment ions which can be assigned as complexes of deprotonated conjugates and lead cation, thus the ions [M−H+Pb]+ (fragment ions formed as a result of HNO3 loss). Interaction between the nitro group (present at the second ortho position to the crown moiety) and the metal cation affects the formation of fragment ion [M−H+Pb]+ (but does not prevent its formation). Ions [M−H+Pb]+ are very rare example of the organometallic species (containing metal-carbon bond), in which metal cation is also complexed by crown ether moiety.

Scorpionate complexes of aza-18-crown-6 containing fluoronitrophenyl substituents as studied by electrospray ionization mass spectrometry.

Coordination of a metal cation by a nitro group is rare and the interaction between them is usually weak. Examples of complexes in which such an interaction takes place are metal complexes of cyclams containing nitrophenyl or nitrobenzyl substituents. It seemed to be interesting to check if in the complexes of the respective crown ether conjugates the interaction can also take place. We synthesized the respective conjugates of aza-18-crown-6, namely fluoro-substituted N-nitrophenylaza-18C6 conjugates. Their complexes with alkali and alkali earth metal cations were generated in the gas phase by electrospray ionization mass spectrometry (ESI-MS). Both collision-induced dissociation 'in-source' and collision-induced dissociation tandem mass spectrometry (ESI-CID-'in-source' and ESI-CID-MS/MS) were used to study the gas-phase stabilities of the generated complexes. The gas-phase decomposition of the studied complexes indicated that the complexes formed by the conjugates containing a nitro group at the ortho position are more stable than those formed by the conjugates with a nitro group at the para position. This indicates that a metal cation complexed by crown ether ring is additionally coordinated by a nitro group in the scorpionate mode. To the best of our knowledge, our finding provide the first example of a complex in which a metal cation complexed by a crown ether ring is additionally coordinated by a nitro group.

Subcritical Water Hydrolysis of Peptides: Amino Acid Side-Chain Modifications

Previously we have shown that subcritical water may be used as an alternative to enzymatic digestion in the proteolysis of proteins for bottom-up proteomics. Subcritical water hydrolysis of proteins was shown to result in protein sequence coverages greater than or equal to that obtained following digestion with trypsin; however, the percentage of peptide spectral matches for the samples treated with trypsin were consistently greater than for those treated with subcritical water. This observation suggests that in addition to cleavage of the peptide bond, subcritical water treatment results in other hydrolysis products, possibly due to modifications of amino acid side chains. Here, a model peptide comprising all common amino acid residues (VQSIKCADFLHYMENPTWGR) and two further model peptides (VCFQYMDRGDR and VQSIKADFLHYENPTWGR) were treated with subcritical water with the aim of probing any induced amino acid side-chain modifications. The hydrolysis products were analyzed by direct infusion electrospray tandem mass spectrometry, either collision-induced dissociation or electron transfer dissociation, and liquid chromatography collision-induced dissociation tandem mass spectrometry. The results show preferential oxidation of cysteine to sulfinic and sulfonic acid, and oxidation of methionine. In the absence of cysteine and methionine, oxidation of tryptophan was observed. In addition, water loss from aspartic acid and C-terminal amidation were observed in harsher subcritical water conditions.Graphical ᅟ

A robust platform based on ultra-high performance liquid chromatography Quadrupole time of flight tandem mass spectrometry with a two-step data mining strategy in the investigation, classification, and identification of chlorogenic acids in Ainsliaea fragrans Champ

The chlorogenic acids are the major bioactive constituents of the whole plant of Ainsliaea fragrans Champ. (Xingxiang Tuerfeng). These compounds are usually present as isomeric forms in Xingxiang Tuerfeng. Therefore, an efficient approach should be developed for the rapid discovery and identification of chlorogenic acids isomers through the fragmentation pathway and rules. In this study, the collision induced dissociation tandem mass spectrometry (CID-MS/MS) fragmentation routes of chlorogenic acids were systematically investigated by UHPLC–QTOF–MS/MS in the negative ion mode using eight chlorogenic acids standards. As a result, diagnostic product ions for rapid discovery and classification of chlorogenic acids isomers were determined according to their MS/MS fragmentation patterns and intensity analysis. Based on these findings, a novel two-step data mining strategy was established. The first key step was to screen different kinds of substitution and the skeleton of the quinic acid using the characteristic product ions and neutral losses. The second key step was to screen and classify different types of chlorogenic acids using their diagnostic product ions. It was apply to the rapid investigation, classification, and identification of chlorogenic acids. And the same carbon skeletons from a complex extract of Ainsliaea fragrans Champ. were effectively identified. 88 constituents, including 14 chlorogenic acids types, were rapidly discovered and identified, and in particular, 12 types of chlorogenic acids, including p-CoQC, FQA, BQC, CQA-Glu, CFQA, p-Co-CQC, di-p-CoQC, BCQA, di-CQA-Glu, PCQA, tri-QCA, and P-di-CQA, were first discovered in Ainsliaea fragrans Champ. In conclusion, UHPLC-QTOF-MS/MS method together with a systematic two-step data mining strategy was established as a feasible, effective, and rational technique for analyzing chlorogenic acids. Additionally, this study laid a foundation for the study of the active substances and quality control of Ainsliaea fragrans Champ.

Negative Polarity Helium Charge Transfer Dissociation Tandem Mass Spectrometry: Radical-Initiated Fragmentation of Complex Polysulfated Anions.

This work provides the first use of helium charge transfer dissociation (He-CTD) tandem mass spectrometry (MS/MS) in negative polarity mode. Three sulfated oligosaccharides of natural origin were chosen as representative structures that are difficult to solve by conventional MS/MS approaches. Negative polarity He-CTD provided a full set of structurally informative fragments, which permitted the unambiguous determination of the complete structures of these molecules, including the characterization of labile sulfated functional groups. Despite close structural features, the three molecules led to distinct fragmentation patterns depending on the position of the sulfate group in the heterocycle. The observed fragments showed a consistent radical-initiated mechanism of dissociation, which shares similarities with fragment types produced in electron detachment dissociation (EDD), negative electron transfer dissociation (NETD), or extreme UV photodissociation (XUV-PD). Short times of data collection and the fact that the technique can be affordably implementable in any standard laboratory and with a classical ion trap mass spectrometer were other remarkable characteristics of negative polarity He-CTD.

Characterization of nitrated sugar alcohols by atmospheric-pressure chemical-ionization mass spectrometry.

The nitrated sugar alcohols mannitol hexanitrate (MHN), sorbitol hexanitrate (SHN) and xylitol pentanitrate (XPN) are in the same class of compounds as the powerful military-grade explosive pentaerythritol tetranitrate (PETN) and the homemade explosive erythritol tetranitrate (ETN) but, unlike for PETN and ETN, ways to detect MHN, SHN and XPN by mass spectrometry (MS) have not been fully investigated. Atmospheric-pressure chemical-ionization mass spectrometry (APCI-MS) was used to detect ions characteristic of nitrated sugar alcohols. APCI time-of-flight mass spectrometry (APCI-TOF MS) and collision-induced dissociation tandem mass spectrometry (CID MS/MS) were used for confirmation of each ion assignment. In addition, the use of the chemical ionization reagent dichloromethane was investigated to improve sensitivity and selectivity for detection of MHN, SHN and XPN. All the nitrated sugar alcohols studied followed similar fragmentation pathways in the APCI source. MHN, SHN and XPN were detectable as fragment ions formed by the loss of NO2 , HNO2 , NO3 , and CH2 NO2 groups, and in the presence of dichloromethane chlorinated adduct ions were observed. It was determined that in MS/MS mode, chlorinated adducts of MHN and SHN had the lowest limits of detection (LODs), while for XPN the lowest LOD was for the [XPN-NO2 ]- fragment ion. Partially nitrated analogs of each of the three compounds were also present in the starting materials, and ions attributable to these compounds versus those formed from in-source fragmentation of MHN, SHN, and XPN were distinguished and assigned using liquid chromatography APCI-MS and ESI-MS. The APCI-MS technique provides a selective and sensitive method for the detection of nitrated sugar alcohols. The methods disclosed here will benefit the area of explosives trace detection for counterterrorism and forensics. Copyright © 2016 John Wiley & Sons, Ltd.