Characterization of alpha- and gamma-glutamyl dipeptides by negative ion collision-induced dissociation

The low-energy CID mass spectra of the [M-H](-) ions of a variety of dipeptides containing glutamic acid have been obtained using cone-voltage collisional activation. Dipeptides with the gamma-linkage, H-Glu(Xxx-OH)-OH, are readily distinguished from those with the alpha-linkage, H-Glu-Xxx-OH, by the much more prominent elimination of H-Xxx-OH from the [M-H](-) ions of the former isomers, resulting in formation of m/z 128, presumably deprotonated pyroglutamic acid. Dipeptides with the reverse linkage, H-Xxx-Glu-OH, show distinctive fragmentation reactions of the [M-H](-) ions including enhanced elimination of CO(2) and formation of deprotonated glutamic acid. Exchange of the labile hydrogens for deuterium has shown that there is considerable interchange of C-bonded hydrogens with labile (N- and O-bonded) hydrogens prior to most fragmentation reactions. All dipeptides show loss of H(2)O from [M-H](-). MS(3) studies show that the [M-H-H(2)O](-) ion derived from H-Glu-Gly-OH has the structure of deprotonated pyroglutamylglycine while the [M-H-H(2)O](-) ions derived from H-Glu(Gly-OH)-OH and H-Gly-Glu-OH show a different fragmentation behaviour indicating distinct structures for the fragment ions.     

Negative ion electrospray ionization mass spectral study of dicarboxylic acids in the presence of halide ions

The negative ion electrospray ionization (ESI) mass spectra of a series of dicarboxylic acids, a pair of isomeric (cis/trans) dicarboxylic acids and two pairs of isomeric (positional) substituted benzoic acids, including a pair of hydroxybenzoic acids, were recorded in the presence of halide ions (F(-), Cl(-), Br(-) and I(-)). The ESI mass spectra contained [M--H](-) and [M+X](-) ions, and formation of these ions is found to be characteristic of both the analyte and the halide ion used. The analytes showed a greater tendency to form adduct ions with Cl(-) under ESI conditions compared with the other halide ions used. The isomeric compounds yielded distinct spectra by which the isomers could be easily distinguished. The collision-induced dissociation mass spectra of [M+X](-) ions reflected the gas-phase basicities of both the halide ion and [M--H](-) ion of the analyte. However, the relative ordering of gas-phase basicities of all analyte [M--H](-) and halide ions could not account for the dominance of chloride ion adducts in ESI mass spectra of the analytes mixed with equimolar quantities of the four halides.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Generation of alkoxide anions from a series of aliphatic diols and alcohols and their ion-molecule reactions with carbon dioxide in the gas phase

Alkoxide anions, [M-H](-) from a series of aliphatic diols and alcohols are generated in the source under negative ion electrospray ionisation conditions by cone-voltage fragmentation of the corresponding [M + F](-) ions. The collision-induced dissociation (CID) spectra of [M-H](-) ions consist of [M-H-2H](-) ions, in addition to the other characteristic fragment ions, and the relative abundance of [M-H-2H(-) ions among the series of diols varies as a function of chain length that could be explained based on their stabilities through intramolecular hydrogen bonding. The reactivity of alkoxide anions is studied through ion-molecule reactions with CO(2) in the collision cell of a triple quadrupole mass spectrometer. All the alkoxide anions reacted with CO(2) and formed corresponding carbonate anions, [M-H + CO(2)](-) ions. The reactivity of alkoxide anions within the series of diols also reflected the stability of their [M-H](-) ions.     

Oligosaccharide analysis using anion attachment in negative mode electrospray mass spectrometry

Eleven different anionic species were able to form adducts with neutral oligosaccharides at low cone voltage in negative ion mode electrospray mass spectrometry. Among them, fluoride and acetate have the ability to significantly enhance the absolute abundance of [M - H](-) for neutral oliogosaccharides, which otherwise have low tendencies to deprotonate due to the lack of a highly acidic group. Evidence shows that the source of high abundances of [M - H](-) for neutral oligosaccharides arises from the decomposition of [M + F](-) and [M + Ac](-) with neutral losses of HF and HAc, respectively. The chloride adducts have the best stability among all the adduct species investigated, and chloride adducts consistently appeared in higher abundances relative to [M - H](-). In tandem mass spectrometry (ES-MS/MS) experiments, upon collision induced dissociation (CID), F(-) and Ac(-) adducts gave purely analyte-related product ions, i.e., no detection of the attaching anion and no incorporation of these anions into decomposition products. Cl(-) adducts produced both Cl(-) and analyte-related product ions. For the above three anions, CID of adduct species may be used for structural determination of neutral oligosaccharides because, in each case, structurally-informative fragment ions were produced. In the presence of F(-) and Ac(-), simultaneous detection of acidic and neutral oligosaccharides was achieved, because the problem of the presence of an acidic group that can impede the deprotonation of a neutral oligosaccharide was minimized. The ratio of Cl(-):non-Cl-containing product ions obtained in CID spectra of chloride adducts of disaccharides was used to differentiate anomeric configurations of disaccharides. Density functional theory (DFT) was employed to evaluate the optimized structures of chloride adducts of disaccharides, and it was found that chloride anions favor close contact with the hydrogen from the anomeric hydroxyl group. Multiple hydrogen bonding further stabilizes the chloride adduct.     

Mass spectrometric study of the ionized C60: (gamma-Cyclodextrin)2 inclusion complex by collision induced dissociation

The water soluble inclusion complex [C(60):(gamma-cyclodextrin)(2)] has been characterized using electrospray tandem mass spectrometry and collision induced dissociation. [C(60):(gamma-cyclodextrin)(2)] ions were detected in the gas phase as doubly deprotonated, doubly protonated and doubly sodiated ions. The absence of monocharged complex ions following electronebulization is a likely consequence of the dimeric nature and structural symmetry of the inclusion complex. The collision induced dissociation of positive ions led exclusively to the observation of the protonated and sodiated cyclodextrin ions as well as their fragments. In negative ion mode the closed shell anion C(60)H(-) was the dominant fragment detected at low collision energies whereas at higher collision energies the signal corresponding to deprotonated cyclodextrin units becomes significant. Since C(60) (2-) has been reported to have a nonnegligible basicity compared to C(60) and C(60) (-), it is likely that the proton transfer involved in the formation of the C(60)H(-) anion occurs following transfer of the two electrons from the deprotonated gamma-cyclodextrins to the fullerene. Finally, the charge state of the inclusion complex ions is also shown to affect the interaction strengths between its subunits. The relative stabilities of the three ionic species studied in gas phase following electronebulization are as follows: [C(60):(gamma-cyclodextrin)(2) + 2H](2+) < [C(60):(gamma-cyclodextrin)(2)- 2H](2-) < [C(60):(gamma-cyclodextrin)(2) + 2Na](2+).     

Effect of substituents and protonation on the mechanism of 1,3-dipolar retro-cycloaddition reaction of pyrrolidino[60]- and [70]fullerenes

The mass spectra of new substituted pyrrolidino[60]- and [70]fullerenes have been obtained using electrospray ionization conditions in the positive and negative mode of detection with two different mass spectrometers, a quadrupole ion trap and a Fourier transform ion cyclotron resonance. Radical anions M(●-) and deprotonated molecules [M-H](-) are formed under negative electrospray ionization mass spectrometry conditions, and the collision-induced dissociations of both ionic species have been studied. Either negative odd-electron ions or negative even-electron ions undergo a retro-cycloaddition process forming the corresponding fullerene product ions C(60)(●-) and C(70)(●-). The generation of fullerene radical anions from deprotonated molecules is a new exception of the "even-electron rule." In contrast, the protonated molecules [M + H](+) obtained from the positive mode of detection do not undergo this cycloreversion reaction, and the MS(n) experiment reveals a variety of eliminations of neutral molecules involving different hydrogen shifts and multiple bond cleavages that lead eventually to substituted methanofullerene fragment ions. The observed fragmentations can be correlated with the electronic character of the substituents attached to the heterocyclic moiety. The results obtained from the thermal reactions of these compounds, carried out under different pH conditions, correlate well with those obtained in gas phase. The different behaviors between protonated and unprotonated molecules and ions can be explained assuming that the retro-cycloaddition reaction takes place only when the nitrogen atom of the pyrrolidine ring (the basic center of the molecule) is unprotonated both in gas and condensed phase. The protonation of the NH group inhibits the cycloreversion process, and therefore different fragmentations take place. The detailed mechanisms of the formation and evolution of the intermediate fragments are described.     

Electron capture negative ion mass spectra of some typical matrix-assisted laser desorption/ionization matrices

A series of seven typical matrix-assisted laser desorption/ionization (MALDI) matrices has been investigated by means of electron capture negative ion mass spectrometry (ECNI-MS). It has been shown that the most effective matrices form deprotonated negative ions predominantly in the low-energy region. Relative dissociative cross sections have been measured for all molecules under investigation. The relative integrated abundance of [M - H](-) ion formation in the series changes by four orders of magnitude. It has been shown that 2,5-DHB (gentisic acid), one of the most effective MALDI matrices, has maximal relative intensity of [M - H](-) formation at the energy approximately equal 0.8 eV. This result is in accordance with a finding of Frankevich and Zenobi [Book of Abstracts, Workshop-school "Mass spectrometry in chemical physics, bio-physics and environmental sciences", Zvenigorod, Russia, April, 25-26, 2002, p. 40] that a probable origin of negative ions in MALDI is the process of low-energy (0.5-1 eV) dissociative electron capture by matrix molecules.     

Characterization of inositol phosphorylceramides from <Emphasis Type="Italic">Leishmania major</Emphasis> by tandem mass spectrometry with electrospray ionization

We describe tandem mass spectrometric approaches, including multiple stage ion-trap and source collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization (ESI) to characterize inositol phosphorylceramide (IPC) species seen as [M - H](-) and [M - 2H + Li](-) ions in the negative-ion mode as well as [M + H](+), [M + Li](+), and [M - H + 2Li](+) ions in the positive-ion mode. Following CAD in an ion-trap or a triple-stage quadrupole instrument, the [M - H](-) ions of IPC yielded fragment ions reflecting only the inositol and the fatty acyl substituent of the molecule. In contrast, the mass spectra from MS(3) of [M - H - Inositol](-) ions contained abundant ions that are readily applicable for assignment of the fatty acid and long-chain base (LCB) moieties. Both the product-ion spectra from MS(2) and MS(3) of the [M - 2H + Alk](-), [M + H](+), [M + Alk](+), and [M - H + 2Alk](+) ions also contained rich fragment ions informative for unambiguous assignment of the fatty acyl substituent and the LCB. However, the sensitivity of the ions observed in the forms of [M - 2H + Alk](-), [M + H](+), [M + Alk](+), and [M - H + 2Alk](+) (Alk = Li, Na) is nearly 10 times less than that observed in the [M - H](-) form. In addition to the major fragmentation pathways leading to elimination of the inositol or inositol monophosphate moiety, several structurally informative ions resulting from rearrangement processes were observed. The fragmentation processes are similar to those previously reported for ceramides. While the tandem mass spectrometric approach using MS(n) (n = 2, 3) permits the structures of the Leishmania major IPCs consisting of two isomeric structures to be unveiled in detail, tandem mass spectra from constant neutral loss scans may provide a simple method for detecting IPC in mixtures.     

Characterization of C-glycosyl quinochalcones in Carthamus tinctorius L. by ultraperformance liquid chromatography coupled with quadrupole-time-of-flight mass spectrometry

C-Glycosyl quinochalcones are unique components in Carthamus tinctorius L. The reported C-glycosyl quinochalcones have the same quinochalcone skeleton with a hydroxyl group at the 5'-position and a glucose linked to this position with a carbon-carbon bond. In this study, the standard hydroxysafflor yellow A and water-extracted fraction of Carthamus tinctorius L. were analyzed by ultraperformance liquid chromatography coupled with quadrupole-time-of-flight mass spectrometry (UPLC/Q-TOFMS) in both positive and negative ion modes. The fragmentation pathways of C-glycosyl quinochalcones were interpreted and validated by accurate mass measurement. Their fragmentation showed a special cleavage at the C-C bond except for the typical internal cleavage at the sugar moiety of other C-glycosyl flavonoids. In positive ion mode, cleavage of the 5'-glucose produced an [M+H-162](+) ion by a neutral loss, while cleavage of the 5'-glucose in negative ion mode led to an [M-H-163](-.) ion by radical cleavage. The cleavage from the carbonyl group produced fragment ions containing an A or a B ring. The fragment ions containing an A ring were common product ions of seven compounds in both ion modes, and fragment ions containing the B ring were used to judge the different substituent groups at the 3'-position. The fragmentation patterns of seven structurally related C-glycosyl quinochalcones were analyzed systematically and the formation of the fragment ions in two modes is explained in detail in this report. UPLC/Q-TOFMS is an effective tool for characterizing a complex sample, which gives higher resolution separation and generates accurate mass measurement of the product ions.     

Structural characterization of iridoid glucosides by ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry

The mass spectral fragmentation behavior of ten iridoid glucosides (IGs) has been studied using electrospray ionization (ESI), collision-induced dissociation (CID), and quadrupole time-of-flight tandem mass spectrometry (Q-TOF MS/MS). In the negative ESI mass spectra, the deprotonated [M-H](-) ion was observed for all of the ten IGs except gardoside methyl ester, while the formate adduct [M+HCOO](-) ion appeared to be favored by the presence of a methyl ester or a lactone group in the C-4 position when formic acid was added to the mobile phase. The CID MS/MS spectra of the [M-H](-) ions have been used for structural elucidation. Ring cleavages of the aglycone moiety have been observed in the MS/MS spectra, corresponding to (1,4)F(-), (2,6)F(-), (2,7)F(-), and (2,7)F(0) (-) ions, based on accurate mass measurements and the elemental compositions of the product ions. These characteristic ions gave valuable information on the basic structural skeletons. Furthermore, on the basis of the relative abundances of the fragment ions (1,4)F(-) and (2,7)F(-), different sub-classes, such as cyclopentane-type and 7,8-cyclopentene-type IGs, can be differentiated. Ring cleavage of the sugar moieties was also observed, yielding useful information for their characterization. In addition, the neutral losses, such as H(2)O, CO(2), CH(3)OH, CH(3)COOH, and glucosidic units, have proved useful for confirming the presence of functional substituents in the structures of the IGs. Based on the fragmentation patterns of these standard IGs, twelve IGs have been characterized in an extract of Hedyotis diffusa Willd. by means of ultra-performance liquid chromatography/Q-TOF MS/MS, of which six have been unambiguously identified and the other six have been tentatively identified.     

Gas-phase smiles rearrangement reactions of deprotonated 2-(4, 6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide and its derivatives in electrospray ionization mass spectrometry

The negative ions of deprotonated 2-(4, 6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide and its derivatives are studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Upon collisional activation, the [M - H](-) ions dissociate in two competitive pathways that can be considered as the gas-phase Smiles rearrangement reactions, giving rise to the characteristic fragment ions [M - H - C(7)H(4)OS](-) and [M - H - C(13)H(8)NSR](-) (R = substituent). Theoretical computations were invoked to shed light on the reaction mechanisms of the representative Compound 1 by the semiempirical PM3 method. These theoretical calculations show that the formation of [M - H - C(13)H(8)NSR](-) (R = H for Compound 1) is more favorable. Furthermore, it is found that the intensities of the two product ions are strongly influenced by the position and the nature of the substituents. For the para-substituted compounds, the ln[(M - H - C(7)H(4)OS(-))/(M - H - C(13)H(8)NSR(-))] values are well correlated with the sigma(p)(-) substituent constants. In addition, the dependence of the intensity ratios of these two ions, ln[(M - H- C(7)H(4)OS(-))/(M - H - C(13)H(8)NSR(-))](R = CH(3)), on the collision energy can be used to distinguish the positional isomers.     

Multiple lithium exchange under lithium cationization of cyclodextrins

Multiple lithium exchange is observed during electrospray ionization of alpha-, beta- and gamma-cyclodextrins from aqueous methanolic solution containing LiOH. Apart from [M + Li](+) and [M + nLi - (n - 1)H](+) ions, abundant multiply lithiated doubly charged ions corresponding to [M + nLi - (n - 2)H](2+) ions were observed. At least six lithium exchanges in alpha-cyclodextrin, seven in beta-cyclodextrin and eight in gamma-cyclodextrin were noted. The propensity of multiply lithiated doubly charged ions is much less in the open-ended maltoheptaose. It appears that during droplet or cluster formation and subsequent desolvation, LiOH trapped in the cavity of cyclodextrin reacts to form multiply lithiated ions. The singly charged [M + Li](+) and doubly charged [M + 2Li](2+) ions fragment by glycosidic cleavages, giving B series of ions, whereas the multiply lithiated ions fragment by cross ring cleavages ((2, 4)A or (O, 2)X) followed by glycosidic cleavage. From the tandem mass spectra, it appears that a maximum of two lithium exchanges occur in one sugar unit in these cyclodextrins.     

Collisionally-induced dissociation of substituted pyrimidine antiviral agents: Mechanisms of ion formation using gas phase hydrogen/deuterium exchange and electrospray ionization tandem mass spectrometry

ESI and CID mass spectra were obtained for four pyrimidine nucleoside antiviral agents and the corresponding compounds in which the labile hydrogens were replaced by deuterium using gas-phase exchange. The number of labile hydrogens, x, was determined from a comparison of ESI spectra obtained with N(2) and with ND(3) as the nebulizer gas. CID mass spectra were obtained for [M + H](+) and [M - H](-) ions and the exchanged analogs, [M(D(x)) + D](+) and [M(D(x)) - D](-), produced by ESI using a SCIEX API-III(plus) mass spectrometer. Protonated pyrimidine antiviral agents dissociate through rearrangement decompositions of base-protonated [M + H](+) ions by cleavage of the glycosidic bonds to give the protonated bases with a sugar moiety as the neutral fragment. Cleavage of the glycosidic bonds with charge retention on the sugar moiety eliminates the base moiety as a neutral molecule and produces characteristic sugar ions. CID of protonated pyrimidine bases, [B + H](+), occurs through three major pathways: (1) elimination of NH(3) (ND(3)), (2) loss of H(2)O (D(2)O), and (3) elimination of HNCO (DNCO). Protonated trifluoromethyl uracil, however, dissociates primarily through elimination of HF followed by the loss of HNCO. CID mass spectra of [M - H](-) ions of all four antiviral agents show NCO(-) as the principal decomposition product. A small amount of deprotonated base is also observed, but no sugar ions. Elimination of HNCO, HN(3), HF, CO, and formation of iodide ion are minor dissociation pathways from [M - H](-) ions.     

Electrospray ionization mass spectrometric studies on the amphoteric surfactant cocamidopropylbetaine.

After liquid chromatographic (LC) separation, electrospray ionization mass spectrometry (ESI-MS) was investigated for the determination of the amphoteric surfactant cocamidopropylbetaine (CAPB). In the positive ion mode the molecule formed the adduct ions [M + H](+), [M + Na](+) and [M + K](+). Adducts of these cations were also detected with decreasing abundance as dimer and trimer clusters. Additionally, doubly charged molecular ions with different combinations of cations were identified. It was noticed that the relative abundances of individual cation adducts were not reproducible, apparently owing to varying contents of alkali metal ions originating from the solvent and the sample. Under negative ionization, the major molecular ion was [M - H](-). Higher clusters formed by two and three surfactant molecules, i.e. [2M - H](-) and [3M - H](-) were likewise registered. The tendency to form clusters in both positive and negative ion modes, even at 0.1 mg l(-1) levels, was attributed to strong electrostatic interactions between the zwitterionic head groups. Further evidence for this assumption was provided by the detection of a fragment formed from [2M - H](-) which contained the two charged head groups. Studies were undertaken in the negative ion mode on the concentration- and orifice voltage-dependent monomer, dimer and trimer formation of C(12)-CAPB in order to evaluate potential issues in using the ion [M - H](-) mode for quantitative analysis. Finally, the established (-)-LC/ESI-MS method was applied to follow up the primary degradation of CAPB in a laboratory-scale fixed-bed bioreactor (FBBR) spiked with a test concentration of 10 mg l(-1). Direct analysis without sample pretreatment revealed that higher alkyl homologues were more prone to adsorption. Primary biodegradation of all alkyl homologues was completed after a period of 4 days. Selected lyophilized FBBR samples were examined for the presence of transient or stable degradation intermediates, but no metabolite could be identified.     

Structural characterization of glycoporphyrins by electrospray tandem mass spectrometry

Porphyrin derivatives having a galactose or a bis(isopropylidene)galactose structural unit, linked by ester or ether bonds, were characterized by electrospray tandem mass spectrometry (ES-MS/MS). The electrospray mass spectra of these glycoporphyrins show the corresponding [M + H](+) ions. For the glycoporphyrins with pyridyl substituents and those having a tetrafluorophenyl spacer, the doubly charged ions [M + 2H](2+) were also observed in ES-MS with high relative abundance. The fragmentation of both [M + H](+) and [M + 2H](2+) ions exhibited common fragmentation pathways for porphyrins with the same sugar residue, independently of the porphyrin structural unit and type of linkage. ES-MS/MS of the [M + H](+) ions of the galactose-substituted porphyrins gave the fragment ions [M + H - C(2)H(4)O(2)](+), [M + H - C(3)H(6)O(3)](+), [M + H - C(4)H(8)O(4)](+) and [M + H - galactose residue](+). The fragmentation of the [M + 2H](2+) ions of the porphyrins with galactose shows the common doubly charged fragment ions [porphyrin + H](2+), [M + 2H - C(2)H(4)O(2)](2+), [M + 2H - C(4)H(8)O(4)](2+), [M + 2H - galactose residue](2+) and the singly charged fragment ions [M + H - C(3)H(6)O(3)](+) and [M + H - galactose residue](+). The fragmentation of the [M + H](+) ions of glycoporphyrins with a protected galactosyl residue leads mainly to the ions [M + H - CO(CH(3))(2)](+), [M + H - 2CO(CH(3))(2)](+), [M + H - 2CO(CH(3))(2) - CO](+), [M + H - C(10)H(16)O(4)](+) and [M + H - protected galactose](+). The doubly charged ions [M + 2H](2+) fragment to give the doubly charged ions [porphyrin + H](2+) and the singly charged ions [M + H - protected galactose residue](+) and [M + H - CO(CH(3))(2)](+). For the porphyrins where the sugar structural unit is linked by an ester bond, [M + 2H](2+), ES-MS/MS showed a major and typical fragmentation corresponding to combined loss of a sugar structural unit and further loss of water, leading to the ion [M + 2H - sugar residue - H(2)O](2+), independently of the structure of the sugar structural unit. These results show that ES-MS/MS can be a powerful tool for the characterization of the sugar structural unit of glycoporphyrins, without the need for chemical hydrolysis.     

Mass spectral behavior of the hydrolysis products of sesqui- and oxy-mustard type chemical warfare agents in atmospheric pressure chemical ionization

Bis(2-hydroxyethylthio)alkanes and bis(2-hydroxyethylthioalkyl)ethers are important biological and environmental degradation products of sulfur mustard analogs known as sesqui- and oxy-mustards. We used atmospheric pressure chemical ionization mass spectrometry (APCI MS) to acquire characteristic spectra of these compounds in positive and negative ionization modes. Positive APCI mass spectra exhibited [M + H](+); negative APCI MS generated [M + O(2)](-), [M - H](-), and [M - 3H](-); and both positive and negative APCI mass spectra contained fragment ions due to in-source collision-induced dissociation. Product ion scans confirmed the origin of fragment ions observed in single-stage MS. Although the spectra of these compounds were very similar, positive and negative APCI mass spectra of the oxy-mustard hydrolysis product, bis(2-hydroxyethylthiomethyl)ether, differed from the spectra of the other compounds in a manner that suggested a rearrangement to the sesqui-mustard hydrolysis product, bis(2-hydroxyethylthio)methane. We evaluated the [M + O(2)](-) adduct ion for quantification via liquid chromatography-MS/MS in the multiple-reaction monitoring (MRM) mode by constructing calibration curves from three precursor/product ion transitions for all the analytes. Analytical figures of merit generated from the calibration curves indicated the stability and suitability of these transitions for quantification at concentrations in the low ng/mL range. Thus, we are the first to propose a quantitative method predicated on the measurement of product ions generated from the superoxide adduct anion of the sesqui-and oxy-mustard hydrolysis products.     

Evaluation of three ionisation modes for the analysis of chlorinated paraffins by gas chromatography/ion-trap mass spectrometry

The applicability of three ionisation modes to the analysis of short-chain chlorinated paraffins (SCCPs) using gas chromatography coupled with mass spectrometry (GC/MS) was evaluated. MS conditions for electron ionisation (EI), positive chemical ionisation (PCI) and electron-capture negative ionisation (ECNI) were optimised using commercially available individual tetra-, penta- and hexachlorodecanes. In addition, mass spectra were studied and fragmentation pathways were proposed for each individual congener. Different fragment ions were evaluated for quantification, and ECNI-MS using [HCl2](-) and [Cl2](-*) ions was selected for the determination of SCCPs. Quality parameters (repeatability, limits of detection and calibration range) were established for the proposed method, which was then applied to the analysis of SCCPs in river sediments.     

A comparison of EDI with solvent-free MALDI and LDI for the analysis of organic pigments

To evaluate the applicability of EDI to material analysis as a new ionization method, a comparison of EDI with solvent-free matrix-assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) was made for the analysis of organic pigments, e.g. Pigment Yellow 93, Pigment Yellow 180, and Pigment Green 36, as test samples, which are poorly soluble in standard solvents. In EDI, the samples were prepared in two ways: deposition of suspended samples in appropriate solvents and dried on the substrate, and the direct deposition of the powder samples on the substrate. No matrices were used. Both sample preparation methods gave similar mass spectra. Equally strong signals of [M + H](+) and [M - H](-) ions were observed with some fragment ions for azo pigments in the respective positive or negative mode of operation. For the powder sample of the phthalocyanine pigment PG36, M(+*) and [M + H](+) in the positive mode and M(-*) in the negative mode of operation were observed as major ions. Positive-mode, solvent-free MALDI gave M(+), [M + H](+) and [M + Na](+) and negative mode gave [M - H](-) depending on the sample preparation. As solvent-free MALDI, EDI was also found to be an easy-to-operate, versatile method for the samples as received.     

Different behavior of dextrans in positive-ion and negative-ion mass spectrometry

A mass spectrometric (MS) comparative study of dextran samples using two different ionization techniques (matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI)) in both positive- and negative-ion modes is reported. The experiments were carried out with two polydisperse dextrans (1000 and 8800 Da) and isomaltotriose. In the positive-ion mode, the expected alkali metal ion adducts of dextrans were observed in both techniques. In contrast, the expected preferential formation of deprotonated molecules [M - H](-) was not confirmed in negative mode MALDI time-of-flight (TOF) MS, where the series of ions [M(x)- H +42](-) or [M(x+1)- H - 120](-), coming either from some addition or fragmentation, were observed. In both ionization techniques, these ions formed the main distributions of dextrans in the negative-ion mode. It seems that the negative molecular ions formed from the alpha1 --> 6 linkage of polyglucose oligomers easily decompose, and the product ions [M - H - 120](-) markedly dominate. The fragmentation experiments and especially the investigation of the fundamental role of the nozzle-skimmer potential in ESI-MS supported our explanation of the observed behavior because its higher values caused higher fragmentation. The experiments with isomaltotriose excluded any addition of 42 Da during the MS procedures, which is not distinguishable from the loss of 120 Da in the case of polydisperse dextrans. MALDI-TOFMS was found to be more sensitive for the detection of higher oligosaccharides and ESI-MS more useful for structural studies.