Collision induced dissociation in a flowing afterglow-tandem mass spectrometer for the selective detection of C5 unsaturated alcohols and isoprene

A flowing afterglow-tandem mass spectrometer (FA-TMS) was used to study a series of C₅ unsaturated alcohols and isoprene. The analytical procedure was validated through collision induced dissociation (CID) experiments on proton hydrates. In the FA, reagent H₃O⁺ ions were used to chemically ionize the alcohols under study and isoprene. Chemical ionization (CI) by H₃O⁺ is widely used, especially in PTR-MS instruments, and produces a main peak at m/z 69 for all studied compounds, implying the impossibility to distinguish them by a simple quadrupole mass filter. The CID of these ions at m/z 69 resulted in daughter ions with the same masses but with different intensities depending on the organic compound, the collision energy and the Ar target gas pressure in the collision cell. From these observations, pentenols were easily distinguished from methylbutenols and 3-methyl-3-buten-1-ol from the other compounds. CID experiments were also performed on the protonated alcohol, which is only a stable ion for 1-penten-3-ol, 2-methyl-3-buten-2-ol and 3-methyl-3-buten-1-ol, showing different CID patterns as a function of the collision energy. The coupling between a FA reactor and a TMS has proven to be a valuable approach to identify C₅ unsaturated alcohols and isoprene.     

Gas phase ion chemistry of molybdenum hexacarbonyl

In an ion cyclotron resonance (ICR) cell, Mo(CO)_n⁺ ions (n = 0–6), generated by electron ionization (EI) with 70 eV electrons, on collisions with Mo(CO)₆ undergo charge exchange (confirmed by isotopic experiments), collision-induced dissociation (CID), and association reactions to produce Mo_m(CO)_n⁺ ions (m = 1–6). Reactions are essentially complete within 9 s at a pressure of 3 × 10⁻⁹ Torr, as recorded by the manifold ion gauge (uncalibrated); Mo(CO)_n⁺ ions with n = 0–5 have been consumed within this time whereas Mo(CO)₆⁺ ions have achieved a steady concentration. All Mo₂(CO)_n⁺ ions (n = 0–11) were observed: the abundances of dimolybdenum-containing ions with n < 7 decrease at extended reaction times, whereas those with n ≥ 7 remain steady or increase slowly, implying that reactivity decreases with increasing CO content. The major dimers have n = 7, 9, and 10. When subjected to CID the Mo₂(CO)₇⁺ ion yields Mo₂(CO)_n⁺ ions (n = 0–6). Most Mo₃(CO)_n⁺ ions (n = 0–13) were observed, those with n = 9 being formed most readily. Similar observations apply to larger clusters, the most abundant ions being those with CO:Mo ratios of 2–3:1. Mo(CO)_n⁺ ions (n = 0, 3–6) formed by EI with 15 eV electrons are unreactive for reaction times of at least 5 s at the same pressure. General reaction sequences are proposed. Negative ions generated with 70 eV electrons (∼ 90% Mo(CO)₅⁻) are much less reactive but also lead to cluster ion formation on reaction with Mo(CO)₆.     

Linkage Determination of Linear Oligosaccharides by MSn (n > 2) Collision-Induced Dissociation of Z1 Ions in the Negative Ion Mode

Obtaining unambiguous linkage information between sugars in oligosaccharides is an important step in their detailed structural analysis. An approach is described that provides greater confidence in linkage determination for linear oligosaccharides based on multiple-stage tandem mass spectrometry (MSⁿ, n >2) and collision-induced dissociation (CID) of Z₁ ions in the negative ion mode. Under low energy CID conditions, disaccharides ¹⁸O-labeled on the reducing carbonyl group gave rise to Z₁ product ions (m/z 163) derived from the reducing sugar, which could be mass-discriminated from other possible structural isomers having m/z 161. MS³ CID of these m/z 163 ions showed distinct fragmentation fingerprints corresponding to the linkage types and largely unaffected by sugar unit identities or their anomeric configurations. This unique property allowed standard CID spectra of Z₁ ions to be generated from a small set of disaccharide samples that were representative of many other possible isomeric structures. With the use of MSⁿ CID (n = 3 – 5), model linear oligosaccharides were dissociated into overlapping disaccharide structures, which were subsequently fragmented to form their corresponding Z₁ ions. CID data of these Z₁ ions were collected and compared with the standard database of Z₁ ion CID using spectra similarity scores for linkage determination. As the proof-of-principle tests demonstrated, we achieved correct determination of individual linkage types along with their locations within two trisaccharides and a pentasaccharide.

Selective parent ion axialization for improved efficiency of collision-induced dissociation in laser desorption-ionization fourier transform ion cyclotron resonance mass spectrometry

We have systematically established the excitation frequency, amplitude, duration, and buffer gas pressure for optimal axialization efficiency and mass selectivity of quadrupolar excitation-collisional cooling for isolation of parent ions for collision-induced dissociation in Fourier transform ion cyclotron resonance mass spectrometry. For example, at high quadrupolar excitation amplitude, ion axialization efficiency and selectivity are optimal when the applied quadrupolar excitation frequency is lower than the unperturbed ion cyclotron frequency by up to several hundred hertz. Moreover, at high buffer gas pressure (10^?6 Torr), quadrupolar excitation duration can be quite short because of efficient collisional cooling of the cyclotron motion produced by magnetron-to-cyclotron conversion. Efficiency, detected signal magnitude, and mass resolving power for collision-induced dissociation (CID) product ions are significantly enhanced by prior parent ion axialization. With this method, we use argon CID to show that C ₉₄ ⁺ (m/z 1128) formed by Nd:YAG laser desorption-ionization behaves as a closed-cage structure.     

Structural chemistry of Ba2CdS3, Ba2CdSe3, BaCdS2, BaCu2S2 and BaCu2Se2

The structures of all compounds were determined from three dimensional single crystal X-ray diffraction data and refined by least squares. Ba₂CdS₃ and Ba₂CdSe₃ are isostructural, Pnma, a = 8.9145(6)Å, b = 4.3356(2)Å, c = 17.2439(9)Å for the former compound and a = 9.2247Å, b = 4,4823(6)Å, c = 17.8706(11)Å for the latter, z = 4, R = 0.0751 and R = 0.0462, respectively. The compounds are isostructural with the previously reported Mn analogues and with K₂AgI₃. Cd ions are in tetrahedral environment and the tetrahedra form infinite linear chains by corner sharing. Ba ions are in 7-fold coordination in which 6 anions form a trigonal prism and 1 anion caps one of the rectangular faces. BaCdS₂, Pnma, a = 7.2781(3)Å, b = 4.1670(1)Å, c = 13.9189(6)Å, z = 4, R = 0.0685. Cd ions can be considered to have a triangular planar coordination with CdS distances of 2.47 and 2.53 Å (twice). Two additional S ions are at 2.89 and 3.22 Å to complete a triangular bipyramidal configuration. Ba is in 7-fold coordination with the anions forming a trigonal prism which is capped on one rectangular face. The compound is isostructural with BaCdO₂ and is related to the structure of BaMnS₂. BaCdSe₂ could not be prepared. BaCu₂S₂ and BaCu₂Se₂ are isostructural, Pnma, a = 9.3081(4)Å, b = 4.0612(3)Å, c = 10.4084(5)Å for the sulfide and a = 9.5944(6)Å, b = 4.2142(4)Å, c = 10.7748(8)Å for the selenide, z = 4, R = 0.0634 and 0.0373, respectively. Ba ions are in the usual 7-fold, capped hexagonal prism, coordination. However, 9 Cu ions also can be considered to form a trigonal prism with all rectangular faces capped, around Ba since the BaCu distances range from 3.24 to 3.54 Å for the sulfide and from 3.37 to 3.67 Å for the selenide. One of the Cu ions is in a very distorted tetrahedral environment and the second one is located in a more regular tetrahedral configuration of the anions. Two independent infinite chains of tetrahedra are present. They are formed by sharing of two adjacent edges of each tetrahedron and then these chains in turn are linked by corner sharing into a three-dimensional network of tetrahedra.     

Mechanism of Formation of the Major Estradiol Product Ions Following Collisional Activation of the Molecular Anion in a Tandem Quadrupole Mass Spectrometer

The importance of the mass spectral product ion structure is highlighted in quantitative assays, which typically use multiple reaction monitoring (MRM), and in the discovery of novel metabolites. Estradiol is an important sex steroid whose quantitation and metabolite identification using tandem mass spectrometry has been widely employed in numerous clinical studies. Negative electrospray ionization tandem mass spectrometry of estradiol (E2) results in several product ions, including the abundant m/z 183 and 169. Although m/z 183 is one of the most abundant product ions used in many quantitative assays, the structure of m/z 183 has not been rigorously examined. We suggest a structure for m/z 183 and a mechanism of formation consistent with collision induced dissociation (CID) of E2 and several stable isotopes ([D₄]-E2, [¹³C₆]-E2, and [D₁]-E2). An additional product ion from E2, namely m/z 169, has also been examined. MS³ experiments indicated that both m/z 183 and m/z 169 originate from only E2 [M – H]^– m/z 271. These ions, m/z 183 and m/z 169, were also present in the collision induced decomposition mass spectra of other prominent estrogens, estrone (E1) and estriol (E3), indicating that these two product ions could be used to elucidate the estrogenic origin of novel metabolites. We propose two fragmentation schemes to explain the CID data and suggest a structure of m/z 183 and m/z 169 consistent with several isotopic variants and high resolution mass spectrometric measurements.      

An ion trap-ion mobility-time of flight mass spectrometer with three ion sources for ion/ion reactions

This instrument combines the capabilities of ion/ion reactions with ion mobility (IM) and time-of-flight (TOF) measurements for conformation studies and top-down analysis of large biomolecules. Ubiquitin ions from either of two electrospray ionization (ESI) sources are stored in a three dimensional (3D) ion trap (IT) and reacted with negative ions from atmospheric sampling glow discharge ionization (ASGDI). The proton transfer reaction products are then separated by IM and analyzed via a TOF mass analyzer. In this way, ubiquitin +7 ions are converted to lower charge states down to +1; the ions in lower charge states tend to be in compact conformations with cross sections down to ～880 Ų. The duration and magnitude of the ion ejection pulse on the IT exit and the entrance voltage on the IM drift tube can affect the measured distribution of conformers for ubiquitin +7 and +6. Alternatively, protein ions are fragmented by collision-induced dissociation (CID) in the IT, followed by ion/ion reactions to reduce the charge states of the CID product ions, thus simplifying assignment of charge states and fragments using the mobility-resolved tandem mass spectrum. Instrument characteristics and the use of a new ion trap controller and software modifications to control the entire instrument are described.     

Molecular species analysis of arachidonate containing glycerophosphocholines by tandem mass spectrometry

Carboxylate anions arising from collision-induced dissociation (CID) of the [M - 15]^- ion produced by fast atom bombardment (FAB) of glycerophosphocholine (GPCho) were previously shown to be produced in an abundance ratio of 1:3 for the carboxylic acids esterified at sn - 1 and sn - 2, respectively. This observation has been confirmed in a series of 13 synthetic GPCho molecular species. A good correlation was found between the isomeric purity of GPCho molecular species as determined by negative-ion FAB/CID analysis and the isomeric purity of the sn - 2 fatty acid using a phospholipase A₂ assay. Negative-ion FAB mass spectra of several 1-0-alkyl-2-acyl-GPCho molecular species were found to be similar to those of diacyl GPCho. However, the cm spectra from the major high-mass ions are different from those of the diacyl species in that the [M - 15]^- ion yields only one carboxylate anion and the [M - 86]^- undergoes a neutral loss of the sn - 2 carboxylic acid as a major decomposition product. These results suggest several rules useful for structural characterization of GPCho molecular species by negative-ion tandem mass spectrometry (MS/MS): (1) For diacyl species, the mass of the two carboxyl anions plus the mass of the GPeho backbone (minus a methyl group) must correspond to the mass of the [M - 15] anion; (2) for diacyl species there is a carboxylate anion ratio approximately 1:3 for the substituents at sn - 1 and sn - 2; and (3) for alkylether species, only one fatty acyl group is present, and the difference between the [M - 15] ion and the GPCho backbone (minus methyl) plus the fatty acyl group at sn - 2 corresponds to an alkylether substituent. (4) Assignment of ether-linked molecular species can be made from the [M - 86]^- ion, which has a strong neutral loss of the sn - 2 fatty acid. Analysis of GPCho isolated from human neutrophils by total lipid extraction and normal-phase HPLC was carried out by negative-ion FABand MS/MS. The major arachidonate-eontaining molecular species, which comprise only 5% of total GPCho, were identified by using precursor ion scans for the arachidonate anion, m/ z 303. Decomposition of identified. precursor ions permitted the assignment of those molecular species of GPCho that contain arachidonate at sn - 2 and identification of the substituent at the sn - 1 position. These results were compared to previously identified molecular species from human neutrophils. Several minor arachidonate-containing molecular species were tentatively identified.     

Synthesis and crystal structure of nickel(II) dinitrate molecular complex with 3,5-lutidine

Complex [Ni(NO₃)₂(3,5-lutidine)₃] (3,5-lutidine, 3,5-Lut-C₇H₉N) was synthesized, and its crystal structure was determined. Complex crystals are monoclinic: space group Cc, a = 14.445(1) Å, b = 10.024(1) Å, c = 17.607(1) Å, β = 108.96(1)°, V = 2411(1) ų, ρ_calcd = 2.389 g/cm³, Z = 4. The structure is built of discrete neutral complexes [Ni(3,5-Lut)₃(NO₃)₂]. The coordination sphere of Ni²⁺ ions is a disordered octahedron, which consists of three crystallographically nonequivalent lutidine ligands (Ni-N, 2.059–2.124 Å) and two oxygen ions of the two nitrate ions (Ni-O, 2.047–2.258 Å).     

Calibration Point for Electron Ionization MS / MS spectra measured with multiquadrupole mass spectrometers

A protocol for establishing standard instrument conditions for measurement of product ion MS/MS spectra from parent ions produced by electron ionization is presented. Within this protocol, the ion at m/z 231 (C₅F₉ ⁺) from perfluorokerosene or perfluorotributylamine is selected as the parent ion and subjected to collision-induced dissociation. The relative intensities of product ions at m/z 69, 131, and 181 are monitored as a function of collision energy while keeping the target gas pressure constant within the range of 10^?4–10^?6 torr (measured), or a beam attenuation of approximately 30-70%. The collision energy at which the ion intensities for product ions at m/z 69 and 181 are equal is defined as the calibration point at that collision gas pressure; the intensity of the ion at m/z 131 is very close to this value as well. Electron ionization MS/MS spectra taken at the calibration point using two different multiquadrupole instruments show good reproducibility for several test compounds. The high degree of similarity may aid in the establishment of a MS/MS spectral library.     

Site of alkylation of N-methyl- and N-ethylaniline in the gas phase: a tandem mass spectrometric study

N-Methylaniline (NMA) was ethylated and N-ethylaniline (NEA) was methylated under chemical ionization conditions using C₂H₅I and CH₃I, respectively, as reagent gases. The structures of the resulting m/z 136 adduct ions have been probed using metastable ion and collision-induced dissociation (CID) methods. From the similarity of the spectra obtained and from the presence of structure-diagnostic ions at m/z 59 (CH₃NHC₂H₅^+•) and m/z 44 (CH₃NHCH₂⁺), it is concluded that predominantly N-alkylation occurs in both systems. This interpretation was aided by the use of C₂D₅I and CD₃I as reagents. Adduct ions of m/z 136 were also formed by ethylation of the isomeric toluidines and by methylation of the ring-ethylanilines. The resulting CID mass spectra were distinctly different from those obtained for the m/z 136 ions obtained by alkylation of NMA and NEA. Protonation of N-ethyl-N-methylaniline using CH₃C(O)CH₃ as Brønsted acid reagent produced an m/z 136 species whose CID mass spectrum also featured intense ion signals at m/z 59 and 44. This observation led to the conclusion that protonation with acetone as reagent results, in this case, in dominant N-protonation. However, the CID mass spectrum of the m/z 136 ion formed when CH₃OH was the protonating agent featured a weak signal at m/z 44 and no signal at m/z 59. Hence it was concluded that the latter m/z 136 ion contains a larger contribution from the ring-protonated adduct. Copyright © 2004 John Wiley & Sons, Ltd.     

Losses of ions during forward and reverse scans in a quadrupole ion trap mass spectrometer and how to reduce them

The higher order fields present in the quadrupole ion trap may have beneficial effects such as increases in mass resolution in the mass-selective instability or resonance ejection modes of operation, but may also result in losses of ions due to nonlinear resonances. In this work, the reduction in ion intensities observed in the mass spectra of polyethylene glycol (PEG 1000) has been utilized to monitor the ion losses resulting from these higher order fields during the rf voltage scans in both the forward and reverse directions. Extensive ion losses were observed in reverse rf voltage scans at q _z=0.64 (a _z=0), which corresponds to octopole resonance at β _z=1/2. The losses depended upon rf voltage scan rate and ion mass being greater for lower scan rates and lower masses. For ions of m/z 877, losses of up to 60% of the stored ions were observed at low scan rates (<1×10⁴ Da/s), but were minimal at higher scan rates. Thus, it is possible to avoid such losses during reverse scans by scanning the region q _z=0.64 at rates in excess of 4×10⁴ Da/s. In forward rf voltage scans, ion storage was considerably more reliable, with significant losses observed only at very high scan rates near the region q _z=0.78 (hexapole resonance at β _z=2/3).     

Observation and implications of high mass-to-charge ratio ions from electrospray ionization mass spectrometry

High mass-to-charge ratio ions (> 4000) from electrospray ionization (ESI) have been observed for several proteins, including bovine cytochrome c (M _r 12,231) and porcine pepsin (M _r 34,584), by using a quadrupole mass spectrometer with an m/z 45,000 range. The ESI mass spectrum for cytochrome c in an aqueous solution gives a charge state distribution that ranges from 12 + to 2 +, with a broad, low-intensity peak in the mass-to-charge ratio region corresponding to the [M + H]⁺ ion. the negative ion ESI mass spectrum for pepsin in 1% acetic acid solution shows a charge state distribution ranging from 7? to 2?. To observe the [M - H]^? ion, harsher desolvation and interface conditions were required. Also observed was the abundant aggregation of the protens with average charge states substantially lower than observed for their monomeric counterparts. The negative ion ESI mass spectrum for cytochrome c in 1–100 mM NH₄OAc solutions showed greater relative abundances for the higher mass-to-charge ratio ions than in acuidic solutions, with an [M - H]^? ion relative abundance approximately 50% that of the most abundant charge state peak. The observation that protein aggregates are formed with charge states comparable to monomeric species (at fower mass-to-charge ratios) suggests that the high mass-to-charge ratio monomers may be formed by the dissociation of aggregate species. The observation of low charge state and aggregate molecular ions concurrently with highly charged species may serve to support a variation of the charged residue model, originally described by Dole and co-workers (Dole, M., et al. J. Chem. Phys. 1968, 49, 2240; Mack, L. L., et al. J. Chem. Phys. 1970, 52, 4977) which involves the Coulombically driven formation of either very highly solvated molecular ions or lower ananometer-diameter droplets.     

Study of the molecular and crystal structure of a new coordination polymer of copper(II) nitrate with 4,4,10,10-tetramethyl-1,3,7,9-tetraazospiro[5.5]undecane-2,8-dione

A coordination polymer {[Cu(C₁₁H₂₀N₄O₂)₂(H₂O)]²⁺·2(NO ₃ ^? )} _n (I) is synthesized and analyzed using single-crystal XRD. The crystals are monoclinic: space group P2/c, a = 12.5237(6) Å, b = 7.3310(3) Å, c = 16.8926(8) Å, β = 92.569(4)°, V = 1549.38(13) ų, ρ_calc = 1.47 g/cm³, and Z = 2. The copper atom is in a special position in the second-order axis. The coordination polyhedron of the metal is a trigonal bipyramid. In the axial direction, it is coordinated by two oxygen atoms O(1) of the organic ligand molecules connected by the symmetry operation [?x, y, 0.5-z]; the angle O(1)Cu(1)O(1) ⁱ is 175.16(15)°. In the equatorial direction, the copper atom is coordinated by oxygen atoms O(2) of two ligand molecules connected with the reference molecule by the symmetry operations [?x, y, 0.5-z] and [x, 1-y, ?0.5+z] and the water molecule O(6) in a special position on the twofold axis. The OCuO angles between the equatorial oxygen atoms are 96.80(16)–131.60(8)°, and those between the axial and equatorial atoms are 87.58(8)-91.60(10)°. It is noteworthy that the length of the Cu-O(1) bond (1.955(2) Å) with the axial carbonyl oxygen atom is somewhat shorter than the bond (2.060(3) Å) with the equatorial atom. Nitrate anions are outside the coordination sphere of the metal. The Cu…Cu distance in the polymer is 8.33 Å. To confirm the purity of the sample of I, the powder X-ray pattern is refined by the Rietveld method; the lattice parameters at room temperature are: a = 12.535(4) Å, b = 7.3161(13) Å, c = 16.841(5) Å, β = 92.11(2)°, and V = 1543.4(7) ų.     

On performing simultaneous electron transfer dissociation and collision-induced dissociation on multiply protonated peptides in a linear ion trap

We propose a tandem mass spectrometry method that combines electron-transfer dissociation (ETD) with simultaneous collision-induced dissociation (CID), termed ETD/CID. This technique can provide more complete sequence coverage of peptide ions, especially those at lower charge states. A selected precursor ion is isolated and subjected to ETD. At the same time, a residual precursor ion is subjected to activation via CID. The specific residual precursor ion selected for activation will depend upon the charge state and m/z of the ETD precursor ion. Residual precursor ions, which include unreacted precursor ions and charge-reduced precursor ions (either by electron-transfer or proton transfer), are often abundant remainders in ETD-only reactions. Preliminary results demonstrate that during an ETD/CID experiment, b, y, c, and z-type ions can be produced in a single experiment and displayed in a single mass spectrum. While some peptides, especially doubly protonated ones, do not fragment well by ETD, ETD/CID alleviates this problem by acting in at least one of three ways: (1) the number of ETD fragment ions are enhanced by CID of residual precursor ions, (2) both ETD and CID-derived fragments are produced, or (3) predominantly CID-derived fragments are produced with little or no improvement in ETD-derived fragment ions. Two interesting scenarios are presented that display the flexibility of the ETD/CID method. For example, smaller peptides that show little response to ETD are fragmented preferentially by CID during the ETD/CID experiment. Conversely, larger peptides with higher charge states are fragmented primarily via ETD. Hence, ETD/CID appears to rely upon the fundamental reactivity of the analyte cations to provide the best fragmentation without implementing any additional logic or MS/MS experiments. In addition to the ETD/CID experiments, we describe a novel dual source interface for providing front-end ETD capabilities on a linear ion trap mass spectrometer.     

Structure Determination of a Low Temperature Phase of Calcium and Strontium Amide by means of Neutron Powder Diffraction on Ca(ND2)2 and Sr(ND2)2

Calcium and Strontium amide are ionic compounds crystallising in a tetragonally distorted anatase structure-type at ambient temperatures. The amide ions (NH₂^–/ND₂^–) resemble water molecules in structure and in charge distribution. By means of temperature dependent neutron diffraction investigations weak super-structure reflections were observed at temperatures below 90 K (Ca(ND₂)₂) and 60 K (Sr(ND₂)₂), respectively, indicating the existence of a so far unknown low-temperature (LT) phase. Using high resolution neutron powder diffraction at temperatures below 10 K the structure was determined for both compounds. The LT-phases are isotypic and crystallise monoclinic in the space group P2₁/c with four formula units within the unit cell: Ca(ND₂)₂ at 10 K a = 7.257(2) Å, b = 7.2434(2) Å, c = 6.300(1) Å, β = 124.73(1)° Sr(ND₂)₂ at 5 K a = 7.6950(1) Å, b = 7.68374(9) Å, c = 6.6324(3) Å, β = 124.917(2)°. Their structure is closely related to the tetragonal HT-phase, but an ordering of the amide ions occurs due to freezing of a lattice mode which is dominated by the librational motion of the amide ions in the {1 0 0} planes of the HT-phase.     

Diagnostic di‐ and triphosphate cyclisation in the negative ion electrospray mass spectra of phosphoSer peptides

It has been shown previously that [M–H]^– anions of small peptides containing two phosphate residues undergo cyclisation of the phosphate groups, following collision‐induced dissociation (CID), to form a characteristic singly charged anion A (H₃P₂O₇^–, m/z 177). In the present study it is shown that the precursor anions derived from the diphosphopeptides of caerin 1.1 [GLLSVLGSVAKHVLPHVVPVIAEHL(NH₂)] and frenatin 3 [GLMSVLGHAVGNVLGGLFKPKS(OH)] also form the characteristic product anion A (m/z 177). Both of the precursor peptides show random structures in water, but partial helices in membrane‐mimicking solvents [e.g. in d₃‐trifluoroethanol/water (1:1)]. In both cases the diphosphopeptide precursor anions must have flexible conformations in order to allow approach of the phosphate groups with consequent formation of A: for example, the two pSer groups of 4,22‐diphosphofrenatin 3 are seventeen residues apart. Finally, CID tandem mass spectrometric (MS/MS) data from the [M–H]^– anion of the model triphosphoSer‐containing peptide GpSGLGpSGLGpSGL(OH) show the presence of both product anions A (m/z 177) and D (m/z 257, H₄P₃O₁₀^–). Ab initio calculations at the HF/6‐31+G(d)//AM1 level of theory suggest that cyclisation of the three phosphate groups occurs by a stepwise cascade mechanism in an energetically favourable reaction (ΔG = ?245 kJ mol^–1) with a maximum barrier of +123 kJ mol^–1. Copyright © 2011 John Wiley & Sons, Ltd.     

New binuclear complex of bis(2,4,6,8-tetramethyl-2,4,6,8- tetraazabicyclo(3.3.0)octane-3,7-dione-<Emphasis Type="Italic">O,O</Emphasis>')-diaquatetrakis(nitrato-<Emphasis Type="Italic">O,O</Emphasis>')-dimanganese(II) monohydrate: Synthesis and crystal structure

The centrosymmetric binuclear manganese(II) nitrate complex with a bicyclic bis-carbamide, namely, 2,4,6,8-tetramethyl-2,4,6,8-tetraazabicyclo(3.3.0)octane-3,7-dione, or mebicar (Mk) [Mn(C₈H₁₄N₄O₂)(H₂O)(NO₃)₂]₂ · H₂O (I), has been synthesized for the first time. The structure of complex I has been solved (CCDC no. 1435139). Crystals of complex I are monoclinic, space group P2₁/c, a = 12.8108(11) Å, _b = 10.0662(2) Å, c = 18.6367(17) Å, β = 136.512(16)°, V = 1654.0(4) ų, ρcalcd = 1.659 g/cm³, Z = 2. Each manganese atom is coordinated to the two oxygen atoms of two Mk molecules related by the symmetry codes (1–x, 2–y, 1–z) and to two bidentate nitrate anions and one water molecule. The coordination polyhedron of the manganese atom is a strongly distorted pentagonal bipyramid. The Mn···Mn distance in the complex is 8.7261(9) Å.     

Gas-Phase Synthesis of Singly and Multiply Charged Polyoxovanadate Anions Employing Electrospray Ionization and Collision Induced Dissociation

Electrospray ionization mass spectrometry (ESI-MS) combined with in-source fragmentation and tandem mass spectrometry (MS/MS) experiments were used to generate a wide range of singly and multiply charged vanadium oxide cluster anions including V_xO_y ^n– and V_xO_yCl^n– ions (x = 1–14, y = 2–36, n = 1–3), protonated clusters, and ligand-bound polyoxovanadate anions. The cluster anions were produced by electrospraying a solution of tetradecavanadate, V₁₄O₃₆Cl(L)₅ (L = Et₄N⁺, tetraethylammonium), in acetonitrile. Under mild source conditions, ESI-MS generates a distribution of doubly and triply charged V_xO_yCl^n– and V_xO_yCl(L)^(n–1)– clusters predominantly containing 14 vanadium atoms as well as their protonated analogs. Accurate mass measurement using a high-resolution LTQ/Orbitrap mass spectrometer (m/Δm = 60,000 at m/z 410) enabled unambiguous assignment of the elemental composition of the majority of peaks in the ESI-MS spectrum. In addition, high-sensitivity mass spectrometry allowed the charge state of the cluster ions to be assigned based on the separation of the major from the much less abundant minor isotope of vanadium. In-source fragmentation resulted in facile formation of smaller V_xO_yCl^(1–2)– and V_xO_y ^(1–2)– anions. Collision-induced dissociation (CID) experiments enabled systematic study of the gas-phase fragmentation pathways of the cluster anions originating from solution and from in-source CID. Surprisingly simple fragmentation patterns were obtained for all singly and doubly charged V_xO_yCl and V_xO_y species generated through multiple MS/MS experiments. In contrast, cluster anions originating directly from solution produced comparatively complex CID spectra. These results are consistent with the formation of more stable structures of V_xO_yCl and V_xO_y anions through low-energy CID. Furthermore, our results demonstrate that solution-phase synthesis of one precursor cluster anion combined with gas-phase CID is an efficient approach for the top-down synthesis of a wide range of singly and multiply charged gas-phase metal oxide cluster anions for subsequent investigations of structure and reactivity using mass spectrometry and ion spectroscopy techniques.      

Effect of methanol and sodium dodecylsulfate on radial profiles of ion abundance in inductively coupled plasma mass spectrometry

The radial profiles of some background ions and analytes spanning the mass range and with a wide range of first ionization potentials were investigated using inductively coupled plasma time-of-flight mass spectrometry. In particular, three different matrices were considered to assess the effect of organic modifiers: 1) 1% HNO₃, 2) 1% HNO₃ with 2%v/v methanol, 3) 1% HNO₃ with 0.2% m/v sodium dodecylsulfate (SDS). Although these concentrations of methanol and SDS induced the same sample transport increase (37%), as measured with a silica gel trap at the exit of the spray chamber, neither of them resulted in a 37% increase in signal across the mass range. In fact, the change in analyte signal as a function of m/z followed opposite trends in these two matrices. With 0.2% m/v SDS, suppression was observed at high m/z with an increasing enhancement as m/z decreased, which was ascribed to electrostatic effects in solution. In contrast, little change or enhancement was seen at high m/z while suppression was evident at lower m/z (with the notable exception of As) with 2% v/v methanol, as a result of a widening of the radial profile, which was inversely dependent on m/z. Although, the total carbon concentration was quite different in these two matrices, i.e. 0.8 M with 2% methanol and 0.08 M with 0.2% SDS, it cannot account for the completely different radial profiles that they produced. Indeed, the same bell-shaped distributions of analyte ions were observed with 0.2% SDS as in 1% HNO₃ alone. However, a bimodal distribution, with maxima on either side of the central axis, resulted in presence of 2% MeOH. This distribution was found to be similar to that of several background ions (C⁺, CO⁺ and ArC⁺), which further substantiates the suggestion that ionization then predominantly occurs through charge transfer with carbon-containing ions. This bimodal distribution also suggests that a volatile organic solvent such as methanol quickly spreads into the surrounding area of the central channel upon sample introduction in the plasma.