Anionic adducts of oligosaccharides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The formation and decomposition (postsource decay, PSD) of anionic adducts in negative ion matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry have been studied. Chloride, a small inorganic anion, has been found to form stable anionic adducts with a variety of neutral oligosaccharides that can survive the MALDI process to give readily identifiable signals (with characteristic isotope patterns) allowing subpicomole detection in the best cases. The matrixes that can aid the formation of chloride adducts of oligosaccharides have gas-phase acidities lower than or close to that of HCl (1373 kJ/mol). In PSD experiments, precursor chloride adducts of oligosaccharides yield fragment ions that retain the charge on the sugar molecule rather than solely forming Cl-, and these fragments can provide structurally informative product ions. In negative ion MALDI, highly acidic oligosaccharides do not form adducts with chloride anions, but mildly acidic saccharides (e.g., containing a carboxylic acid group) form both deprotonated molecules and chloride adducts, and each may provide structural information concerning the oligosaccharide upon decomposition.     

Effects of mobile-phase additives, solution pH, ionization constant, and analyte concentration on the sensitivities and electrospray ionization mass spectra of nucleoside antiviral agents

The effects of various mobile-phase additives, solution pH, pKa, and analyte concentration on electrospray ionization mass spectra of a series of purine and pyrimidine nucleoside antiviral agents were studied in both positive and negative ion models. The use of 1% acetic acid resulted in good HPLC separation and the greatest sensitivity for [M + H]+ ions. In the negative ion mode, 50 mM ammonium hydroxide gave the greatest sensitivity for [M - H]- ions. The sensitivities as [M + H]+ ions were significantly larger than the sensitivities as [M - H]- ions for purine antiviral agents. Vidarabine monophosphate and pyrimidine antiviral agents, however, showed comparable or greater sensitivities as [M - H]- ions. The sensitivity as [M + H]+ showed no systematic variation with pH; however, the sensitivity as [M - H]- did increase with increasing pH. At constant pH, the ion intensity of the protonated species increased with increasing pKa. At higher analyte concentrations, dimer (M2H+) and trimer (M3H+) ions were observed. [M + Na]+ adducts were the dominant ions with 0.5 mM sodium salts for these compounds. The spectra of the more basic purine antiviral agents showed no [M + NH4]+ adduct ions, but [M + NH4]+ ions were the major peaks in the spectra of the less basic pyrimidine antiviral agents with ammonium salts. The ammonium adduct ion was formed preferentially when the proton affinity of the analyte was close to that of NH3. Abundant [M + OAc]- ions were observed for all of the antiviral agents except vidarabine monophosphate from solutions with added HOAc, NaOAc, and NH4OAc. The utility of mobile phases containing 1% HOAc or 50 mM NH4OH was demonstrated for chromatographic separations.     

An ion-selective electrode for acetate based on a urea-functionalized porphyrin as a hydrogen-bonding ionophore

An ion-selective electrode for acetate based on (α,α,α,α)-5,10,15,20-tetrakis[2-(4-fluorophenylureylene)phenyl]porphyrin as an ionophore that has no metal center and forms hydrogen bonds to the analyte is described. At pH 7.0 (0.1 M HEPES-NaOH buffer), the electrode based on this ionophore and cationic sites (50 mol % relative to the ionophore) responds to acetate in a linear range from 1.58 × 10(-)(4) to 1.58 × 10(-)(2) M with a slope of -54.8 ± 0.8 mV/decade and a detection limit of (3.06 ± 1.15) × 10(-)(5) M. Selectivity coefficients determined with the separate solution method (SSM) indicate that interferences of hydrophobic inorganic anions are relatively small (log[Formula: see text] (SSM): NO(3)(-), +0.68; SCN(-), +0.60; NO(2)(-), +0.22; I(-), +0.20; ClO(4)(-), +0.12; Br(-), -0.13). Responses to anions that are good hydrogen bond acceptors, i.e., Cl(-), HSO(3)(-), and HCO(3)(-), were Nernstian and were weaker than the response to acetate (log[Formula: see text] (SSM): -0.54, -0.56, and -1.34, respectively). Negligibly small responses were observed for very hydrophilic anions, i.e., F(-), SO(4)(2)(-), and H(2)PO(4)(-)/HPO(4)(2)(-). While aliphatic carboxylates such as formate, propanoate, pyruvate, and lactate gave Nernstian responses similar to acetate, interferences of salicylate and benzoate were considerably decreased in comparison with electrodes based on cationic sites only. Concentrations of acetic acid in vinegar samples were determined by direct potentiometry and agreed with values determined by a standard enzymatic method.     

Ion-pairing ability,chemical stability,and selectivity behavior of halogenated dodecacarborane cation exchangers in neutral carrier-based ion-selective electrodes

Recently, it has been discovered that carba-closo-dodecaborates can be used as cation exchangers in neutral carrier-based ion-selective chemical sensors. Because of their inherent chemical stability and versatile functionalization chemistries, they offer many advantages that may potentially be exploited for ion analyses that require nontraditional sample conditions, including strongly acidic media. In this work, trimethylammonium salts of undecachlorinated (UCC), undecabrominated (UBC), hexabrominated (HBC), and undecaiodinated (UIC) carborane anions were prepared and evaluated for their potential use in solvent polymeric membrane-based sensors. Computational methods including Natural population analysis and electrostatic mapping were used to predict the ion-exchanging ability of each lipophilic anion. In addition, the sandwich membrane technique was used to evaluate the ion-pairing ability of each carborane anion in situ (i.e., within bis(2-ethylhexyl) sebacate (DOS)- and 2-nitrophenyl octyl ether (o-NPOE)-plasticized ISE membranes). The results of the computational and potentiometric studies found that binding affinity of the anions followed the generalized trend HBC > UCC > UBC > UIC. PVC-DOS bulk optode thin films containing the chromoionophore ETH 5315 and a respective anion were used to determine the chemical stability/lipophilicity of the carboranes and tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (TFPB) in acidic media (0.2 M HOAc) under flowing conditions. The studies found that in terms of stability/lipophilicity UIC > UBC > TFPB approximately UCC > HBC. Electrodes containing a Pb(2+)-selective ionophore, tert-butylcalix[4]arene-tetrakis(N,N-dimethylthioacetamide)(lead IV), were used to evaluate the functionality of each cation exchanger. An evaluation of response characteristics such as slope and selectivity found that UIC and UBC were quite comparable to the behavior of TFPB. Interestingly, both UIC and UBC showed a marked selectivity improvement over cadmium, with log K(pot)(pb),(Cd) values of -7.19 and -7.29, respectively, with TFPB giving a value of -5.89. Demonstrating excellent stability and suitable electrostatic properties, the carboranes, UIC in particular, are a very promising alternative to the tetraphenylborates and should find widespread application in the field of chemical sensors.     

In-source and postsource decay in negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of neutral oligosaccharides

Cross-ring cleavage ions produced by in-source decay (ISD), as well as deprotonated molecular ions [M - H]-, are invariably observed in negative-ion linear-mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry spectra of neutral oligosaccharides with 9H-pyrido[3,4-b]indole (norharman) as a matrix. The patterns of ISD ions depend on the oligosaccharide linkage type; thus, these ions are potentially useful in linkage analysis. In postsource decay (PSD) spectra from chlorinated molecular ions [M + Cl]-, all PSD ions are observed in the deprotonated form, although no deprotonated molecular ions are detected. In oligosaccharides having an alditol at the reducing end, deprotonated molecular ions [M - H]- are clearly seen in linear-mode mass spectra and survive in the PSD measurements. These results indicate that the deprotonation process drives ISD and PSD of oligosaccharides and that keto-enol tautomerization at the reducing terminal promotes ISD and PSD processes.     

A new nanostructured material based on fluorophosphate incorporated into a zinc–aluminium layered double hydroxide by ion exchange

Layered double hydroxides (LDHs), also called anionic clays, consist of cationic brucite-like layers and exchangeable interlayer anions. These hydrotalcite-like compounds, with Zn and Al in the layers and chloride in the interlayer space, were prepared following the coprecipitation method at constant pH. The effect of pH, aging time and anion concentration on the intercalation of fluorophosphate \((\hbox {PO}_{3}\hbox {F}^{2-}\), FP) in the [Zn–Al] LDH was investigated. The best crystalline material, with high exchange extent, was obtained by carrying out the exchange at 25\({^{\circ }}\hbox {C}\) in a 0.03 M FP solution at pH 7 with at least 42 h of aging time. A mechanism for the FP intercalation was confirmed by X-ray diffraction, infrared spectroscopy and thermogravimetry (TG) analyses (TG and DTG curves).     

Coupled Electron- and Proton-Transfer Processes in the Reduction of α-[P(2)W(18)O(62)](6)(-) and α-[H(2)W(12)O(40)](6)(-) As Revealed by Simulation of Cyclic Voltammograms

Quantitative analysis of the complex problem of coupled electron- and proton-transfer steps during reduction of the polyoxo anions α-[P(2)W(18)O(62)](6)(-) and α-[H(2)W(12)O(40)](6)(-) in aqueous NaCl (0.5 M) has been achieved by simulation of cyclic voltammograms (Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66, 589A) over wide ranges of anion concentration, pH, and scan rate. Since there are too many unknown parameters to attempt a one-step global form of simulation, a systematic, stepwise approach has been adopted by progressively accessing regimes of increasing voltammetric complexity. This protocol allows experimental behavior in each system over 5 orders of magnitude in proton concentration to be simulated by estimation of three protonation constants combined with experimentally determined reversible half-wave potentials for the two one-electron processes involved. Fast electron transfer and protonation kinetics are assumed. The importance of the values chosen for the diffusion coefficients of the proton and polyoxo anion species is considered. The simulations account for the fact that pairs of one-electron processes coalesce to give an apparent two-electron process in the pH range 1-6 for reduction of both anions.     

Desorption and ionization mechanisms in desorption atmospheric pressure photoionization

The factors influencing desorption and ionization in newly developed desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS) were studied. Redirecting the DAPPI spray was observed to further improve the versatility of the technique: for dilute samples, parallel spray with increased analyte signal was found to be the best suited, while for more concentrated samples, the orthogonal spray with less risk for contamination is recommended. The suitability of various spray solvents and sampling surface materials was tested for a variety of analytes with different polarities and molecular weights. As in atmospheric pressure photoionization, the analytes formed [M + H](+), [M - H](-), M(+*), M(-*), [M - H + O](-), or [M - 2H + 2O](-) ions depending on the analyte, spray solvent, and ionization mode. In positive ion mode, anisole and toluene as spray solvents promoted the formation of M(+*) ions and were therefore best suited for the analysis of nonpolar compounds (anthracene, benzo[a]pyrene, and tetracyclone). Acetone and hexane were optimal spray solvents for polar compounds (MDMA, testosterone, and verapamil) since they produced intensive [M + H](+) ion peaks of the analytes. In negative ion mode, the type of spray solvent affected the signal intensity, but not the ion composition. M(-*) ions were formed from 1,4-dinitrobenzene, and [M - H + O](-) and [M - 2H + 2O](-) ions from 1,4-naphthoquinone, whereas acidic compounds (naphthoic acid and paracetamol) formed [M - H](-) ions. The tested sampling surfaces included various materials with different thermal conductivities. The materials with low thermal conductivity, i.e., polymers like poly(methyl methacrylate) and poly(tetrafluoroethylene) (Teflon) were found to be the best, since they enable localized heating of the sampling surface, which was found to be essential for efficient analyte desorption. Nevertheless, the sampling surface material did not affect the ionization mechanisms.     

Charged polaron-enhanced circular dichroism of optically nonlinear 3-nitroaniline crystal

The circular dichroism (CD) spectra in visible range of the 3-nitroaniline (m-NA) single crystal, of its solution in chloroform, and in the KBr pellet were recorded. Neither the molecular (m or C_s) nor the crystal (mm2 or C_2v) point groups belong to the chiral groups. The DFT calculation of CD spectrum of the m-NA isolated neutral molecule confirmed its chirality. The red shifted bands in the calculated CD spectrum of m-NA radical anion (charged polaron), as compared with the neutral molecule, resemble better the spectra of solids than that of the solution. It seems that these facts corroborate qualitatively the “hop and turn” model explaining the m-NA optical nonlinearity and electric conductivity proposed in Szostak et al. [M.M. Szostak, H. Chojnacki, E. Staryga, M. D?u?niewski, G. B?k, Chem. Phys. 365 (2009) 44-52].     

Solvation energy and gas-phase stability influences on alkali metal cluster ion formation in electrospray ionization mass spectrometry

The ability to observe abundant gas-phase metal cluster ions in electrospray ionization mass spectrometry (ESI-MS) is highly dependent on experimental conditions. Alkali halides (MX) and other alkali metal salts were used to investigate the formation of cluster ions in ESI-MS. All compounds were found to give cluster ions of the form (M(n)(+1)X(n))(+) and (M(n)X(n+1))(-), with only two alkali salts yielding doubly charged cluster ions. In homologous alkali halide series, the relative abundances of cluster ions increased with increasing size of either the cation (positive ion mode) or the anion (negative ion mode). Calculations using an electrostatic model show that the gas-phase stability of cluster ions is greater for smaller cations or anions when a fixed counterion is employed. This stability calculation goes in a direction just opposite to the trend in cluster ion abundances observed in ESI-MS. Studies of equimolar mixtures consisting of two alkali halides reveal two distinct trends. When the equimolar mixture was composed of differing ions that participate in the droplet charge excess with the same counterion, the less solvated ions were found to form more abundant cluster ions. When the ions participating in the charge excess were fixed, the preferred counterion in observed clusters was the one that is more solvated in solution and forms more stable clusters in the gas phase. These observations can be rationalized by an extended form of the charged residue model where the weakly solvated ions that are part of the charge excess are preferentially enriched in offspring droplets during uneven fission. By contrast, transfer of a particular counterion located in the bulk of the droplets to the offspring droplets is not disfavored when this counterion is strongly solvated.     

Sensitive analyses of neutral N-glycans using anion-doped liquid matrix G3CA by negative-ion matrix-assisted laser desorption/ionization mass spectrometry

Negative-ion fragmentation of N-glycans has been proven to be more informative than that of positive-ion. In particular, it defines structural features such as the specific composition of the two antennae and the location of fucose. However, negative-ion formation of neutral N-glycans by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) remains a challenging task, and the detection limit of N-glycans in negative-ion mode is merely at the subpicomole level. Thus, practical applications are limited. In this study, combinations of five liquid matrices and nine anions were used to ionize N-glycans as anionic adducts, and their performances for sensitive analyses were evaluated. The best results were obtained with anion-doped liquid matrix G(3)CA, which consists of p-coumaric acid and 1,1,3,3-tetramethylguanidine; the detection limits of anion adducted N-glycans were 1 fmol/well for NO(3)(-), and 100 amol/well for BF(4)(-). Negative-ion MS(2) spectra of 1 fmol N-glycans were successfully acquired with a sufficient signal-to-noise ratio and were quite useful for MS-based structural determination. The anion-doped G(3)CA matrix opens the way for sensitive and rapid analysis of neutral N-glycans in negative-ion MALDI at a low femtomole level.     

Quantum chemical treatment of cyanogen azide and its univalent and divalent ionic forms

An explosive material, cyanogen azide (CN4) and its univalent and divalent anionic and cationic forms have been studied quantum chemically by using different theoretical approaches. In this study, the structures considered have been screened for their relative stabilities. Also, they have been investigated whether the charged forms play a role in the usual explosion process or any electrical charging during storage cause explosion. Various quantum chemical properties are obtained and discussed. It has been found that the univalent cation and anion and divalent cation formations do not cause much change in the molecular structure as compared to the neutral cyanogen azide molecule, whereas the divalent anionic form exhibits drastic changes in the geometry, resulting in bond cleavage to eliminate nitrogen molecule.     

Detection of in vivo formed DNA adducts at the part-per-billion level by capillary liquid chromatography/microelectrospray mass spectrometry

Capillary liquid chromatography/microelectrospray mass spectrometry has been applied to the detection of deoxyribonucleoside adducts of the food-derived mutagen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) from in vivo sources. Adjustments were made to a previously described methodology such that analyte detection could be improved by nearly 2 orders of magnitude. These adjustments included changing the electrospray ionization sprayer configuration, increasing the sample injection volume, improving the solid-phase extraction procedure, and increasing peak efficiency by modifying chromatographic conditions. While this scheme for improving analyte detection was targeted for DNA adducts, it could be applied to almost any LC/MS methodology where sensitive analysis is the primary objective. Selective reaction monitoring) techniques with a triple quadrupole mass spectrometer enabled sensitive and specific detection of IQ adducts, with detection limits approaching 1 adduct in 10(9) unmodified bases using approximately 500 microg of DNA. The DNA adducts N-(2'-deoxyguanosin-8-yl)-2-amino-3-methylimidazo[4,5-f]quinoline and 5-(2'-deoxyguanosin-N2-yl)-2-amino-3-methylimidazo[4,5-f]quinoline were detected in pancreas tissue of a cynomolgus monkey sacrificed 24 h after a single administration of 10 mg/kg carcinogen. The LC/MS results were consistent with previously published 32P-postlabeling data (Turesky et al. Chem Res. Toxicol. 1996, 9, 403-408). Thus, capillary tandem LC/MS is a highly sensitive technique, which can be used to screen for DNA adducts in vivo.     

四硼酸锂晶体解理的形成机理

本文从结晶化学角度研究了Ｌｉ２Ｂ４Ｏ７晶体结构，根据晶体的结构基元为［Ｂ４Ｏ９］＾６－络阴离子，并用计算机绘出了该络阴离子在｛００１｝，｛１００｝，｛１１０｝面上的投影，由［Ｂ４Ｏ９］＾６－络阴离子的结晶方位和分布确定了晶体的开裂是沿着络阴离子之间相连接的桥氧部位所断裂的。     

Laser desorption-atmospheric pressure chemical ionization mass spectrometry for the analysis of peptides from aqueous solutions

A recently reported ionization method, comprising an infrared (IR) laser pulse to desorb (LD) analyte species, followed by atmospheric pressure chemical ionization (APCI) with a corona discharge (LD-APCI) to effect ionization of the desorbed neutral analyte molecules, is described for the direct analysis of aqueous peptide solutions. The source employs a heated capillary atmospheric pressure (AP) inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. By use of the corona discharge, signals of the atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI)-generated analyte protonated molecule were enhanced by factors as large as 1400. In addition, the acid modifier trifluoroacetic acid (TFA) was found to improve the AP-IR-MALDI-generated signal by a factor of approximately 10, whereas the LD-APCI generated signal yielded a 100-fold increase. In this study, the use of the corona discharge is described to enhance the analyte signal generated via AP-IR-MALDI and, as a tool, to probe the gas-phase neutral molecule population generated by the MALDI process. Finally, through the decoupling of desorption from ionization, implications regarding the application of LD-APCI for the direct analysis of numerous new analyte containing matrixes (e.g., polyacrylamide gel electrophoresis (PAGE), tissue, etc.) are discussed.     

Charge-remote fragmentation of odd-electron peptide ions

Comparison between the gas-phase fragmentation of odd-electron M+*, [M + H]2+*, and [M - 2H]-* ions of model peptides suggests that charge-remote radical-driven fragmentation pathways play an important role in the dissociation of odd-electron peptide ions. We have found that charge-remote processes are responsible for a variety of side-chain losses from the precursor ion and some backbone fragmentation. These fragmentation pathways most likely involve hydrogen abstraction by the radical site that initiates subsequent cleavages. These findings are generally relevant to our understanding of the fragmentation patterns of odd-electron peptide ions produced through various approaches including the capture of low-energy electrons, electron detachment, and electron transfer.     

The effect of substituents on molecular electronic junctions

Previous reports have demonstrated that molecular electronic junctions exhibiting negative differential resistance show a marked substituent effect. We show that this substituent effect correlates with stability of the anionic form of the junction molecule. A sufficiently stable anion gives rise to a double potential barrier to electron transport across the molecular junction. The effect of thermal motion of the junction molecule on the electron transport is also considered.     

Absorption of water by room-temperature ionic liquids: effect of anions on concentration and state of water

Near-infrared (NIR) spectrometry was successfully used for the non-invasive and in situ determination of concentrations and structure of water absorbed by room-temperature ionic liquids (RTILs). It was found that RTILs based on 1-butyl-3-methylimidazolium, namely, [BuMIm]+ [BF4]-, [BuMIm]+ [bis((trifluoromethyl)sulfonyl)amide, or Tf2N]- and [BuMIm]+ [PF6]-, are hydroscopic and can quickly absorb water when they are exposed to air. Absorbed water interacts with the anions of the RTILs, and these interactions lead to changes in the structure of water. Among the RTILs studied, [BF4]- provides the strongest interactions and [PF6]- the weakest. In 24 hours, [Bu-MIm]+ [BF4]- can absorb up to 0.320 M of water, whereas [Bu-MIm]+ [PF6]- can only absorb 8.3 x 10(-2) M of water. It seems that higher amounts of water can be absorbed when the anion of the RTIL can strongly interact and hence stabilize absorbed water molecules by forming hydrogen bonds with them or inducing hydrogen bonds among water molecules. More importantly, the NIR technique can be sensitively used for the noninvasive, in situ determination of absorbed water in RTILs, without any pretreatment, and at limits of detection as low as 3.20 x 10(-3) M.     

A flexible interpenetrating coordination framework with a bimodal porous functionality

Introducing a functional part into open-framework materials that tunes the pore size/shape and overall porous activity will open new routes in framework engineering and in the fabrication of new materials. We have designed and synthesized a bimodal microporous twofold interpenetrating network {[Ni(bpe)2(N(CN)2)](N(CN)2)(5H2O)}n (1), with two types of channel for anionic N(CN)2- (dicyanamide) and neutral water molecules, respectively. The dehydrated framework provides a dual function of specific anion exchange of free N(CN)2- for the smaller N3- anions and selective gas sorption. The N3-exchanged framework leads to a dislocation of the mutual positions of the two interpenetrating frameworks, resulting in an increase in the effective pore size in one of the counterparts of the channels and a higher accommodation of adsorbate than in the as-synthesized framework (1), showing the first case of controlled sorption properties in flexible porous frameworks.     

MALDI-TOF MS of phosphorylated lipids in biological fluids using immobilized metal affinity chromatography and a solid ionic crystal matrix

When targeting a certain class of analytes, such as the phosphorylated lipids in complex biological extracts, interfering species can pose challenges to qualitative and quantitative analyses. Two aspects of lipid analysis were optimized to simplify the isolation and characterization of phosphorylated lipids in biological extracts. A new solid ionic crystal MALDI matrix was synthesized which combined the lipid response enhancing UV-absorber p-nitroaniline with the protonating agent butyric acid. Mass spectra of the extracts containing phosphorylated lipids were simplified by revealing only protonated molecules [M + H]+ of the zwitterionic phosphatidylcholine (PC) headgroup-containing lipids, such as lyso-PC, PC, and platelet-activating factor. For the anionic phosphorylated lipids, such as phosphatidylglycerol, phosphatidic acid, and phosphatidylserine, further spectrum simplification is obtained by the appearance of only the monosodium adducts [M + Na]+ as the major molecular ions, in preference to the double sodium adducts [M + 2Na - H]+. In addition, a new extraction, isolation, and cleanup procedure has been developed to prepare the phosphorylated lipids for MALDI-TOF analysis by the use of immobilized metal ion affinity chromatography media (i.e., ZipTip). The latter procedure was successfully applied to a complex biological tear film lipid layer extract in preparation for MALDI-TOF analysis and phospholipid characterization.