Positive mode electrospray ionization mass spectrometry of bisphosphonates using dicationic and tricationic ion-pairing agents

A general method for detecting bisphosphonate drugs by ESI-MS and LC-ESI-MS as positive ions has been developed. Bisphosphonates can have multiple negative charges in solution. Tricationic ion-pairing reagents were paired with bisphosphonates to form a positively charged complex. It was clear that this facile pairing method worked. However, an appreciable presence of −1 bisphosphonate species were observed in positive mode ESI-MS (i.e. as the +2 complex with tricationic reagents). This led to an extended investigation on the use of dicationic pairing agents. The use of dicationic reagents improved the detection sensitivity for all of the bisphosphonates. Tandem mass spectrometry also improved the limits of detection for most of the bisphosphonates using both the tricationic and dicationic pairing reagents. A tricationic reagent also was used as an ion-pairing reagent in chromatography experiments. Thus the addition of a single reagent produced benefits in that it increased chromatographic retention and enhanced the ESI-MS detection of bisphosphonates.     

Imidazolium-Based Dicationic Cyclophanes. Solid-State Aggregates with Unconventional (C–H)+···Cl− Hydrogen Bonding Revealed by X-ray Diffraction

The first single-crystal X-ray crystallographic diffraction analysis of a dicationic heterophane showed a non-classic (C–H)⁺···Cl^?  hydrogen bond between the imidazolium rings and halide anions and the formation of unconventional charged assisted hydrogen bonds, which were the non-covalent forces driving the anion interactions shown by the dications 4·2X. Here is reported the halide-templated controlled synthesis and chemical response in basic media of 4·2X. Their structural properties were examined at the gas phase by electrospray ionization mass spectrometry in the negative-ion mode and in the solid-state by X-ray crystallography. Thus, the negative-ion ESI-MS response showed that the formation of non-covalent self-aggregates of macrocyclic dications is a consequence of hydrogen-bonded complexes with halide anions. Notably, X-ray diffraction of dication 4a·2Cl·2H₂O provides evidence for the H-bonding network, which has a crucial role in crystal packing. The solid-state aggregates showed that chloride anions and water molecules formed channels among dications 4a⁺.     

The evaluation and comparison of trigonal and linear tricationic ion-pairing reagents for the detection of anions in positive mode ESI-MS

A general and sensitive method for detecting divalent anions by ESI-MS and LC/ESI-MS as positive ions has been developed. The anions are paired with tricationic reagents to form positively charged complexes. In this study, four tricationic reagents, 2 trigonal and 2 linear, were used to study a wide variety of anions, such as disulfonates, dicarboxylates, and inorganic anions. The limits of detection for many of the anions studied were often improved by tandem mass spectrometry. Tricationic pairing agents can also be used with chromatography to enhance the detection of anions. The tricationic reagents were also used to detect monovalent anions by monitoring the doubly charged positive complex. The limits of detection for some of the divalent anions by this method are substantially lower than by other current analytical techniques.     

Analysis of neutral drugs in human plasma by fluoride attachment in liquid chromatography/negative ion electrospray tandem mass spectrometry

The analysis of several neutral drugs, mephenesin, guaifenesin, simvastatin, podophyllotoxin and inositol, was accomplished by negative ion electrospray ionization mass spectrometry (ESI-MS) using adduct formation with three different halide ions. The fluoride, chloride and bromide adducts of the selected drugs exhibited intense signals in negative ion ESI. Under collision-induced dissociation, the major product ions of bromide and chloride adducts were the nonspecific bromide and chloride anions, respectively. In contrast, fluoride adducts produced strong [M--H](-) ions as well as product ions with good intensity. Fluoride attachment liquid chromatography/negative ion electrospray tandem mass spectrometry (LC/ESI-MS/MS) was applied to the analysis of the selected neutral drugs in human plasma. Detection limits in the range of 0.025-0.05 ng/mL were achieved using 0.5 mL plasma. Good linearity was observed for each of the drugs examined in human plasma over the range of 0.05-50 ng/mL.     

Use of ion pairing reagents for sensitive detection and separation of phospholipids in the positive ion mode LC-ESI-MS

Phospholipids make up one of the more important classes of biological molecules. Because of their amphipathic nature and their charge state (e.g., negatively charged or zwitterionic) detection of trace levels of these compounds can be problematic. Electrospray ionization mass spectrometry (ESI-MS) is used in this study to detect very small amounts of these analytes by using the positive ion mode and pairing them with fifteen different cationic ion pairing reagents. The phospholipids used in this analysis were phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidic acid (PA), 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC), cardiolipin (CA) and sphingosyl phosphoethanolamine (SPE). The analysis of these molecules was carried out in the single ion monitoring (SIM) positive mode. In addition to their detection, a high performance liquid chromatography and mass spectrometry (HPLC-MS) method was developed in which the phospholipids were separated and detected simultaneously within a very short period of time. Separation of phospholipids was developed in the reverse phase mode and in the hydrophilic interaction liquid chromatography (HILIC) mode HPLC. Their differences and impact on the sensitivity of the analytes are compared and discussed further in the paper. With this technique, limits of detection (LODs) were very easily recorded at low ppt (ng L(-1)) levels with many of the cationic ion pairing reagents used in this study.     

Comparison of flow injection analysis electrospray mass spectrometry and tandem mass spectrometry and electrospray high-field asymmetric waveform ion mobility mass spectrometry and tandem mass spectrometry for the determination of underivatized amino acids

Twenty proteinogenic amino acids (AAs) were determined without derivatization using flow injection analysis followed by electrospray ionization mass spectrometry and tandem mass spectrometry (ESI-MS and ESI-MS/MS) and electrospray ionization high-field asymmetric waveform ion mobility mass spectrometry and tandem mass spectrometry (ESI-FAIMS-MS and ESI-FAIMS-MS/MS), in positive and negative ionization modes. Three separate sets of ESI-FAIMS conditions were used for the separation and detection of the 20 AAs. Typically ESI-FAIMS-MS showed somewhat improved sensitivity and significantly better signal-to-noise ratios than ESI-MS mainly due to the elimination of background noise. However, the difference between ESI-FAIMS-MS and ESI-MS/MS was significantly less. ESI-FAIMS was able to partially or completely resolve all the isobaric amino acid overlaps such as leucine, isoleucine and hydroxyproline or lysine and glutamine. Detection limits for the amino acids in ESI-FAIMS-MS mode ranged from 2 ng/mL for proline to 200 ng/mL for aspartic acid. Overall, ESI-FAIMS-MS is the preferred method for the quantitative analysis of AAs in a hydrolyzed yeast matrix.     

Evaluation of tetracationic salts as gas‐phase ion‐pairing agents for the detection of trivalent anions in positive mode electrospray ionization mass spectrometry

In previous studies, new electrospray ionization mass spectrometry (ESI‐MS) approaches were developed for the highly sensitive detection of singly and doubly charged anions in positive mode ESI‐MS by using specially synthesized dicationic and tricationic ion‐pairing agents, respectively. By detecting the positively charged ion complex in the positive mode, limits of detection (LODs) for the anions can be lowered by several magnitudes. In this work, we used eighteen newly synthesized tetracationic ion‐pairing agents, constructed with different geometries, linkages and cation moieties, for the detection of eighteen triply charged anions of different structural motifs. The LODs for these anions were from ten to several thousand times lower in the positive selective ion monitoring (SIM) mode than in the negative mode. These tetracationic agents also were shown to be useful for the detection of ?1 and ?2 anions. In addition, the LODs for ?3 anions can be further lowered by monitoring the product fragments of the ion‐pairing complexes in the single reaction monitoring (SRM) mode. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrostatic and Non‐covalent Interactions in Dicationic Imidazolium–Sulfonium Salts with Mixed Anions

A series of thioether‐functionalised imidazolium salts have been prepared and characterized. Subsequent reaction of the thioether‐functionalised imidazolium salts with iodomethane affords imidazolium–sulfonium salts composed of doubly charged cations and two different anions. Imidazolium–sulfonium salts containing a single anion type are obtained either by a solvent extraction method or by anion exchange. The imidazolium–sulfonium salts undergo a methyl‐transfer reaction on exposure to water, giving rise to a new, singly charged imidazolium salt with iodide introduced at the 2‐position of the imidazolium ring. Crystal structures of some of the imidazolium–sulfonium salts were determined by X‐ray crystallography providing the topology of the interactions between the dications and the anions. Electrospray ionization mass spectrometry and quantum‐chemical calculations were used to rationalise the relative strength of these interactions.     

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.      

CE-ESI-MS separation of divalent organic and inorganic anions using a tricationic complexing reagent

A tricationic ion-pairing reagent, 1,3,5-1-butyl-3-methyl-1H-imidazol-3-ium-2,4,6-trimethylbenzene, was used to form complexes with doubly charged anions for their subsequent analysis by capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) in the positive-ion mode. This methodology offers the advantages of greater versatility and sensitivity relative to direct detection of the anions in negative-ion mode, and it can be realized by a number of possible complexation strategies, including pre-column, on-column, and post-column modes. Three model anions, sulfate [SO?2?], thiosulfate [TSFA, S?O?2?], and benzenedisulfonate [BZDS, C?H?(SO?)?2?], were amenable to complexation with the tricationic reagent, yielding singly charged cations with greater mass-to-charge (m/z) ratios than the native analytes. By utilizing optimized parameters obtained through previous work with dicationic reagents and singly charged anions, including the CE separation buffer composition and pH, the concentration of the dicationic reagent, the mode of complexation, the nebulizing gas pressure, and the sheath liquid composition, it was possible to develop a robust CE-ESI-MS method appropriate for the analysis of divalent anions in a mixture.     

Reduced matrix effects for anionic compounds with paired ion electrospray ionization mass spectrometry

It is well-known that matrix effects in high performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC-ESI-MS) can seriously compromise quantitative analysis and affect method reproducibility. Paired ion electrospray ionization (PIESI) mass spectrometry is an approach for analyzing ultra-low levels of anions in the positive ion mode. This approach uses a structurally optimized ion pairing reagent to post-column associate with the anionic analyte, subsequently forming positively charged complexes. These newly formed complex ions are often more surface-active as compared to either the native anion or the ion pairing reagent. No studies have examined whether or not the PIESI approach mitigates matrix effects. Consequently, a controlled study was done using five analytes in highly controlled and reproducible synthetic groundwater and urine matrices. In addition, two different mass spectrometers (linear ion trap and triple quadrupole) were used. Compared to the negative ion mode, the PIESI-MS approach was less susceptible to matrix effects when performed on two different MS platforms. Using PIESI-MS, less dilution of the sample is needed to eliminate ionization suppression which, in turn, permits lower limits of detection and quantitation.     

Separation of N-nitrosoamino acids by C18 reversed-phase ion-pair high-performance liquid chromatography and compatible detection by electrospray ionization mass spectrometry.

Four non-volatile N-nitrosoamino acids, namely N-nitrososarcosine, N-nitrosoproline, N-nitrosothiazolidine-4-carboxylic acid and N-nitroso-2-methyl-thiazolidine-4-carboxylic acid were separated by C18 reversed-phase ion-pair high-performance liquid chromatography (HPLC) using 1.4 mM C16-cetyltrimethylammonium chloride in methanol-water-acetonitrile (60:35:5, v/v) as the mobile phase. The N-nitrosoamino acids were sensitively detected by negative electrospray ionization mass spectrometry (ESI-MS) in the form of the deprotonated carboxylate anion, [M-H]-. Compatibility problems associated with HPLC separation and ESI-MS detection, such as formation of solvent cluster ions and the effects of co-eluting anions of the ion-pairing reagent, were systematically investigated. The optimized experimental conditions for separation and detection of N-nitrosoamino acids were described.     

Determination of trace inorganic anions in anionic surfactants by single-pump column-switching ion chromatography

An ion chromatography method has been proposed for the determination of three common inorganic anions(chloride,nitrate and sulfate) in anionic surfactants using a single pump system.The new system consists of an ion exclusion column,a concentrator column,and an anion exchange column connected in series via two 6-ports valves in a Dionex ICS-2000 ion chromatograph.The valves were switched several times for removing surfactants,concentrating and separating the three anions.The chromatographic conditions were optimized.Detection limits(S/N = 3) were in the range of 0.10-0.68μg/L.The relative standard deviations(RSDs) of peak area were less than 4.6%.The recoveries were in the range of 84.1-112.6%.     

Arsenic speciation by ion chromatography with inductively coupled plasma mass spectrometric detection.

Four As compounds were successfully separated and detected by single-column ion chromatography with inductively coupled plasma (ICP) mass spectrometric detection. The mass spectral interferent ArCl+ was reduced by chromatographically resolving chloride from the negatively charged arsenic species. Determination of four As species was investigated in urine, club soda and wine. Detection limits of 0.16 ng of As(III), 0.26 ng of As(v), 0.073 ng of dimethylarsinic acid (DMA) and 0.18 ng of methylarsonic acid (MMA) in wine were obtained. Sensitivity was further improved by using an He-Ar mixed gas ICP as the ionization source. However, the intensity of the ArCl+ interference was also increased using this plasma. Detection limits of 0.063 ng of As(III), 0.037 ng of As(v), 0.032 ng of DMA and 0.080 ng of MMA in club soda were achieved using the He-Ar plasma source. Similar limits of detection were found in urine and wine.     

Localization of cyclopropyl groups and alkenes within glycerophospholipids using gas-phase ion/ion chemistry

Shotgun lipid analysis using electrospray ionization tandem mass spectrometry (ESI-MS/MS) is a common approach for the identification and characterization of glycerophohspholipids GPs. ESI-MS/MS, with the aid of collision-induced dissociation (CID), enables the characterization of GP species at the headgroup and fatty acyl sum compositional levels. However, important structural features that are often present, such as carbon–carbon double bond(s) and cyclopropane ring(s), can be difficult to determine. Here, we report the use of gas-phase charge inversion reactions that, in combination with CID, allow for more detailed structural elucidation of GPs. CID of a singly deprotonated GP, [GP − H]⁻, generates FA anions, [FA − H]⁻. The fatty acid anions can then react with doubly charged cationic magnesium tris-phenanthroline complex, [Mg(Phen)₃]²⁺, to form charge inverted complex cations of the form [FA − H + MgPhen₂]⁺. CID of the complex generates product ion spectral patterns that allow for the identification of carbon–carbon double bond position(s) as well as the sites of cyclopropyl position(s) in unsaturated lipids. This approach to determining both double bond and cyclopropane positions is demonstrated with GPs for the first time using standards and is applied to lipids extracted from Escherichia coli.     

Separation of multiply charged anions by capillary electrophoresis using alkyl phosphonium pairing agents

Two newly developed UV transparent phosphonium-based cationic reagents were evaluated as background electrolyte additives for capillary electrophoresis for the separation of multiply charged anions, including several complex anions. These cationic reagents showed moderate suppression of the electroosmotic flow, interacted with the analytes to improve their separation and often improved the peak shape. The effects of the additives and their concentration on the separation were studied, as well as the buffer type, pH, and voltage. The dicationic reagent effectively separated eight divalent anions within 17 min and the tetracationic reagent best separated nine trivalent anions, as well as a mixture of all the anions.     

Structural characterization of poly(N-alkyl-4-vinylpyridinium) triflates using pyrolysis/tandem mass spectrometry

Desorption chemical ionization (DCI) and desorption electron ionization (DEI) of homo- and co-polymers of N-alkyl-4-vinylpyridinium triflates having ethyl, n-hexyl and n-dodecyl groups as N-alkyl substituents, produce mass spectra that display oligomeric ions. These positively charged ions are singly-charged and result from cleavage of the polymer into neutral oligomers and the loss of a single triflate anion per oligomer. Analogous negatively charged ions, in which each neutral oligomer carries an extra triflate anion, are observed in the desorption chemical ionization mass spectra. Each oligomer within the available mass range is represented in the mass spectra. The formation of cluster ions in which a single, multiply-charged cation is associated with a number of singly-charged anions, as observed for these ammonium polysalts, is unusual. Five major and three minor series of positively charged ions are observed in DCI and DEI methods of ionization. Ions in the different series correspond either to cleavage at different bonds between the constituent monomers or to hydrogen transfer in different directions. Unique and structurally diagnostic fragmentation processes are observed in tandem mass spectrometry (MS/MS) experiments performed using collision activated dissociation of mass-selected oligomeric ions.     

新化合物五氧化铀的电喷雾串联质谱法制备与表征

采用醋酸铀酰为原料在气相中制备了HOUO2OH负离子, 在一定条件下再与O2反应, 从而制备了UO5负离子, 并采用串联质谱法对UO5进行了表征, 考察了生成条件对制备UO5负离子的影响. 初步实验表明在不同能量作用下UO5负离子能释放出O2. 探讨了气相中负离子与分子反应生成UO5负离子的可能机理, 指出通过调控分子与离子的碰撞时间和碰撞能量可以控制离子的电子活动半径, 合适的能量可使电子云的半径落在rinert≤rReactiverreactive≤rdecomposition之间, 从而促进某些化学反应的进行, 以合成某些通常状况下难以生成的物质.     

溶液中蛋白质的气液荷电萃取电离质谱研究

利用自主研制的气液荷电萃取电离装置实现了溶液中蛋白组分的荷电萃取电离直接质谱分析.系统考察了所用气体种类、气泡路径长度、溶液电压、气压等条件对溶液中溶菌酶等蛋白荷电萃取电离的影响,以期得到最佳蛋白质信号.在以CO2为萃取气体、气泡路径长度32 cm、溶液电压+2 kV、气压0.05 MPa条件下,溶菌酶在水、纯水稀释200倍尿液、未稀释尿液中的浓度分别为1×10-8 mol/L、1×10-7 mol/L、1×10-5 mol/L时获得谱图信噪比(S/N≥3)类似,信号强度不受样品管内径大小影响,所用尿液最小体积为6 mL,但对珍稀样品可进一步减少用量.与ESI-MS相比,本方法获得更多低价态蛋白质离子,对溶液中的小分子基体、无机盐具有更强的耐受性,且辣根过氧化物酶经气液苛电萃取电离能保留53.9%活度.本方法具有无需复杂样品预处理、无化学试剂污染的特点,有望为分析复杂基质中蛋白质提供一种新方法.     

Anion binding by bambus[6]uril probed in the gas phase and in solution

Electrospray ionization mass spectrometry (ESI-MS) is used to probe the binding of small anions to the macrocycle of bambus[6]uril. For the halide ions, the experimental patterns suggest F(-) < Cl(-) < Br(-) < I(-), which is consistent with the order of anion binding found in the condensed phase. Parallel equilibrium studies in the condensed phase establish the association constants of halide anions and bambus[6]uril in mixed solvents. A detailed analysis of the mass spectrometric data is used to shed light on the correlations between the binding constants in the condensed phase and the ion abundances observed using ESI-MS. From the analysis it becomes apparent that ESI-MS can indeed represent the situation in solution to some extent, but the sampling in the gas-phase experiment is not 1:1 compared to that in solution.