Determinations of geniposide using LC/MS/MS methods via forming ammonium and acetate adducts

In this paper, we report method development work to determine geniposide using LC/MS/MS via the formation of positive and negative ion adducts. Geniposide, which has been recognized to have choleretic effects, is the major iridoid glycoside component of Gardenia herbs. To enhance the sensitivity of LC/MS detection of geniposide, a small amount of volatile additives such as ammonium acetate and acetic acid are added into mobile phase solvents to form positive and negative adducts, which can then ionize via electrospray processes. The formation of positive adducts is due to the complexation between geniposide and ammonium ions ([M + NH₄]⁺). The formation of anionic adducts [M + CH₃COO]⁻ is believed to occur via hydrogen bonds bridging acetate ions and glucose groups on the geniposide molecule. Mobile phase solvents containing acetonitrile and aqueous solution (0.2 mM ammonium acetate or 0.1% acetic acid) at the ratio 15: 85 are employed to elute geniposide using C8 reverse phase liquid chromatography columns with electrospray tandem mass spectrometry determinations. Using geniposide standards, the methods are validated at the concentration ranges of 5 to 1000 ng/mL and 20 to 5000 ng/mL using ammonium and acetate adducts respectively. The correlation coefficients of the standard curves are 0.9999 using both ammonium and acetate adducts. The detection limits of using ammonium and acetate adducts are 1 and 5 ng/mL respectively. The measurement accuracy and precision of using ammonium adducts are within 12% and 3% respectively, whereas the accuracy and precision are within 6 and 11% respectively using acetate adducts. When the validated calibration curves of the ammonium adduct of geniposide are used to determine spiked control samples in rat blood dialysates, the determination errors of accuracy and precision are within 12% and 10% respectively.     

Production and characterization of thermally-annealed large clusters by high-pressure laser-vaporization technique

We have developed a high-pressure laser-vaporization cluster source, which enables us to produce large, annealed clusters. Large carbon clusters up to C ₂₀₀₀ ⁺ , annealed sodium fluoride clusters of Na⁺(NaF)_n, and ammonium iodide clusters of NH ₄ ⁺ (NH₄I)_n have been produced by this technique. Annealed ammonium iodide clusters show a transition from hydrogen bonded complexes to ionic crystals as the cluster size increases. The characterization of the present technique and the production processes of the large, annealed clusters are discussed.     

Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan‐containing dipeptides complexed with Ca2+ and 18‐crown‐6 in the gas phase

The relationship between peptide structure and electron transfer dissociation (ETD) is important for structural analysis by mass spectrometry. In the present study, the formation, structure and reactivity of the reaction intermediate in the ETD process were examined using a quadrupole ion trap mass spectrometer equipped with an electrospray ionization source. ETD product ions of zwitterionic tryptophan (Trp) and Trp‐containing dipeptides (Trp‐Gly and Gly‐Trp) were detected without reionization using non‐covalent analyte complexes with Ca²⁺ and 18‐crown‐6 (18C6). In the collision‐induced dissociation, NH₃ loss was the main dissociation pathway, and loss related to the dissociation of the carboxyl group was not observed. This indicated that Trp and its dipeptides on Ca²⁺(18C6) adopted a zwitterionic structure with an NH₃⁺ group and bonded to Ca²⁺(18C6) through the COO^? group. Hydrogen atom loss observed in the ETD spectra indicated that intermolecular electron transfer from a molecular anion to the NH₃⁺ group formed a hypervalent ammonium radical, R‐NH₃, as a reaction intermediate, which was unstable and dissociated rapidly through N–H bond cleavage. In addition, N–C_α bond cleavage forming the z₁ ion was observed in the ETD spectra of Trp‐GlyCa²⁺(18C6) and Gly‐TrpCa²⁺(18C6). This dissociation was induced by transfer of a hydrogen atom in the cluster formed via an N–H bond cleavage of the hypervalent ammonium radical and was in competition with the hydrogen atom loss. The results showed that a hypervalent radical intermediate, forming a delocalized hydrogen atom, contributes to the backbone cleavages of peptides in ETD. Copyright © 2015 John Wiley & Sons, Ltd.     

Primary-particle-beam-induced reduction of silver acetate in glycerol solutions to form a silver mirror

The liquid SIMS mass spectra of silver acetate dissolved in a glycerol matrix is discussed, with emphasis on the formation of a ‘silver mirror’ on the surface of the glycerol droplet owing to reduction of the silver acetate. Silver clusters containing up to three silver atoms have been observed from this mirrored surface; Ag₃⁺ cluster ions are not observed in the spectrum when conditions are such that the mirror is not formed. For example, use of a slightly oxidizing matrix (o-nitrophenyl octyl ether or m-nitrobenzylalcohol) prevents formation of the ‘mirror’; only Ag⁺ is sputtered from this surface.     

Effect of method of ion preparation on low-energy collision-induced dissociation mass spectra

The collision-induced dissociation (CID) mass spectra of protonated cocaine and protonated heroin have been measured using a triple quadrupole mass spectrometer at 50 eV ion/neutral collision energy for protonated molecules prepared by different protonating agents. The CID mass spectra of protonated cocaine using H⁺(H₂O)_n, H⁺(NH₃)_n and H⁺((CH₃)₂NH)_n as protonating agents are essentially identical and it is concluded that, regardless of the initial site of protonation, the fragmentation reactions occurring on collisional activation are identical. By contrast, protonated heorin prepared with H⁺(H₂O)_n and H⁺(NH₃)_n as protonating agents show substantial differences. That formed by reaction of H⁺(H₂O)_n shows a much more abundant peak corresponding to loss of CH₃CO₂H. From a comparison with model compounds, and from a consideration of the three-dimensional structure of heroin, it is concluded that with H⁺(H₂O)_n as protonating agent significant protonation occurs at the acetate group attached to the alicyclic ring, leading to acetic acid loss on collisional activation, but that reaction of H⁺(NH₃)_n leads to protonation at the nitrogen function. The proton attached to nitrogen cannot interact with the acetate group and, consequently, the probability of loss of acetic acid on collislional activation is greatly reduced.     

Molecular beam multiphoton-ionization studies on ammonia—water binary clusters

Two-photon ionization mass spectra are obtained for NH₃H₂O binary clusters both with a nozzle beam and an ArF excimer laser. The detected major ions are H⁺(NH₃)_n(H₂O)_m(1 <m + n < 9). The results suggest that ammonia molecules constitute an inner shell which is surrounted by water molecules.     

A liquid chromatography–mass spectrometric method for the quantification of azithromycin in human plasma

A liquid chromatographic mass spectrometric assay for the quantification of azithromycin in human plasma was developed. Azithromycin and imipramine (as internal standard, IS) were extracted from 0.5 mL human plasma using extraction with diethyl ether under alkaline conditions. Chromatographic separation of drug and IS was performed using a C₁₈ column at room temperature. A mobile phase consisting of methanol, water, ammonium hydroxide and ammonium acetate was pumped at 0.2 mL/min. The mass spectrometer was operated in positive ion mode and selected ion recording acquisition mode. The ions utilized for quantification of azithromycin and IS were m/z 749.6 (M + H) ⁺ and m/z 591.4 (fragment) for azithromycin, and 281.1 m/z for internal standard; retention times were 6.9 and 3.4 min, respectively. The calibration curves were linear (r² > 0.999) in the concentration ranges of 10–1000 ng/mL. The mean absolute recoveries for 50 and 500 ng/mL azithromycin and 1 µg/ mL IS were >75%. The percentage coefficient of variation and mean error were <11%. Based on validation data, the lower limit of quantification was 10 ng/mL. The present method was successfully applied to determine azithromycin pharmacokinetic parameters in two obese volunteers. The assay had applicability for use in pharmacokinetic studies. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthese und Kristallstruktur zweier Formen von Ammoniummonomolybdat (NH4)2MoO4

Synthesis and Structure of Two Forms of Ammonium Monomolybdate (NH₄)₂MoO₄ Ammonium monomolybdate (NH₄)₂MoO₄ exists in two different polymorphic forms which differ in their lattice constants and in the arrangement of the ammonium cations relative to the molybdate anions. The ammonium molybdates (NH₄)₂MoO₄(mS60)¹⁾ and (NH₄)₂MoO₄(mP60)²⁾ are synthesized by the reaction of ammonia and (NH₄)₆[Mo₇O₂₄] · 4 H₂O. (NH₄)₂MoO₄(mS60) crystallizes isostructural to the potassium compound in space group C2/m (Nr. 12) and lattice constants a = 1263.6(3), b = 652.2(1) pm, c = 776.4(2) pm and β = 117.36(1)° (V = 568.3(2) · 10⁶ pm³) containig four formula units per unit cell (R = 0.0250). (NH₄)₂MoO₄(mP60) crystallizes monoclinic in space group P2₁/n (Nr. 14) and lattice constants a = 622.8(2), b = 777.0(1) pm, c = 1118.8(4) pm and β = 98.09(2)° (V = 536.0(3) · 10⁶ pm³) (R = 0.0205). The different arrangements of the polyhedra within the unit cell is caused by hydrogen bridges. A transition point was not yet determined.     

A theoretical investigation on the structures of (NH3)·(H2SO4)·(H2O)0-14 clusters

Ternary clusters (NH₃)·(H₂SO₄)·(H₂O)_n have been widely studied. However, the structures and binding energies of relatively larger cluster (n > 6) remain unclear, which hinders the study of other interesting properties. Ternary clusters of (NH₃)·(H₂SO₄)·(H₂O)_n, n = 0-14, were investigated using MD simulations and quantum chemical calculations. For n = 1, a proton was transferred from H₂SO₄ to NH₃. For n = 10, both protons of H₂SO₄ were transferred to NH₃ and H₂O, respectively. The NH₄⁺ and HSO₄⁻ formed a contact ion-pair [NH₄⁺-HSO₄⁻] for n = 1-6 and a solvent separated ion-pair [NH₄⁺-H₂O-HSO₄⁻] for n = 7-9. Therefore, we observed two obvious transitions from neutral to single protonation (from H₂SO₄ to NH₃) to double protonation (from H₂SO₄ to NH₃ and H₂O) with increasing n. In general, the structures with single protonation and solvated ion-pair were higher in entropy than those with double protonation and contact ion-pair of single protonation and were thus preferred at higher temperature. As a result, the inversion between single and double protonated clusters was postponed until n = 12 according to the average binding Gibbs free energy at the normal condition. These results can serve as a good start point for studies of the other properties of these clusters and as a model for the solvation of the [H₂SO₄-NH₃] complex in bulk water.     

Synthesis, structures, and properties of molybdenum and tungsten chalcogenide cubane complexes (NH4)6[M4Q4(CN)12]·6H2O (M=Mo or W; Q=S or Se)

Four new chalcogenide molybdenum and tungsten cubane clusters (NH₄)₆[M₄Q₄(CN)₁₂]·6H₂O (M=Mo or W; Q=S or Se) were prepared by high-temperature reactions of the triangular M₃O₇Br₄ complexes with KCN at 430 °C followed by crystallization from aqueous solutions of ammonium acetate. The molecular and crystal structures of (NH₄)₆[Mo₄S₄(CN)₁₂]·6H₂O, (NH₄)₆[W₄S₄(CN)₁₂]·6H₂O, and (NH₄)₆[W₄Se₄(CN)₁₂]·6H₂O were established by X-ray diffraction analysis. The mixed-valence cubane clusters are diamagnetic and isostructural and have the symmetryT _d . The clusters were characterized by IR and electronic spectroscopy. The data of cyclic voltammetry demonstrated that the [M₄Q₄(CN)₁₂] ⁿ⁻ clusters exist in three oxidation states from the most oxidized (n=6; 10 cluster electrons) to the most reduced electron-precise 12-electron species (n=8). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 18–24, January, 2000.     

Acetic acid cluster ions for tuning and calibration in thermospray liquid chromotography/mass spectrometry

Combined liquid chromatography and mass spectrometry is frequently performed with the thermospray interface nowadays. With thermospray a special ion source has to be installed. Tuning and calibration of that special source is still a problem which has not been adequately solved, Reference compounds commonly used in conventional electron impact or chemical ionization mass spectrometry cannot be used in the thermospray source. Most of the solutions to this problem reported in the literature cause a rapid contamination of the ion source or may result in memory effects. In this paper the use of acetic acid ammonia cluster ions is proposed for tuning and calibration in thermospray. Abundant cluster ions can be observed over the mass range between m/z = 100 and 1000 when an eluent containing ammonium acetate and 0.5% acetic acid is used. Features of the mass spectra obtained in both filament-off, filament-on and discharge-on modes will be discussed.     

Muttiphoton Ionization of Benzene-Ammonia Clusters: Intracluster Reaction and Cluster Ion Stability

Neutral benzene-ammonia clusters, prepared in a supersonic expansion, were ionized using multiphoton ionization. The cluster ions were investigated with a time-of-flight mass spectrometer. The observed major cluster ions, under 355-nm laser irradiation, resulting from prompt intracluster ion-molecule reaction and fragmentation following ionization are (C₆H₆)_m(NH₃)_nH⁺, m = 1–6, n = 1–4 and (C₆H₆)_m⁺, m = 1–3. The results of isotopic labeling experiments clearly indicate that C₆H₆ does not participate in intracluster ion-molecule reactions to form (C₆H₆)_m(NH₃)_nH⁺. A local maximum appears at n = 2 in the intensity distribution of (C₆H₆)_m(NH₃) _nH⁺ for each value of m under all experimental conditions. This finding indicates that (C₆H₆)_m(NH₃)₂H⁺ is more stable than any other (C₆H₅)_m(NH₃)_mH⁺ (n = 1,3,4) for m = 1–6.     

Ammonia in the ion source. II. Clustering with aromatic nitro compounds

Under positive ion chemical ionization conditions with ammonla at relatively low pressure, aromatic nitro compounds do not form [M + H]⁺ ions but often form ionic clusters [M + NH₄]⁺ and [M + N₂H₇]⁺. Nitrobenzene forms a cluster [2M + NH₄]⁺ and aniline, formed by nucleophilic substitution, leads to a cluster [anilinium ion + nitrobenzene]⁺. The dinitrobenzenes form [M + NH₄]⁺ clusters and show evidence of nitroaniline formation and clustering. 1,3,5-Trinitrobenzene gives little indication of clustering or of substitution. The six isomers of trinitrotoluene appear to be stabilized by the methyl group and form clusters up to [M + N₃H₁₀]⁺. Nucleophilic substitution leads to dinitrotoluidines, which also form clusters with ammonium ions.     

Observations of magic numbers in gas phase hydrates of alkylammonium ions

Hydration of alkylammonium ions under nonanalytical electrospray ionization conditions has been found to yield cluster ions with more than 20 water molecules associated with the central ion. These cluster ion species are taken to be an approximation of the conditions in liquid water. Many of the alkylammonium cation mass spectra exhibit water cluster numbers that appear to be particularly favorable, i.e., “magic number clusters” (MNC). We have found MNC in hydrates of mono- and tetra-alkyl ammonium ions, NH₃(C _m H_2m+1)⁺(H₂O) _n , m=1–8 and N(C _m H_2m+1) ₄ ⁺ (H₂O) _n , m=2–8. In contrast, NH₂(CH₃) ₂ ⁺ (H₂O) _n , NH(CH₃) ₃ ⁺ (H₂O) _n1 and N(CH₃) ₄ ⁺ (H₂O) _n do not exhibit any MNC. We conjecture that the structures of these magic number clusters correspond to exohedral structures in which the ion is situated on the surface of the water cage in contrast to the widely accepted caged ion structures of H₃O⁺(H₂O) _n and NH ₄ ⁺ (H₂O) _n .     

松香基季铵盐为模板剂有序超微孔二氧化硅的合成

以松香基季铵盐(脱氢枞基三甲基溴化铵,标记为DTAB)为模板剂、正硅酸乙酯为硅源、氨水为碱性介质成功合成出具有纳米片状形貌的六方有序超微孔二氧化硅材料。采用X射线衍射、N2吸附-脱附、透射电镜、扫描电镜等手段对样品进行表征,结果表明,体系中模板剂添加量、硅源添加量、碱性介质添加量、晶化温度、搅拌时间对前驱体的有序度有着较大的影响。当物质的量之比为nSi O2∶nDTAB∶nNH3·H2O∶nH2O=1.0∶0.1∶11.3∶924.0,晶化温度为373 K,搅拌时间为24 h,所得样品有序度最高。经煅烧后样品具有较大的比表面积(1 024 m2·g-1)和孔容(0.56 cm3·g-1),以及狭窄的孔径分布(集中于1.80 nm)。     

Liquid chromatography/thermospray mass spectrometry of monosaccharides and their derivatives

Thermospray (TSP) mass spectrometry has proved to be a useful technique for analysing various highly polar compounds. The use of a quadrupole mass spectrometer equipped with a thermospray/plasmaspray interface in the analysis of more than 30 monosaccharides and sugar derivatives is described. A 1:1 mixture of methanol or acetonitrile and 0.1 M aqueous ammonium acetate as eluent at 1 cm³ min^?1 affords the best results. The correct setting of the capillary tip temperature and repeller voltage was fundamental for the ionization of carbohydrates. The/optimum values of these parameters were 240–250°C and 220–240 V, respectively. The thermospray mass spectra of most carbohydrates exhibit strong [M + NH₄]⁺ ions which provide molecular mass information. The underivatized and derivatized monosaccharides could be grouped according to the presence or absence and the relative abundances of [M + NH₄ ? H₂O]⁺, [M + H]⁺ and [M + NH₄]⁺ ions.     

Unique thermospray mass spectrometry of 1,4-benzoquinone and analogs in ammonium acetate solutions

In general, ions corresponding to [M + H]⁺ and/or [M + NH₄]⁺ are observed in thermospray mass spectrometry (TSMS) when using ammonium acetate in the liquid carrier. For several quinones investigated, unique thermospray mass spectra were detected with a mass spectral peak corresponding to an [M + 16]⁺ ion being observed in aqueous ammonium acetate solutions. Investigation of l,4-benzoquinone (BQU) and structurally analogous quinones indicated that amine conjugate formation with BQU and similar quinones was the origin of the unique [M + 16]⁺ ion in TSMS. When methanol was added to the liquid carrier, ions corresponding to methoxy conjugation were detected. High-performance liquid chromatography followed by TSMS or electrochemical detection gave evidence that this amine and methoxy conjugate formation was occurring in the thermospray source area.     

Sputtered ammonium dinitramide: Tandem mass spectrometry of a new ionic nitramine

The recently synthesized ammonium dinitramide (ADN) is an ionic compound containing the ammonium ion and a new oxide of nitrogen, the dinitramide anion (O₂N? N? NO₂^?). ADN has been investigated using high-energy xenon atoms to sputter ions directly from the surface of the neat crystalline solid. Tandem mass spectrometric techniques were used to study dissociation pathways and products of the sputtered ions. Among the sputtered ionic products were NH₄⁺, NO⁺, NO₂^?, N₂O₂^?, N₂O, N₃O₄^? and an unexpected high abundance of NO₃^?. Tandem mass spectra of the dinitramide anion reveal the uncommon situation where a product ion (NO₃^?) is formed in high relative abundance from metastable parent ions but is formed in very low relative abundance from collisionally activated parent ions. It is proposed that the nitrate anion is formed in the gas phase by a rate-determining isomerization of the dinitramide anion that proceeds through a four-centered transition state. The formation of the strong gas-phase acid, dinitraminic acid (HN₃O₄), the conjugate acid of the dinitramide anion, was observed to occur by dissociation of protonated ADN and by dissociation of ADN aggregate ions with the general formula [NH₄(N(NO₂)₂)_n] NH₄⁺, where n = 1–30.