Quasi-molecular Ions Research Articles

Ion-trap SIMS analysis of pinacolyl methylphosphonic acid on soil

The minimum detection limit and semi-quantitative determination of surface coverages of pinacolyl methylphosphonic acid (PMPA) on soil by static secondary-ion mass spectrometry (SIMS) are reported. The soil was exposed to aqueous solutions of PMPA and then analyzed on an ion-trap SIMS instrument. Mass spectrometry/mass spectrometry was utilized to discriminate against the chemical background inherent in environmental samples (such as soil). The quasimolecular ion [PMPA − H] − at m/ z = 179 was trapped and then subsequently fragmented to form an ion at m/ z = 95, which is interpreted as a loss of the pinacolyl olefin (C 6H 12) to form the methylphosphonic acid anion. The product ion at m/ z = 95 was used to investigate the surface coverage of PMPA on the soil. The m/ z = 95 product-ion abundance was observed to be linearly related to the PMPA surface coverage between 2 and 0.002 monolayers. The minimum detection limit is estimated at 0.008 monolayer (approximately 12 pg mm −2, three standard deviations of the blank). These data were compared with analyses performed by using a quadrupole SIMS instrument, which indicates an improvement in sensitivity by the ion-trap SIMS of a factor of 250. The results of this study demonstrate that ion-trap SIMS is a facile approach for determination of phosphonates on soil.

Kiloelectron volt cluster impacts: prospects for cluster-SIMS

This paper describes part of our continuing efforts in examining the ability of polyatomic or ‘cluster’ projectiles to improve secondary ion yields in SIMS. Negative secondary ion yields resulting from keV bombardment of lanthanum and bismuth oxide projectiles (composition (M 2O 3) mMO +) were measured and compared for both organic and inorganic targets. Each metal oxide projectile produces an increase in secondary ion yield when compared to the same number of Cs + projectiles at similar energies. In addition, the nonlinear increase in secondary ion yield per number of constituent atoms in the primary ion is compared for polyatomic projectiles within a given metal oxide series. The high secondary ion yields combined with efficient time-of-flight mass analysis translate into prospects for SIMS under ‘superstatic’ conditions, which should be particularly attractive for the detection of organics. In considering analytical applications it must be recognized that when a projectile becomes more efficient at ejecting molecular or quasimolecular ions (intact emission), it may also become more efficient at ‘manufacturing’ ions (recombination processes). Moreover, the number of molecules destroyed per analyte-specific secondary ions detected varies with projectile characteristics. The challenge for the practice of ‘cluster-SIMS’ is to optimize the projectiles for maximizing sensitivity and eventually limit-of-detection.

Enantiomeric separation and detection by high-performance liquid chromatography-mass spectrometry of 2-arylpropionic acids derivatized with benzofurazan fluorescent reagents.

The enantiomneric separation and the detection of 2-arylpropionic acids after derivatization with the fluorescent reagents with a benzofurazan structure, (S)-(+)-4-(N,N- dimethylaminosulphonyl)-7-(3-aminopyrrolidin-1-yl)-2,1,3-ben zoxadiazole ((S)-DBD-Apy), (R)-(-)-4-nitro-7-(3-aminopyrrolidin-1-yl)-2,1,3- benzoxadiazole ((R)-NBD-Apy), 4-N,N-dimethylaminosulphonyl-7-piperazino-2,1,3-benzoxadi zole (DBD-PZ) and N-hydrazinoformylmethyl-N-methylamino-4,4- N,N-dimethylaminosulphonyl-2,1,3-benzoxadiazole (DBD-CO-Hz) by high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS) were examined. The diastereomeric derivatization with (S)-DBD-Apy or (R)-NBD-Apy and the separation on the reversed phase column afforded the high sensitivity. The separation on chiral stationary phase after non-chiral derivatization with DBD-PZ or DBD-CO-Hz provided less sensitivity. The signal-to-noise ratio of (S)-DBD-Apy-(S)-ketoprofen of 200:1 was observed for 12.5 picomole (pmol) injection and selected ion monitoring (SIM) of the quasi-molecular ion after splitting 1:7 before entering into the electrospray ion sources. As a result, the usefulness of these reagents for MS detection has been demonstrated.

LC/MSにおける誘導体化

LC/MS is now considered to be the most promising analytical method for the determination of biological substances, especially nonvolatile or high molecular weight substances. But one of the drawbacks of this method is its low sensitivity in comparison with GC/MS. In order to overcome this problem, the derivatization methods are used in LC/MS. This report overviews the detection-oriented derivatization methods in LC/MS including our results. The typical examples are as follows. The derivatization of vitamin D with a Cookson-type reagent (analog of 4-phenyl-1, 2, 4-triazoline-3, 5-dione) gave satisfactory sensitivity, which was also observed on the derivatization of oxosteroids with O-methylhydroxylamine. Although the oxosteroid fatty acid esters gave a de-esterified ion as a base peak in atmospheric pressure chemical-ionization MS, the methyloxime of which gave a quasimolecular ion as a base peak, which is helpful to identify the compounds in the brain.

Analyses of iminodipeptides containing C-terminal proline or hydroxyproline in biological samples by liquid chromatography/mass spectrometry

Analyses of authentic iminodipeptides containing an N-terminal proline (Pro-X) or C-terminal proline (X-Pro), and iminodipeptides in the urine and serum of patients with prolidase deficiency, have been demonstrated using liquid chromatography/mass spectrometry with an atmospheric pressure ionization interface system. The separation of iminodipeptides was carried out on a reversed phase high-performance liquid Chromatographic column using 0.1% aqueous trifluoroacetic acid-methanol (75:25, v/v) as mobile phase. Very intense protonated molecular ions [ M+ H] + of various synthetic iminodipeptides, Pro-Gly, Gly-Pro, Pro-Ala, Ala-Pro, Pro-Val, Val-Pro, Pro-Leu and Leu-Pro, were observed. ProGly (Pro-X) and Gly-Pro (X-Pro) have the same protonated molecular ion (m/z 173), but the peaks of these compounds on the mass chromatograms were clearly distinguished by the differences of the retention times and mass spectra. The quasimolecular ions [ M+ H] + of various iminodipeptides were observed in the urine samples of patients with prolidase deficiency, and Gly-Pro, Ala-Pro, Val-Pro, Leu-Pro, Ile-Pro, Ser-Pro, Thr-Pro, Glu-Pro, Asp-Pro, His-Pro, Lys-Pro, Pro-Pro and Tyr-Pro as iminodipeptides containing proline with C-terminal residues and Glu-Hyp, Pro-Hyp, Ile-Hyp and Gly-Hyp as iminodipeptides containing hydroxyproline with C-terminal residues were identified in the urine of patients with prolidase deficiency. The quasi-molecular ions [ M+ H] + of various iminodipeptides containing with C-terminal proline and hydroxyproline also were observed in the sera of patients with prolidase deficiency, using selected ion monitoring. The quasi-molecular ions [ M+ H] + of iminodipeptides containing C-terminal proline were not observed in the sera of normal subjects or the patients' mother, but the latter contained various iminodipeptides containing C-terminal hydroxyproline. This method proved useful for the determination of iminodipeptides in the sera of patients with prolidase deficiency.

Determination of naphthodianthrones in plant extracts from Hypericum perforatum L. by liquid chromatography–electrospray mass spectrometry

The determination of the naphthodianthrone constituents in extracts of dried blossoms of Hypericum perforatum L. by combined HPLC–electrospray mass spectrometry is described. Hypericin (1), pseudohypericin (2) and their precursor compounds produce intensive negative quasi-molecular ions by deprotonation provided a non-acidic eluent system is used in the HPLC separation. From the [M–H] − ions formed in the electrospray ionization process characteristic daughter ion spectra can be obtained by collisional activation which have been studied by tandem mass spectrometry.

Characterization of Neutral Oligosaccharide-Alditols fromXenopus laevisEgg Jelly Coats by Matrix-Assisted Laser Desorption Fourier Transform Mass Spectrometry

Neutral oligosaccharides were released by alkaline sodium borohydride reduction of the jelly coating from the South African clawed toad,Xenopus laevis.The oligosaccharides were isolated by HPLC and analyzed by matrix-assisted laser desorption ionization (MALDI)-Fourier transform mass spectrometry (FTMS). The mass spectrometry analysis allowed confirmation of 12 structures first proposed by Streckeret al.using nuclear magnetic resonance. In addition, seven new oligosaccharides with weak abundances were found and characterized by mass spectrometry. A method for discriminating metastable fragments from quasimolecular ions is described. It involves doping the sample with cesium chloride. Cesium-coordinated oligosaccharides do not fragment as readily as those coordinated to sodium. Tandem MS experiments are performed on an unknown oligosaccharide illustrating the potential of MALDI-collision-induced dissociation-FTMS.

Structure of the Monophosphoryl Lipid A Moiety Obtained from the Lipopolysaccharide of Chlamydia trachomatis

Monophosphoryl lipid A was prepared from the lipopolysaccharide of Chlamydia trachomatis, converted to the methyl ester, and fractionated by reverse-phase high-performance liquid chromatography. The peak fractions were collected and analyzed by mass spectrometry. Matrix-assisted laser desorption/ionization and liquid secondary ion mass spectrometry of the first of two major high-performance liquid chromatographic fractions showed multiple quasi-molecular ions of MNa+ at m/z 1780, 1794, 1808, 1822, and 1836. The positive-ion liquid secondary ion mass spectrometry spectrum also showed a minor series of peaks at m/z 1916, 1930, 1944, 1958, and 1971, consistent with the formation of matrix adducts with 3-nitrobenzyl alcohol. Oxonium ions representing the distal subunit were observed at m/z 1057, 1071, 1085, 1099, and 1113. The second fraction was similarly analyzed and found to contain structural homologs of the first fraction. Based on this study, the major lipid A component of chlamydial lipopolysaccharide is a glucosamine disaccharide that contains five fatty acids and a phosphate in the distal segment. Three fatty acyl groups are in the distal segment, and two are in the reducing end segment. The acyloxyacyl group is located in the distal segment in amide linkage. Two structural series, differing by 14 atomic mass units in the reducing subunit, were observed. Chlamydial lipid A is complex and consists of at least 20 homologous structural components. The relatively low potency of Chlamydia trachomatis lipopolysaccharide in activating lipopolysaccharide-responsive cells might be related to the unusual fatty acid composition of the lipid A moiety.

Cationization Processes in Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry: Attachment of Divalent and Trivalent Metal Ions

In this paper, divalent and trivalent metal ions have been used as cationization reagents in matrix-assisted laser desorption/ionization (MALDI) experiments. Analytes studied include polyethylene glycol, polyethylene oxide dimethyl ether, cyclic polyethylene oxide and α-cyclodextrin. Independent of the valence of the metal ions and the nature of the analyte molecules studied, singly charged quasimolecular ions were generated in all cases. These quasimolecular ions were shown to be generated in different forms depending on the type of the di- or trivalent metal salts and the chemical properties of the analyte molecules. For Cu2+ ions, the quasimolecular ions were formed by the adduction of the reduced metal ions, i.e. Cu+. For Pb2+, Cd2+, Ba2+, Sr2+ and La3+ ions, singly charged quasimolecular ions were formed by adduction of the metal ions with either simultaneous deprotonation of the analyte molecule or adduction of an additional deprotonated matrix moiety onto the analyte molecule. Of these two choices, deprotonation of the analyte molecule is always preferred if labile protons, such as those in hydroxyl groups, are available in the analyte molecule. A model of ion formation from MALDI based on solid-state molecular pre-arrangement is used to account for the formation of such complex singly charged quasimolecular ions. © 1997 John Wiley & Sons, Ltd.

Cationization Processes in Matrix-assisted Laser Desorption/Ionization Mass Spectrometry: Attachment of Divalent and Trivalent Metal Ions

In this paper, divalent and trivalent metal ions have been used as cationization reagents in matrix-assisted laser desorption/ionization (MALDI) experiments. Analytes studied include polyethylene glycol, polyethylene oxide dimethyl ether, cyclic polyethylene oxide and α-cyclodextrin. Independent of the valence of the metal ions and the nature of the analyte molecules studied, singly charged quasimolecular ions were generated in all cases. These quasimolecular ions were shown to be generated in different forms depending on the type of the di- or trivalent metal salts and the chemical properties of the analyte molecules. For Cu2+ ions, the quasimolecular ions were formed by the adduction of the reduced metal ions, i.e. Cu+. For Pb2+, Cd2+, Ba2+, Sr2+ and La3+ ions, singly charged quasimolecular ions were formed by adduction of the metal ions with either simultaneous deprotonation of the analyte molecule or adduction of an additional deprotonated matrix moiety onto the analyte molecule. Of these two choices, deprotonation of the analyte molecule is always preferred if labile protons, such as those in hydroxyl groups, are available in the analyte molecule. A model of ion formation from MALDI based on solid-state molecular pre-arrangement is used to account for the formation of such complex singly charged quasimolecular ions. © 1997 John Wiley & Sons, Ltd.

Electron Impact- and Chemical Ionization-Mass Spectrometry of Naturally Occurring Sesquiterpene Lactones fromAchillea spp.

The electron impact-mass spectra as well as the complementary chemical ionization-mass spectra (with the reactant gas ammonia) of various structural types of sesquiterpene lactones isolated from Achillea species are discussed in respect to the important information either gained by fragmentation patterns or by the quasimolecular ions. © 1997 by John Wiley & Sons, Ltd.

The reduction and oxidation reactions of [Ru(CN)2(bpy)2] observed in liquid secondary ion mass spectra with a thioglycerol matrix

Liquid secondary ion mass spectra (LSIMS) of [Ru(CN)2(bpy)2] were measured with thioglycerol containing 15.6 wt% of glycerol. An isotopic cluster in the region of the molecular ion (M+) was observed with a mass range of more than 10 in LSIMS mass spectra. Isotopic peaks of the molecular ion and quasi-molecular ions, that were formed by the redox reaction of the molecule during the ionization process, were contained in the isotopic cluster. The identification of chemical species in the cluster was performed by the least-squares method. The difference between the calculated and observed relative intensities was small. The values of standard deviation obtained in the calculation were less than 0.01 in six runs out of 11. The main chemical species in the cluster were M+, (M + H)+, (M + 2H)+ and (M + 3H)+. The results regarding these chemical species agreed with the results regarding the characteristics of the complex in the solution obtained by the measurement of redox potentials.

Recent progress in atmospheric pressure ionization mass spectrometry

Atmospheric pressure ionization (API) is used as an interface in liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry. In API, quasi-molecular ions of biological molecules in solution are produced at atmospheric pressure. There have recently been several significant advances in spray ionization techniques, which use a solution spraying process. This article gives an overview of three spray ionization techniques: electrospray, ion spray, and sonic spray. It presents their characteristic features and describes the ion formation processes from charged droplets. Furthermore, the charged droplet formation process for each technique is described in terms of the non-uniformity in positive and negative ions in solution as well as in terms of the atomization process.

Application of liquid chromatography-atmospheric pressure ionization mass spectrometry in natural product analysis evaluation and optimization of electrospray and heated nebulizer interfaces

A representative set of 46 secondary metabolites of plant and microbial origin was examined with electrospray and atmospheric pressure chemical ionization MS in both positive and negative ion mode, in order to evaluate the applicability of atmospheric pressure ionization LCMS as a tool for rapid identification and dereplication of bioactive compounds in the drug screening process. MS characteristics of different compound classes were determined on the basis of quasi-molecular ion and other adduct ion responses observed with the two LCMC interfaces. Interface parameters and LC mobile phase composition were optimized and an analytical protocol applicable to a wide range of secondary metabolites is proposed.

Laser-induced Fluorescence of Ba+ Ions Trapped and Mass-selected in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

We present the design and preliminary results from a Fourier transform ion cyclotron resonance (ICR) mass spectrometer developed for the direct detection of UV/visible laser-induced fluorescence of trapped, mass-selected, gas-phase ions. A 3 T superconducting magnet and an open-ended multi-section cylindrical Penning trap capture and confine ions created by electron impact or laser desorption. Azimuthal quadrupolar excitation in the presence of ion/neutral collisions cools, axializes and mass selects ions as they fill the trap. A pulsed dye laser pumped by an Nd:YAG laser provides electronic energy excitation. A Brewster window and baffles on each side of the vacuum chamber reduce the scattered light from the excitation laser. Laser-induced fluorescence is collected from mirrors and lenses and directed through a quartz window and fiber-optic bundle to a photomultiplier. The ICR and optical events are controlled by a modular ICR data station and GPIB and RS-232 interfaces. An excitation spectrum is demonstrated for atomic Ba+ ions, and should extend to laser-induced fluorescence of virtually any stable positive or negative gas-phase ions of arbitrary molecular weight: molecular or quasimolecular ions, fragment ions, adduct ions, and ions formed from ion/molecule reactions.

Fourier transform ion cyclotron resonance mass spectrometry in a 20 T resistive magnet.

We present Fourier transform ion cyclotron resonance (FTICR) mass spectra at a magnetic field of 20 T; more than twice the highest field previously used for FTICR. Our instrument is based on a resistive magnet installed at the National High Magnetic Field Laboratory. The magnet has a 50 mm diameter bore and spatial inhomogeneity of approximately 1000 ppm over a 1 cm diameter spherical volume. However, FTICR mass resolving power far in excess of magnet homogeneity is achieved routinely for ions produced by either electron ionization (EI) or matrix-assisted laser desorption/ionization (MALDI). As examples, we show a MALDI mass spectrum of [M + H] quasimolecular ions of the peptide, human luteinizing hormone-releasing hormone (monoisotopic molecular weight, 1181.6 Da) at mass resolving power, m/delta m > 10,000; and an EI mass spectrum of molecular ions of the platinum cluster compound, Pt4(PF3)8 (average molecular weight, 1484 Da at mass resolving power, m/delta m approximately 20,000. Much better FTICR MS performance is predicted for future NHMFL resistive magnets of higher spatial and temporal homogeneity.

Chemical ionization mass spectrometry of hydrofluorocarbons, hydrofluorocarbon ethers and perfluoroalkenes

Positive and negative chemical ionization (CI) mass spectrometry is studied for hydrofluorocarbons (HFCs), hydrofluorocarbon ethers (HFEs), and perfluoroalkenes (PFCs) using various kinds of reagent gas. While no quasi-molecular ion was observed under electron impact ionization for saturated MFCs, [M-F] + is detected under CI conditions using methane as a reagent gas. Mechanisms for the generation of [M-F] + are discussed. Furthermore, nitrogen monoxide can be used as a reagent gas to observe [M + NO] + for many HFCs and HFEs. In negative mode chloroform is also available to generate [M + Cl] − for HFCs and HFEs containing -CHF- groups.

Molecular Surface Imaging of Particles Using Microprobe Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

The objective of this research is to develop imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) to characterize ultra-thin organic films on microscopic particles. An initial application is to evaluate the surface chemistry of polycyclic organic matter (POM) on combustion-generated particles as an area of fundamental interest in the assessment of the environmental fate and impact of carcinogenic pollutants.Controlled deposition of POM monolayers was achieved using either gas or solution phase coating on model particles such as silica, as well as on authentic environmental particles such as coal flyash or soot. Another aspect of the work was to monitor surface transformations of adsorbed POM involving photochemical degradation or reactions with gaseous pollutants such as nitrogen oxides. For the first time, variations in POM adsorption and reactivity have been probed as a function of particle type by the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to perform surface analysis on single particles. Results using a pulsed gallium microbeam source on a TOF-SIMS indicated that 0.1 monolayer coverages of individual POM species can be detected as quasimolecular ions from single particles with diameters typically in the 5 μm range. Primary ion doses were <1013 ions/cm2 to minimize surface damage during a typical 10 min spectrum acquisition from an 40x40 ftm image field. Correlation of in situ measurements using TOF-SIMS with traditional solvent extraction and chromatographic results, including LC or GC-MS, allowed for more detailed assessments of the sensitivity and quantitative capabilities of TOF-SIMS. The combination of monolayer analysis with microanalysis creates severe challenges to sensitivity since the total number of molecules within the analytical volume is so small (< 107 POM molecules on a lμm2 particle area)

Determination of parts per trillion levels of organophosphorus pesticides in groundwater by automated on-line liquid-solid extraction followed by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation.

Liquid chromatography/atmospheric pressure chemical ionization mass spectrometry with positive and negative ion modes of operations was used for the trace determination of several organophosphorus pesticides, (E)- and (Z)-mevinphos, dichlorvos, azinphos-methyl, azinphosethyl, parathion-methyl, parathion-ethyl, malathion, fenitrothion, fenthion, chlorfenvinphos, and diazinon, in groundwater. This method required only 100 mL of water, and it was combined with a prior automated online liquid-solid extraction step using an OSP-2 autosampler containing C18 cartridges. The limit of quantitation (LOQ) varied between 5 and 37 ng/L in positive ion (PI) mode. Under negative ion (NI) mode of operation, only the parathion group (both parathions and fenitrothion) had a better sensitivity as compared to that in PI mode, with a LOQ of 5-15 ng/L, whereas the rest of pesticides had 2-4 times higher LOQs as compared to those in PI mode. Selected ion monitoring of the group-specific fragment of the organophosphorus pesticides, e.g., [(CH3O)2PO2]- or the [M + H]+ ions, under NI or PI mode, respectively, was used. Sample cone voltage varied from 10 to 130 V. This parameter influenced the transmission and fragmentation of quasi-molecular ions, and it was optimized to achieve identification capabilities with the highest sensitivity. At 20 V, good fragmentation was obtained for most of the studied analytes. The system was used for the certification of a groundwater sample spiked at the nanograms per liter level with organophosphorus pesticides provided by Aquachek.

Rapid screening of taxol metabolites in human microsomes by liquid chromatography/electrospray ionization-mass spectrometry.

Liquid chromatography with an electrospray ionization (ESI) interface has been applied to study the anticancer drug taxol and its metabolites after incubation with human hepatic microsomes. The parent drug and its metabolites were monitored in the positive-ionization mode. Since ESI gave only quasi-molecular ions for taxol and its analogues, collision-induced dissociation experiments were carried out in order to generate fragment ions, by increasing the cone voltage at the ESI source. The product-ion mass spectra of taxol and its metabolites contained diagnostic fragment ions, which enabled the presence of hydroxylated and deacetylated metabolites of taxol to be established.