Quasi-molecular Ions Research Articles

Proton transfer chemical ionization mass spectrometry of fatty acid methyl esters separated by gas chromatography: quantitative aspects

AbstractThe propensity of esters to accept protons in the gas phase during chemical ionization (CI) mass spectrometry (MS) has been well known for some time. This proton transfer CI almost exclusively produces the [M+H]+ quasimolecular ion for most esters. While it has been suggested that this could serve as the basis for particularly sensitive quantitation of fatty acid methyl esters (FAME) separated by gas chromatography (GC), the application of this approach to quantitate FAME has not been fully explored. Here, we briefly address some considerations pertinent to the application of GC with proton transfer CI‐MS to quantitative analysis of FAME. Limits of detection and relative response factors (RF) for a number of FAME separated by GC and detected by CI‐MS are presented and discussed. While the detection limits are comparable or superior to other detection methods, it was noted that the RF for unsaturated FAME were dramatically reduced with increasing degree of unsaturation.

Direct CIMS for the determination of mixtures of fatty alcohols from octanol to octadecanol

The possibility of using direct chemical-ionization mass spectrometry to determine fatty alcohols in technical mixtures was examined. CH 4, i-C 4H 10, H 2S, NH 3 and (CH 3) 3− n NH n were examined as reactant gases. Methane, because its proton affinity is significantly lower than those of the alcohols, and isobutane lead to an overlapping pattern of fragment ions, which prevents evaluation of the spectra. H 2S does not form such interfering fragment ions; it forms only the quasi-molecular ions [ M − 17] +, which serve for the quantitative detection, and [ M − 1] + at low intensity. The detection limits of the alcohols in methanol are between 0.007 and 0.025%. NH 3 and the amines (CH 3) 3− n NH n also generate no fragment ions. Instead, they form adduct ions of the form [ROH···H n+1 N(CH 3) 3− n ] +, where n = 0–3. The catalytic action of the fatty alcohols in the formation of the dimers ((CH 3) 3− n NH n ) 2H + does interfere here, however. The detection limits by NH 3-CIMS of the alcohols in a mixture are between 0.01 and 0.02%.

ESI-MS quantitation of increased sphingomyelin in Niemann-Pick disease type B HDL

HDLs have been proposed to have antiatherogenic properties because of their role in reverse cholesterol transport as lipid acceptors. To elucidate the phospholipid profile of these particles, we used electrospray ionization mass spectrometry to examine the phosphatidylcholine (PC) and sphingomyelin (SM) composition of HDLs purified from plasma and nascently generated in vitro from fibroblasts. We also quantitatively compared the phospholipids present in these lipoproteins between normal and Niemann-Pick disease type B (NPD-B) subjects characterized by sphingomyelinase (SMase) deficiency. We demonstrated that plasma HDLs from NPD-B were significantly enriched in SM by an average of 28%, particularly the palmitoyl SM (with an increase of 95%), which accounted for approximately 25-44% of total SM molecular species. Similarly, we observed an increase of approximately 63% in total SM levels in nascent HDLs prepared from NPD-B fibroblasts. Although PC levels in nascent HDLs were comparable between control and NPD-B cells, there was a 95% increase in total PC levels similar to that of SM in plasma HDLs extracted from NPD-B subjects. These data provide insight into the structure of HDLs and identify potential new roles for SMase in lipoprotein metabolism.

Collisional and thermal ionization of sodium Rydberg atoms: II. Theory for nS, nP and nD states with n = 5–25

A stochastic model of associative ionization in collisions of Rydberg atoms with ground-state atoms is presented. The conventional Duman–Shmatov–Mihajlov–Janev (DSMJ) model treats the ionization as excitation of Rydberg electron to the continuum by the electric-dipole field generated by exchange interaction within the quasi-molecular ion. The stochastic model essentially extends this treatment by taking into account redistribution of population over a range of Rydberg states prior to ionization, which is caused by non-adiabatic processes in overlapping multiple level crossings of quasi-molecular Rydberg states. The redistribution is modelled as diffusion of electrons in the Rydberg energy spectrum using a Fokker–Planck-type equation. The process of l-mixing of Rydberg states at large internuclear distances and twisting of the collision trajectories on attractive potentials are taken into account. The choice of the collision velocity distribution is also shown to be important. Associative ionization rates have been calculated for Na**(nl) + Na collisions with n = 5–25 and l = 0, 1, 2, and compared with the available experimental data and the calculations performed using the nonlinear DSMJ model. At relatively low n the stochastic model yields a substantially better agreement with the experimental data than the DSMJ model, while the results of both models converge at large n.

Analysis of Polyethylene by Using Cyclopentadienyl Cobalt Chemical Ionization Combined with Laser-Induced Acoustic Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A new mass spectrometric method has been developed for the analysis of low molecular weight polyethylene (PE). Laser-induced acoustic desorption (LIAD), combined with chemical ionization by the cyclopentadienyl cobalt radical cation (CpCo.+) in a Fourier transform ion cyclotron resonance mass spectrometer, produces predominantly a quasimolecular ion, (R + CpCo - 2H2).+, for each PE oligomer (R). An examination of artificial alkane mixtures revealed no mass bias for alkanes of differing molecular weights. However, the success of the LIAD/CpCo.+ CI technique depends greatly upon the LIAD sample preparation method used. Several sample preparation methods were evaluated, and pneumatically assisted spin coating was concluded to provide the best mass spectra as a result of its ability to provide uniform PE coverage on the LIAD foils. The molecular weight distributions measured for several low molecular weight PE samples (200-655) were found to be in good agreement with manufacturers' values as determined by gel permeation chromatography.

Insecticidal compounds of the biofilm‐forming cyanobacterium Fischerella sp. (ATCC 43239)

Cyanobacterial biofilms are grazed by many different benthic invertebrates. In particular, larvae of insects are often present in great numbers and exert strong grazing pressure on cyanobacteria. Along with morphological adaptations, allelochemicals may have been developed as defense mechanisms against insect larvae. To investigate the chemical defense of biofilm-forming cyanobacteria, larvae of Chironomus sp., a widely distributed genus in this habitat, were used. Ten artificial biofilms of axenic and nonaxenic cyanobacteria were screened for insecticidal activity against Chironomus sp. Fischerella 43239 was the cyanobacterium that exhibited the highest acute toxicity. A bioassay-guided isolation procedure was used to study the compounds responsible for toxicity in more detail. A toxic fraction was obtained when the 60% methanolic extract of Fischerella 43239 was separated on a C18 HPLC column. Electrospray mass spectrometry indicated the presence of several compounds in this fraction. The successful separation into individual compounds was achieved by HPLC on a cyanopropyl column. The heavily clustered quasimolecular ions observed on an electrospray mass spectrometer and the absorption spectra of the separated compounds were indicative of indole derivatives. The existence in the benthic cyanobacterium Fischerella 43239 of strong insecticidal metabolites that serve as chemical protection agents against insect larvae is supported by the data.

Direct molecular mass determination of trehalose monomycolate from 11 species of mycobacteria by MALDI-TOF mass spectrometry

Direct estimation of the molecular mass of single molecular species of trehalose 6-monomycolate (TMM), a ubiquitous cell-wall component of mycobacteria, was performed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. When less than 1 microg TMM was analysed by MALDI-TOF mass spectrometry, quasimolecular ions [M+Na]+ of each molecular species were demonstrated and the numbers of carbons and double bonds (or cyclopropane rings) were determined. Since the introduction of oxygen atoms such as carbonyl, methoxy and ester groups yielded the appropriate shift of mass ions, the major subclasses of mycolic acid (alpha, methoxy, keto and wax ester) were identified without resorting to hydrolytic procedures. The results showed a marked difference in the molecular species composition of TMM among mycobacterial species. Unexpectedly, differing from other mycoloyl glycolipids, TMM from Mycobacterium tuberculosis showed a distinctive mass pattern, with abundant odd-carbon-numbered monocyclopropanoic (or monoenoic) alpha-mycolates besides dicyclopropanoic mycolate, ranging from C75 to C85, odd- and even-carbon-numbered methoxymycolates ranging from C83 to C94 and even- and odd-carbon-numbered ketomycolates ranging from C83 to C90. In contrast, TMM from Mycobacterium bovis (wild strain and BCG substrains) possessed even-carbon-numbered dicyclopropanoic alpha-mycolates. BCG Connaught strain lacked methoxymycolates almost completely. These results were confirmed by MALDI-TOF mass analysis of mycolic acid methyl esters liberated by alkaline hydrolysis and methylation of the original TMM. Wax ester-mycoloyl TMM molecular species were demonstrated for the first time as an intact form in the Mycobacterium avium-intracellulare group, M. phlei and M. flavescens. The M. avium-intracellulare group possessed predominantly C85 and C87 wax ester-mycoloyl TMM, while M. phlei and the rapid growers tested contained C80, C81, C82 and C83 wax ester-mycoloyl TMM. This technique has marked advantages in the rapid analysis of not only intact glycolipid TMM, but also the mycolic acid composition of each mycobacterial species, since it does not require any degradation process.

Mass Spectral Analysis of Chloropicrin under Negative Ion Chemical Ionization Conditions

A chemical ionization (CI) method is developed for the first time to obtain molecular weight information for chloropicrin (CP), which is used as a chemical warfare agent and as an insecticide. The study includes a detailed investigation on the behavior of CP under electron impact (EI) and CI. Reagent gases of different nature, i.e., methane, isobutane, ammonia, hydrogen, and carbon dioxide, were used for CI analysis. Negative ion mode is found more sensitive than positive ion mode for the EI/CI mass spectrometric analysis of CP, but none of the methods provided molecular weight information, except negative ion CI using ammonia as the reagent gas (NICI (NH3)). The NICI (NH3) showed formation of the quasi-molecular ion, [M + H]-, in addition to other adduct ions. The [M + H]- abundance critically depends on the source temperature, reagent gas pressure, and concentration of the analyte, and it can be 13% under optimized conditions by which CP can be confirmed unambiguously. This method meets the criteria used in official proficiency tests conducted by OPCW for confirming the molecular weight of the unknowns.

Mechanism of insulin emission in matrix assisted laser ionization

In the present paper the emission of intact insulin quasi-molecular ion [M+H]+ during laser ablation (MALDI) is studied. It was observed in particular that the insulin TOF molecular peak increases as the laser power increases. The DE-Vestal method for the initial velocity measurements was improved theoretically including the distance (d0) until the free expansion regime can be considered. According to the present analysis, the v0 parameter given by the DE-Vestal method is interpreted as the initial velocity that the desorbed ion would have if no collision occurs in the plasma. The improved method interprets v0 as the "final" initial velocity, i.e., as the velocity that the desorbed ions have when the plasma free expansion starts and, effectively, collisions no longer occur. The new method allows also the determination of d0, the distance to the solid when the free expansion starts. The data fitting shows that the distance (d0) has a linear dependence on the laser's intensity. Extrapolation of these values gives I = 0.69 G W cm-2 as the minimum energy density necessary to produce high density plasma during the insulin ions desorption when using alphaCHCA matrix.

Identification and Comparative Determination of Senkyunolide A in Traditional Chinese Medicinal Plants Ligusticum chuanxiong and Angelica sinensis by HPLC Coupled with DAD and ESI-MS

Using the HPLC/DAD/ESI/MS method, the qualitative and quantitative analysis of senkyunolide A (SA) in the rhizomes of Ligusticum chuanxiong (Rhizoma chuanxiong; CX) and roots of Angelica sinensis (DG) was established. As a result, it was found that SA is a characteristic standard compound for the quality evaluation and chemical differentiation between CX and DG. Methanol was chosen in the preparation of standard solutions and extraction of samples based on the stability data. The identity of SA in CX and DG was unambiguously determined based on the quasimolecular ions in ESI-MS. A comprehensive validation of the method, including sensitivity, linearity, reproducibility and recovery, was conducted using the optimized chromatographic conditions. The linear calibration curve was acquired with R2>0.999 and limit of detection (S/N=3) was estimated to be 12.5 mug/g. The reproducibility was evaluated by repeated sample injection and replicated analysis of samples with the relative standard deviation (RSD) value found within 0.68%. The recovery rates of SA varied within the range of 96.91-101.50% with RSD less than 2.38%. In the present work, the contents of SA were quantified within 3.94-9.14 mg/g and 0.108-0.588 mg/g for 12 batches each of CX and DG. The results demonstrated that SA is a useful standard compound for the quality evaluation and chemical differentiation between CX and DG. The analytical procedure is precise and reproducible and thus suitable for the analysis of a large number of samples.

Delayed extraction experiments using a repulsing potential before ion extraction: evidence of non‐covalent clusters as ion precursor in UV matrix‐assisted laser desorption/ionization. Part II—Dynamic effects with α‐cyano‐4‐hydroxycinnamic acid matrix

Delayed extraction experiments were undertaken to gain a better insight into the dynamic effects involved in the ion formation in UV matrix-assisted laser desorption/ionization. Part I1 was devoted to a 2,5-dihydroxybenzoic (2,5-DHB) matrix. The results clearly demonstrated the existence and the role of high-mass precursors corresponding to a non-covalent matrix-analyte association in ion formation. In this complementary study, ion flight time and abundance were studied as a function of the delay extraction time using the matrix alpha-cyano-4-hydroxycinnamic acid (HCCA). Under our instrumental conditions, where ejected ions experienced a low repulsing electric field before extraction, two main results were obtained: (i) two ion components are observed in the peak profiles depending on the repulsing field, a first, major component (I) similar to that observed for 2,5-DHB and a second, minor component (II) apparently triggered by the delayed extraction pulse, and (ii) ion time-of-flight variation vs delay time remained lower than that noted with 2,5-DHB matrix, indicating that the initial axial velocity is smaller. The initial kinetic energy of matrix and low molecular mass peptide ions for the component I is not high enough to overcome the repulsing potential in the delay time range (200-2200 ns) and we have to assume that ions have non-covalent clusters as precursors. Complete desolvation of these clusters-aggregates would be achieved through the extraction step. Simulations of the ion time-of-flight as a function of the delay time allow the determination of the average size of the precursors, typically 4500, 40000 and 50000 u for HCCA, ACTH 7-38 and bovine insulin quasi-molecular ion, respectively, assuming that the precursors are singly charged. The size of these ion precursors is greater than that of those generated for 2,5-DHB. For component II, ions are probably not solvated and they are directly desorbed from the target. Taking into account the results on HCCA and 2,5-DHB matrices and other results from the literature, a general model for ion formation based on clusters as ion precursors is proposed and discussed.

Molecular Weight Evaluation of Poly(dimethylsiloxane) on Solid Surfaces Using Silver Deposition/TOF-SIMS

Molecular ions include information about end groups, functional groups and molecular weight. A method for the direct detection of these in the high mass range (m/z > 1000) from poly(dimethylsiloxane) (PDMS) on a solid surface was investigated. It was found that a TOF-SIMS analysis of silver-deposited surfaces (silver deposition/TOF-SIMS) is useful for this purpose. Using the silver-deposition/TOF-SIMS method, silver-cationized quasi-molecular ions were clearly detected from PDMS on solid surfaces, and their structure and molecular weight were evaluated. In addition, their images were observed without the interference of deposited silver. By applying to the analysis of paint defects etc., it was confirmed that this technique is useful to analyze actual industrial materials. Silver-cationized ions were detected not only from PDMS, but also from other organic materials, such as lubricant additives and oils on solid surfaces. Therefore, the silver deposition/TOF-SIMS method was proved to be useful for the analysis of ultrathin substances on solid surfaces.

Mass spectral behavior of some homoleptic and mixed aryldichalcogenide bis(diphenylphosphino)ferrocenenickel(II), palladium(II), and platinum(II), and bis(diisopropylphosphino)ferrocenepalladium(II) complexes

The mass spectral behavior of a number of organometallic complexes containing the Group 10 metals Ni, Pd, and Pt, together with various thiolate ligands were studied. For Pd, two main types of complexes, differing by the substituents on the phosphorus atom were studied. Types I and II were substituted with bis(diphenylphosphino)ferrocene and bis(diisopropylphosphino)ferrocene ligands, respectively. The Ni complexes, except for one, and the Pd Type I complexes had no molecular radical cations (M(+.)) in their EI spectra. On the other hand, all the Pt complexes showed intense M(+.) ions in their EI spectra indicating that these complexes were more stable as radical cations than those of Ni and Pd. The FAB and MALDI spectra of all the complexes displayed intense quasi-molecular ions (MH(+)) and the fragmentations in both modes were similar. The MALDI spectra of several complexes displayed only M(+.) ions while one gave evidence of both MH(+) and M(+.) ions. Several Pd Type II complexes yielded intense M(+.) in their EI spectra.

Detection of quasimolecular ion of Cu(hfac)(tmvs) by ion attachment mass spectrometry

A molecular ion of (trimethylvinylsilane)(1,1,1,5,5,5-hexafluoroacetyl-acetonato)Cu(I) [Cu(hfac)(tmvs)], which could not be detected by mass spectrometry with conventional ionization techniques such as electron impact ionization because of weak coordination bonds, has been successfully detected as a quasimolecular ion, Cu(hfac)(tmvs)Li+, for the first time by recently developed ion-attachment mass spectrometry (IAMS). Cu(hfac)(tmvs) is a material widely used in the metal organic chemical vapor deposition (MOCVD) process. In the thermal decomposition of Cu(hfac)(tmvs), trimethylvinylsilane (tmvs) and hexafluoroacetylacetone (Hhfac) were observed by IAMS as the reaction products. We also evaluated the dependence of the decomposition process on temperature. We found IAMS to be an effective tool in monitoring and clarifying MOCVD processes.

Tandem mass spectrometric investigation of acylpolyamines of spider venoms and their 15N-labeled derivatives

The fragmentation mechanism of the acylpentamine toxins 1-4 found in the venom of the spider Agelenopsis aperta has been investigated in detail. To identify the origin of the two doublets of unexpected fragment ions at m/z 129/112 and m/z 115/98, three synthetic 15N-labeled analogs 5-7 have been prepared and subjected to CID fragmentation on a triple quadrupole mass spectrometer. It appears that the unexpected doublet of fragment ions arises from an internal portion of the polyamine backbone after either a transaminative Zip reaction or a sequential fragmentation of the quasi-molecular ion. The second option has been proven by in-source CID experiments. The detailed knowledge of acylpentamine fragmentation mechanisms is essential for the correct characterization of isomeric compounds, particularly for coeluting compounds within complex mixtures such as spider venoms.

Collision-induced dissociation of a series of coumarins studied by positive- and negative-ion electrospray triple quadrupole tandem mass spectrometry.

Aryl-substituted 4-hydroxycoumarins (1-57) were investigated by electrospray ionisation (ESI) mass spectrometry. Their fragmentation in the ion source or in the collision cell of a triple quadrupole mass spectrometer was investigated. The effect of the substituents in the aromatic ring on the fragmentation of the 4-hydroxycoumarin derivatives is shown. The influence of the tautomerism on the formation of quasimolecular ions and mass spectral fragmentation was explained. Mass spectral studies on some deuterated compounds proved some of the proposed fragmentation pathways. Results obtained are very useful in the process of detection and characterisation of 4-hydroxycoumarins, as well as for structural elucidation of their more complex derivatives.

Quantification of ligustilides in the roots of Angelica sinensis and related umbelliferous medicinal plants by high-performance liquid chromatography and liquid chromatography–mass spectrometry

A reversed phase high-performance liquid chromatographic method was developed for quantifying E-ligustilide ( 1) and Z-ligustilide ( 3) in the roots of Angelica sinensis (Oliv.) Diels with confirmation using UV, atmospheric pressure chemical ionisation (APCI) MS and APCI-MS–MS techniques. Based on the UV spectra of compounds 1, E-butylidenephthalide ( 2), 3 and Z-butylidenephthalide ( 4), the absorption at 350 nm was chosen as measuring wavelength in which baseline separation of compounds 1 and 3 could be obtained but avoided the interference of compounds 2 and 4. The identity of compounds 1 and 3 in samples were unambiguously determined by the respective quasi-molecular ions ([ M+H] +) in APCI-MS. According to the stability data, acetonitrile was chosen for the preparation of standard solutions in order to minimize the isomerization of compound 3. Compounds 1 and 3 were qualitatively and quantitatively analyzed in seven samples of the roots of Angelica sinensis (Oliv.) Diels, Angelica acutiloba Kitagawa, Angelica acutiloba Kitagawa var. sugiyamae Hikino and the rhizome of Ligusticum chuanxiong Hort. Analysis of an extract from a sample root of Angelica gigas Nakai using LC–MS for the first time could not detect the presence of ligustilide in this herb. The overall analytical procedure is rapid and reproducible which is considered suitable for quantitative analysis of large number of samples.

Mass spectrometric imaging of lipids in brain tissue.

The spatial distributions of various specific lipids in freeze-dried mouse brain sections were monitored using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Mouse brain sections were prepared by cryosectioning, rinsing in 0.15 M NH3HCOO, and freeze-drying, after which the samples were analyzed directly by TOF-SIMS, using Au3+ ions as primary ions. Positive and negative TOF-SIMS spectra of the tissue surface contained peaks from quasimolecular ions of a variety of specific lipids, including cholesterol, sulfatides, phosphatidylinositols, and phosphatidylcholines. Images showing the spatial signal intensity distributions of specific ions were recorded across analysis areas ranging from 100 x 100 microm(2) to 9 x 9 mm(2). The results demonstrate a highly complementary localization of cholesterol and phosphatidylcholine over dimensions from millimeter to micrometer range. Characteristic spatial distributions of several other lipids, including sulfatides and phosphatidylinositols, were observed. Principal component analysis was used to localize regions of the sample surface that show common spectral features. Spectra from different such regions showed large variations in lipid ion signals, indicating large variations in the lipid composition in different regions.

Interest of silver and gold metallization for molecular SIMS and SIMS imaging

The metallization procedure, recently proposed for signal improvement in organic secondary ion mass spectrometry (metal assisted-SIMS or MetA-SIMS), has been quantitatively evaluated for Irganox 1010 and polystyrene fragment and quasimolecular ions. In addition to gold, we investigate the effect of silver evaporation as a sample treatment prior to static SIMS analysis. Ion yields and yield enhancement factors are compared for Ag and Au-metallized molecular films, pristine coatings on silicon and sub-monolayers of the same molecules adsorbed on silver and gold. The results are sample-dependent but, as an example, the yield enhancement calculated for metallized polymer additive (Irganox 1010) films with respect to untreated coatings is larger than two orders of magnitude for the quasimolecular ion and a factor between 1 and 10 for characteristic fragments. The interest of the method for imaging SIMS applications is illustrated by the study of a non-uniform coating of polystyrene oligomers on a 100 μm polypropylene film.

Interest of silver and gold metallization for molecular SIMS and SIMS imaging

The metallization procedure, recently proposed for signal improvement in organic secondary ion mass spectrometry (metal assisted-SIMS or MetA-SIMS), has been quantitatively evaluated for Irganox 1010 and polystyrene fragment and quasimolecular ions. In addition to gold, we investigate the effect of silver evaporation as a sample treatment prior to static SIMS analysis. Ion yields and yield enhancement factors are compared for Ag and Au-metallized molecular films, pristine coatings on silicon and sub-monolayers of the same molecules adsorbed on silver and gold. The results are sample-dependent but, as an example, the yield enhancement calculated for metallized polymer additive (Irganox 1010) films with respect to untreated coatings is larger than two orders of magnitude for the quasimolecular ion and a factor between 1 and 10 for characteristic fragments. The interest of the method for imaging SIMS applications is illustrated by the study of a non-uniform coating of polystyrene oligomers on a 100 μm polypropylene film.