Molecular Intensity Research Articles

Extensive rearrangement reactions observed for differently N-substituted 2,5-dimethylpyrroles under electron ionization

The 70 eV electron ionization mass spectra of fourteen differently N-substituted 2,5-dimethylpyrroles were studied. In accord with the aromatic character of the pyrrole ring, all the compounds studied gave rise to an intense molecular ion peak. Regardless of the functional group at the 1-substituent, for all the compounds the principal fragmentation products were practically the same, although the relative peak intensities varied depending on the structure of the substituent. In addition, an amino group substituent prompted intense radical site initiated cleavages which were minor or totally absent with other compounds. The data showed that extensive rearrangement reactions took place. Some of these were seen directly from the mass spectra, and some only from closer examination of the ion structures. The structures of the ions at m/z 108 and 94, formally representing the [M−1]+ ions of 1,2,5-trimethylpyrrole and 2,5-dimethylpyrrole, respectively, were carefully studied using the collision induced dissociation (CID) technique. Measurements showed that ring expansion reactions took place, protonated 2,5-lutidine and protonated 2-picoline being the most likely isomers formed for the m/z 108 and 94 ions, respectively. The results obtained from the CID experiments were supported by semi-empirical calculations of heats of formation. © 1998 John Wiley & Sons, Ltd.

Fundamentals and features of analytical laser mass spectrometry with ultrashort laser pulses

Among the various ionization techniques used in mass spectrometry, resonant multiphoton ionization (REMPI) is the method of choice if the analysis demands high temporal resolution together with the ability to detect several preselected compounds simulta neously in a complex mixture or matrix. However, REMPI can only fulfill these demands if the excited molecular state(s) involved in the multiphoton ionization process are not significantly depleted by fast energy relaxation or by fragmentation processes during the duration of the ionizing laser pulse. Up to now, only lasers with pulse lengths in the nanosecond range were available for analytical applications of laser mass spectrometry. It turns out that, under these conditions, no useful mass spectra can be obtained for a broad variety of compounds such as organometallic compounds, biomolecules, chemical warfare agents and explosives. A possibility of extending the advantageous features of REMPI to these important groups of substances in particular, and to quickly relaxing molecules in general, is the application of ultrashort but intense laser pulses, which complete the ionization process so quickly that relaxation processes cannot become important. The mass spectra obtained in this way are characterized by an intense molecular ion signal accompanied by structure-specific fragments. Using selected examples of environmentally relevant substances, the influence of laser pulse duration on the features of the laser mass spectra is discussed. Furthermore, it is demonstrated that, even under the high laser intensities used in this experiment, non-resonant processes do not play a significant role and selective ionization is still po ssible.

Structure elucidation of everninomicin-6, a new oligosaccharide antibiotic, by chemical degradation and FAB-MS methods

The structural characterization of a new oligosaccharide antibiotic, Everninomicin-6 (EV-6), is described. Detailed fast-atom bombardment mass spectrometry (FAB-MS) studies along with NMR and chemical degradation methods were conducted to elucidate the structure of EV-6. The effects of the use of various matrices, including salt addition, on the quality of the FAB-MS were explored. The use of 3-nitro benzyl alcohol, dimethyl sulfoxide (DMSO), and NaCl produced the best results: an intense sodiated molecular ion plus structurely informative fragmentation. FAB-MS yields information providing the complete sugar sequence information for everninomicins, which is quite valuable to the elucidation of the structure of this complex oligosaccharide antibiotic. In addition, the results of accurate mass work with the molecular ion are consistent with the assigned structure. The use of electrospray ionization mass spectrometry (ESI-MS) and ESI-MS/MS for the study of EV-6 was investigated and was found to produce an abundant molecular ion with limited structural information. These results revealed that EV-6 resembled EV-D quite closely except for the absence of the nitrosugar and the replacement on ring g of the -CH2OCH3 group with a -CH2OH group.

Investigations on cluster and molecular ion formation by plasma mass spectrometry

The formation of molecular and cluster ions of different inorganic materials in plasma mass spectrometry – spark source mass spectrometry (SSMS), radiofrequency glow discharge mass spectrometry (rf GDMS), laser ionization mass spectrometry (LIMS), inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) – was investigated and compared. Similar abundance distributions of cluster ions were observed for a graphite sample, for boron nitride/ graphite and for metal oxide/graphite mixtures using different plasma mass spectrometric methods. A correlation of intensities of metal argide ions in ICP-MS with their bond dissociation energies was used to estimate unknown dissociation energies of molecular ionic species. For the elements of the 2nd or 3rd period in the periodic table, the intensities of most argon molecular ions (ArX+) measured by ICP-MS rise with increasing atomic number in a similar manner to the theoretically calculated bond dissociation energies of argon molecular ions.

ToF‐SIMS quantification of polystyrene spectra based on principal component analysis (PCA)†

Monodisperses polystyrenes (PS) with molecular weight (Mn) ranging from 850 to 20000 g mol-1 and with different butyl end-groups have been analysed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Only the fingerprint part of the spectra (m/z<200) has been considered. This part comes from the fragmentation of the macromolecules and this is the only contribution in the secondary ion spectra of bulk polymers. In a former study, it has been shown that Mn has a significant influence on the PS secondary molecular ion intensities. It was found that not only the end-group characteristic ion intensities are modified for Mn<50000 g mol-1, but also those related to main chain fragments. In this study, the possibility to use this effect to quantify the molecular weight of the macrochains at the surface is investigated. Because the quantification method should preferably include all the information contained in the spectra (over 150 peaks), statistical methods are needed. Principal component analysis is applied to these ToF-SIMS data. Principal component analysis builds the best linear combination of the variables, namely the principal components that describe the greatest amount of variation of the sample set. It is found that only one principal component is required to account for the molecular weight variations and a second one allows the samples to be discriminated depending on their butyl end-groups. Whatever the PS end-groups (linear, secondary or tertiary butyl), the correlation found between the first principal component and the sample molecular weight leads to the definition of a universal curve that allows the molecular weight determination for an unknown monodisperse PS sample. © 1997 John Wiley & Sons, Ltd.

ToF-SIMS quantification of polystyrene spectra based on principal component analysis (PCA)†

Monodisperses polystyrenes (PS) with molecular weight (M-n) ranging from 850 to 20 000 g mol(-1) and with different butyl end-groups have been analysed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Only the fingerprint part of the spectra (m/z < 200) has been considered. This part comes from the fragmentation of the macromolecules and this is the only contribution in the secondary ion spectra of bulk polymers. In a former study, it has been shown that M-n has a significant influence on the PS secondary molecular ion intensities, It was found that not only the end-group characteristic ion intensities are modified for M-n < 50 000 g mol(-1), but also those related to main chain fragments. In this study, the possibility to use this effect to quantify the molecular weight of the macrochains at the surface is investigated. Because the quantification method should preferably include all the information contained in the spectra (over 150 peaks), statistical methods are needed, Principal component analysis is applied to these ToF-SIMS data. Principal component analysis builds the best linear combination of the variables, namely the principal components that describe the greatest amount of variation of the sample set, It is found that only one principal component is required to account for the molecular weight variations and a second one allows the samples to be discriminated depending on their butyl end-groups. Whatever the PS end-groups (linear, secondary or tertiary butyl), the correlation found between the first principal component and the sample molecular weight leads to the definition of a universal curve that allows the molecular weight determination for an unknown monodisperse PS sample. (C) 1997 by John Wiley & Sons, Ltd.

Mass spectrometric behaviour of 1‐imino‐substituted pyrazolo[1,2‐a]‐[1,2,4]triazoles under electron and chemical ionization

AbstractThe spectra of five pharmacologically interesting substituted pyrazolo[1,2‐a][1,2,4]triazole hydroiodides were measured under electron and chemical ionization. In the electron ionization spectra, in addition to the intense molecular ion peak of the free base (M+*), there was also a relatively intense molecular ion peak of the hydroiodide form, which is unusual since the hydroiodides are rarely so stable. The phenylimino and phenylamino substituents of the triazole ring affected the fragmentation behaviour of the compounds very much. The chemical ionization reagent gases used in this work were methane, isobutane, deuterated ammonia and acetone. In all the cases practically only [M+H]+ ions were observed, the only exception being acetone which also gave rise to intense [M+C2H3O]+ and [M+C3H7O]+ adduct ions. None of the reagent gases used was able to cause any fragmentation.

Origin of the thiazole ring of camalexin, a phytoalexin from Arabidopsis thaliana.

The principal phytoalexin that accumulates in Arabidopsis thaliana after infection by fungi or bacteria is 3-thiazol-2'-yl-indole (camalexin). Detached noninoculated leaves of Arabidopsis and leaves inoculated with the fungus Cochliobolus carbonum were fed [35S]cysteine (Cys) and [35S]methionine. Inoculated leaves incorporated more than a 200-fold greater amount of radioactivity from [35S]Cys into camalexin, as compared with noninoculated leaves. The amount of radioactivity from [35S]Cys that was incorporated into camalexin from inoculated Arabidopsis leaves was 10-fold greater than the amount of radioactivity that was incorporated into camalexin from [35S]methionine. Additional labeling experiments were performed to determine whether other atoms of Cys are incorporated into camalexin. [14C]Cys and [35S]Cys were incorporated into camalexin with approximately the same efficiency. Cys labeled either with deuterium (D3-Cys[2,3,3]) or 13C and 15N ([U-13C,15N]Cys) was also fed to inoculated leaves of Arabidopsis; camalexin was analyzed by mass spectroscopic analysis. The average ratio of molecular ion intensities of 203/200 for [U-13C,15N]Cys-labeled camalexin was 4.22, as compared with 0.607 for the average 203/200 ratio for unlabeled camalexin. The mass fragment-ion intensity ratios of 60/58 (thiazole ring ion fragment) and 143/142 were also higher for [U-13C,15N]Cys-labeled camalexin, as compared with unlabeled camalexin. The 59/58 and 201/200 ratios were higher for D3-Cys-labeled camalexin as compared with unlabeled camalexin. These data are consistent with the predicted formation of the thiazole ring of camalexin from Cys.

Analysis of ruthenium carbonyl–porphyrin complexes: a comparison of matrix-assisted laser desorption/ionisation time-of-flight, fast-atom bombardment and field desorption mass spectrometry

The mass spectrometric characterisation of ruthenium carbonyl‐porphyrin complexes and the corresponding porphyrins is described. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS), fast-atom bombardment mass spectrometry (FAB-MS), in both low-resolution (LR) and high-resolution (HR) modes, and field desorption mass spectrometry (FD-MS) have been used and compared in detail. MALDI-TOF-MS turned out to be ideal for rapid screening of potential synthetic pathways. FAB-MS proved to be especially useful in establishing elemental compositions by HR measurement. Whereas the porphyrins gave very good molecular ion intensities, the ruthenium carbonyl complexes exhibited abundant carbon monoxide (CO) loss in MALDI-TOF-MS and still a measurable CO loss in FAB-MS. Only FD-MS yielded the molecular ion as the base peak of the spectrum in all cases. Optimised FD emitter heating conditions allow the remaining portion of the thermally-induced CO loss to be significantly reduced.

Dissociative scattering of low-energy SiF 3+ and SiF + ions (5–200 eV) on Cu(100) surface

Dissociative scattering of molecular SiF 3 + and SiF + ions from a Cu(100) single crystal surface has been investigated in the incident energy range from 5 eV to 200 eV with a scattering angle of 77°. The scattered ion intensity of dissociative ions and parent molecular ions were measured as a function of incident ion energy. The observed data show that onset energies of dissociation for SiF 3 + and SiF + ions are 30 eV and 40 eV, respectively. The obtained threshold energies are consistent with a impulsive collision model where the dissociation of incident ion is caused by vibrational excitation during collision.

Use of ammonium halides as co-matrices for matrix-assisted laser desorption/ionization studies of oligonucleotides.

Four ammonium salts, NH4F, NH4Cl, NH4Br and NH4I were tested as co-matrices for the matrix-assisted laser desorption/ionization (MALDI) analysis of a number of DNA homopolymers, including phosphorylated d(T)8, d(A)4, d(C)4 and non-phosphorylated d(G)4, using 2-amino-5-nitropyridine (ANP) matrix. In the present study, all the ammonium halides displayed significant enhancement effects on the signal intensities of the intact molecular ions of the DNA homopolymers. Among the halides used, NH4F was found to exhibit the greatest enhancement effects. By comparison with results obtained using the corresponding isomorphous potassium halides as co-matrices, it is postulated that both the cationic and anionic portions of the co-matrix molecule play important roles in the desorption/ionization of the oligonucleotides under typical MALDI conditions. It was also demonstrated that the ANP matrix exhibits a strong inhibitory effect on the formation of alkali-metal adducts in oligonucleotide analysis.

Liquid Gold-Antimony Ion Sources

The mass spectra of ion beams extracted from the impregnated-electrode type liquid gold-antimony ion sources have been investigated. Either eutectic alloy Au66Sb34 or intermetallic compound AuSb2 was used as the source material. The mass spectra were studied as a function of source current or as a function of source temperature, to investigate the behavior of Sb atoms. Analysis of liquid flow and evaporation within the ion source demonstrated that the alloy composition at the ionization point was determined by selective evaporation of Sb atoms, especially for the AuSb2 ion source. Relative intensities of multiply charged monomer ions strongly depended on the estimated concentration of Sb at the ionization point, which determined the evaporation field strength. Also, relative intensities of molecular ions were almost proportional to the estimated concentration of Sb. The present results indicated that ion formation in the liquid gold-antimony ion sources was dominated by the alloy composition at the ionization point. As a result, Au66Sb34 was found to be the preferred material.

Effect of the eluent pH on the thermospray molecular ion intensity of nucleosides

The protonated molecular ion intensities of 15 nucleosides obtained by thermospray ionisation have been measured using 0.1 M ammonium acetate mobile phase at neutral and acidic pH. To explain the dependence of the molecular ion intensity on the mobile phase pH, the hydrophobicity, the pK a values and reversed-phase high-performance liquid chromatographic retention data (log k′) at neutral and acidic pH values were studied. Significant correlations (above 95% probability level) were found between the change in the protonated molecular ion intensity and the hydrophobicity as well as the p K a values of the compounds. The reversed-phase chromatographic retention parameter (log k′) obtained at pH 3.5, showed significant correlation together with the p K a values to the molecular ion intensity change caused by decreasing the mobile phase pH. None of the investigated nucleosides showed an increased molecular io intensity change at low pH when more than 5% methanol was present in the mobile phase.

Mass spectrometric and theoretical investigations into the formation of argon molecular ions in plasma mass spectrometry

The abundance and distribution of argon molecular ions (e.g., ArH+, ArO+, ArN+, Ar2+ and MAr+; M = metal) in plasma MS (ICP-MS, LA-ICP-MS and rf-GDMS) were investigated and compared. In ICP-MS the non-metal argon molecular ions were formed with higher intensity compared with the metal argide ions. This could be explained by theoretical calculations of the binding energies. The ArH+ ion can be viewed as an isoelectronic system, comparable to HCl. The intensities of diatomic metal argide ions relative to metal ions in ICP-MS are less than 10–4. A correlation between the intensities of metal argide ions with the bond dissociation energies of diatomic ions was found. The highest intensity of metal argide ions, of the order of per cent. values, were observed in rf-GDMS. The intensity of the argon molecular ions in an rf-GD varied by up to three orders of magnitude as a function of the plasma parameters (e.g., argon pressure in the GD ion source). The characteristic distribution of diatomic argide ions of REEs in ICP-MS was found to be comparable to the distribution of rare earth oxide ions.

Mass spectrometry of perfluoroalkylated Buckminsterfullerene: a case for sequential gas phase double electron capture

Electron attachment mass spectra of the products of fluoroalkyl radical additions to C 60 show intense molecular ions as well as ions produced by attachment of two electrons to the gas phase molecules. Remarkably, little fragmentation is observed for these fluorocarbon derivatized fullerenes, suggesting that electron attachment is occurring on the highly electron deficient C 60 rather than the fluorocarbon side chains. Perfluoroalkylated C 60 compounds are sufficiently volatile to be used as mass references for high-mass electron attachment mass spectrometry. These compounds are also found to produce molecular ions by negative ionization matrix assisted laser desorption ionization mass spectrometry.

Utilization of electrospray ionization to gain information on the processes involved in [M − H] + formation in FAB/LSIMS

Some cyclic acetals and derivatized monosaccharides which yielded anomalous [M − H] + in FAB/LSIMS were analyzed by electrospray ionization (ESI) to gain further information on the condensed phase processes suggested to be responsible for the formation of [M − H] + in FAB/LSIMS. For all compounds, [M + H] + was the only significant ion present in the molecular ion region in ESI. Since ionization in ESI is believed to reflect predominantly condensed phase equilibrium processes, the absence of [M − H] + in ESI suggests that these processes are not responsible for hydride abstraction in FAB/LSIMS and that processes other than the pre-formation of ions in solution have to be invoked. Fast atom/ion-induced processes are thus considered the most likely origin of [M − H] + in FAB/LSIMS and would explain the difference in ESI and FAB/LSIMS relative molecular ion intensities. The proposed occurrence of particle-induced ionization processes in the condensed phase in FAB/LSIMS is consistent with previous experimental evidence from this laboratory, and many chemical reactions which take place in FAB/LSIMS have been explained in like manner. The [M + H] + peak in ESI for the compounds studied is attributed to the presence of NH 4 + in solution as an impurity and the resulting condensed phase equilibrium processes. The presence of [M + NH 4] + in all ESI mass spectra is consistent with this interpretation. An [M + Na] + adduct is also observed in ESI for the derivatized monosaccharides which suggests that condensed phase equilibrium processes are responsible for this adduct when observed in FAB/LSIMS.

An investigation of the ionization processes occurring in fast atom bombardment (FAB) for perhydro-3a,6a,9a-triazaphenalene, a compound with high proton affinity/basicity and hydride donor properties

The ionization processes which take place in fast atom bombardment (FAB) mass spectrometry for perhydro-3a,6a,9a-triazaphenalene, a compound with strong proton affinity/basicity and hydride donor ability, are investigated. Chemical ionization (CI) and electrospray ionization (ESI) were also performed to facilitate elucidation of the FAB ionization mechanisms. At the onset of fast atom bombardment, protonation is favored over hydride abstraction, but upon continued bombardment the intensity of [M − H] + increases relative to [M + H] + and hydride abstraction becomes the dominant process. In CI, protonation is strongly favored with the relative intensity of [M − H] + much lower than that obtained in FAB. Since the reactive ionic species in CI have greater gas phase hydride ion affinities than those proposed for FAB, hydride abstraction in FAB cannot be described predominantly in gas phase terms. A condensed phase ionization model for FAB, where protonation and hydride abstraction occur in solution, can instead explain the difference in relative molecular ion intensities obtained in FAB and CI. ESI data of perhydro-3a,6a,9a-triazaphenalene further enhances the condensed phase ionization model proposed for FAB, as the negligible presence of [M − H] + suggests that beam-induced processes are responsible for the formation of this species in FAB. This interpretation would explain the increase in relative [M − H] + intensity upon continued fast atom bombardment, through the accumulation of beam-induced reaction products in solution. The protonation of perhydro-3a,6a,9a-triazaphenalene in FAB is also proposed to occur predominantly through beam-induced processes, as equilibrium processes in solution should not account for the absolute ion intensities obtained in FAB. The presence of a strong [M + H] + peak in ESI of perhydro-3a,6a,9a-triazaphenalene with methanol as solvent, is thought to be a consequence of the presence of NH 4 + in solution and not to reflect the predominant formation of this species in FAB through equilibrium processes in solution. ESI work with ethylene glycol as solvent supports this interpretation.

Differentiation of olefin isomers by survivor‐ion mass spectrometry

AbstractSurvivor‐ion mass spectrometry is a method that relies on the selective monitoring of non‐dissociating ions that underwent collisional neutralization and reionization. These charge‐permutation processes are found to modulate the relative intensities of precursor ions, both molecular ions and fragments, formed by electron impact from C6H10, C6H12, C6H12O, and xylene isomers. The relative intensities of highly‐unsaturated ions, such as C5H5+, C4H, C3H3+, and C2H, and enol ions are enhanced in the survivor‐ion spectra in dependence on the neutralization gas. Isomer differentiation through survivor‐ion spectra is achieved for conjugated hexadienes, 1,4‐hexadiene and cyclohexene, but not for hexenols and xylenes.

Coumarin laser dyes as matrices for matrix assisted UV laser desorption/ionization mass spectrometry

We describe the performance of laser dyes, coumarin 2, coumarin 47, coumarin 120, and coumarin 152, as matrices, for UV laser desorption ionization mass spectrometry and report their use in the analysis of a variety of macromolecules including peptides, proteins, deoxynucleotides and polymers with masses extending up to about 150 000 u. Mass spectra with intense molecular ion signals have been obtained with subpicomole sensitivity and resolutions in the region of M/Δ M ≈ 450 (FWHM; M < 6000 u), in both the positive and the negative ion modes using 337 nm N 2 laser radiation for sample desorption. We have also studied the influence of laser fluence and matrix concentrations on the mass resolutions of the molecular ion signals and the formation of matrix related adduct ions. Scanning electron microscope inspection of the samples reveals a rather non-homogeneous surface structure and the methods adopted for improving the sample homogeneity are discussed.

GC—MS analysis of carboxylic acids in petroleum after esterification with fluoroalcohols

AbstractThe GC–MS characteristics of carboxylic acid esters prepared from fluorine‐containing alcohols were compared to those of methyl esters. The GC retention of 2,2,2‐trifluoroethyl (TFE) esters was less than, and 2,2,3,3,4,4,4‐heptafluoro‐1‐butyl (HFB) esters was approximately equivalent to that of methyl esters. Mass spectra of TFE and HFB aliphatic esters show significantly more intense molecular and key fragment ions than those of methyl esters. Also, owing to their significantly higher molecular weights, TFE or HFB ester molecular ions and most fragment ions of interest occur at significantly higher m/z values than most potential interfering ions.Data for about 70 individual TFE and HFB esters are reported. Application of the methodology to a petroleum‐derived carboxylic acid concentrate resulted in identification of straight chain, isoprenoid, methyl‐substituted straight chain (2‐, 3‐, 5‐,10‐, 12‐positions along chain), and dimethyl‐substituted straight chain acids containing from 11 to 22 carbons. Benzoic acid and homologs with up to 3‐carbons in alkyl substitutents were minor components in the sample.The procedure provided for forming TFE and HFB esters from free acids requires less time and effort than a previously reported method, while retaining its capability for achieving essentially quantitative conversion. Free hydroxyl groups in alcohols and phenols are converted to trifluoroacetate esters concurrently with formation of TFE/HFB carboxylic acid esters. The reaction products, including compounds with two functional groups (diacids, salicylic acid, etc.), chromatograph well on conventional nonpolar GC stationary phases.