Regioselective ion-molecule reactions for the mass spectrometric differentiation of protonated isomeric aromatic diamines

A mass spectrometric method utilizing regioselective ion-molecule reactions has been developed for the differentiation of protonated isomeric aromatic diamines in FT-ICR, linear quadrupole ion trap and triple quadrupole mass spectrometers.     

Coupling of ion-molecule reactions with liquid chromatography on a quadrupole ion trap mass spectrometer

We report for the first time a coupling of gas-phase ion-molecule reactions with chromatographic separations on a quadrupole ion trap mass spectrometer. The interface was accomplished by using a pulsed valve for the introduction of a volatile neutral into the ion trap. The pulsed valve controller is synchronized with the mass spectrometer software. The setup requires some minor modifications to the vacuum system of the commercial quadrupole ion trap but most of the modifications are external to the mass spectrometer. Two applications of this interface are described: differentiation between two phosphoglucose positional isomers and detection of a phosphopeptide in a peptide mixture. Both applications are using the reactivity of trimethoxyborate towards a phosphate moiety in the negative ion mode. The detection of phosphopeptides hinges on our findings that non-phosphorylated peptide anions do not react with trimethoxyborate. This LC/MS detection can be easily visualized in terms of selected reaction monitoring.     

Probing the mechanisms of the self ion-molecule reactions of dopamine in an ion trap mass spectrometer

Our previous work was the first to report [M+CH](+) and [M+C(2)H(3)](+) ions in the self ion-molecule reactions (SIMR) of two aza-crown ethers in an ion trap mass spectrometer (ITMS). In this study, the CH and C(2)H(3) addition ions were also found in the SIMR of dopamine. The SIMR of dopamine lead to the formation of the protonated molecules ([M+H](+)), of adduct ions ([M+F](+), where F represents fragment ions), and of [M+CH](+), [M+C(2)H(3)](+) and [2M+H](+) ions. Based on the combination of the results of isolation experiments and semi-empirical calculations, the reactive site for the formation of the [M+H](+) and [M+CH](+) ions of dopamine is proposed to be the amino group.     

Stereochemical differentiation of cyclic glycols by ion-molecule reactions in a quadrupole mass spectrometer using dimethyl ether acetonitrile and 2-S-pyrrolidinemethanol

In this study, ion-molecule reactions using chemical ionization in the positive ion mode using dimethyl ether, acetonitrile and 2-S-pyrrolidinemethanol as reagent gases have been used to distinguish between cis- and trans-1,2-cyclopentanediol and cis- and trans-1,2-cyclohexanediol. In the gas phase, the stereospecific ion-molecule reaction gives characteristic ions and substantial differences are observed in their relative abundances from which the diastereoisomers of those cyclic glycols can be clearly differentiated. Copyright 2000 John Wiley & Sons, Ltd.     

Formation of solvated ions in the atmospheric interface of an electrospray ionization triple-quadrupole mass spectrometer

A simple method capable of generating and investigating various solvent clusters and solvated ions was developed. The technique opens a door to studying these complexes on commercially available instruments. Formation of the desired solvated ion in the gas phase was achieved by introducing the appropriate volatile solvent vapour into the curtain gas stream. Capabilities of the technique are illustrated by generating alkali, alkaline earth and transition metal cations solvated by various volatile compounds such as water, methanol and acetonitrile. Depending on the ligands and on the experimental conditions, clusters of 2-100 molecules may be observed. Isotope labelling suggests that these are formed by a re-solvation process in the curtain gas region.     

Using a triple-quadrupole mass spectrometer in accurate mass mode and an ion correlation program to identify compounds

Atomic masses and isotopic abundances are independent and complementary properties for discriminating among ion compositions. The number of possible ion compositions is greatly reduced by accurately measuring exact masses of monoisotopic ions and the relative isotopic abundances (RIAs) of the ions greater in mass by +1 Da and +2 Da. When both properties are measured, a mass error limit of 6-10 mDa (< 31 ppm at 320 Da) and an RIA error limit of 10% are generally adequate for determining unique ion compositions for precursor and fragment ions produced from small molecules (less than 320 Da in this study). 'Inherent interferences', i.e., mass peaks seen in the product ion mass spectrum of the monoisotopic [M+H]+ ion of an analyte that are -2, -1, +1, or +2 Da different in mass from monoisotopic fragment ion masses, distort measured RIAs. This problem is overcome using an ion correlation program to compare the numbers of atoms of each element in a precursor ion to the sum of those in each fragment ion and its corresponding neutral loss. Synergy occurs when accurate measurement of only one pair of +1 Da and +2 Da RIAs for the precursor ion or a fragment ion rejects all but one possible ion composition for that ion, thereby indirectly rejecting all but one fragment ion-neutral loss combination for other exact masses. A triple-quadrupole mass spectrometer with accurate mass capability, using atmospheric pressure chemical ionization (APCI), was used to measure masses and RIAs of precursor and fragment ions. Nine chemicals were investigated as simulated unknowns. Mass accuracy and RIA accuracy were sufficient to determine unique compositions for all precursor ions and all but two of 40 fragment ions, and the two corresponding neutral losses. Interrogation of the chemical literature provided between one and three possible compounds for each of the nine analytes. This approach for identifying compounds compensates for the lack of commercial ESI and APCI mass spectral libraries, which precludes making tentative identifications based on spectral matches.     

High-sensitivity quantitation of cabergoline and pergolide using a triple-quadrupole mass spectrometer with enhanced mass-resolution capabilities

Quantitative analysis of pharmaceuticals with low systemic plasma levels requires the utmost in sensitivity and selectivity from the analytical method used. A recently introduced triple-quadrupole mass spectrometer with unique enhanced mass-resolution capability was evaluated in the analysis of two such drugs, cabergoline and pergolide, in plasma. Liquid chromatographic/electrospray ionization selected reaction monitoring determination of cabergoline in plasma at unit mass-resolution demonstrated improved sensitivity (50 fg on-column), coupled with suitable accuracy and precision over a broad linear dynamic range covering five orders of magnitude (50 fg to 5 ng on-column). Liquid chromatographic/atmospheric pressure chemical ionization selective reaction monitoring determination of pergolide in plasma also attained a high level of sensitivity (500 fg on-column) at unit mass-resolution, with accuracy and precision values well within pharmaceutical industry standards. Again, a linear dynamic range covering five orders of magnitude (500 fg to 50 ng on-column) was achieved for the assay. Utility of the enhanced mass-resolution feature of the triple-quadrupole mass spectrometer in the determination of pergolide resulted in an improvement in analyte sensitivity (250 fg on-column) and linear dynamic range (250 fg to 50 ng on-column).     

Differentiation of isomeric cyclic diamides by electron impact mass spectra

The behaviour under electron impact of two series of isomeric cyclopentane- and cyclohexane-1, 2- and -1, 3-dicarboxylic acid dipiperidides was studied. Diagnostic fragmentation pathways were found to differentiate the isomeric diamides. Additional evidence was obtained from the metastable transitions.     

Traceability study of hay with a mass spectrometer based on ion-molecule reactions of krypton,xenon and mercury using multivariate data analysis

The application of mountain hay for wellness purposes has led to a substantial valorisation. To assure the quality associated with the high standards of production, which are often related to a characteristic distribution of plants and a variety of essential oils, a mass spectrometer, based on ion-molecule-reactions (IMR) of mercury, krypton and xenon, was employed to analyse characteristic VOCs of hay such as coumarin or typical monoterpenes and then used to develop an approach for the traceability of single hay samples based on concepts of multivariate statistics. The application of the primary gases to aqueous solutions of the pure compounds shows their suitability to deal with this problem, reveals important factors for the creation of a measurement set-up of such gas mixtures and indicates different mechanisms for the fragmentation, as shown for coumarin. The limit of determination (3*s_R/SEN) for aqueous solutions of p-cymene is 0.13?mg?L^?1 using PLS1 and the presented combined mercury and xenon set-up, which confirms that this strategy is appropriate for an integration of compounds, which are present in low concentrations only, into a qualitative model. The results of the principal component analyses (PCA) of 136 hay samples were verified for the suitability to characterize single types of hay using three measurements of nine mountain hay samples, three normal hay samples and three aftermath samples for evaluation and applying SIMCA for classification at a significance level of 5%. The traceability of mountain hay samples is good (93% correctly classified) and can be used to protect these valuable samples.     

The use of ion-molecule reactions in the mass spectrometric location of double bonds

A method for the location of the positions of carbon-carbon double bonds using high pressure mass spectrometry is proposed. A known olefinic molecular ion reacts with a second, unknown olefin to form a four-centre complex, which fragments with retention of the structural identity of methylene and substituted methylene groups to eliminate a new olefin molecule and to form an unsaturated ion from which the position of the double bond in the unknown olefin can be inferred. Vinyl methyl ether proved to be a convenient reagent gas and its molecular ion undergoes the required reaction with several classes of olefinic compound. Conjugated dienes and unsaturated compounds containing electronegative groups do not undergo this reaction.     

Effect of ion-molecule collisions in the vacuum chamber of an electrospray time-of-flight mass spectrometer on mass spectra of proteins

Electrospray ionization spectra of positive multiply charged ions of several proteins with molecular weight from 8565 to 339,100 u were recorded at different pressures of residual gas in the vacuum chamber of an electrospray time-of-flight mass spectrometer. The pressure was varied in the range (0.2 to 5) × 10^?6 torr. The effect of pressure was found to be significant even for the lightest protein investigated (ubiquitin), which resulted in a decrease of both sensitivity and resolution. Investigations of the arrival-time distributions and the energy distributions of ions showed that collision-induced dissociation (CID) of the protein ions in the drift region is the main process responsible for the effect. Several CID cross sections of proteins were estimated from a series of mass spectra recorded at different pressures in the reflecting mode: 1150 Ų for cytochrome c (averaged over charge states z = 14–18), 800 Ų for lysozyme (z = 8–10), 1840 Ų for apomyoglobin (z = 12–25), 800 Ų for holomyoglobin (z = 8), and 2500 Ų for carbonic anhydrase II (z = 22–35). Several experiments with large proteins in their native conformations and low charge states (m/z 0,000) demonstrate that these ions are less sensitive to high residual gas pressure.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

Metal-metal chelate exchange reactions in the ion source of a mass spectrometer

In the mass spectra of several transition metal chelates with diethyldithiocarbamate as a ligand, intense peaks are present which could be assigned to extraneous metal (particularly nickel) containing ions. Using an Au-Rh coated ion source, evidence was obtained for the occurrence of exchange reactions in the region of the filament assembly.     

Differentiation of isomeric compounds by two-stage proton transfer reaction time-of-flight mass spectrometry

We investigated a two-stage ion source for proton transfer reaction (PTR) ionization to achieve more selective mass spectrometric (MS) detection of selected volatile organic compounds (VOCs) than that achieved with commonly used PTR-MS instruments, which are based on single-step PTR ionization with H3O+. The two-stage PTR ion source generated reagent ions other than H3O+ by an initial PTR between H3O+ and a selected VOC, and then a second PTR ionization occurred only for VOCs with proton affinities larger than the affinity of the reagent VOC. Acetone and acetonitrile were useful as reagent VOCs because they provided dominant peaks as a protonated form. Using two-stage PTR-MS, we differentiated isomeric VOCs (for example, ethyl acetate and 1,4-dioxane) by means of differences in their proton affinities; protonated acetone formed the [M + H]+ ion from ethyl acetate but not from 1,4-dioxane. The PTR-MS-derived concentrations agreed quantitatively with those independently determined by Fourier transform infrared spectroscopy (FT-IR) at parts per million by volume (ppmv) levels. In addition, interfering fragment ions formed from alkyl benzenes at m/z 79 (C6H7+) could be distinguished from the m/z 79 ion arising from protonation of benzene, and therefore this method would prevent overestimation of benzene concentrations in air samples in which both benzene and alkyl benzenes are present. This two-stage PTR ionization may be useful for distinguishing various isomeric species, including aldehydes and ketones, if appropriate reagent ions are selected.     

Conformational effects in the chemical ionization spectra of derivatives of isomeric 2,3-dimethyl-1,4-cyclopentanediols

The chemical ionization mass spectra of the trimethylsilyl and methylboronate derivatives of some isomeric 2,3-dimethyl-1,4-cyclopentanediols have been determinded using methane and isobutane as reagent gases. The degree of loss of TMSOH or CH₃B(OH)₂ appears to be strongly dependent upon the stereochemistry of the 2,3-dimethyl-1,4-cyclopentanediol moiety and permits differentiation between the structural isomers. The results obtained from the chemical ionization (isobutane) spectra of the methylboronates are in excellent agreement with those derived from infrared data.     

Identification of N-gluconyl [corrected] ethanolamine in wine by negative electrospray ionization with post-column chloride attachment and accurate mass determination on a triple-quadrupole mass spectrometer

In this study a specific taste modulating flavour-ingredient, N-glucosyl ethanolamine, was determined in two Beerenauslese wines using two different LC-MS techniques. For a first screening LC-MS(2) on an ion-trap mass spectrometer with negative electrospray ionization (ESI(-)) was applied. Sensitivity (and selectivity) was successfully increased approx. 10-fold by post-column addition of chloroform to form [M+Cl](-) species. In a second step LC-MS(2) on a triple-quadrupole mass spectrometer in accurate mass mode confirmed the presence of N-glucosyl ethanolamine in wine. The application of the right MS(2) transitions for an unambiguous identification is discussed. N-Glucosyl ethanolamine concentrations in the wines were found to be 1.1 and 4.0 microg/l.     

Ion-molecule reactions studied at non-zero scattering angles in a mass spectrometer

A method is described by which ion-molecule reactions which involve non-zero scattering angles can be studied in a double-focusing mass spectrometer. Reactions in which there is no alteration in the charge on the reactant ion, while the target species is ionized or excited, have been examined at relative translational energies of 1-10 keV. The method allows the measurement of kinetic energy losses.Interference due to the main beam of non-reacting or elastically scattered ions is minimized relative to the zero scattering angle case. In addition to applications for the determination of ionization potentials and other quantities of thermochemical interest, it is suggested that the study of collisional excitation might be facilitated.     

Fragmentation of isomeric intrastrand crosslink lesions of DNA in an ion-trap mass spectrometer

The collision-induced dissociation pathways of isomeric cytosine-guanine and cytosine-adenine intrastrand crosslink-containing dinucleoside monophosphates were investigated with the stable isotope-labeled compounds to gain insights into the effects of chemical structure on the fragmentation pathways of these DNA modifications. A Dimroth-like rearrangement, which was reported for protonated 2′-deoxycytidine and involved the switching of the exocyclic N4 with the ring N3 nitrogen atom, was also observed for the cytosine component in the protonated ions of C[5–8]G, C[5–2]A, and C[5–8]A, but not C[5-N ²]G or C[5-N ⁶]A. In these two sets of crosslinks, the C5 of cytosine is covalently bonded with its neighboring purine base via a carbon atom on the aromatic ring and an exocyclic nitrogen atom, respectively. On the contrary, the rearrangement could occur for the deprotonated ions of C[5-N ²]G, C[5-N ⁶]A, and unmodified cytosine, but not C[5–8]G, C[5–2]A, or C[5–8]A. In addition, ammonia could be lost more readily from C[5-N ²]G and C[5-N ⁶]A than from C[5–8]G, C[5–2]A, and C[5–8]A. The results from the present study afforded important guidance for the application of mass spectrometry for the structure elucidation of other intrastrand/interstrand crosslink lesions.