J.-C. Tabet

Proton affinities of the commonly occuring L-amino acids by using electrospray ionization-ion trap mass spectrometry

In this study, the use of an electrospray ionization (ESI)-ion trap mass spectrometry to perform thermochemical determinations using the kinetic method is shown. In this method, competitive dissociations of selected proton-bound heterodimeric [BiHB]+ ions are investigated and particularly, the product ion abundance [BiH+] / [BH+] ratio is accurately measured and compared to the proton affinity of each neutral partner (i.e. the Bi and B). The proton affinities of the amino acids are well known and these compounds were chosen to investigate the ion trap potentiality for such purpose, particularly, when externally prepared ions were injected. It appears that, in spite of the low abundance of homo and heterodimer ions, constant and reproducible [BiH+] / [BH+] ratios are obtained when sequential tandem mass spectrometry (MS/MS) experiments are performed within similar ion trapping conditions. Such conditions are reached for a constant ion excitation qz value. The resulting ln([BiH+] / [BH+]) dependence upon the PABi and PAB values (from the literature) is linear and yields a reproducible slope after several measurements are made to determine the accuracy of this approach. Under various excitation conditions, in spite of the change in effective temperature, neither the scale order nor the PA values are changed. Furthermore, entropy variation (δΔS0) between leucine and other amino acids was evaluated by several methods.

Kinetic Analysis of the Reaction Between d(TTGGCCAA) and [Pt(NH3)3(H2O)]2+ by Enzymatic Degradation of the Products and ESI and MALDI Mass Spectrometries

The kinetics of the reaction between the octanucleotide d(TTGGCCAA) in the single-stranded form in pure water and the platinum complex [Pt(NH3)3(H2O)]2+ was investigated by electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) mass spectrometries coupled with enzymatic degradation of the adducts. These methods led to the determination of specific rate constants of platination. The global rate constant characteristic of the formation of adducts on each 5- or 3-guanine were measured by electrospray ionization analysis. The ratios between the 5- and 3-adducts were determined from enzymatic degradation of the final reaction mixture and MALDI analysis. The platination in water is approximately eight times faster than in 0.1 M NaClO4. The selectivity of platination is a factor of 2 in favor of the 5-guanine, and similar to that observed for the reaction between d(CTGGCTCA) and [Pt(NH3)3(H2O)]2+ in 0.1 M NaClO4.

Study of ghost peaks resulting from space charge and non-linear fields in an ion trap mass spectrometer

Ion trap improvements require some geometrical modifications which can alter significantly the relative linear and non-linear field contributions. In this work, where the ion trap was interfaced with an electrospray source, it was shown that under particular ion trapping conditions due to space charge, artefacts called ghost peaks are displayed in the mass spectra. They are characterized by particular width and shape and appear at twice the m/z ratio of the expected peaks (for positive and negative ions) when the analyzed ions are ejected at β x = 1/3 (extended mass range mode). They were unambiguously identified by using isolation sequences. The various experimental factors governing the observation of ghost peaks were explored in order to rationalize their origin. Especially the capillary exit voltage, ion accumulation time and low m/z cut-off of the analytical scan, which modify the space charge strength, influence the ghost peak intensities. Furtbermore, it is evidenced that only the injected ions which are located at q z ≤ 0.25 (β x ≤ 2/11) at the beginning of the analytical scan step give rise to the formation of such artefact peaks. Under tbese experimental conditions, the ion cloud defocused by space charge is subjected to high-order multipole fields (e.g. 22-pole). Then the motion of the analyzed ions is destabilized, affecting the ion ejection. It seems that two ion populations can co-exist: (i) one submitted to a normal ion ejection at q z = 0.45, where the axial modulation is applied, i.e. normal peaks, and (ii) a second ejected at the natural q z = 0.908 boundary, leading to ghost peaks at twice the m/z value of normal signals.

Investigation of Space Charge Interactions Which Arise During Simultaneous Confinement of Positive and Negative Ions in an Ion Trap Mass Spectrometer

While positive and negative ions are created simultaneously in a quadrupole ion trap during electron ionization, there is a marked preponderance of the former species. With prolonged ionization and a relatively high concentration of substrate molecules, positive ion species of low mass/charge ratio can give rise to space charge potentials that can neutralize and exceed the normal trapping potentials for positive ions of high mass/charge ratio, leading to the ejection of positive ions, but not negative ions, from the ion trap. The onset of space charge is manifested by the ejection of positive ions ofm/z>600; as the space charge potential is increased incrementally, ejection of positive ions is extended to those of lower mass/charge ratio, that is, tom/z≈300. Negative ions are not affected by the space charge induced by the positive ions; within the limits of experimentation, negative ions in the mass/charge ratio rangem/z352–633 remain confined within the ion trap and can be ejected during a normal analytical scan. Under the influence of positive ion-induced space charge, there is an enhancement of the storage of negative ions having low values of the trapping parameterqz. These observations can be rationalized using a theoretical approach based on a pseudo-potential well model. Although not investigated here, the simultaneous storage of positive ions and negative ions permits the interactions between these species to be studied.

Simulation of ion beam and optimization of orthogonal tandem ion trap/reflector time-of-flight mass spectrometry

The aim of the present work is to design an orthogonal coupling between a quadrupole ion trap and a reflector time-of-flight (reTOF) analyzer. According to the characteristics of the orthogonal ion injection in the reTOF analyzer, the incident ion beam properties have been determined. Different ion ejection modes from the trap cell, consistent with orthogonal-reflector time-of-flight mass spectrometry (O-reTOF/MS), have been investigated by simulations (SIMION 6.0) and experimental exploration. Especially, the following scanning modes have been studied: (1) dc scan applied to the ring electrode, (2) dc pulse applied to the ring electrode, and (3) dc pulse applied to the end-cap electrodes with switching off the drive rf voltage, under various storage conditions and for different m/z values. Distributions of ion velocities, kinetic energies, and ion ejection times, obtained by simulations and experiments, were compared using the ejection mode (2). Results from simulations of ions orthogonally injected into the reTOF/MS, show that the incident ion beam must be mono-energetic, with initial radial KE<2 eV to be efficiently deflected and detected. The simulations and experiments of original scanning from ion trap enlighten that dc ejection with rf voltage was not compatible with O-reTOF analysis (high energetic and noncontinuous ion beam). Appropriate dc ejection, with switching off the rf voltage, was consistent with O-reTOF analysis. For enhancing the duty cycle of the proposed system, the challenge is then to optimize ion ejection to get an ion beam as continuous as possible.

Characterization of zirconium complexes of interest in spent nuclear fuel reprocessing by electrospray ionization mass spectrometry

Liquid-liquid extraction of zirconium, one of the most important fission products, was followed using electrospray ionization mass spectrometry under conditions simulating reprocessing of nuclear spent fuel. Zr(IV) can precipitate from the organic phase after extraction by dibutylphosphoric acid (HDBP), the most common degradation product of tributylphosphate (TBP) radiolysis. Different complexes were detected with electrospray used in positive or negative ion modes, according to the extraction conditions such as the ligand/metal ratio. Stoichiometry of the Zr(IV) complexes was determined by combining isotopic labeling [H(15)NO(3)] of the aqueous phase in the extraction system and tandem mass spectrometry experiments. These results were compared with the species observed using other techniques reported in the literature. The mechanisms of ionization/desorption of these complexes are proposed depending on the organic ligand character (neutral (L) such as TBP, or acidic (HL) such as HDBP), and the ionization mode used. Copyright 2000 John Wiley & Sons, Ltd.

Influence of trapping parameters on ion injection and dissociation efficiencies in a quadrupole mass filter/ion trap tandem instrument

Investigations on ion injection into a laboratory-built quadrupole/ion trap tandem instrument (Nermag R10-10/ITMS Finnigan) with a 90° geometry are described. The quadrupole dual electron impact/chemical ionization (EI/CI) source was used as an external source to prepare ions under El (or CI) conditions. They were selected by the quadrupole analyzer and injected into the ion trap by using an electrostatic quadrupolar deflector. The effects of major trapping parameters (i.e. ion injection r.f. level and helium pressure) were considered. The injection efficiency of the stable CF 3 + (m/z 69) fragment ion from FC43 and also those of the m/z 40 and 84 atomic cations of argon and krypton depend upon the injection r.f. level conditions. As expected, effects of non-linear resonances were observed during these experiments. Under certain injection and trapping conditions, ion motion can be strongly affected, yielding ion ejection, because non-linear fields (resulting from device imperfections) are present in addition to the quadrupolar potential required for ion trapping. The influences of non-linear effects according to their strength were investigated to show the best and worst trapping conditions. The non-linear resonances are characterized by the expected β x values. In order to determine the average internal energy deposited on the selected polyatomic ions during injection, butylbenzene molecular ions were chosen as a model for internal energy measurement. The effects of the r.f. injection level and helium pressure on the injected ion fragmentation were studied. The M +. ions were only observed through a narrow injection q s window. At low q s values, only low critical energy m/z 92 ions were detected, while the higher critical energy m/z 91 ions were observed at higher q s values.

Investigation of the reactivity of arylamines, organo-hydrazines and tolylisocyanate towards [PW12−xMxO40]n− Keggin anions

Reaction of K7[A,alpha-PW9Mo2O39] with Na2MoO4.2H2O in a mixture of water/dioxane/hydrochloric acid and further precipitation with (Bu4N)Br provided (Bu4N)3[A,alpha-PW9Mo3O40](3). Analogous reaction with K7-xNax[alpha-PW11O39] is an alternative to the synthesis of (Bu4N)3[alpha-PW11O39{MoVIO}]2. Multinuclear NMR and ESI mass spectrometry have been used to interpret the reaction of (Bu4N)x[alpha-PW11O39{ReO}](x=3 1; x=4 1I), (Bu4N)x[alpha-PW11O39{MoO}](x=3 2; x=4 2I) and (Bu4N)3[A,alpha-PW9Mo3O40]3 by organohydrazines, arylamines, tolylisocyanate and tetraphenylphosphine imide.

Peptide toxin identification by liquid chromatography/ mass spectrometry using an external electrospray ionization source combined with ion trap mass spectrometry

Three mutated synthetic peptides, analogs of the channel toxin BGK, were studied by liquid chromatography/mass spectrometry n (LC/MS n ) using an external electrospray ionization (ESI) source. The reduction of disulfide bridges, combined with alkylation of the free cysteine residues, is shown to be a useful tool to determine their number as well as their location in conjunction with proteolysis of the peptides. The modified toxins were identified by LC/MS, while LC/MS/MS experiments were conducted to determine and locate modifications. Low-energy collision-induced dissociation spectra of S-alkyl cysteine derivative display ions (MH − 91)+ attributed to the loss of HS–CH2–CONH2 as confirmed by sequential MS3 experiments. Identification and localization of cysteine residues can be achieved providing definitive identification for unknown proteins containing S-carbamidomethylcysteine.