Internal energy distributions in desorption electrospray ionization (DESI)

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the "survival yield" method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7-1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves "droplet pick-up" (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters.     

Comparison of the internal energy deposition of venturi-assisted electrospray ionization and a venturi-assisted array of micromachined ultrasonic electrosprays (AMUSE)

The internal energy deposition of a Venturi-assisted array of micromachined ultrasonic electrosprays (AMUSE), with and without the application of a DC charging potential, is compared with equivalent experiments for Venturi-assisted electrospray ionization (ESI) using the "survival yield" method on a series of para-substituted benzylpyridinium salts. Under conditions previously shown to provide maximum ion yields for standard compounds, the observed mean internal energies were nearly identical (1.93-2.01 eV). Operation of AMUSE without nitrogen flow to sustain the air amplifier focusing effect generated energetically colder ions with mean internal energies that were up to 39% lower than those for ESI. A balance between improved ion transfer, adequate desolvation, and favorable ion energetics was achieved by selection of optimum operational ranges for the parameters that most strongly influence the ion population: the air amplifier gas flow rate and API capillary temperature. Examination of the energy landscapes obtained for combinations of these parameters showed that a low internal energy region (相似文献   

Manipulating internal energy of protonated biomolecules in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

The internal energy of protonated leucine enkephalin has been manipulated in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry with two newly designed pump-probe experiments. Blackbody infrared radiation was applied to pump an ion population into a well-defined internal energy distribution below the dissociation threshold. Following this pumping stage, the internal energy distribution was probed using on-resonance collisional activation to dissociate the ions. These pump-probe experiments were carried out in two different ways: (a) using on-resonance collisional activation with variable kinetic energies to dissociate the ions at a constant initial ion temperature (determining the precursor ion survival percentage as a function of kinetic energy) and (b) using on-resonance collisional activation with a constant kinetic energy to dissociate the ions at variable initial ion temperatures (to investigate the ion survival yield-initial ion temperature dependence). Using this approach, a detailed study of the effects of the initial ion temperature, the probing kinetic energy and the internal energy loss rate on the effective conversion efficiency of (laboratory-frame) kinetic energy to internal energy was conducted. This conversion efficiency was found to be dependent on the initial ion temperature. Depending on the experimental conditions the conversion efficiency (for collisions with argon) was estimated to be about 4.0 +/- 1.7%, which agrees with that obtained from a theoretical modeling. Finally, the reconstructed curves of the ion survival yield versus the mode of the (final) total internal energy distribution of the activated ion population (after pump and probe events) at different pump-probe conditions reveal the internal energy content of the activated ions.     

Mass spectral fragmentation reactions of a therapeutic 4-azasteroid and related compounds

Mass spectra were acquired for a therapeutic 4-azasteroid (dutasteride), and some related compounds, using various ionization conditions (EI, CI, APCI and ESI) in both positive and negative ion modes. The ionization and fragmentation behavior of the compound dutasteride, its precursors and several analogs is reported. Positive atmospheric pressure chemical ionization (APCI+) and positive electrospray ionization (ESI+) produced distinctive collision-induced dissociation (CID) spectra for the respective [MH]+ ions of dutasteride. The spectral differences are attributed to ion populations having either different structures or different internal energy distributions (as a consequence of the method of ionization). Irrespective of their origin, the protonated molecules undergo interesting fragmentation reactions when collisionally activated. The identity of the major fragmentation products was confirmed by accurate mass measurement. The negative APCI mass spectrum of dutasteride displays extensive dehydrohalogenation, apparently due to the thermal component of the APCI process. Some of the resulting radical anions display remarkable stability toward collisional decomposition. Details of the fragmentation behavior for the negative ion species and their relationship to the positive ion results are discussed.     

Comparison of the internal energy deposition of direct analysis in real time and electrospray ionization time-of-flight mass spectrometry

The internal energy (E_int) distributions of a series of p-substituted benzylpyridinium ions generated by both direct analysis in real time (DART) and electrospray ionization (ESI) were compared using the “survival yield” method. DART mean E_int values at gas flow rates of 2, 4, and 6 L min⁻¹, and at set temperatures of 175, 250, and 325 °C were in the 1.92–2.21 eV range. ESI mean E_int at identical temperatures in aqueous and 50% methanol solutions ranged between 1.71 and 1.96 eV, and 1.53 and 1.63 eV, respectively. Although the results indicated that ESI is a “softer” ionization technique than DART, there was overlap between the two techniques for the particular time-of-flight mass spectrometer used. As a whole, there was an increase in E_int with increasing reactive and drying gas temperatures for DART and ESI, respectively, indicating thermal ion activation. Three dimensional computational fluid dynamic simulations in combination with direct temperature measurements within the DART ionization region revealed complex inversely coupled fluid-thermal phenomena affecting ion E_int values during atmospheric transport. Primarily, that DART gas temperature in the ionization region was appreciably less than the set gas temperature of DART due to the set gas flow rates. There was no evidence of E_int deposition pathways from metastable-stimulated desorption, but fragmentation induced by high-energy helium metastables was observed at the highest gas flow rates and temperatures.     

Determination of Internal Energy Distributions of Laser Electrospray Mass Spectrometry using Thermometer Ions and Other Biomolecules

The internal energy distributions for dried and liquid samples that were vaporized with femtosecond duration laser pulses centered at 800 nm and postionized by electrospray ionization-mass spectrometry (LEMS) were measured and compared with conventional electrospray ionization mass spectrometry (ESI-MS). The internal energies of the mass spectral techniques were determined by plotting the ratio of the intact parent molecular features to all integrated ion intensities of the fragments as a function of collisional energy using benzylpyridinium salts and peptides. Measurements of dried p-substituted benzylpyridinium salts using LEMS resulted in a greater extent of fragmentation in addition to the benzyl cation. The mean relative internal energies, int> were determined to be 1.62?±?0.06, 2.0?±?0.5, and 1.6?±?0.3 eV for ESI-MS, dried LEMS, and liquid LEMS studies, respectively. Two-photon resonances with the laser pulses likely caused lower survival yields in LEMS analyses of dried samples but not liquid samples. In studies with larger biomolecules, LEMS analyses of dried samples from glass showed a decrease in survival yield compared with conventional ESI-MS for leucine enkephalin and bradykinin of ~15% and 11%, respectively. The survival yields for liquid LEMS analyses were comparable to or better than ESI-MS for benzylpyridinium salts and large biomolecules. Figure

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Internal energy transfer in laser desorption/ionization from silicon nanowires

Laser-induced desorption/ionization from silicon nanowires (SiNW) is an emerging method for mass spectrometry of small to medium-size molecules. In this new technique, we examined the internal energy transfer to seven benzylpyridinium thermometer ions and extracted the corresponding internal energy distributions. To explore the effect of the energy-deposition rate on the internal energy transfer, two lasers with significantly different pulse lengths (4 ns vs 22 ps) were utilized as excitation sources. A comparison of ion yields indicated that the SiNW substrates required 5-8 times less laser fluence for ion production than either matrix-assisted laser desorption/ionization (MALDI) or desorption/ionization on silicon (DIOS). In contrast however, the survival yield (SY) values showed that the internal energy transferred to the thermometer ions was more than (ps laser) or comparable to (ns laser) MALDI but it was significantly less than in DIOS. The internal energy transfer was only slightly dependent on laser fluence and on wire density. These effects were rationalized in terms of the confinement of thermal energy in the nanowires and of unimpeded three-dimensional plume expansion. Unlike in MALDI from CHCA and in perfluorophenyl-derivatized DIOS, for desorption from SiNWs the effect of laser pulse length on the internal energy transfer was found to be negligible.     

Qualitative and quantitative determination of poloxamer surfactants by mass spectrometry

Poloxamers are polyethylene-polypropylene glycol linear co-polymers. A simple matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) method has been developed for the determination of the average molecular weight of poloxamers. The molecular mass of five standard poloxamers determined by MALDI closely corresponds to that specified by the manufacturers, and no mass distribution effects were observed. Quantitation of distributions based on the molecular mass envelope using electrospray (ES) ionization was unsuccessful. To overcome this problem, quantitation was based on fragment ions (m/z 45 and 59) which gave reproducible signals using a very high orifice voltage ( approximately 200 eV). Poloxamer concentrations were determined accurately with a good linear response using the standard addition method. We believe that the use of very small fragment ions for quantitation of polymers may become a widely applicable general technique.     

Fragmentation of benzylpyridinium “thermometer” ions and its effect on the accuracy of internal energy calibration

Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical method to study biomolecules and noncovalent complexes. The prerequisite for their intact observation is soft ionization. In ESI, the internal energy of ions is primarily influenced by collisional activation in the source. The survival yield method is frequently used to probe the energy deposition in ions during the electrospray process. In the present work, we investigate the fragmentation pathways of para-substituted benzylpyridinium ions, the most widely used “thermometer ions” in the survival yield method. In addition to the C-N bond cleavage, alternative fragmentation channels were found for the compounds studied. We consider these pathways to result from intramolecular rearrangements. The effect of these additional fragments on the accuracy of the internal energy calibration is estimated for both collision-cell and in-source collision-induced dissociation (CID). Altogether, results presented suggest that a correction of the energy scale is necessary for the method based on benzylpyridinium ions to precisely quantify ion internal energies.     

Ion internal energy distributions validate the charge residue model for small molecule ion formation by spray methods

This paper reports a detailed study of the internal energy distribution of ions formed by four electrospray ionization (ESI)-related ionization methods, with particular emphasis on electrosonic spray ionization (ESSI). Substituted benzylpyridinium ions were used as thermometer ions to probe the internal energy distribution. The influence of different instrumental parameters was studied. Cone and skimmer voltages as well as the collision energy were found to strongly affect the ion internal energy distribution, whereas the distance between the emitter and the inlet of the mass spectrometer, the nebulizing gas pressure or the flow rate showed no influence. The internal energy distribution obtained with an ESSI source was compared with those obtained for electrospray (ESI), nanoelectrospray (nanoESI) and sonic spray ionization (SSI) on the same mass spectrometer with the same instrumental parameters. No clear differences were observed. As the charge residue model is the only ion formation mechanism possible for SSI, we conclude that benzylpyridinium ions are formed by the pathway suggested by this model.     

Collision cell pressure effect on CID spectra pattern using triple quadrupole instruments: a RRKM modeling

Control of the ion internal energy in mass spectrometry is needed to establish a workable mass spectral library. The purpose of this study is to understand and to compare the pressure effects on the collision‐induced dissociation (CID) spectrum pattern recorded using triple quadrupole instruments. The monoprotonated Leucine enkephalin [YGGFL, H⁺] was used as a thermometer molecule to calibrate the electrospray ionization (ESI) and the CID internal energies deposited on the molecular species and the time scale of ion decompositions. The survival yield and the ratio of a₄/b₄ fragment ions were mainly monitored. The energy uptake for the ESI source geometry used in our study has no impact on the CID spectrum fingerprint. The collision cell pressure for the [YGGFL, H⁺] has a major influence on the SY curves slope and on the experimental time scale. To demonstrate the pressure effect on internal energy distribution, three models (threshold, thermal and collisional) based on RRKM theory were built using the Masskinetics software. As a result, the limit of each model is discussed, and the investigation demonstrates that the thermal model, using truncated Maxwell‐Boltzmann internal energy distribution, is well‐suited for simulating the experimental data at high pressure widely used in the analytical conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Internal energy deposition and ion fragmentation in atmospheric-pressure mid-infrared laser ablation electrospray ionization

Mid-infrared laser ablation of water-rich targets at the maximum of the 2.94 μm absorption band is a two-step process initiated by phase explosion followed by recoil pressure induced material ejection. Particulates and/or droplets ejected by this high temperature high pressure process can be ionized for mass spectrometry by charged droplets from an electrospray. In order to gauge the internal energy introduced in this laser ablation electrospray ionization (LAESI?) process, we apply the survival yield method and compare the results with electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). The results indicate that LAESI yields ions with internal energies indistinguishable from those produced by ESI. This finding is consistent with the recoil pressure induced ejection of low micrometre droplets that does not significantly change the internal energy of solute molecules.     

Thin‐layer chromatography and mass spectrometry coupled using proximal probe thermal desorption with electrospray or atmospheric pressure chemical ionization

An atmospheric pressure proximal probe thermal desorption sampling method coupled with secondary ionization by electrospray or atmospheric pressure chemical ionization was demonstrated for the mass spectrometric analysis of a diverse set of compounds (dyestuffs, pharmaceuticals, explosives and pesticides) separated on various high‐performance thin‐layer chromatography plates. Line scans along or through development lanes on the plates were carried out by moving the plate relative to a stationary heated probe positioned close to or just touching the stationary phase surface. Vapors of the compounds thermally desorbed from the surface were drawn into the ionization region of a combined electrospray ionization/atmospheric pressure chemical ionization source where they merged with reagent ions and/or charged droplets from a corona discharge or an electrospray emitter and were ionized. The ionized components were then drawn through the atmospheric pressure sampling orifice into the vacuum region of a triple quadrupole mass spectrometer and detected using full scan, single ion monitoring, or selected reaction monitoring mode. Studies of variable parameters and performance metrics including the proximal probe temperature, gas flow rate into the ionization region, surface scan speed, read‐out resolution, detection limits, and surface type are discussed. Published in 2010 by John Wiley & Sons, Ltd.     

Electrospray ionization-multistage tandem mass spectrometry of complex multitin organometallic compounds

The series of 14 complex organotin(IV) compounds containing many tin atoms and noncovalent bonds in the structure was characterized by electrospray ionization multistage tandem mass spectrometry (ESI-MS(n)). The mass spectra were measured in both polarity modes to obtain complementary structural information. The characteristic pattern of ten natural tin isotopes allowed the determination of the number of tin atoms in the molecular adducts and fragment ions by comparing theoretical and experimental isotopic distributions. Positive ion ESI spectra show unusual adduct formation depending on the type of organic solvent used for the direct infusion analysis owing to the ion-molecule reactions in the ion source. On the basis of the detailed spectral interpretation of organotin(IV) compounds, the fragmentation patterns of multitin organometallic compounds have been proposed. Noncovalent bonds in polymeric complexes are fragmented first, which is then followed by characteristic neutral losses in monomeric units.     

Surface Acoustic Wave Nebulization Produces Ions with Lower Internal Energy than Electrospray Ionization

Surface acoustic wave nebulization (SAWN) has recently been reported as a novel method to transfer non-volatile analytes directly from solution to the gas phase for mass spectrometric analysis. Here we present a comparison of the survival yield of SAWN versus electrospray ionization (ESI) produced ions. A series of substituted benzylpyridinium (BzPy) compounds were utilized to measure ion survival yield from which ion energetics were inferred. We also estimated bond dissociation energies using higher level quantum chemical calculations than previously reported for BzPy ions. Additionally, the effects on BzPy precursor ion survival of SAWN operational parameters such as inlet capillary temperature and solution flow-rate were investigated. Under all conditions tested, SAWN-generated BzPy ions displayed a higher tendency for survival and thus have lower internal energies than those formed by ESI.     

Collision‐induced dissociation study of poly(2‐ethyl‐2‐oxazoline) using survival yields and breakdown curves

Poly(2‐ethyl‐2‐oxazoline), a synthetic polymer was analysed by mass spectrometry using different ion sources. Two distributions could be identified in the mass spectra which related to two different polymer series (one with hydrogen and hydroxyl end‐groups and the other with methyl and hydroxyl end‐groups). The fragmentation behaviour of the protonated oligomers was studied in a quadrupole time‐of‐flight mass spectrometer (MS) with electrospray, atmospheric pressure chemical ionization and direct analysis in real time soft ionization techniques. Three product ion series were identified in the MS/MS spectra independently of the ion source used. Based on the results, a mechanism was proposed for the dissociation by means of the accurate mass of the product ions, pseudo MS³ experiments and the energy dependence of the product ion intensity, i.e. breakdown curves. The survival yield method was used to highlight the correlation between the size of the oligomers and the laboratory frame collision energy. Copyright © 2012 John Wiley & Sons, Ltd.     

A comparative study of the fragmentation of neutral lactooligosaccharides in negative-ion mode by UV-MALDI-TOF and UV-MALDI ion-trap/TOF mass spectrometry

Structure analyses of underivatized neutral lacto oligosaccharides are systematically performed by ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI TOF MS) and UV-MALDI ion-trap time-of-flight mass spectrometry (ion-trap/TOF MS) acquired in negative-ion mode. Interestingly, their fragmentation significantly differ each other. In postsource decay (PSD) in UV-MALDI TOF MS, cross-ring cleavage at the reducing terminal predominates. On the other hand, glycosyl bond cleavage (C-type fragmentation) takes place preferentially in collision induced dissociation (CID) in UV-MALDI ion-trap/TOF MS. The cross-ring cleavage in PSD similar to that in in-source decay occurs via a prompt reaction path characteristic of the UV-MALDI process itself. The product ion spectra of UV-MALDI ion-trap/TOF MS are similar to the electrospray ionization (ESI) ion-trap or quadrupole/TOF CID product ion spectra. During ion-trap/TOF MS experiments, the deprotonated molecular ions survive for several tens of milliseconds after CID event because the high internal energy chlorinated precursor ions are cooled by collisional cooling in the ion trap. The results obtained suggest that the PSD from the chlorinated precursor ion in UV-MALDI TOF MS might proceed as a two-step reaction; in the first, a high internal energy deprotonated molecular ion is generated as a reaction intermediate during the flight in the drift tube, and in the second, the rapid decomposition from the deprotonated molecular ion takes place.     

Surface modification and laser pulse length effects on internal energy transfer in DIOS

Benzyl-substituted benzylpyridinium (BP) chloride salts were used as a source of thermometer ions to probe the internal energy (IE) transfer in desorption/ionization on porous silicon (DIOS). To modify their wetting properties and the interaction energies with the thermometer ions, the DIOS surfaces were silylated to produce trimethylsilyl- (TMS), amine- (NH2), perfluoroalkyl- (PFA), and perfluorophenyl-derivatized (PFP) surfaces. Two laser sources--a nitrogen laser with pulse length of 4 ns and a mode locked 3 x omega Nd:YAG laser with a pulse length of 22 ps--were utilized to induce desorption/ionization and fragmentation at various laser fluence levels. The corresponding survival yields were determined as indicators of the IE transfer and the IE distributions were extracted. In most cases, with increasing the laser fluence in a broad range (approximately 20 mJ/cm2), no change in IE transfer was observed. For ns excitation, this was in remarkable contrast with MALDI, where increasing the laser fluence resulted in sharply (within approximately 5 mJ/cm2) declining survival yields. Derivatization of the porous silicon surface did not affect the survival yields significantly but had a discernible effect on the threshold fluence for ion production. The IE distributions determined for DIOS and MALDI from alpha-cyano-4-hydroxycinnamic acid reveal that the mean IE value is always lower for the latter. Using the ps laser, the IE distribution is always narrower for DIOS, whereas for ns laser excitation the width depends on surface modification. Most of the differences between MALDI and DIOS described here are compatible with the different dimensionality of the plume expansion and the differences in the activation energy of desorption due to surface modifications.     

Internal energy distribution in electrospray ionization

Internal energies and energy distributions were studied using the 'survival yield' method developed previously. In addition to conventional benzylpyridinium salts, protonated esters (fragmenting by rearrangement) and protonated leucine enkephalin were also used, extending the validity of the technique. Fragmentation processes were studied in the cone voltage region and modeled by the RRKM-based MassKinetics program. The results show that the shapes of the energy distributions are similar to thermal distributions. The mean internal energies are very similar for all compound classes studied, and show a linear increase with collision energy in the 10-50 eV region.     

Comparison of the effects of ionization mechanism, analyte concentration, and ion “cool-times” on the internal energies of peptide ions produced by electrospray and atmospheric pressure matrix-assisted laser desorption ionization

The propensities of a series of peptide ions produced by both electrospray and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) to fragment in an ion trap mass spectrometer under various conditions were studied in detail by measuring the extent of fragmentation of precursor ions by collision induced dissociation (CID) as a function of applied resonance excitation RF voltage. For the most basic peptides, the energy required to fragment MH+ ions generated by electrospray exceeded that required to fragment equivalent AP-MALDI ions under identical instrumental conditions; the reverse was observed for a peptide incorporating no basic residues, while peptides of intermediate basicity showed little difference between the ionization methods. This correlation between peptide basicity and the difference in the energy required to induce fragmentation of MH+ ions generated by AP-MALDI and electrospray is attributed primarily to a trend in the internal energies of the ions generated by AP-MALDI (the greater the difference in gas-phase basicities between the matrix and the analyte the greater the internal energy of the analyte ions produced). Furthermore the internal energies of ions produced by AP-MALDI, but not the equivalent ions formed by electrospray, were observed to decrease with decreasing analyte concentration. We attribute this finding to the cooling effect of endothermic dissociation of analyte ion/matrix molecule clusters following the matrix assisted laser desorption step. Time-resolved analyses (measurement of extent of fragmentation of precursor ions by CID as a function of pre-CID "cool times") revealed that cooling periods in excess of 250 ms were required to achieve internal energy equilibrium through cooling collisions with the helium buffer gas. Furthermore, these analyses demonstrated that, even after these extended cooling times, equivalent ions formed by the two ionization techniques showed different propensities to fragment. We conclude that the two different ionization techniques produce ion populations that may differ in their three-dimensional structure.