Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.     

Polarization dependent fragmentation of ions produced by laser desorption from nanopost arrays

Tailored silicon nanopost arrays (NAPA) enable controlled and resonant ion production in laser desorption ionization experiments and have been termed nanophotonic ion sources (Walker et al., J. Phys. Chem. C, 2010, 114, 4835-4840). As the post dimensions are comparable to or smaller than the laser wavelength, near-field effects and localized electromagnetic fields are present in their vicinity. In this contribution, we explore the desorption and ionization mechanism by studying how surface derivatization affects ion yields and fragmentation. We demonstrate that by increasing the laser fluence on derivatized NAPA with less polar surfaces that have decreased interaction energy between the structured silicon substrate and the adsorbate, the spectrum changes from exhibiting primarily molecular ions to showing a growing variety and abundance of fragments. The polarization angle of the laser beam had been shown to dramatically affect the ion yields of adsorbates. For the first time, we report that by rotating the plane of polarization of the desorption laser, the internal energy of the adsorbate can also be modulated resulting in polarization dependent fragmentation. This polarization effect also resulted in selective fragmentation of vitamin B(12). To explore the internal energy of NAPA generated ions, the effect of the post aspect ratios on the laser desorption thresholds and on the internal energy of a preformed ion was studied. Elevated surface temperatures and enhanced near fields in the vicinity of high aspect ratio posts are thought to contribute to desorption and ionization from NAPA. Comparison of the fluence dependence of the internal energies of ions produced from nanoporous silicon and NAPA substrates indicates that surface restructuring or transient melting by the desorption laser is a prerequisite for the former but not for the latter.     

Competing ion decomposition channels in matrix-assisted laser desorption ionization

We gauged the internal energy transfer for two dissociative ion decomposition channels in matrix-assisted laser desorption ionization (MALDI) using the benzyltriphenylphosphonium (BTP) thermometer ion [PhCH 2PPh 3] (+). Common MALDI matrixes [alpha-cyano-4-hydroxycinnamic acid (CHCA), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA), and 2,5-dihydroxycinnamic acid (DHB)] were studied with nitrogen laser (4 ns pulse length) and mode-locked 3 x omega Nd:YAG laser (22 ps pulse length) excitation. Despite the higher fluence required to initiate fragmentation, BTP ions indicated lower internal energy transfer with the picosecond laser in all three matrixes. These differences can be rationalized in terms of phase explosion induced by the nanosecond laser vs a stress-confinement-driven desorption mechanism for the picosecond laser. For the two ion production channels of the BTP thermometer ion, breaking a single bond can result in the formation of benzyl/tropylium ions, F1, or triphenylphosphine ions, F2. In SA and DHB, as well as in CHCA at low fluence levels, the efficiency of these channels (expressed by the branching ratio I F1/ I F2) is moderately in favor of producing tropylium ions, 1 < I F1/ I F2 < 6. As the laser fluence is increased, for CHCA, there is a dramatic shift in favor of the tropylium ion production, with I F1/ I F2 approximately 30 for the nanosecond and the picosecond laser, respectively. This change is correlated with the sudden increase in the BTP internal energies in CHCA in the same laser fluence range. The large changes observed in internal energy deposition for CHCA with laser fluence can account for its ability to induce fragmentation in peptides more readily than SA and DHB.     

Ultraviolet photodissociation of protonated pharmaceuticals in a pressurized linear quadrupole ion trap

Ultraviolet photodissociation (UVPD) was evaluated as a technique for generating ion fragmentation information that is alternative and/or complementary to the information obtained by collision‐induced dissociation (CID). Ions trapped in a pressurized linear ion trap were dissociated using a 355 nm or a 266 nm pulsed laser. Comparisons of UVPD and CID spectra using a set of aromatic chromophore‐containing compounds (desmethyl bosentan, haloperidol, nelfinavir) demonstrated distinct characteristic fragmentation patterns resulting from photodissociation. The wavelength of light and the pressure of the buffer gas in the UVPD cell are important parameters that control fragmentation pathways. The wavelength effect is related to the absorption cross section, location of the chromophore and the energy carried by one photon. Thus, UV irradiation wavelength affects fragmentation pathways as well as the fragmentation rate. The pressure effect can be explained by collisional quenching of ‘slow’ fragmentation pathways. We observed that higher pressure of the buffer gas during UVPD experiments highlights unique fragment ions by suppressing slow fragmentation pathways responsible for CID‐like fragmentation patterns. Copyright © 2010 John Wiley & Sons, Ltd.     

Factors that influence the observed fast fragmentation of peptides in matrix-assisted laser desorption

Fragmentation processes that occur very early during matrix-assisted laser desorption ionization (MALDI) of peptides are examined by utilization of delayed pulsed ion extraction with a linear time-of-flight mass spectrometer. The oxidized B chain of bovine insulin (MW=3495. 95 u), which produces a wide range of fragment ions, is utilized as a probe to examine the effects of several experimental parameters on this process. Experimental evidence suggests that this MALDI process is not prompt fragmentation and involves metastable ion decay that is quite different from that which is observed with postsource decay experiments. This conclusion is based upon the significant differences observed in the fragmentation products produced by the two techniques. This metastable ion decay process also appears to be over within the minimum pulse delay period (320 ns) that is possible with the current pulsed ion extraction hardware. These two observations suggest that either different activation processes are involved in the two techniques or that the much different time frame of the methods influences the observed ion decay pathways. This fast MALDI metastable ion fragmentation also is shown to be influenced by both the MALDI matrix and the laser fluence.     

Effects of polarization of 1.4 microm femtosecond laser pulses on the formation and fragmentation of naphthalene molecular ions compared at the same effective ionization intensity

Naphthalene was ionized with 130 fs pulses of different polarizations at 1.4 microm. In contrast to the results of ionization by 0.8 microm pulses, fragmentation was dramatically suppressed and naphthalene molecular ions of up to 3+ were produced. The use of this simple model of ionization and large electron kinetic energy enabled us to study the electron-recollision-induced fragmentation and/or double ionization more precisely. The failure of the theoretical prediction of ion yield for the case of naphthalene prevented us from judging the electron recollision solely by a comparison with theoretical curves. Therefore, the effects of laser polarization on the ratios between differently charged states and between molecular and total ions were compared at the same effective (peak) intensity instead of average intensity. Comparison under the same effective intensity enabled us to identify the effects of ellipticity clearly. Evidence of the electron recollision was found in the doubly charged molecular ion formation but not in the fragmentation. The single-electron recollision event was not sufficient to induce fragmentation because of its low energy transfer efficiency. We concluded that the fragmentation originated in the unstable nature of the highly charged molecular ion itself and in the Coulomb explosion in the case of naphthalene.     

二乙胺分子的多光子电离："梯转换"过程

The multiphoton ionization (MPI) of diethylamine is first reported in this paper. A time-of-flight mass spectrometer was used in the experiment under collision-free condition. MPI mass spectra were measured in the region of 464 ～486 nm using a dye laser and at double frequency of a Nd:YAG laser. Molecular ion was created through resonance enhanced multiphoton ionization (REMPI) via Rydberg states. Fragment, ion distribution was formed through “ladder switching” process. The experimental demonstration of the process is reported for the first time by the discussion of dependence of ion relative, abundance on laser wavelength and energy. The competition between further up-pumping and fragmentation of Parent ion also exists, and higher laser intensity favors the former.     

Application of femtosecond laser mass spectrometry to the analysis of volatile organic compounds

Femtosecond (fs) lasers have high intensity and ultrashort pulse duration. Tunneling ionization occurs for molecules subject to such intense laser fields. We have studied the mass spectra of a variety of molecules irradiated by intense fs laser pulses. These molecules include some typical volatile organic compounds contained in human breath and in the atmosphere. The results demonstrate that all of these molecules can be ionized by intense fs laser pulses. Dominant parent ion and some characteristic ionic fragments are observed for each molecule. The degree of fragmentation can be controlled by adjusting the laser intensity. Moreover, saturation ionization can occur for each molecule by increasing the laser intensity. These features indicate that fs laser mass spectrometry can be a sensitive tool to identify and quantify volatile organic compounds in human breath.     

Pseudo-MS3 in a MALDI orthogonal quadrupole-time of flight mass spectrometer

Both the matrix selected and the laser fluence play important roles in MALDI-quadrupole/time of flight (QqTOF) fragmentation processes. "Hot" matrices, such as alpha-cyano4-hydroxycinnamic acid (HCCA), can increase fragmentation in MS spectra. Higher laser fluence also increases fragmentation. Typical peptide fragment ions observed in the QqTOF are a, b, and y ion series, which resemble low-energy CID product ions. This fragmentation may occur in the high-pressure region before the first mass-analyzing quadrupole. Fragment ions can be selected by the first quadrupole (Q1), and further sequenced by conventional MS/MS. This allows pseudo-MS3 experiments to be performed. For peptides of higher molecular weight, pseudo-MS3 can extend the mass range beyond what is usually accessible for sequencing, by allowing one to sequence a fragment ion of lower molecular weight instead of the full-length peptide. Peptides that predominantly show a single product ion after MS/MS yield improved sequence information when this technique is applied. This method was applied to the analysis of an in vitro phosphorylated peptide, where the intact enzymatically-generated peptide showed poor dissociation via MS/MS. Sequencing a fragment ion from the phosphopeptide enabled the phosphorylation site to be unambiguously determined.     

Elucidation of the initial step of oligonucleotide fragmentation in matrix-assisted laser desorption/ionization using modified nucleic acids

To probe the mechanism of gas-phase oligonucleotide ion fragmentation, modified oligonucleotides were studied using matrix-assisted laser desorption/ionization. The oligonucleotides were of the form 5'-TTTTXTTTTT, where X was a modified nucleotide. Modifications included substitution of hydroxy, methoxy, amino, and allyl groups at the 2'-position of the deoxyribose. The modified ribose contained adenine, guanine, cytosine, or uracil bases. For comparison, we studied oligomers where X was an unmodified adenosine, guanosine, cytidine, thymidine, or uridine deoxyribonucleotide. We found a very strong dependence of the matrix-to-analyte ratio on fragmentation for these oligomers. Analysis of these modifications suggests that the initial fragmentation step in MALDI-MS involves a two-step (E1) elimination of the base.     

Multiphoton ionization mass spectroscopy of p-xylene at 193 and 248 nm

Multiphoton ionization (MPI) fragmentation pattern of p-xylene is investigated as a function of laser power at 193 and 248 nm. The mass pattern at 193 nm suggests the presence of two reaction sequences and the occurrence of drastic rearrangement in the parent ion and also in the superexcited xylene before fragmentation.     

Fragmentation of deuterated rhodamine B derivates by laser and collisional activation in an FT-ICR mass spectrometer

Xanthene dyes such as rhodamine B undergo an interesting mass-spectrometric fragmentation reaction that eliminates small neutral alkanes such as propane. This fragmentation reaction has been investigated in a Fourier transform ion cyclotron mass spectrometer by means of laser photodissociation with visible light as well as by collision-induced dissociation. Different isotopically labeled decarboxyrhodamine B compounds were used to investigate the fragmentation mechanism. The results support a concerted mechanism for the formation of the alkanes instead of a two-step radical mechanism.     

LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.     

Internal energy build-up in matrix-assisted laser desorption/ionization

This paper reports detailed studies on the internal energy of ions formed in matrix-assisted laser desorption/ionization (MALDI) using delayed extraction MALDI-time-of-flight (TOF) and atmospheric pressure (AP) MALDI mass spectrometric (MS) methods. We use benzylpyridinium cations as internal energy probes. Our study reveals three distinct contributions to internal energy build-up in vacuum-MALDI (classical MALDI-TOF), each having different effects on ion fragmentation. Some fragments are formed before ion extraction (i.e. no more than 100 ns after the laser impact), and they are therefore well resolved and recorded as sharp signals in the MALDI-TOFMS scan. This prompt fragmentation can have two origins: (i) in-plume thermal activation, presumably always present, and (ii) in-plume chemical activation, in the course of reactions with hydrogen radicals. In addition to early internal energy build-up associated with these well-resolved promptly formed fragments, a broad peak slightly offset to higher masses could be detected corresponding to fragments formed after the extraction has started. This second signal corresponds to a third source of internal energy in MALDI ions, (iii) the extraction-induced collisional activation of the ions with the neutral components of the plume. These three contributions are difficult to quantify in vacuum-MALDI, because of the combined influence of several parameters (nature of the matrix, spot-to-spot variability, total laser exposure, delay time, acceleration voltage) on extraction-induced fragmentation. AP-MALDI, on the other hand, has two advantages for comparative studies of analyte fragmentation. First, extraction-induced fragmentation is absent, and only the contributions of early plume activation remain. Second, the reproducibility is far better than in vacuum-MALDI. AP-MALDI is therefore expected to shed new light on the early steps of the MALDI process.     

Enhancement of anthracene fragmentation by circularly polarized intense femtosecond laser pulse

The authors compared circularly and linearly polarized lights in the ionization and fragmentation of anthracene, using 800 nm femtosecond laser pulses at intensities of 10(13)-10(15) W cm-2. Singly and doubly charged intact molecular ions as well as numerous fragment ions were observed in the mass spectra, which were investigated as a function of laser intensity and polarization. At comparable intensities above the saturation threshold for complete ionization, the fragmentation pathways are enhanced with a circularly polarized field compared to a linearly polarized field. Resonant excitation of the molecular cation through the 2Au<--2Bg transition is proposed to be the initial step to ion fragmentation. The circularly polarized field interacts with a larger fraction of the randomly oriented molecules than the linearly polarized field, and this is considered to be the reason for the enhanced fragmentation brought about by circularly polarized light.     

Comparison of laser-induced dissociation and high-energy collision-induced dissociation using matrix-assisted laser desorption/ionization tandem time-of-flight (MALDI-TOF/TOF) for peptide and protein identification

The fragmentation of peptides under laser-induced dissociation (LID) as well as high-energy collision-induced dissociation (CID) conditions has been investigated. The effect of the different fragmentation mechanisms on the formation of specific fragment ion types and the usability of the resulting spectra, e.g. for high-throughput protein identification, has been evaluated. Also, basic investigations on the influence of the matrix, as well as laser fluence, on the fragment ion formation and the consequences in the spectral appearance are discussed. The preconditions for obtaining 'pure' CID spectra on matrix-assisted laser desorption/ionization tandem time-of-flight (MALDI-TOF/TOF) instruments are evaluated and discussed as well as the differences between LID and CID in the resulting fragment ion types. While containing a wealth of information due to additional fragment ions in comparison with LID, CID spectra are significantly more complex than LID spectra and, due to different fragmentation patterns, the CID spectra are of limited use for protein identification, even under optimized parameter settings, due to significantly lower scores for the individual spectra. Conditions for optimal results regarding protein identification using MALDI-TOF/TOF instruments have been evaluated. For database searches using tandem mass spectrometric data, the use of LID as fragmentation technique in combination with parameter settings supporting the use of internal fragment ions turned out to yield the optimal results.     

Laser desorption ionization and MALDI time-of-flight mass spectrometry for low molecular mass polyethylene analysis

Polyethylene's inert nature and difficulty to dissolve in conventional solvents at room temperature present special problems for sample preparation and ionization in mass spectrometric analysis. We present a study of ionization behavior of several polyethylene samples with molecular masses up to 4000 Da in laser desorption ionization (LDI) time-of-flight mass spectrometers equipped with a 337 nm laser beam. We demonstrate unequivocally that silver or copper ion attachment to saturated polyethylene can occur in the gas phase during the UV LDI process. In LDI spectra of polyethylene with molecular masses above approximately 1000 Da, low mass ions corresponding to metal-alkene structures are observed in addition to the principal distribution. By interrogating a well-characterized polyethylene sample and a long chain alkane, C94H190, these low mass ions are determined to be the fragmentation products of the intact metal-polyethylene adduct ions. It is further illustrated that fragmentation can be reduced by adding matrix molecules to the sample preparation.     

High Efficiency Photo-Induced Dissociation of Precursor Ions in a Tandem Time-of-Flight Mass Spectrometer

High efficiency photo-induced dissociation (PID) has been demonstrated in a tandem time-of-flight mass spectrometer. This instrument focuses isomass ion packets to temporal and spatial dimensions similar to those of the focused laser pulses from a high power excimer laser. This high density overlap of photons and ions yields highly efficient fragmentation and also provides high resolution selection of specific precursor ion mass-to-charge ratio values. Using 193 nm photon excitation of the molecular ion of bromobenzene (m/z = 1561, fragmentation, collection, and PID efficiencies af 79%, 132%, and 104%, respectively, were obtained. Characteristic fragmentations of toluene, nitrobenzene, acetophenone, triethylamine, N,N-diethylformamide, N-methylacetamide, and cyclohexene have also been demonstrated.     

MALDI TOF mass spectrometry for the investigation of the structure of ruthenium carborane complexes

The results of investigation of ruthenium carborane complexes with chelating diphosphine ligands by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry are presented. The influence of matrix, power of laser irradiation, and analysis mode on the substrate fragmentation and spectra is analyzed. It is shown that the best spectra are recorded in the negative ion mode with DCTB as a matrix.