Femtosecond Laser Mass Spectrometry Research Articles

Systematic discrimination of heptane isomers with femtosecond laser linear time of flight mass spectrometry

The dissociative ionization processes of heptane molecule with nine isomers have been investigated and presented experimentally in detail by courtesy of femtosecond laser mass spectrometry technique with a fundamental wavelength of 800 nm and 90 fs pulse duration in this study. It has been aimed to show that the classification of heptane isomers using factor analysis as statistical method reducing the dimension of obtained data. The main mass ratio of heptane isomers is known as m/z 100 which is obtained. The dissociation process on the parent molecule was dominant at the laser intensity about 1.42 x 1014 W/cm2 used for all isomers. The ions obtained by the breakdown of the main ion peak were primarily determined as factors which were clearly distinguished from each other according to factor 1 and factor 2 axes by applying factor analysis.

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling.

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3-NO2 structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+. On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

Chiral discrimination by recollision enhanced femtosecond laser mass spectrometry

Chiral molecules and their interactions are critical in a variety of chemical and biological processes. Circular dichroism (CD) is the most widely used optical technique to study chirality, often performed in a solution phase. However, CD has low-efficiency on the order of 0.01–1%. Therefore, there is a growing need to develop high-efficiency chiroptical techniques, especially in gas-phase, to gain background-free in-depth insight into chiral interactions. By using mass spectrometry and strong-field ionization of limonene with elliptically polarized light, we demonstrate an efficient chiral discrimination method that produces a chiral signal of one to two orders of magnitude higher than the conventional CD. The chiral response exhibits a strong dependence on wavelength in the range of 1,300–2,400 nm, where the relative abundance of the ion yields alternates between the two enantiomers. The origin of enhanced enantio-sensitivity in intense laser fields is attributed to two mechanisms that rely on the recollision dynamics in a chiral system: (1) the excited ionic state dynamics mediated either by the laser field or by the recollision process, and (2) non-dipole effects that alter the electron’s trajectories. Our results can serve as a benchmark for testing and developing theoretical tools involving non-dipole effects in strong-field ionization of molecules.

Atomic-resolution mapping of transcription factor-DNA interactions by femtosecond laser crosslinking and mass spectrometry

Transcription factors (TFs) regulate target genes by specific interactions with DNA sequences. Detecting and understanding these interactions at the molecular level is of fundamental importance in biological and clinical contexts. Crosslinking mass spectrometry is a powerful tool to assist the structure prediction of protein complexes but has been limited to the study of protein-protein and protein-RNA interactions. Here, we present a femtosecond laser-induced crosslinking mass spectrometry (fliX-MS) workflow, which allows the mapping of protein-DNA contacts at single nucleotide and up to single amino acid resolution. Applied to recombinant histone octamers, NF1, and TBP in complex with DNA, our method is highly specific for the mapping of DNA binding domains. Identified crosslinks are in close agreement with previous biochemical data on DNA binding and mostly fit known complex structures. Applying fliX-MS to cells identifies several bona fide crosslinks on DNA binding domains, paving the way for future large scale ex vivo experiments.

Characterization of Ultraviolet Photoreactions in Therapeutic Peptides by Femtosecond Laser Catalysis and Mass Spectrometry.

Peptides and proteins have diverse ultraviolet (UV) photoreaction pathways that can be activated by the energy of the UV photons absorbed. Simple light sources such as lamps are conventionally used to study these photoreactions in solution. This work provides a proof of concept that femtosecond laser technology can function as a highly potent UV source in rapidly conducting UV photostability studies of peptides. Correspondingly, sufficient quantities of photoproducts were generated in 1 min or less, allowing for identification of known and new photomodifications in the therapeutic peptides somatostatin-14 and arginine vasopressin. Identical photoproducts were also generated with a conventional continuous source. The major modifications included N-formylkynurenine, a cross-link between Trp and Phe, a Tyr product with an NH3 loss, and disruption of an unstable disulfide bond into a complex mixture of a trisulfide bond and multiple scrambled dimeric products. In conclusion, femtosecond lasers are extremely useful to drive fast UV-induced reactions for high throughput screening of photostability and modifications in amino acid polymers.

Real-Time Distinguishing of the Xylene Isomers Using Photoionization and Dissociation Mass Spectra Obtained by Femtosecond Laser Mass Spectrometry (FLMS)

Distinguishing chemicals and improvement on analytical methods has a direct impact on modern chemical analysis. In this work, the dissociative ionization of xylene isomers was investigated using a femtosecond laser mass spectrometry (FLMS) method with a custom-built linear time-of-flight (TOF) instrument. Laser beams at 800 nm and 400 nm were used and intensity-dependent analysis of the obtained mass spectra was performed using principal component analysis (PCA) to distinguish the xylene isomers, which give identical mass spectra in appearance that cannot be distinguished using normal mass spectrometry methods. The results show that there is a statistically highly significant difference between the xylene isomers for two principal components (1 − α > 99.99%) and minimal information loss (<5%) took place during the PCA procedure. Also, the use of the k-medoid clustering method showed that the isomers may be distinguished in real-time for a wide range of ionization laser pulse powers with approximately 99% accuracy. The results suggest that real-time isomer analysis by the FLMS method is suitable for mass spectral identification applications. The FLMS method has been shown to be an important alternative to other mass spectrometric methods that use different ionization mechanisms.

A New Method for Investigation of Different Tissues Using Femtosecond Laser Mass Spectrometry.

Femtosecond laser mass spectrometry (FLMS) has become an important tool for investigation of chemical and biological materials in many areas from medical to industrial. In medicine, the morphological examination of tissues is determined by performing pathological investigations under microscope. However, some novel improvements or developments must be performed for much faster diagnosis of the tissue during the operation when patient is under anesthesia. The information obtained from the tissue under the microscope remains very limited because it cannot reveal characteristics of the whole molecules. For this reason, some novel methods for analysis of tissues are important issues to be achieved. This process can be performed using FLMS much quicker than traditional techniques. The aim of this study is to develop a new procedure for interpretation of mass spectra obtained from different types of muscle tissues, such as lamb, bones, and beef obtained from the butcher. The results obtained in this study are believed to open a new window for these kinds of applications for cancer diagnosis on human tissue studies, as a faster analysis technique to give some concrete contributions to pathological examinations. Both, principal component analysis statistical approach and FLMS technique offer a great opportunity to identify the biological materials from mass spectra. In conclusion, this present study interprets a great data from bone, beef, and lamb, which show that we can distinguish these different types of materials using FLMS data and statistical approaches. Eventually, the experimental results obtained from our group studies present that these types of tissues can easily be distinguished using small m/q peaks in the lower region (m/z ≤100 amu) of the mass spectra by courtesy of FLMS.

Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers

Structural isomers, molecules having the same chemical formula but with atoms bonded in different order, are hard to identify using conventional spectroscopy and mass spectrometry. They exhibit virtually indistinguishable mass spectra when ionized by electrons. Laser mass spectrometry based on photoionization of the isomers has emerged as a promising alternative but requires shaped ultrafast laser pulses. Here we use transform limited femtosecond pulses to distinguish the isomers using two methods. First, we probe doubly charged parent ions with circularly polarized light. We show that the yield of doubly charged ortho-xylene decreases while para-xylene increases over a range of laser intensities when the laser polarization is changed from linear to circular. Second, we probe high harmonic generation from randomly oriented isomer molecules subjected to an intense laser field. We show that the yield of high-order harmonics varies with the positioning of the methyl group in xylene isomers (ortho-, para- and meta-) and is due to differences in the strength of tunnel ionization and the overlap between the angular peaks of ionization and photo-recombination.

Imaging of alignment and structural changes of carbon disulfide molecules using ultrafast electron diffraction.

Imaging the structure of molecules in transient-excited states remains a challenge due to the extreme requirements for spatial and temporal resolution. Ultrafast electron diffraction from aligned molecules provides atomic resolution and allows for the retrieval of structural information without the need to rely on theoretical models. Here we use ultrafast electron diffraction from aligned molecules and femtosecond laser mass spectrometry to investigate the dynamics in carbon disulfide following the interaction with an intense femtosecond laser pulse. We observe that the degree of alignment reaches an upper limit at laser intensities below the ionization threshold, and find evidence of structural deformation, dissociation and ionization at higher laser intensities.

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.

Femtosecond pulse shaping adds a new dimension to mass spectrometry

Phase-shaped femtosecond laser pulses and mass spectrometry were implemented as a tool for improving molecular identification. We demonstrate that the specific lines in the mass spectra of several chemical warfare simulants are sensitive to the phase characteristics of the incident laser field. The deviation in the relative yield of fragment ions observed upon pulse shaping (enhancement or suppression) adds a new dimension to mass spectrometry that improves molecular identification and can be used to quantitatively analyze mixtures of isomers.

A High Mass Resolution Study of the Interaction of Aromatic and Nitro-Aromatic Molecules with Intense Laser Fields

The technique of femtosecond laser mass spectrometry has been applied to benzene, nitrobenzene, toluene and nitrotoluene using pulses of 80 fs and λ = 800 nm (1014 − 1016 W cm-2). The ultrafast laser pulses used were able to largely defeat the dissociation pathways associated with nanosecond ionization and produce a molecular ion for both the aromatics and the two photounstable nitro-aromatics. The high mass resolution (m/Δm = 800) permitted, for the first time, the observation of various doubly charged species and allowed a study of the effect of the substituent NO2 group on the multiple ionization process. It was found that the femtosecond laser irradiation of benzene and toluene enabled the production of a doubly charged cation envelope in each case along with an additional doubly ionized contribution from certain lower mass fragments. Doubly ionized species were also observed for the nitro-aromatics including, most notably the loss of NO2 doubly charged ion ([M−NO2]2+) although a doubly charged parent was not observed. In addition, an NO2+ ion was detected for both nitro-aromatics which was thought to be evidence of a “charge-separation” process involving a transient doubly charged molecular ion.

Angular distributions of fragment ions arising from tetrahedral CH3I and isomer identification using intense laser fields

Utilizing an ultraintense (1016 W cm−2) fs laser, the laser/matter interaction of the tetrahedral CH3I molecule is investigated. A mass spectrum and the angular distributions of fragment ions arising from Coulomb explosion of molecular ions, obtained with linearly polarized light, are presented. The distributions for In+ (n ≤ 7), CHm+ (m ≤ 3), Cp+ (p ≤ 4) and H+ ions are all anisotropic and maximal when the polarization lies along the spectrometer axis. The molecule hence seems to behave as a diatomic, with the fragment ions being ejected along the field direction. Also presented are mass spectra of the isomers 1- and 2-nitropropane, which are explosive species, taken for horizontal and vertical polarizations at both 375 and 750 nm. It is shown that femtosecond laser mass spectrometry (FLMS) can be used to distinguish between these two isomers through their differing dissociation patterns. Isomer identification is important for many different applications and FLMS may provide a means of achieving this for a wide range of molecules.

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.

Multiply Charged Ions from Aromatic Molecules Following Irradiation in Intense Laser Fields

It is well-known that aromatic molecules, when irradiated by UV laser beams in the nanosecond pulse width regime, yield exclusive parent ions at laser intensities of 107 W cm-2 (soft ionization). As the laser intensities increase up to 109 W cm-2, however, extensive fragmentation takes place such that small mass fragments dominate the spectra at the expense of parent signature ions. The reason for this is that the dissociative lifetimes are shorter than the laser pulse width and ladder-switching fragmentation takes place. With the development of high power femtosecond lasers, these dissociative lifetimes can often be bypassed. Presently, at laser intensities up to the order of 1015 W cm-2 with pulse widths as short as 50 fs at near-infrared (IR) wavelengths (790 nm), soft ionization again takes place. This has been seen using a technique known as femtosecond laser mass spectrometry (FLMS). Under such conditions, stable multiply charged parent and adjacent satellite ions are observed and the fragmentation ...

Time-of-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams

The recent introduction of femtosecond technology to pulsed lasers has led to the development of femtosecond laser mass spectrometry (FLMS). The present paper describes an FLMS investigation of the aromatic molecules, benzene, toluene and naphthalene. Wavelengths of 750 and 375 nm were used with beam intensities up to 4 × 1014 W cm−2. Pulse widths were of the order of 50–90 fs. The laser system was coupled to a linear time-of-flight mass spectrometer. This experimental method of chemical analysis is gaining momentum, often replacing its nanosecond forerunner, resonant enhanced multiphoton ionization. For the said molecules, predominant parent ion production is found, making identification unambiguous. In fact this characteristic is being consistently attained in small to medium mass molecules irradiated under similar conditions, leading to the conclusion that a universal chemical detection system is a possibility. Such soft ionization is particularly evident at longer wavelengths (∼750 nm) with less relative fragmentation, daughter ion formation, compared to results at shorter wavelengths (∼375 nm). In terms of parent ion formation, similar numbers are produced with laser intensities around 1014 W cm−2 for both wavelengths. It has also been shown that at a threshold of about 5 × 1013 W cm−2, double ionized molecules appear for the 750 nm wavelength. These interesting new mass spectra display intense single, double and even triple ionized peaks without significantly increased dissociation. Such effects are less pronounced at 375 nm. © 1998 John Wiley & Sons, Ltd.

Ionization and Dissociation of Benzaldehyde Using Short Intense Laser Pulses

In a recent series of experiments, femtosecond laser mass spectrometry (FLMS) was applied to benzaldehyde utilizing laser pulse widths in the range 90 fs to 2.7 ps. Beam intensities up to 2 × 1014 W cm-2 were used with wavelengths of 750 and 375 nm. Different ionization−dissociation channels were found compared with previous studies by other authors based in the nanosecond regime. The general theme emerging is one of predominant above-threshold ionization−dissociation (ID) in which predissociative states are largely bypassed via rapid optical up-pumping to the molecular ionization continuum. Above the parent ionization threshold, ladder-switching is seen to be a function of laser pulse width, intensity, and wavelength with exclusive parent ion formation being achievable in the lower intensity regions at all pulse widths. Increasing fragmentation occurs as the laser intensity increases, although parent supremacy remains. At 750 nm however, the increase of fragmentation with intensity is greatly reduced compared to 375 nm, leading to the conclusion that FLMS at longer wavelengths is preferred for chemical analysis. Moreover at 750 nm and at laser intensities close to 1014 W cm-2 C7H6O2+ becomes evident. It is also shown that the C7H6O+/C7H5O+ ratio is strongly dependent on the pulse width, suggesting that a hydrogen loss pathway has a dissociation time of about a picosecond.

Multiphoton Ionization and Dissociation of Nitromethane Using Femtosecond Laser Pulses at 375 and 750 nm

The photochemistry of nitromethane has been studied extensively for many years. Although it is generally agreed that the principal photodissociative process is cleavage of the C−N bond to yield the methyl radical and nitrogen dioxide, there is some evidence of minor competing dissociation channels. A number of different groups have used lasers of different wavelengths, but the results of these studies vary considerably and no clear picture of the minor dissociative channels has yet emerged. The use of femtosecond (fs) duration laser pulses for photoionization of molecules is currently an area of considerable interest, since the process can lead to the efficient production of intact molecular ions. It was felt that femtosecond laser mass spectrometry (FLMS) could provide added information on the dissociation pathways of nitromethane. Laser pulses of 90 fs time duration at wavelengths of 375 and 750 nm, coupled to a time-of-flight mass spectrometer, have been used in this study, and contrary to photoexcitation using nanosecond (ns) pulses, a large parent ion, 61 (CH3NO2+), is detected together with strong peaks at m/e = 15 (CH3+), 30 (NO+), 46 (NO2+) as well as a number of other minor peaks. This fragmentation pattern can be explained by a predominantly ID (ionization followed by dissociation) route.

A comparison of femtosecond and nanosecond multiphoton ionization and dissociation for some nitro‐molecules

AbstractResonance‐enhanced multiphoton ionization spectrometry (REMPI), using time‐of‐flight mass spectrometers and tuned lasers, has proved an important ultra‐sensitive analytical technique. Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states. In the case of thermally labile molecules, which include the nitro‐molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible. Femtosecond laser mass spectrometry (FLMS) can often ‘defeat’ these dissociative states, resulting in large parent or high‐mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time‐of‐flight mass spectrometry for NO2 gas and a number of different nitro‐molecules: nitromethane, nitrobenzene, m‐nitrotoluene, dinitrotoluene and trinitrotoluene.