Influence of the laser fluence in infrared matrix-assisted laser desorption/ionization with a 2.94 microm Er : YAG laser and a flat-top beam profile

The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry using a flat-top laser beam profile. The beam of an Er : YAG laser (wavelength, 2.94 microm; pulse width, 90 ns) was coupled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser spot sizes of 175, 350 and 700 microm diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few thousand J m(-2), depending on the matrix and the size of the irradiated area. In the MALDI-typical fluence range, above the detection threshold ion signals increase strongly with fluence for all matrices, with a dependence similar to that for UV-MALDI. Despite the strongly different absorption coefficients of the tested matrices, varying by more than an order of magnitude at the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dependence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to about 100 kJ mol(-1) for glycerol.     

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization

A quantitative model of primary ionization in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) is presented. It includes not only photochemical processes such as exciton pooling, but also the effects of the desorption event. The interplay of these two is found to be a crucial aspect of the MALDI process. The desorbing plume is modeled as an adiabatic expansion with entrained clusters. The parameters in the model are defined as much as possible via experiment or by analogy with known effects. The model was applied to the matrix 2,5-dihydroxybenzoic acid and found to reproduce the fluence dependence of the fluorescence yield and key features of the picosecond two-pulse ion generation efficiency curves. In addition, the model correctly predicts a fluence rather than irradiance threshold, the magnitude of the threshold, the magnitude of the ion yield, laser wavelength effects, plume temperatures, plume expansion velocities and the spot size effect.     

Wavelength-dependent fragmentation and clustering observed after femtosecond laser ablation of solid C60

We report here the resonance effect in femtosecond laser ablation of solid C60 by investigating wavelength and fluence dependence of product ion species. When the ablation laser wavelength is far from the molecular absorption band of C60, we observe both C60-2n+ fragment ions and C60+2n+ cluster ions as well as C60+ parent ion. Delayed ionization of C60 is not significant. When the ablation laser wavelength is near resonant with the molecular absorption, we observe C60+ and some amount of C60-2n+ fragment ions depending on the laser fluence. Delayed ionization of C60 is significant in this case, which indicates high internal energy of C60 molecule. From the observations, we confirm the strong coupling of femtosecond laser energy with C60 molecule when the molecular absorption is high at the ablation laser wavelength.     

Atmospheric pressure laser desorption/ionization using a 6–7 µm‐band mid‐infrared tunable laser and liquid water matrix

Due to the characteristic absorption peaks in the IR region, various molecules can be used as a matrix for infrared matrix‐assisted laser desorption/ionization (IR‐MALDI). Especially in the 6–7 µm‐band IR region, solvents used as the mobile phase for liquid chromatography have absorption peaks that correspond to their functional groups, such as O–H, CO, and CH₃. Additionally, atmospheric pressure (AP) IR‐MALDI, which is applicable to liquid‐state samples, is a promising technique to directly analyze untreated samples. Herein we perform AP‐IR‐MALDI mass spectrometry of a peptide, angiotensin II, using a mid‐IR tunable laser with a tunable wavelength range of 5.50–10.00 µm and several different matrices. The wavelength dependences of the ion signal intensity of [M + H]⁺ of the peptide are measured using a conventional solid matrix, α‐cyano‐4‐hydroxycinnamic acid (CHCA) and a liquid matrix composed of CHCA and 3‐aminoquinoline. Other than the O–H stretching and bending vibration modes, the characteristic absorption peaks are useful for AP‐IR‐MALDI. Peptide ions are also observed from an aqueous solution of the peptide without an additional matrix, and the highest peak intensity of [M + H]⁺ is at 6.00 µm, which is somewhat shorter than the absorption peak wavelength of liquid water corresponding to the O–H bending vibration mode. Moreover, long‐lasting and stable ion signals are obtained from the aqueous solution. AP‐IR‐MALDI using a 6–7 µm‐band IR tunable laser and solvents as the matrix may provide a novel on‐line interface between liquid chromatography and mass spectrometry. Copyright © 2015 John Wiley & Sons, Ltd.     

Multiphoton ionization of molecules in selectively excited rovibrational states

Reported are observations of multiphoton ionization of a molecule (NO) due to sequential excitation by a tunable infrared laser and a tunable dye laser. These double-resonance experiments yield multiphoton ionization spectra of specific rotational levels, selectively populated by direct IR absorption. This technique simplifies complex multiphoton ionization spectra and offers a means of sensitively and selectively detecting IR absorption of molecules.     

Deep‐ultraviolet laser ablation electrospray ionization mass spectrometry

A 193‐nm wavelength deep ultraviolet laser was used for ambient laser ablation electrospray ionization mass spectrometry of biological samples. A pulsed ArF excimer laser was used to ablate solid samples, and the resulting plume of the desorbed material merged with charged electrospray droplets to form ions that were detected with a quadrupole time‐of‐flight mass spectrometer. Solutions containing peptide and protein standards up to 66‐kDa molecular weight were deposited on a metal target, dried, and analyzed. No fragmentation was observed from peptides and proteins as well as from the more easily fragmented vitamin B₁₂ molecule. The mass spectra contained peaks from multiply charged ions that were identical to conventional electrospray. Deep UV laser ablation of tissue allowed detection of lipids from untreated tissue. The mechanism of ionization is postulated to involve absorption of laser energy by a fraction of the analyte molecules that act as a sacrificial matrix or by residual water in the sample.     

Influence of the laser intensity and spot size on the desorption of molecules and ions in matrix-assisted laser desorption/ionization with a uniform beam profile

The dependence of the number of desorbed particles on laser fluence has been investigated for matrix-assisted laser desorption/ionization (MALDI) of analyte and matrix ions as well as for (photoionized) neutral matrix molecules using a homogeneous “flat-top” laser profile. Laser spot diameters ranging from 10 to 200 μm in size have been used. 2,5-Dihydroxybenzoic acid (DHB) and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid) have been tested as matrices. The threshold (for ion detection) is higher and the dependence of the ion signal upon higher-than-threshold fluences is stronger for directly desorbed ions than for photoionized neutral molecules. Directly desorbed analyte ions exhibit the same dependence on fluence as the matrix ions with only minor differences between the two matrices tested, so both have approximately the same detection threshold. For both ions and photoionized neutral molecules, the fluence threshold increases with decreasing spot size while the slope of the intensity/fluence curves decreases. A quasi-thermal, sublimation/desportion model was found to describe the experimental results with excellent precision. For a complete explanation, non-equilibrium effects had to be taken into account.     

Matrix-free infrared soft laser desorption/ionization

Infrared soft laser desorption/ionization was performed using a 2.94 µm Er : YAG laser and a commercial reflectron time-of-flight mass spectrometer. The instrument was modified so that a 337 nm nitrogen laser could be used concurrently with the IR laser to interrogate samples. Matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization and desorption/ionization on silicon with UV and IR lasers were compared. Various target materials were tested for IR soft desorption ionization, including stainless steel, aluminum, copper, silicon, porous silicon and polyethylene. Silicon surfaces gave the best performance in terms of signal level and low-mass interference. The internal energy resultant of the desorption/ionization was assessed using the easily fragmented vitamin B₁₂ molecule. IR ionization produced more analyte fragmentation than UV-MALDI analysis. Fragmentation from matrix-free IR desorption from silicon was comparable to that from IR-MALDI. The results are interpreted as soft laser desorption and ionization resulting from the absorption of the IR laser energy by the analyte and associated solvent molecules. Copyright © 2004 John Wiley & Sons, Ltd.     

Quantitative reproducibility of mass spectra in matrix‐assisted laser desorption ionization and unraveling of the mechanism for gas‐phase peptide ion formation

In a previous study on matrix‐assisted laser desorption ionization (MALDI) of peptides using α‐cyano‐4‐hydroxycinnamic acid (CHCA) as a matrix, we found that the patterns of single‐shot spectra obtained under different experimental conditions became similar upon temperature selection. In this paper, we report that absolute ion abundances are also similar in temperature‐selected MALDI spectra, even when laser fluence is varied. The result that has been obtained using CHCA and 2,5‐dihydroxybenzoic acid as matrices is in disagreement with the hypothesis of laser‐induced ionization of matrix as the mechanism for primary ion formation in MALDI. We also report that the total number of ions in such a spectrum is unaffected by the identity, concentration and number of analytes, i.e. it is the same as that in the spectrum of pure matrix. We propose that the generation of gas‐phase ions in MALDI can be explained in terms of two thermal reactions, i.e. the autoprotolysis of matrix molecules and the matrix‐to‐analyte proton transfer, both of which are in quasi‐equilibrium in the early matrix plume. Copyright © 2013 John Wiley & Sons, Ltd.     

Laser two-photon ionization of organic molecules in dielectric liquids

The ionization threshold of fluoranthene and of TMPD in n-pentane was determined by laser two-photon ionization (TPI) and found to be 4.50 ± 0.05 and 3.88 ± 0.05 eV respectively. For both molecules the TPI spectra show distinct structure due to autoionization. For fluoranthene the TPI spectrum suggests that the molecule dissociates via the first excited singlet state.     

Fingerprint vibrational spectra of protonated methyl esters of amino acids in the gas phase

Infrared spectra of the protonated monomers of glycine, alanine, valine, and leucine methyl esters are presented. These protonated species are generated in the gas phase via matrix assisted laser desorption ionization (MALDI) within the cell of a Fourier transform ion cyclotron resonance spectrometer (FTICR) where they are subsequently mass selected as the only species trapped in the FTICR cell. Alternatively, they have also been generated by electrospray ionization and transferred to a Paul ion-trap mass spectrometer where they are similarly isolated. In both cases IR spectra are then derived from the frequency dependence of the infrared multiple photon dissociation (IRMPD) in the mid-infrared region (1000-2200 cm(-1)), using the free electron laser facility Centre de Laser Infrarouge d'Orsay (CLIO). IR bands are assigned by comparison with the calculated vibrational spectra of the lowest energy isomers using density functional theory (DFT) calculations. There is in general good agreement between experimental IRMPD spectra and calculated IR absorption spectra for the lowest energy conformer which provides evidence for conformational preferences. The two different approaches to ion generation and trapping yield IRMPD spectra that are in excellent agreement.     

Singlet-singlet annihilation in ultraviolet matrix-assisted laser desorption/ionization studied by fluorescence spectroscopy

Laser-induced fluorescence spectroscopy was carried out on microcrystalline samples of three typical matrices under conditions of matrix-assisted laser desorption/ionization (MALDI). The emitted fluorescence intensity was determined as a function of incident laser fluence and a sublinear increase of the fluorescence intensity with laser fluence was found. A very good fit was obtained when the experimental fluorescence vs. fluence data were compared with a numerical model assuming that under typical MALDI fluence conditions a large fraction of molecules in the excited singlet state undergoes singlet-singlet annihilation. Throughout the fluence range relevant for MALDI, however, the experimental data could not be fit well to a model assuming resonant two-photon absorption as the process depopulating the singlet state. In a separate set of experiments, the singlet lifetimes of several typical crystalline MALDI matrices were determined and found to be considerably shorter than previously reported. While both singlet-singlet annihilation and resonant two-photon absorption have been discussed in the literature as candidates for pathways to primary matrix ion generation in MALDI, the data presented here suggest that singlet-singlet annihilation is the dominant mechanism for depopulating the singlet state in a matrix crystal excited at typical MALDI fluences.     

OCS分子的共振多光子电离光谱

OCS分子的真空紫外吸收光谱早有研究,但有关OCS分子的多光子电离光谱尚未见报导。我们利用可调脉冲激光器,研究了OCS分子的共振多光子电离光谱。首次观察到OCS分子在277—285nm波长范围内的多光子电离光谱。OCS由硫氰酸铵和硫酸反应制取,经多次低温蒸馏纯化。产物通过FT-IR红外光谱和     

Synthesis of Triply Fused Porphyrin‐Nanographene Conjugates

Two unprecedented porphyrin fused nanographene molecules, 1 and 2 , have been synthesized by the Scholl reaction from tailor‐made precursors based on benzo[m]tetraphene‐substituted porphyrins. The chemical structures were validated by a combination of high‐resolution matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (HR MALDI‐TOF MS), IR and Raman spectroscopy, and scanning tunnelling microscopy (STM). The UV‐vis‐near infrared absorption spectroscopy of 1 and 2 demonstrated broad and largely red‐shifted absorption spectra extending up to 1000 and 1400 nm, respectively, marking the significant extension of the π‐conjugated systems.     

Insight into absorption of radiation/energy transfer in infrared matrix-assisted laser desorption/ionization: the roles of matrices, water and metal substrates.

Although the ionization/desorption mechanisms in matrix-assisted laser desorption/ionization (MALDI) remain poorly understood, there is a clear difference between the energy absorption processes in the ultraviolet (UV) and infrared (IR) modes of operation. UV-MALDI demands an on-resonance electronic transition in the matrix compound, whereas results presented here support earlier work showing that a corresponding resonant vibrational transition is not a requirement for IR-MALDI. In fact, data from the present study suggest that significant absorption of radiant energy by a potential matrix impairs its performance, although this observation is at variance with some other reports. For example, sinapinic acid, with an IR absorption maximum close to the 2.94 micrometer wavelength of the Er-YAG laser, has been little used as an IR-MALDI matrix. By contrast, succinic acid, with much lower IR absorption and no history of use in UV-MALDI as it has no UV absorption at the wavelength of common UV lasers, has become widely recognized as a good general purpose matrix for IR-MALDI. Despite reports by others that glycerol is an effective matrix for IR-MALDI, we find that glycerol, which also absorbs strongly at 2.94 micrometer, is useful only if applied as a very thin film. Thus the cumulative evidence for the role of the matrix in IR-MALDI appears confusing and often contradictory. Water has been postulated to be a major contributor to the absorption of energy in IR-MALDI. Consistent with this, we find that samples dried from D(2)O, which does not absorb at 2.94 micrometer, give spectra of inferior quality compared with the same samples from H(2)O. Similarly, samples dried under vacuum, that probably contain less water than those dried in the open laboratory, give weaker and more erratic spectra. Another potential participant in energy absorption and energy transfer is the surface of the metal support, an alternative mechanism for IR-MALDI, for which some evidence is presented here.     

Mass spectrometry of N-linked oligosaccharides using atmospheric pressure infrared laser ionization from solution

An atmospheric pressure (AP) infrared (IR) laser ionization technique, implemented on a quadrupole ion trap mass spectrometer, was used to analyze underivatized, N-linked oligosaccharides in solution. Experiments were conducted on an atmospheric pressure infrared ionization from solution (AP-IRIS) ion source which differed from previous AP IR matrix-assisted laser desorption/ionization (MALDI) interfaces in that the ion source operated in the absence of an extraction electric field with a higher power 2.94 microm IR laser. The general term 'IRIS' is used as the mechanism of ionization differs from that of MALDI, and is yet to be fully elucidated. The AP-IRIS ion source demonstrated femtomole-level sensitivity for branched oligosaccharides. AP-IRIS showed approximately 16 times improved sensitivity for oligomannose-6 and the core-fucosylated glycan M3N2F over optimal results obtainable on a AP UV-MALDI with a 2,4,6-trihydroxyacetophenone matrix. Comparison between IR and UV cases also showed less fragmentation in the IR spectrum for a glycan with a conserved trimannosyl core, core-substituted with fucose. A mixture of complex, high-mannose and sialylated glycans resulted in positive ion mass spectra with molecular ion peaks for each sugar. Tandem mass spectrometry of the sodiated molecular ions in a mixture of glycans revealed primarily glycosidic (B, Y) cleavages. The reported results show the practical utility of AP-IRIS while the ionization mechanism is still under investigation.     

Infrared multiphoton dissociation of CHFCl2

We have studied the dependence of the IR laser induced multiphoton dissociation of CHFCl₂ on laser wavelength and fluence, number of laser pulses and initial CHFCl₂ pressure. Multiphoton dissociation spectrum presents a broad maximum centered at 9.342 μm. Dissociation has a strongly dependence on initial CHFCl₂ pressure and laser fluence. There exists a pressure lower limit (≈ 0.4 mb) below which no significant dissociation occurs. Up to 2.1 J/cm² no unimolecular dissociation takes place in a significant proportion. The only radical we have been able to clearly detect by real time optical absorption technique is CFCl. This supports the mechanism reaction CHFCl₂ + nhν → CFCl + HCl.     

Photolysis and desorption of adsorbed halomethane molecules under long-wavelength radiation of excimer lasers

The photolysis and desorption of CH₂I₂, CH₃I, CCl₄, CHCl₃, CH₂Cl₂, CF₂Cl₂, and CHF₂Cl molecules adsorbed on fused silica under the action of XeF and XeCl laser radiation absorbed by these molecules have been studied. The desorption of molecules that occurs due to expansion to the long-wavelength region of the absorption spectrum of molecules in the adsorbed state, compared to the gas phase, predominates. The laser desorption is characterized by a strong nonlinear dependence on the density of radiation energy. Depending on the relationship between the laser radiation wavelength and the spectrum of electronic states of molecules, photolysis is observed upon absorption of either one or two photons. At an increased fluence of laser radiation energy, the one-photon detachment of the primary CH₂I fragment from the CH₂I₂ molecule changes into the three-photon process. A similar behavior is revealed for the desorption of CH₃I molecules from clusters formed on the surface in multilayer adsorption coverages.