A Dual Ionization Source with Laser Desorption Sample Introduction in a Time-of-Flight Mass Spectrometer

Abstract

A new mass spectrometric detection system has been developed for the analysis of non-volatile chemical species such as biochemicals. This system involves the use of a novel dual ionization source, consisting of laser-induced electron ionization (EI) and multiphoton ionization (MPI), to provide versatile ionization for a wide range of chemicals in a time-of-flight mass spectrometer. Laser desorption is incorporated into the system for sample vaporization.

The experimental design of this system is described. It is demonstrated that two ionization modes can be interchanged very readily by simply adjusting the position and focus of the laser beam. EI and MPI mass spectra of a number of amino acids and small peptides are shown to illustrate the detection capability of the system. For chemicals amenable to both ionization methods, EI and MPI can provide sensitive detection with different fragmentation patterns. Thus, the operation in a dual ionization mode provides enhanced structural information. In addition, laser-induced EI is shown to be a more general ionization method while MPI provides selective ionization with controlled fragmentation. The combination of these unique properties is demonstrated in the study of laser desorption process of an organic salt.     

Laser ablation with resonance-enhanced multiphoton ionization time-of-flight mass spectrometry for determining aromatic lignin volatilization products from biomass

We have designed and developed a laser ablation∕pulsed sample introduction∕mass spectrometry platform that integrates pyrolysis (py) and∕or laser ablation (LA) with resonance-enhanced multiphoton ionization (REMPI) reflectron time-of-flight mass spectrometry (TOFMS). Using this apparatus, we measured lignin volatilization products of untreated biomass materials. Biomass vapors are produced by either a custom-built hot stage pyrolysis reactor or laser ablation using the third harmonic of an Nd:YAG laser (355 nm). The resulting vapors are entrained in a free jet expansion of He, then skimmed and introduced into an ionization region. One color resonance-enhanced multiphoton ionization (1+1 REMPI) is used, resulting in highly selective detection of lignin subunits from complex vapors of biomass materials. The spectra obtained by py-REMPI-TOFMS and LA-REMPI-TOFMS display high selectivity and decreased fragmentation compared to spectra recorded by an electron impact ionization molecular beam mass spectrometer (EI-MBMS). The laser ablation method demonstrates the ability to selectively isolate and volatilize specific tissues within the same plant material and then detect lignin-based products from the vapors with enhanced sensitivity. The identification of select products observed in the LA-REMPI-TOFMS experiment is confirmed by comparing their REMPI wavelength scans with that of known standards.     

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.     

Activation of large ions in FT-ICR mass spectrometry

The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition--the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI)--the simplest and most robust means of introducing the multiple collision activation process--is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition--instantaneous decomposition of the ion on the surface--observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID.     

Construction and development of a time-resolved x-ray magnetic circular dichroism-photoelectron emission microscopy system using femtosecond laser pulses at BL25SU SPring-8

A femtosecond pulsed laser system has been installed at the BL25SU soft x-ray beamline at SPring-8 for time-resolved pump-probe experiments with synchronization of the laser pulses to the circularly polarized x-ray pulses. There are four different apparatuses situated at the beamline; for photoemission spectroscopy, two-dimensional display photoelectron diffraction, x-ray magnetic circular dichroism (XMCD) with electromagnetic coils, and photoelectron emission microscopy (PEEM). All four can be used for time-resolved experiments, and preliminary investigations have been carried out using the PEEM apparatus to observe magnetization dynamics in combination with XMCD. In this article, we describe the details of the stroboscopic pump-probe XMCD-PEEM experiment, and present preliminary data. The repetition rate of the laser pulses is set using a pulse selector to match the single bunches of SPring-8's hybrid filling pattern, which consists of several single bunches and a continuous bunch train. Electrons ejected during the bunch train, which do not provide time-resolved signal, are eliminated by periodically reducing the channel plate voltage using a custom-built power supply. The pulsed laser is used to create 300 ps long magnetic field pulses, which cause magnetic excitations in micron-sized magnetic elements which contain magnetic vortex structures. The observed frequency of the motion is consistent with previously reported observations and simulations.     

Angle-resolved photoemission spectroscopy with a femtosecond high harmonic light source using a two-dimensional imaging electron analyzer

An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a two-dimensional electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85 eV) from the harmonic spectrum of the light source is achieved with a multilayer MoSi double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 3 fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6 degrees (=0.03 A(-1) at E(kin,e)=36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real time.     

Time-of-flight mass spectrometry for time-resolved measurements

A time-resolved time-of-flight mass spectrometer (TOF-MS) that can simultaneously monitor multiple species on the millisecond time scale has been constructed. A pulsed photolysis laser is used to initiate reaction, and then via a pinhole the reaction mixture is sampled by the TOF-MS. The ions are created by photoionization via either a discharge lamp or a pulsed laser. Comparison between the two ionization sources showed that the laser is at least an order of magnitude more efficient, based on the time to accumulate the data. Also, unlike the continuous lamp the pulsed laser is not mass limited. Frequency tripling the 355 nm output of a Nd:YAG laser provided a convenient laser ionization source. However, using a dye laser provided an equally intense laser ionization source with the ability to tune the vacuum ultraviolet (vuv) light. To show the versatility of the system the kinetics of the reaction of SO and ClSO radicals with NO(2) were simultaneously measured, and using the dye laser the vuv light was tuned to 114 nm in order to observe H(2)CO being formed from the reaction between CH(3)CO and O(2).     

VACUUM ULTRAVIOLET AND EXTREME ULTRAVIOLET LASERS: PRINCIPLES,INSTRUMENTATION, AND APPLICATIONS

The foundations of coherent vacuum ultraviolet (VUV), and extreme ultraviolet (XUV) generation by non resonant tripling and two-photon resonance enhanced four wave mixing are presented. An experimental system, currently in operation at the University of Western Ontario, that combines VUV generation, time-of-flight mass spectrometry, supersonic jet cooling, laser-induced fluorescence detection, and dispersive photoelectron spectroscopy capabilities is detailed, as are several applications in high resolution molecular spectroscopy and the soft ionization and detection of organic molecules.     

An ultrashort-duration, high-repetition-rate pulse source for laser ionization/mass spectrometry

This paper describes a sample inlet system with several advantages over other pulsed valves, as applied to resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry. The nozzle is based on online concentration by analyte adsorption/laser desorption (online COLD), where a capillary column with a narrowly synthesized tip is employed for sample introduction. The analyte molecules adsorbed at the tip are desorbed by a pulsed laser and are injected into a mass spectrometer as a packet. The online COLD nozzle can produce very short gas pulses on the order of 1 μs. Moreover, this nozzle is capable of operating over a wide range of repetition rates from 1 Hz to 1 kHz. In addition, this nozzle intrinsically possesses several unique characteristics; for instance, it can be heated to very high temperatures and has nearly zero dead volume. Therefore, the present sample introduction technique offers an ideal and versatile nozzle for laser ionization/mass spectrometry.     

单次扫描检测大表面痕量赖氨酸和黑索今的质谱分析方法

为了进一步提高大表面样品的分析速度,减少给定样品表面的扫描次数,建立了一种单次扫描即可检测分布于大表面样品上任意位置痕量待测物的质谱分析方法。以人造革表面（4 cm×4 cm）添加的赖氨酸和黑索今为代表性检测对象,用一块无毛刺等边三角形金属铜电极（边长为8.6 cm）紧贴在样品表面,将甲醇-水（3∶7,V/V）电离试剂涂洒在金属电极表面,让其与样品表面保持湿润接触,并使三角形电极尖端对准质谱仪的离子入口;然后在金属电极上施加+5.8 kV高压,在电场的驱动下,湿润表面的待测物朝着质谱入口移动,并在三角形电极尖端形成待测物离子进入质谱仪检测。结果表明,该方法可单次扫描检测随机分布在皮革样品表面上任意位置的非均匀分布的赖氨酸和黑索今,检测限可达6.2×10^-7 μg/cm²,分析单个样品表面的时间不到2 s;与采用电喷雾解吸电离质谱等顺次扫描检测（采样面积不到1 mm²）的方法相比,本方法的分析速度提高了1000倍。     

A High-Resolution Time-of-Flight Mass Spectrometer With Laser Desorption and a Laser Ionization Source

ABSTRACT

Details of the novel method of laser desorption with a low powered IR-laser and resonance-enhanced multiphoton ionization (REMPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Different features of the method are discussed. Some results of mass spectrometric investigations of biomolecules are shown.     

Photodesorption of molecular nitrogen from the glass surface

Results of experiments on photodesorption of molecular nitrogen from the glass surface are presented. The study is performed in vacuum glass cells whose internal surface is illuminated by a powerful pulsed lamp. The dynamics of the nitrogen density in the cell due to photodesorption is studied with the use of a mass spectrometer and a vacuum ionization lamp. A qualitative comparison of experimental data with results predicted by a developed theoretical model is performed.     

Infrared multiphoton dissociation tandem charge detection-mass spectrometry of single megadalton electrosprayed ions

This work presents the implementation of tandem mass spectrometry for experiments on single electrosprayed ions from compounds of megadalton (MDa) molecular weight, using two charge detection devices. The first mass spectrometry stage (first charge detection device) combined with an ion gate allows both mass-to-charge ratio and charge selections of the megadalton ion of interest. The second stage is based on an electrostatic ion trap and consists of an image charge detection tube mounted between two ion mirrors. Single MDa ions can be stored for several dozen milliseconds. During the trapping time, single ions can be irradiated by a continuous wavelength CO(2) laser. We observe stepwise changes in the charge of a single trapped ion owing to multiphoton activation. Illustration of infrared multiphoton dissociation tandem mass spectrometry are given for single megadalton ions of poly(ethylene oxide)s and DNAs.     

Chemical ionization tandem mass spectrometer for the in situ measurement of methyl hydrogen peroxide

A new approach for measuring gas-phase methyl hydrogen peroxide [(MHP) CH(3)OOH] utilizing chemical ionization mass spectrometry is presented. Tandem mass spectrometry is used to avoid mass interferences that hindered previous attempts to measure atmospheric CH(3)OOH with CF(3)O(-) clustering chemistry. CH(3)OOH has been successfully measured in situ using this technique during both airborne and ground-based campaigns. The accuracy and precision for the MHP measurement are a function of water vapor mixing ratio. Typical precision at 500 pptv MHP and 100 ppmv H(2)O is ±80 pptv (2 sigma) for a 1 s integration period. The accuracy at 100 ppmv H(2)O is estimated to be better than ±40%. Chemical ionization tandem mass spectrometry shows considerable promise for the determination of in situ atmospheric trace gas mixing ratios where isobaric compounds or mass interferences impede accurate measurements.     

Radical approaches to probe protein structure, folding, and interactions by mass spectrometry

This review describes mass spectrometry-based strategies and investigations to determine protein structure, folding dynamics, and protein-protein interactions in solution through the use of radical reagents. The radicals are generated in high flux within microseconds from synchrotron radiation and discharge sources, and react with proteins on time scales that are less than those often attributed to structural reorganization and folding. The oxygen-based radicals generated in aqueous solution react with proteins to effect limited oxidation at specific amino acids throughout the sequence of the protein. The extent of oxidation at these residue markers is highly influenced by the accessibility of the reaction site to the bulk solvent. The extent of oxidation allows protection levels to be measured based on the degree to which a reaction occurs. A map of a protein's three-dimensional structure is subsequently assembled as in a footprinting experiment. Protein solutions that contain various concentrations of substrates that either promote or disrupt dynamic structural transitions can be investigated to facilitate site-specific equilibrium and time-resolved studies of protein folding. The radical-based strategies can also be employed in the study of protein-protein interactions to provide a new avenue for investigating protein complexes and assemblies with high structural resolution. The urea-induced unfolding of apomyoglobin and the binding of gelsolin to actin are among the systems presented to illustrate the approach.     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with approximately 150 nm lateral resolution, approximately 250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump-probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AIGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within approximately 1 microm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

A reversible time-of-flight detector for use in pseudocontinuous resonance enhanced multiphoton (pc-REMPI) detection

A time-of-flight coincidence detector is demonstrated. This detector is optimized for use in a pseudocontinuous resonance enhanced multiphoton ionization scheme that requires photoelectrons and photoions to be detected in coincidence. The detector utilizes two simultaneously operating charged particle detectors, one for the detection of electrons and the other for the detection of ions. In order to allow for field reversal, the detectors are physically identical, differing only by the value of applied voltages. Particular attention is given to the implementation of a charge-to-voltage transducer that allows for subnanosecond detection of both electrons and ions.     

Femtosecond kinetics of reflection of mirrors with saturable absorption

The kinetics of mirrors with saturable absorption is investigated by a pump-probe singlefrequency technique in reflection of femtosecond pulse radiation with the central wavelength of 1040 nm. The double modulation method with probe radiation detection at the summary frequency allows suppressing the scattered pump radiation contribution and reaching the reflection change sensitivity at a level of 10^?5. The kinetics of recovery of the linear reflectivity of the mirror including resonant quantum wells with nanostructured barriers is studied for the surface density of photons in pump pulses of (0.3–5.8) · 10¹⁴ cm^?2. The time of electron-hole recombination (7.8 ps) is found to be appreciably shorter than in samples with quantum wells separated by thick barriers; the time of ionization of excitons localized in quantum wells (0.2–0.4 ps) increases with the pump intensity.     

Multiphoton fluorescence lifetime imaging of human hair

In vivo and in vitro multiphoton imaging was used to perform high resolution optical sectioning of human hair by nonlinear excitation of endogenous as well as exogenous fluorophores. Multiphoton fluorescence lifetime imaging (FLIM) based on time-resolved single photon counting and near-infrared femtosecond laser pulse excitation was employed to analyze the various fluorescent hair components. Time-resolved multiphoton imaging of intratissue pigments has the potential (i) to identify endogenous keratin and melanin, (ii) to obtain information on intrahair dye accumulation, (iii) to study bleaching effects, and (iv) to monitor the intratissue diffusion of pharmaceutical and cosmetical components along hair shafts.     

激光辐射损伤阈值与激光脉宽相互关系的模拟计算

描述了一个超短脉冲引起的电介质激光损伤的理论模型,这个模型考虑了多光子电离、雪崩电离、电子-离子复合和电子扩散。通过假设激光脉冲为高斯型,数值计算得到了激光脉冲宽度与激光辐射损伤间的依赖关系。对于大于10ps的脉冲宽度,数值结果与平方根关系符合很好,对于亚皮秒激光脉冲,通过适当选择损伤的评价标准,我们的模型解释了Du等和Stuart等的相对实验结果。