Surface-assisted laser desorption/ionization mass spectrometry for the detection of low-molecular-weight and oligomeric products of 1,1-dimethylhydrazine transformation on the surfaces of construction materials

Surface-assisted laser desorption/ionization mass spectrometry (SALDI) was applied to study the products of 1,1-dimethylhydrazine transformation on the surfaces of construction materials. The approach proposed makes it possible to distinguish the transformation products of 1,1-dimethylhydrazine from the low-molecular-weight compounds and oligomers utilized in the production of construction materials, which undergo different transformations in the course of operational processes.     

Cobalt coated substrate for matrix-free analysis of small molecules by laser desorption/ionization mass spectrometry

Small molecule analysis is one of the most challenging issues in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. We have developed a cobalt coated substrate as a target for matrix-free analysis of small molecules in laser desorption/ionization mass spectrometry. Cobalt coating of 60-70 nm thickness has been characterized by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and laser induced breakdown spectroscopy. This target facilitates hundreds of samples to be spotted and analyzed without mixing any matrices, in a very short time. This can save a lot of time and money and can be a very practical approach for the analysis of small molecules by laser desorption/ionization mass spectrometry.     

Efficient fabrication of substrates for surface-assisted laser desorption/ionization mass spectrometry using laser ablation in liquids

Substrates for the surface-assisted laser desorption ionization (SALDI) technique were prepared using electrophoresis of gold nanoparticles produced by laser ablation in liquids. Throughout the preparation, no supplemental reagent was added for the stabilization and deposition of nanoparticles. Nanoparticles were deposited more uniformly using the electrophoresis technique than using dropping of the solution. Results demonstrated that the higher uniformity of the deposition of nanoparticles improved the reproducibility of SALDI measurements. Furthermore, the thickness of the deposited nanoparticles influences the SALDI efficiency.     

Preparation, IR spectroscopy, and time-of-flight mass spectrometry of halogenated and methylated Si(111)

The preparation of chlorine-, bromine-, and iodine-terminated silicon surfaces (Si(111):Cl, Br, and I) using atomically flat Si(111)-(1×1):H is described. The halogenated surfaces were obtained by photochemically induced radical substitution reactions with the corresponding dihalogen in a Schlenk tube by conventional inert gas chemistry. The nucleophilic substitution of the Si-Cl functionality with the Grignard reagent (CH₃MgCl) resulted in the unreconstructed methylated Si(111)-(1×1):CH₃ surface. The halogenated and methylated silicon surfaces were characterized by Fourier transform infrared (FTIR) spectroscopy and laser-induced desorption of monolayers (LIDOM). Calibration of the desorption temperature via analysis of time-of-flight (TOF) distributions as a function of laser fluence allowed the determination of the originally emitted neutral fragments by TOF mass spectrometry using electron-impact ionization. The halogens were desorbed atomically and as SiX _n (X = Cl, Br) clusters. The methyl groups mainly desorbed as methyl and ethyl fragments and a small amount of ⁺SiCH₃.     

Photoionization and Dissociative Photoionization Study of Cholesterol by IR Laser Desorption/Tunable Synchrotron VUV Photoionization Mass Spectrometr

介绍了一种将红外激光解吸/真空紫外光电离质谱技术应用于分析胆固醇的新方法. 由于近阈值单光子电离作用,可以在低能量下只产生纯净的胆固醇分子离子峰;增加光子能量则可以使碎片离子峰大量出现. 为了验证碎片离子的归属,利用商用高分辨电子轰击电离-飞行时间质谱仪分析并指认了胆固醇主要的碎片峰. 此外,采用量子化学从头算的方法研究了胆固醇母体离子和碎片构型,并讨论了部分主要的光解离机理     

Probing the mechanisms of ambient ionization by laser-induced fluorescence spectroscopy

The ionization mechanisms of several atmospheric pressure ion sources based on desorption and ionization of samples deposited on a surface were studied. Home-built desorption electrospray ionization (DESI), laserspray ionization (LSI), and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) sources were characterized using low-molecular-weight compounds, in particular fluorescent dyes. Detection of the desorbed and ionized species was performed by laser-induced fluorescence and ion cyclotron resonance mass spectrometry. The dependences of the signal intensities on various experimental parameters were studied. The data obtained reveals common features, such as formation of solvated species and clusters in the ionization processes, in all of the techniques considered.     

Silver nanoparticles as matrix for laser desorption/ionization mass spectrometry of peptides

Silver nanoparticle synthesized from chemical reduction has been successfully utilized as a matrix in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) of peptides. Acting as a substrate to adsorb analytes, as well as a transmission medium for UV laser, silver nanoparticle was found to assist in the desorption/ionization of peptides with little or no induced fragmentation. The size of the nanoparticle was typically in the range of 160 ± 20 nm. One of the key advantages of silver nanoparticle for peptides analysis is its simple step for on-probe sample preparation. In addition, it also minimizes the interferences of sodium dodecyl sulfate (SDS) surfactant background signal, resulting in cleaner mass spectra and more sensitive signal, when compared to α–cyano-4-hydroxycinnamic acid (CCA) matrix.     

MALDI-MS of drugs: Profiling,imaging, and steps towards quantitative analysis

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique which can be used in mass spectrometry to produce ions from biomolecules without inducing the fragmentation associated with traditional methods of ionization. When used with small molecules, the lack of fragmentation allows identification of specific molecules against a background of alternative signals; thus, for example, the presence of drug molecules and metabolites can be distinguished from a range of alternative biomolecules present within a tissue sample. Using highly collimated lasers in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) allows imaging of a tissue sample whereby the laser is rastered across the sample and individual mass spectra are collected in a serial manner. Thus, the distribution of the molecules within the tissue sample can be presented in the form of a 2D image. While the detection of specific drug molecules and metabolites within biological samples has its uses, quantification of those same molecules would be of greater benefit in a clinical setting. However, accurate quantification presents additional challenges. We present an overview of the MALDI-MS technique followed by recent progress in profiling drugs and their metabolites through imaging drug distributions within tissues and finish with recent developments in the quantification of drugs in tissues by MALDI-MSI.     

Stability and fragmentation of organic silicon functionalizations studied by laser desorption mass spectrometry

A method is introduced to investigate organic functionalizations on silicon by laser-induced thermal desorption (LITD), where well-ordered Si(1 1 1)-(1 × 1):H(D) surfaces are used to determine the desorption temperature as a function of laser fluence. To demonstrate the potential of this technique silicon surfaces with ultrathin oxide layers were functionalized with organic end groups. The species desorbed with focused XeCl laser pulses were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer after electron impact ionization. By assuming a negligible contribution of the oxide and organic layers to the heating effect, the TOF temperatures measured for Si(1 1 1)-(1 × 1):H(D) could be used to determine the mass of the desorbed species. Detailed results are presented for dimethylsilyl (DMS), bromomethyldimethylsilyl (BMDMS), and chloromethyldimethylsilyl (CMDMS) terminated surfaces which were prepared by silanization with suitable chloro and disilazane compounds. While for the DMS termination dimethylsilanol (76 u) is desorbed as a single species, clearly identifying the terminating group, in the case of BMDMS and CMDMS further fragmentation of the end group occurs at the surface.     

Coupling of desorption and photoionization processes in two-step laser mass spectrometry of polycyclic aromatic hydrocarbons

The response of polycyclic aromatic hydrocarbons (PAHs) to different desorption and ionization fluences has been investigated in a laser desorption/multiphoton ionization/time-of-flight mass spectrometry scheme. The results evidence an intricate relationship between the desorption and ionization steps, tentatively attributed to the amount of internal energy acquired by the desorbed molecules. Different behaviors have been found for the various PAHs considered, leading to a parametric “signature” for each species. Moreover, some insights on the fragmentation mechanism of the desorbed PAHs have been obtained, with possible interpretation in the frame of a “ladder-switching” model.     

Chemical reactivity in matrix-assisted laser desorption/ionization mass spectrometry

During the control of a multistep organic synthesis on a soluble polymer (PEG) by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, a chemical reactivity was encountered when the matrix was acidic, for the samples where the amino moiety of the anchored compounds was protected as a Schiff base. Such imine hydrolysis was proven to be solely mediated by the acidic matrix during analyses since the expected protected structures were detected when the experiments were duplicated with a non-acidic matrix. Even if MALDI mass spectrometry was found to be more convenient than electrospray ionization mass spectrometry for the monitoring of liquid phase organic syntheses, the chemical reactivity imparted by the use of a matrix must be taken into account to avoid erroneous spectra interpretations. Copyright 1999 John Wiley & Sons, Ltd.     

Field ion and field desorption mass spectrometry of inorganic compounds

The review on recent developments in field ionization mass spectrometry of inorganic compounds concerns the different mechanisms of ion formation at surfaces and under the influence of extremely high electric fields, field dependent chemical reactivity at surfaces and ion desorption from surfaces for mass spectrometric analysis. Applications in various areas are discussed, where this method has been used to identify surface compounds or to study kinetic phenomena at interfaces.     

VUV laser photoionization of laser-stimulated desorbed species

We report here first results about single-photon VUV laser photoionization of desorbed species from a silicon surface irradiated by a pulsed and tunable UV laser (290-300 nm). The combination of VUV photoionization at 10 eV with laser-induced surface desorption offers a largely non-destructive and sensitive method for quantitative analysis. Indeed it allows mass spectrometry measurements with uniform sensitivity and without breaking the chemical bonds in the probed species. The energy of the VUV photons (9.91 eV) is above the ionization limits of a number of molecules and fragments. Moreover, adjustment of the delay between the desorbing and the probe lasers allows the measurement of the time-of- flight distribution of the ejected species. Data extracted from these measurements are fundamental for a better understanding of laser-surface interaction phenomena.     

近场光学用于激光解吸飞行时间质谱的亚微米级空间分辨成像

基于激光离子源的飞行时间质谱法作为一门新兴的成像方法,已经被广泛应用于材料、地质、环境、药物和生命科学领域中。但受限于光学衍射极限、聚焦透镜的焦距和数值孔径等因素,使其难以实现亚微米尺寸的高空间分辨率成像。近场技术的引入成功地解决了光学衍射极限的限制,将近场技术与激光电离技术相结合,可以实现对固体样品表面纳米级弹坑的剥蚀。此外,传统的质谱成像技术常常假设样品表面是平整的,忽略其表面形貌的高低起伏,但这往往会导致信号强度不稳定和成像假象。为此,不仅需要获得样品中的化学组成与空间分布,还需同时获得样品表面的形貌信息,才能实现多功能的原位表征。在自行研制的激光解吸/电离飞行时间质谱的基础上,采用近场纳米有孔针尖离子源代替传统的远场激光聚焦,以532 nm波长激光为第一束解析激光,355 nm波长激光为后电离激光,音叉式原子力显微镜控制系统针尖与样品之间的距离维持在近场范围内,对酞菁铜镀层样品表面进行了弹坑剥蚀实验,获得了直径为550～850 nm的弹坑点阵;并对7.5 μm×7.5 μm的标准酞菁铜网格样品进行了铜离子亚微米级的高分辨率成像;此外,纳米有孔针尖离子源作为原子力显微镜的一种变体,还可同时获得成像区域的表面形貌信息,这一结合优势大大拓展了质谱技术在微纳尺度下的原位表征能力。     

Molecular histology analysis by matrix-assisted laser desorption/ionization imaging mass spectrometry using gold nanoparticles as matrix

Gold nanoparticles (AuNPs) were applied and optimized as matrix for matrix-assisted laser desorption/ionization mass spectrometry analysis of animal tissues, and enabled histological analysis of animal tissues at molecular level by imaging mass spectrometry (IMS). AuNPs were coated on animal tissue in a solvent-free manner via argon ion sputtering. Metabolites, including neurotransmitters, fatty acids and nucleobases, were directly detected from mouse brain tissue. Based on region-specific chemical profiles, fine histological features of mouse brain tissue and heterogeneous regions of tumor tissue were both revealed.     

Detection of hydrogen isotope atoms by mass spectrometry combined with resonant ionization

We examined the application of mass spectrometric methods using resonant ionization by a tunable laser and proposed its use for analyzing hydrogen isotopes. We conducted resonance ionization mass spectrometry (RIMS) to detect gas-phase hydrogen isotope atoms. The ionization efficiency was increased by more than 1000 times that obtained with conventional methods using nonresonant ionization. Resonant laser ablation mass spectrometry (RLAMS) was applied for deuterium detection in solid samples. A graphite substrate implanted with deuterium was used for ordinary laser ablation mass spectrometry (LAMS) and RLAMS. The deuterium signal was observed very clearly by RLAMS, in contrast to LAMS. Mass spectrometry combined with resonance ionization was very useful for hydrogen isotope detection, because components with equal mass numbers were resolved and the method demonstrated higher ionization efficiency. Received: 4 November 1998 / Revised version: 12 January 1999 / Published online: 7 April 1999     

MALDI-TOF MS对难溶聚芳酰胺PA6T的分析表征

选用五种样品试剂三氟乙酸、四氢呋喃、甲酸、乙腈、60%乙腈/5%三氟乙酸和两种不同的制样方法,采用基质辅助激光解吸电离飞行时间质谱(MALDI-TOF MS)表征难溶性聚合物聚对苯二甲酸己二酰胺(PA6T)的结构和分子量。发现以60%乙腈/5%三氟乙酸为样品试剂所得谱图的分子质量范围和信噪比最大,且溶剂法一步点样优于无溶剂法制样。获得了离子液体中合成产物PA6T的链结构,表明其主链为线性,未发现环状聚合物的存在,存在三种端基结构聚合体： 两端均为氨基或羧基、或一端为氨基另一端为羧基。采用MALDI-TOF MS和SEC两种方法对比测量了PA6T的平均分子量和分子量分布指数,表明MALDI-OF MS测得的平均分子量低于SEC法,分子量分布指数也低于SEC法,这是由于MS处理数据时忽略了低分子量部分,且高分子量部分因难以离子化而无法得到引起的。对于难溶的聚芳酰胺类聚合物的结构和分子量及其分布表征,MALDI-TOF MS仍不失为一个好方法。     

Vapor deposition of intact polyethylene glycol thin films

Thin films of polyethylene glycol (PEG) of average molecular weight, 1400 amu, were deposited by both matrix-assisted pulsed laser evaporation (MAPLE) and pulsed laser deposition (PLD). The deposition was carried out in vacuum (∼10^-6 Torr) with an ArF (λ=193 nm) laser at a fluence between 150 and 300 mJ/cm². Films were deposited on NaCl plates, Si(111) wafers, and glass slides. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy (FTIR), electrospray ionization (ESI) mass spectrometry, and matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometry. The results show that the MAPLE films nearly identically resemble the starting material, whereas the PLD films do not. These results are discussed within the context of biomedical applications such as drug delivery coatings and in vivo applications where there is a need for transfer of polymeric coatings of PEG without significant chemical modification. Received: 2 March 2001 / Accepted: 5 March 2001 / Published online: 23 May 2001     

The analysis of alkyl-capped alcohol ethoxylates and alcohol ethoxycarboxylates from alcohol ethoxylates by atmospheric pressure chemical ionization mass spectrometry

Alcohol ethoxylates (AEs) are nonionic surfactants. They are industrially important compounds that have historically been difficult to analyze, with the best results to date achieved through derivatization (e.g., silylation) followed by analysis by gas chromatography/mass spectrometry (GC/MS). Recently, mass spectrometric techniques such as field desorption (FD), time-of-flight secondary ion mass spectrometry (TOF-SIMS), fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) have been employed to analyze surfynol(R) 4xx. In an effort to produce low-cost alkyl-capped AEs and anionic detergents from AEs, a fast and reliable measure of the product yields and conversions from AEs is required in research. We found that the product yields and conversions from reactions of AEs, obtained by the employment of atmospheric pressure chemical ionization (APCI), were in good agreement with those obtained from proton nuclear magnetic resonance spectroscopy ((1)H-NMR). Therefore, APCI can be used as a validated tool for studying AE reactions. Mixtures that contain either silylated or unsilylated ethoxylates and/or carboxylates yield the same APCI mass spectra. Copyright -Copyright 1999 John Wiley & Sons, Ltd.