Silicon nitride nanoparticles for surface-assisted laser desorption/ionization of small molecules

Conventional matrix-assisted laser desorption/ionization mass spectrometry is limited to analyses of higher molecular weight compounds due to high background noise generated by the matrix in the lower mass region. Surface-assisted laser desorption/ionization (SALDI) mass spectrometry is an alternative solution to this problem. Nanoparticles, structured silicon surfaces and carbon allotropes are commonly used as SALDI surfaces. Here, for the first time, we demonstrate the application of silicon nitride nanoparticles as a suitable medium for laser desorption/ionization of small drug molecules.     

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.     

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.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

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.     

Growth of thin films of low molecular weight proteins by matrix assisted pulsed laser evaporation (MAPLE)

Thin films of lysozyme and myoglobin grown by matrix assisted pulsed laser evaporation (MAPLE) from a water ice matrix have been investigated. The deposition rate of these two low molecular weight proteins (lysozyme: 14307 amu and myoglobin: 17083 amu) exhibits a maximum of about 1–2 ng/cm² per pulse at a fluence of 1–2 J/cm² and decreases slowly with increasing fluence. This rate is presumably determined by the matrix rather by the proteins. A significant fraction of the proteins are intact in the film as determined by MALDI (Matrix assisted laser desorption ionization) spectrometry. The results for lysozyme demonstrate that the fragmentation rate of the proteins during the MAPLE process is not influenced by the pH of the water solution prior to freezing.     

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     

延时提取正交发射MALDI技术研究

在反射式飞行时间质谱仪上,采用脉冲电场延时提取技术及离子正交发射采样方式,进行了基质辅助激光解吸附电离技术(MALDI)的研究.实验所用激光波长为266nm,基质为2,5二羟基苯甲酸,样品为孔雀石绿和多肽.得到了上述样品在MALDI条件下离子的质谱图和初速度分布.质谱测量的分辨率达到4000,质量测量的线性相关度好于0.9999,平均误差小于0.0075%.     

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.     

Naturally grown Ag nanoparticles on quartz substrates as SERS substrate excited by a 488 nm diode laser system for SERDS

A silver nanoparticle ensemble was prepared under ultrahigh vacuum (UHV) conditions by Volmer–Weber growth on a quartz substrate for surface-enhanced Raman scattering (SERS) investigations of pyrene molecules. To tune the surface plasmon resonance frequency in the vicinity of the excitation wavelength of 488 nm of the diode laser, the morphology of the silver nanoparticles was optimized. The substrates were mounted in a flow-through cell as part of the optical Raman set-up. A microsystem diode laser generates two slightly different emission wavelengths (λ=487.61 nm and λ=487.91 nm) with a spectral width <10 pm and an optical power of 20 mW, i.e. SERS experiments are possible but also shifted excitation Raman difference spectroscopy (SERDS) can be carried out. For trace analysis of pyrene in water we demonstrate SERS/SERDS experiments which lead to a limit of detection of 2 nmol/l for pyrene. These results suggest that with silver nanoparticle ensembles excited at their plasmon resonance at 488 nm combined SERS/SERDS measurements can be effectively performed for in-situ trace analysis of pollutant chemicals in water.     

Interaction between explosive and analyte layers in explosive matrix-assisted plasma desorption mass spectrometry

An HMX/insulin two-layer system was chosen as a model for further investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular ions. Copyright 1999 John Wiley & Sons, Ltd.     

High fluence deposition of polyethylene glycol films at 1064 nm by matrix assisted pulsed laser evaporation (MAPLE)

Matrix assisted pulsed laser evaporation (MAPLE) has been applied for deposition of thin polyethylene glycol (PEG) films with infrared laser light at 1064 nm. We have irradiated frozen targets (of 1 wt.% PEG dissolved in water) and measured the deposition rate in situ with a quartz crystal microbalance. The laser fluence needed to produce PEG films turned out to be unexpectedly high with a threshold of 9 J/cm², and the deposition rate was much lower than that with laser light at 355 nm. Results from matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis demonstrate that the chemistry, molecular weight and polydispersity of the PEG films were identical to the starting material. Studies of the film surface with scanning electron microscopy (SEM) indicate that the Si-substrate is covered by a relatively homogenous PEG film with few bare spots.     

Mass spectrometric characterization of phosphorylated peptides using MALDI in-source decay via redox reactions

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) has been used for characterization of a phosphorylated peptides and proteins because labile phosphate group is not lost during the MALDI-ISD process. The conventional MALDI-ISD is initiated by the hydrogen transfer from reducing matrix molecules to peptide backbone, leading to c'- and z'-series ions. In contrast, when an oxidizing chemical 5-nitrosalicylic acid (5-NSA) is served as the MALDI-ISD matrix, a- and x-series ions are specifically generated by hydrogen abstraction from peptide backbone to matrix molecule. The 5-NSA provides useful complementary information to the conventional MALDI-ISD for the analysis of amino acid sequencing and site localization of phosphorylation in peptides. The MALDI-ISD with reducing and oxidizing matrix could be a useful method for the de novo peptide sequencing.     

On-plate deposition of oxidized proteins to facilitate protein footprinting studies by radical probe mass spectrometry

The on-plate deposition of oxidized proteins is described to advance footprinting applications by radical probe mass spectrometry (RP-MS). An electrospray ionization (ESI) needle assembly mounted vertically over a 384-target matrix-assisted laser desorption/ionization (MALDI) plate enabled the limited oxidation of proteins as they were released in the charged droplets ahead of their deposition on the plate. This method combined with on-plate proteolytic digestion protocols expedites the analysis of proteins oxidized by RP-MS, and avoids the need to collect and reconstitute samples prior to analysis by MALDI mass spectrometry. Oxidation of peptides from solutions in water as well as an ammonium bicarbonate solution was investigated to test the optimal conditions required for on-plate oxidation of proteins. These comprised of peptides with a wide range of reactive amino acids including Phe, Tyr, Pro, His, Leu, Met and Lys that were previously shown to oxidize in both electrospray discharge and synchrotron radiolysis based footprinting experiments. The on-plate deposition of lysozyme oxidized at electrospray needle voltages of 6 and 9 kV were carried out to demonstrate conditions suitable for footprinting experiments as well as those that induce the onset of protein damage. Copyright ? 2012 John Wiley & Sons, Ltd.     

On-target separation of analyte with 3-aminoquinoline/α-cyano-4-hydroxycinnamic acid liquid matrix for matrix-assisted laser desorption/ionization mass spectrometry

3-Aminoquinoline/α-cyano-4-hydroxycinnamic acid (3AQ/CHCA) is a liquid matrix (LM), which was reported by Kumar et al. in 1996 for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. It is a viscous liquid and has some advantages of durability of ion generation by a self-healing surface and quantitative performance. In this study, we found a novel aspect of 3AQ/CHCA as a MALDI matrix, which converges hydrophilic material into the center of the droplet of analyte-3AQ/CHCA mixture on a MALDI sample target well during the process of evaporation of water derived from analyte solvent. This feature made it possible to separate not only the buffer components, but also the peptides and oligosaccharides from one another within 3AQ/CHCA. The MALDI imaging analyses of the analyte-3AQ/CHCA droplet indicated that the oligosaccharides and the peptides were distributed in the center and in the whole area around the center of 3AQ/CHCA, respectively. This 'on-target separation' effect was also applicable to glycoprotein digests such as ribonuclease B. These features of 3AQ/CHCA liquid matrix eliminate the requirement for pretreatment, and reduce sample handling losses thus resulting in the improvement of throughput and sensitivity.     

Ionization of a multilevel atom by ultrashort laser pulses

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude.     

On-line matrix addition for detecting aerosol particles

Single aerosol particles were measured by matrix-assisted laser desorption/ionization (MALDI) with an aerosol time-of-flight mass spectrometer (ATOFMS). The inlet to the ATOFMS was coupled with an evaporation/condensation flow cell that allowed matrix addition by condensation onto the particles. The coated particles entered the ion source through three-stage differentially pumped capillary inlet and were then ionized by a focused 266 nm Nd:YAG laser. The mass spectra and aerodynamic size of the single particles can be obtained simultaneously. The on-line matrix addition technique makes it possible to identify biological aerosols in real-time.     

Exploration of gold nanoparticle beams for matter wave interferometry

We investigate molecular beam methods for gold nanoparticles. They are based on electrospray, matrix assisted laser desorption and thermal laser desorption in combination with mass spectroscopy and multi-photon ionization. These techniques are analyzed with respect to their potential for coherent matter wave experiments.     

Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.     

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。