Internal energy build-up in matrix-assisted laser desorption/ionization

This paper reports detailed studies on the internal energy of ions formed in matrix-assisted laser desorption/ionization (MALDI) using delayed extraction MALDI-time-of-flight (TOF) and atmospheric pressure (AP) MALDI mass spectrometric (MS) methods. We use benzylpyridinium cations as internal energy probes. Our study reveals three distinct contributions to internal energy build-up in vacuum-MALDI (classical MALDI-TOF), each having different effects on ion fragmentation. Some fragments are formed before ion extraction (i.e. no more than 100 ns after the laser impact), and they are therefore well resolved and recorded as sharp signals in the MALDI-TOFMS scan. This prompt fragmentation can have two origins: (i) in-plume thermal activation, presumably always present, and (ii) in-plume chemical activation, in the course of reactions with hydrogen radicals. In addition to early internal energy build-up associated with these well-resolved promptly formed fragments, a broad peak slightly offset to higher masses could be detected corresponding to fragments formed after the extraction has started. This second signal corresponds to a third source of internal energy in MALDI ions, (iii) the extraction-induced collisional activation of the ions with the neutral components of the plume. These three contributions are difficult to quantify in vacuum-MALDI, because of the combined influence of several parameters (nature of the matrix, spot-to-spot variability, total laser exposure, delay time, acceleration voltage) on extraction-induced fragmentation. AP-MALDI, on the other hand, has two advantages for comparative studies of analyte fragmentation. First, extraction-induced fragmentation is absent, and only the contributions of early plume activation remain. Second, the reproducibility is far better than in vacuum-MALDI. AP-MALDI is therefore expected to shed new light on the early steps of the MALDI process.     

Interfaces for coupled liquid-phase separation/mass spectrometry techniques. An update on recent developments

An update is presented covering the latest developments in the interfacing of liquid-phase separation systems and mass spectrometers. The interfacing devices presented are those developed for continuous-flow matrix-assisted laser desorption/ionization, micro- and nano-liquid chromatography/masspectrometry (MS), capillary electrophoresis/MS and on-chip separation technologies/MS. From the information that can be found in the most recent literature on the topic, it is evident that the trend towards the miniaturization of separation and interface devices is gaining ground. This can be rationalized by the substantial gains in sensitivity for the detection and study of extremely low levels of analytes and especially of high molecular mass biopolymers.     

A differential game of approach with two pursuers and one evader

A differential game of approach with one evader and two pursuers with a nonconvex payoff function is considered. The duration of the game is fixed. The payoff functional is the distance between the object being pursued and the pursuer closest to it when the game terminates. An explicit form of the game value is found for all possible game positions. The paper is closely related to Refs. 1–12.The authors would like to thank Professor A. I. Subbotin for his valuable advice and encouragement.     

The Essential Oil of Pistacia vera L. at Various Temperatures of Direct Thermal Desorption Using Comprehensive Gas Chromatography Coupled with Time-of-Flight Mass Spectrometry

The essential oil obtained from the hulls of Pistacia vera fruits using direct thermal desorption was resolved using comprehensive gas chromatography (GC×GC) coupled with time-of-flight mass spectrometry (TOF-MS). A range of temperatures (100, 150, 200, 250 °C) for thermal desorption were applied to extract volatile organic compounds from around 10mg of raw sample. Although different species profiles were produced at each temperature (notably browning products above 200 °C), -pinene and (Z)--terpineol were found to be the major components at all temperatures studied. The number of compounds observed increased with increasing temperature. A temperature of 150 °C is recommended as optimum for the direct thermal desorption of volatiles from matrices such as fruit hulls. The GC×GC separation chromatographically resolved several hundred components which when coupled for detection with TOF-MS gave high probability identifications for around 100 compounds. The comprehensive separation also separated a number of components which remained unresolved on a single GC column, such as camphene with butyrolactone and dodecane with p-cymene.     

Light-induced sodium desorption from paraffin film

Sodium photoejection from a paraffin coated cell has been observed and analyzed in detail. The effect has been detected by observing the fluorescence induced by a cw dye laser. A maximum Na density of about 2.9 ×10⁸ atoms/cm³ was obtained by shining the cell with 1.05 W Ar⁺ laser green light. A progressive slowing of the dynamics of the process is reported, probably due to a change in the chemical rearrangement of the Na atoms in the coating under intense light exposure. This aging effect represents a limitation in the practical application of this kind of materials.     

紫外光诱导原子脱附技术在单腔磁阱装载中的应用

基于光诱导的原子脱附技术，采用脉冲紫外光剥离出玻璃池壁上吸附的铷原子以形成可快速开启和关断的脉冲铷原子源，成功地解决了单真空腔系统中磁阱的原子数和寿命之间的矛盾，突破了激光冷却和囚禁技术向小型化乃至微型化发展过程中的一个瓶颈.脉冲光源由390nm的LED阵列组成.实验结果表明它能够在1s内使真空铷原子气体分压提高近30倍,并且当紫外光关闭后系统的真空恢复到平衡状态的时间非常短，约120ms.测量了不同铷原子分压下磁光阱所俘获的最大原子数和装载时间,并由此得出系统的背景真空和磁光阱所能俘获的极限原子数，进一步得出磁阱的原子数-寿命积与磁光阱保持阶段时间的变化关系，结果显示在约1.25s处出现极大值，与无磁光阱保持阶段而直接进行磁阱装载情况相比提高了约0.3倍. 关键词： 激光冷却 磁光阱 光诱导原子脱附 原子数-寿命积     

The adsorption of 1,3-butadiene on Ag(1 1 1): A TPD/IRAS study and importance of lateral interactions

Temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) have been used to study the adsorption, desorption, molecular orientation and conformation of 1,3-butadiene on Ag(1 1 1) at 80 K. Butadiene adsorbs weakly as an s-trans conformer with the first layer oriented parallel to the silver surface and desorbs without decomposition. A very narrow line shape of the out-of-plane modes at low submonolayer coverage indicates molecular ordering within the diluted adsorbed layer, presumably through weak π-bonding interaction with the surface and intermolecular repulsive interaction. Compression within the first layer at coverages above 0.5 ML is driven by repulsive interaction as seen in both TPD and IRAS data. The IR intensity rollover and peak broadening, together with a significant shift in the TPD peak to lower temperature, indicate a reorientation of the butadiene molecule. Adsorption in the second- and multilayer is characterized by distinct IR frequency shifts and crystal field splitting effects similar to those reported for solid butadiene.     

Adsorbate-substrate bonding and the growth of naphthalene thin films on Ag(1 1 1)

The adsorption kinetics, energetics and growth of naphthalene thin films, from submonolayer to about 10 layers, on a Ag(1 1 1) surface at low temperature in a ultrahigh vacuum chamber are examined by using temperature programmed desorption spectroscopy. The first layer adsorption occurs with a desorption energy of 85 ± 5 kJ/mole and results in an interface dipole of 5 ± 1 D, from charge transfer of approximately 0.2 e from naphthalene to Ag. The surface dipole induced inter-adsorbate repulsion causes the lowering of the adsorption energy within the first layer near the saturation coverage so that the second layer deposition begins before the completion of the first layer. The second layer is a metastable phase with desorption energy, 74 ± 3 kJ/mole, smaller than the multilayer desorption energy of 79 ± 5 kJ/mole. Fractional order desorption kinetics were found for both the metastable and the multilayer phases, suggesting desorption from 2-D islanding and 3-D islanding, respectively.     

Angular distributions of desorbing N2 in thermal N2O decomposition on Rh(1 0 0)

The angular distribution of desorbing N₂ was studied in the decomposition of N₂O(a) on Rh(1 0 0) at 60-140 K by means of angle-resolved temperature-programmed desorption. N₂ desorption shows two peaks at around 80 K and 110 K. At low N₂O coverage, the former collimates far from the surface normal toward the [0 0 1] direction, whereas at high coverage, the desorption sharply collimates along the surface normal. The adsorption form of N₂O and its dissociation were also examined by DFT-GGA calculations. Dissociating N₂O is proposed to be lying along the [0 0 1] direction at low coverage and to change to an upright form bonding through the terminal oxygen at high coverage.     

Comparison of the adsorption of N2 on Ru(1 0 9) and Ru(0 0 1) - A detailed look at the role of atomic step and terrace sites

We present a direct side-by-side comparison of the adsorption and desorption of nitrogen on the atomically-stepped Ru(1 0 9) surface and the atomically-flat Ru(0 0 1) surface. Both infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) are employed in this study, along with density functional theory (DFT). We find that the chemisorptive terminal binding of N₂ is stronger on the atomic step sites than on the terrace sites of Ru(1 0 9) as indicated by TPD and by a reduction of the singleton vibrational frequency, ν(N₂), by ∼9 cm⁻¹, comparing steps to terraces. In addition, we find that metal-metal compression effects on the terrace sites of Ru(1 0 9) cause stronger binding of N₂ than found on the Ru(0 0 1) surface, as indicated by a reduction of the terrace-N₂ singleton vibrational frequency by ∼11 cm⁻¹ when compared to the singleton N₂ mode on Ru(0 0 1). These spectroscopic results, comparing compressed terrace sites to Ru(0 0 1) sites and confirmed by TPD and DFT, indicate that N₂ bonds primarily as a σ-donor to Ru. Using equimolar ¹⁵N₂ and ¹⁴N₂, it is found that dynamic dipole coupling effects present at higher N₂ coverages may be partially eliminated by isotopically detuning neighbor oscillators. These experiments, considered together, indicate that the order of the bonding strength for terminal-N₂ on Ru is: atomic steps > atomic terraces > Ru(0 0 1). DFT calculations also show that 4-fold coordinated N₂ may be stabilized in several structures on the double-atom wide steps of Ru(1 0 9) and that this form of bonding produces substantial decreases in the N₂ vibrational frequency and increases in the binding energy, compared to terminally-bound N₂. These highly coordinated N₂ species are not observed by IRAS.