Electron ionization | EPFL Graph Search

Electron ionization (EI, formerly known as electron impact ionization and electron bombardment ionization) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of the first ionization techniques developed for mass spectrometry. However, this method is still a popular ionization technique. This technique is considered a hard (high fragmentation) ionization method, since it uses highly energetic electrons to produce ions. This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds. EI is the most useful for organic compounds which have a molecular weight below 600. Also, several other thermally stable and volatile compounds in solid, liquid and gas states can be detected with the use of this technique when coupled with various separation methods. History Electron ionization was first described in 1918 by Canadian-American Physicist Arthur J. Demp

A method to numerically determine the secondary electron yield considering effects of the surface morphology

Guang-Yu Sun, Ning Yang

Secondary electron emission (SEE) of solid materials due to electron bombardment is influenced by numerous properties of materials, where the surface condition plays a critical role in the value of secondary electron yield (SEY). Here, a 3D random microstructure surface model is established to simulate realistic surface morphology and study its effects on SEY by implementing a path tracing algorithm and finite element method. It is found that electron collision frequency on surfaces is strongly affected by local surface geometry parameters, namely the vertical height and the distance between similar features along the horizontal direction of random microstructure surfaces. Manipulating the interaction angle and the inter-barrier collision frequency could quantitively suppress or intensify SEE, allowing for functional design of solid material surfaces under various contexts. In addition, empirical roughness parameters (Ra, Rz) lack certain microscopic information. A method is proposed to estimate secondary electron yield numerically for a given material surface geometry. It provides copious utilities in practical SEE-related applications.

2021

New ionization and tagging methods for mass spectrometry

Xiaoqin Zhong

Mass spectrometry (MS) is an essential detection tool in bioanalytical chemistry owing to its exceptional selectivity and sensitivity, paired with rapid analyte identification and quantification. In this thesis, two classical MS and a newly developed ionization method are employed, including matrix-assisted laser desorption/ionization (MALDI) MS, electrospray ionization (ESI) MS and electrostatic spray ionization (ESTASI) MS. To meet the emerging challenges in bioanalytical chemistry, from disease diagnosis to drug development, new MS-based analytical methods ought to be developed with respect to high sensitivity, throughput, speed, as well as sample consumption and experimental simplification. For these purposes, this thesis presents four analytical strategies combined with MS detection for improved analytical performance in different research fields. Two classical analytical tools, thin layer chromatography (TLC) and 384-well plate, were coupled with ESTASI-MS respectively. In ESTASI-TLC-MS, ESTASI was applied to extract and identify a wide range of molecules of different polarities and chemical structures from both hydrophilic or hydrophobic silica plates with high sensitivity and minimal sample consumption in the femtomole range. In 384-well plate ESTASI-MS, the commercial 384-well plate could work as a container and an emitter for sample spray ionization, without any liquid delivery system or any additional interface. This approach provides fast and high throughput analyses for the large batches of reactions, such as enzyme assay and drug metabolism. Tyrosinase-catalyzed tyrosine oxidation in the presence or absence of inhibitors and cytochrome P450-catalyzed metabolic reactions of two drugs were studied respectively. To improve analysis sensitivity, two strategies, mass barcoded gold nanoparticles (Mb-AuNPs) for MALDI-MS signal amplification and microfabricated on-chip spyhole (Ø 10-12 ¿m) emitter for nanoelectrospray (spyhole-nanoESI), were developed, leading to a low sample consumption and high analysis efficiency. Mb-AuNPs combined with magnetic separation were applied for multiplex cow¿s milk allergy diagnosis in a component-resolved manner. IgE antibodies (Abs) could be extracted from a patient¿s blood serum by the formation of a sandwich structure between allergenic proteins-coated Mb-AuNPs and anti-human IgE Abs-functionalized magnetic beads (MBs). Detection of Mb-AuNPs by MALDI-MS provides a limit of detection (LOD) down to picograms per milliliter level for specific IgE Abs from only 1 ¿L of patient¿s blood serum. To take advantages of microfluidics in low sample consumption and easy integration, a novel interface of spyhole-nanoESI was designed for coupling microfluidics with MS and showed an improved sensitivity than standard ESI. This disposable microchip coupled with MS/MS shows potential application in cancer diagnosis by the successful detection of a small cell lung cancer biomarker in 1 ¿L of human serum at the extensive stage, without any complicated sample preparation steps.

EPFL2017

A Study of Synthesis & [and] Performance of Flame-Made Semiconducting BiVO4 Nanoparticles for the Photocatalytic Degradation of Aqueous Organic Pollutants

Nikola Castillo

Semiconductor photocatalysis is an Advanced Oxidation Technology that uses light as a source of energy to accelerate the degradation of harmful organic pollutants. Current research addresses the development of photocatalysts capable of harvesting visible light (Chapter 1). Bismuth vanadate (BiVO4) is a semiconductor that can absorb visible light up to 500 nm. It has been successfully tested for the oxidation of water and some dyes. So far, this material has been synthesized with low specific surface areas (SSA). Flame Spray Synthesis is a cost-effective method for the synthesis of nanoparticles and can be easily scaled up. The Flame Spray Synthesis of BiVO4 nanoparticles is described in detail. Careful adjustment of the process parameters allowed to control particle size and crystallinity of the as-prepared BiVO4 powders. Temperature of the collection site was found to hold a critical role on the control of these two properties (Chapter 3). Photocatalytic activity of the as-synthesized powders was then evaluated for the degradation of the Methylene Blue dye under visible-light irradiation. Photocatalyst properties controlled by synthesis parameters were confronted with their photocatalytic activity. It was found that both crystalline and amorphous powders could carry out the N-demethylation of Methylene Blue. Higher SSA lead to higher activity. A crystalline sample with moderate SSA was also found to degrade phenol to some extent at acidic pH but was subject to deactivation. Addition of different electron acceptors improved its photocatalytic activity. In particular, hydrogen peroxide (H2O2) could be used as a powerful yet rapidly consumed electron scavenger that enabled the degradation of phenol and dichloroacetate at neutral pH (Chapter 4). Advanced electrochemical and photochemical methods gave evidence of the poor reducing power of conduction-band electrons of BiVO4. Besides, the presence of deep traps associated with the change of coordination geometry of vanadium surface atoms upon photoexcitation has been correlated with the apparent photocatalytic activity of flame-made BiVO4 at pH 2. Finally, a mechanism involving the regneration of MB after its reduction when used as an electron acceptor has been proposed (Chapter 5). The flame spray synthesis of BiVO4 might be developed to produce high-SSA amorphous photocatalysts for the selective N-demethylation of Methylene Blue. Regarding the development of novel photocatalysts, the understanding of the role of the atomic surface structure should become a priority to increase their activity.

EPFL2011