Energetic Ion Bombardment of Ag Surfaces by C60+ and Ga+ Projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations

Elemental speciation using pulsed Glow Discharge Time-of-Flight Mass Spectrometry

Elemental speciation analysis is very important because the toxicity of the elements is dependent on their chemical structures. The coupling of Gas Chromatography (GC) with pulsed Glow Discharge Time-of-Flight Mass Spectrometry (GDToFMS) provides real time speciation of volatile organic selenium compounds. The millisecond pulsed glow discharge is a versatile ion source for the determination of elemental composition, molecular structure, and molecular identification. The utility of this ion source to monitor a mixture of the selenium compounds dimethyl selenide (DMeSe) and dimethyl diselenide (DMeDSe) is illustrated in this work. Following the separation by GC, the compounds are readily characterized by pulsed GDToFMS. Time-gated acquisition provides unique information from three separate time regimes---elemental composition from the prepeak, structural fragments from the plateau, and intact molecular identification from the afterpeak.;Distinguishing and quantifying the specific oxidation states of elements are very hard to achieve for solid samples since specific extraction procedures tend to change the speciation. Therefore direct speciation in solid materials is highly desired. In the present work, a pulsed glow discharge time of flight mass spectrometry method is developed for the direct speciation of chromium in solid state samples. Careful tuning of the operating parameters yields the plasma chemistry that favors cluster ion formation. Unique mass spectral features permit differentiation between the trivalent and hexavalent forms of chromium, (CrIII) and (CrVI) respectively, in chromium oxide samples. Specifically, signals at 104 and 120 m/z corresponding to the Cr2 + and Cr2O+ cluster ions are indicative of the presence of Cr(III) in the sample, whereas signal at 100 m/z corresponding to the CrO3+ cluster ion is indicative of the presence of Cr(VI). The impact of glow discharge operating conditions on the appearance of these characteristic cluster ions is discussed.;Also, pulsed glow discharge Time-of-Flight mass spectrometry is used to differentiate between manganese (IV) dioxide and manganese (II) monoxide in solid sample directly. Under certain conditions, the specific cluster ions Mn2O3+ can be produced in the manganese (IV) dioxide glow discharge plasma. The spatial and temporal characteristics of glow discharge source are evaluated and optimized. The glow discharge operating parameters, such as the glow discharge gas pressure and the glow discharge operating power are studied and optimized. After optimizing all the related parameters, the two oxides can be differentiated easily and quantitative analysis was performed successfully

Interrogation of drug effects on the lipid composition of single cells and Drosophila brain using ToF-SIMS imaging

Lipids are essential for all living organisms on Earth. The most important function of lipids is that they act as the building blocks of cellular membranes. Lipids consist of polar head groups and non-polar tail groups and assemble into bilayer structures to create cell and organelle membranes. The plasma membrane of a cell provides a barrier which segregates cellular internal constituents from the external environment. In addition to acting as a barrier, membrane lipids are involved in many cellular processes including membrane trafficking, signal transduction, fission and fusion. Therefore, various conditions in the central nervous system involving lipid deficiencies can lead to function deficit. There are several drugs that induce the dysregulation of lipid metabolism linked to the impairment or enhancement of cognitive function. Hence, I have studied the lipid alterations in brain induced by drugs with regards to their effects on cognitive processes: cognitive impairing drugs (cocaine and zinc deficiency) and cognitive enhancing drugs (methylphenidate and fatty acids). Much work has been done to investigate the link between lipid metabolisms and these drugs. A powerful technique for lipid analysis is mass spectrometry imaging (MSI). MSI is a surface sensitive method which enables label-free detection of molecules in complex biological systems. In addition, MSI provides the relative composition as well as allows imaging of intact species with high spatial resolution in single experiments. One of the most common MSI techniques is time-of-flight secondary ion mass spectrometry (ToF-SIMS), which achieves high spatial resolution using a focused ion beam to eject and ionize molecules in the sample surface. Recently, gas cluster ion beams have been introduced to reduce the chemical damage during sampling of surfaces and to achieve enhancement of lipid signals. In our studies, ToF-SIMS has been applied to lipids in the membranes of cells and Drosophila melanogaster brain to get a better understanding about the effect of drugs in lipid mechanisms related to neuronal signal transmission. The papers included in this thesis describe the application of ToF-SIMS in biological samples to reveal the alterations of lipids after drug treatments. In paper I, the alterations in lipid distribution and composition induced by cocaine and methylphenidate, which cause the impairment and enhancement in cognitive performance respectively, were investigated. ToF-SIMS data were used to show that cocaine and methylphenidate have opposite effects on the relative levels of lipids in the central fly brain. To enhance our understanding about the lipid mechanisms, in paper II, I used stable deuterium-labeled omega-3 and -6 fatty acids as lipid precursors to analyze the synthesis and transportation of lipids into the plasma membrane of PC12 cells. The use of isotope-labeled fatty acids provided a tool to track the lipid turn-over as well as to measure their relative amounts. Paper III continued the work done in paper I, where experiments were performed to investigate the recovery of lipids after cocaine removal. In addition, the cognitive-enhancing drug, methylphenidate, was used to treat cocaine removal from flies to investigate the reversal of lipid changes in the brain caused by repeated-cocaine exposure. Zinc deficiency in the diet, which causes a decrease in cognitive function, was also studied in fly brain. ToF-SIMS data obtained reveal that the lipid types that change are similar to those when treated with cocaine as seen in paper IV.ToF-SIMS opens a new approach to visualize and relatively quantify phospholipids in biological tissues and cells. In the biological model systems studied here, cognition-affecting drugs show that alterations in the distribution and composition of specific lipids is altered differently based on whether the drug enhances versus diminishes cognition. These results provide new possible targets for lipid-modifying therapies to improve the cognitive decline in drug abuse and diseases

Surface characterization of biomass by imaging mass spectrometry

Lignocellulosic biomass (e.g., non food-based agricultural resides and forestry wastes) has recently been promoted for use as a source of bioethanol instead of food-based materials (e.g., corn and sugar cane), however to fully realize these benefits an improved understanding of lignocellulosic recalcitrance must be developed. The primary goal of this thesis is to gain fundamental knowledge about the surface of the plant cell wall, which is to be integrated into understanding biomass recalcitrance. Imaging mass spectrometry by TOF-SIMS and MALDI-IMS is applied to understand detailed spatial and lateral changes of major components in the surface of biomass under submicron scale. Using TOF-SIMS analysis, we have demonstrated a dilute acid pretreated poplar stem represented chemical differences between surface and bulk compositions. Especially, abundance of xylan was observed on the surface while sugar profile data showed most xylan (ca. 90%) removed from the bulk composition. Water only flowthrough pretreated poplar also represented difference chemistry between surface and bulk, which more cellulose revealed on the surface compared to bulk composition. In order to gain the spatial chemical distribution of biomass, 3-dimensional (3D) analysis of biomass using TOF-SIMS has been firstly introduced in the specific application of understanding recalcitrance. MALDI-IMS was also applied to visualize different molecular weight (e.g., DP) of cellulose oligomers on the surface of biomass.PhDCommittee Chair: Art J. Ragauskas, Advisor; Committee Member: Charles L. Liotta; Committee Member: David M. Collard; Committee Member: Stefan France; Committee Member: Yulin Den

Tip enhanced laser ablation sample transfer for mass spectrometry

© 2015 Materials Research Society. Mass spectrometry is one of the primary analysis techniques for biological analysis but there are technological barriers in sampling scale that must be overcome for it to be used to its full potential on the size scale of single cells. Current mass spectrometry imaging methods are limited in spatial resolution when analyzing large biomolecules. The goal of this project is to use atomic force microscope (AFM) tip enhanced laser ablation to remove material from cells and tissue and capture it for subsequent mass spectrometry analysis. The laser ablation sample transfer system uses an AFM stage to hold the metal-coated tip at a distance of approximately 10 nm from a sample surface. The metal tip acts as an antenna for the electromagnetic radiation and enables the ablation of the sample with a spot size much smaller than a laser focused with a conventional lens system. A pulsed nanosecond UV or visible wavelength laser is focused onto the gold-coated silicon tip at an angle nearly parallel with the surface, which results in the removal of material from a spot between 500 nm and 1 um in diameter and 200 and 500 nm deep. This corresponds to a few picograms of ablated material, which can be captured on a metal surface for MALDI analysis. We have used this approach to transfer small peptides and proteins from a thin film for analysis by mass spectrometry as a first step toward high spatial resolution imaging

Tip-Enhanced Laser Ablation Sample Transfer for Mass Spectrometry

In this research, atomic force microscope tip-enhanced laser ablation mass spectrometry (AFM TELA-MS), an ambient sub-micrometer scale sampling method for offline MS was developed. AFM TELA was used to transfer molecules from thin films to a suspended silver wire for off-line mass spectrometry using laser desorption ionization (LDI) and matrix-assisted laser desorption ionization (MALDI). An AFM with a 30 nm radius gold-coated silicon tip was used to image the sample and to hold the tip 15 nm from the surface for material removal using a pulsed Nd:YAG laser, which provides output at wavelengths of 532 nm in the visible, 1064 nm in the near IR, or the 355 nm UV wavelength. The laser is mildly focused onto the AFM tip and the fluence is set just below the far-field ablation threshold to irradiate the AFM tip for material removal with a smaller spot size than a laser focused with a conventional lens system. The AFM is used to image ablation craters and place the tip at the area being analyzed. For small molecules, approximately 100 fg of material was ablated from each of the 1 µm ablation spots and transferred with approximately 3% efficiency. AFM-TELA of large biomolecules was also demonstrated at 3% efficiency and a mass range up to 600 Da. AFM-TELA studies with different laser parameters indicated that the tip-enhanced material ejection depends on laser wavelength, polarization, fluence, and number of laser shots used for material ejection, but not on the absorption of the sample itself. The utility of AFM-TELA was applied to sampling of rat brain tissue. The ability of producing sub-micrometer scale craters, capture on a suspended silver wire and detection of lipids were demonstrated using off-line MALDI MS

Ion Current Rectification in Nano/Micro-Fluidic Interfaces and Pulsed Glow Discharge Time-of-Flight Mass Spectrometric Chemical Speciation

Microfluidics continues to be of interest for analyzing chemical and biological samples because of the disposability, portability, low sample consumption, fast analysis time, and parallel analysis potential for multiple samples in a single device. To improve microfluidic device functionality, integrated systems-nanofluidic/microfluidic interfaces (NMIs) have been fabricated for concentrating samples and performing as molecular gates. Ion current rectification has been confirmed in NMI with an asymmetric system. In chapter 2, the asymmetry of the NMI is systematically altered by varying the inner diameter of the nanocapillary membrane (NCM) reservoir, and the current rectification factor is observed to increase as the inner diameter of NCM reservoir increases. The data provide a new approach to tune the ion current rectification of NMIs and strengthen the fundamental knowledge of how these devices function.;Glow discharge mass spectrometry (GDMS) is a well-established technique for the direct analysis of elements in solid samples. The introduction of pulsed glow discharge makes the internal energy of GD plasma tunable so that the specific desired ion signal profiles can be obtained and used for chemical speciation. For example, the elemental, structural, and molecular information of organic molecules have been obtained nearly simultaneously using glow discharge time-of-flight mass spectrometry (GDToFMS) coupled with gas chromatography. With careful control of operating parameters of GDToFMS, specific cluster ions or ion abundance ratios can be used for speciation of chromium oxides, manganese oxides, and iron oxides.;Chapters 3 and 4 focus on extending the application of pulsed glow discharge time-of-flight mass spectrometry for chemical speciation. Chapter 3 is the analysis of cysteine using pulsed glow discharge time-of-flight mass spectrometry. The characteristic fragment ion at m/z 76 is used for the quantitative analysis for cysteine. The calibration curve for cysteine standards obtained exhibits good linearity. In chapter 4, the application of pulsed glow discharge mass spectrometry is extended to direct NbxOy speciation. The effect of variations in temporal and spatial sampling along with variations in operating power on analyte ion signal distributions are studied and discussed. Under optimized conditions, the differentiation of three niobium oxides is achieved by comparison of ion abundance ratios

Ion Current Rectification in NanoMicro-Fluidic Interfaces and Pulsed Glow Discharge Time-of-Flight Mass Spectrometric Chemical Speciation

Microfluidics continues to be of interest for analyzing chemical and biological samples because of the disposability, portability, low sample consumption, fast analysis time, and parallel analysis potential for multiple samples in a single device. To improve microfluidic device functionality, integrated systems-nanofluidic/microfluidic interfaces (NMIs) have been fabricated for concentrating samples and performing as molecular gates. Ion current rectification has been confirmed in NMI with an asymmetric system. In chapter 2, the asymmetry of the NMI is systematically altered by varying the inner diameter of the nanocapillary membrane (NCM) reservoir, and the current rectification factor is observed to increase as the inner diameter of NCM reservoir increases. The data provide a new approach to tune the ion current rectification of NMIs and strengthen the fundamental knowledge of how these devices function.;Glow discharge mass spectrometry (GDMS) is a well-established technique for the direct analysis of elements in solid samples. The introduction of pulsed glow discharge makes the internal energy of GD plasma tunable so that the specific desired ion signal profiles can be obtained and used for chemical speciation. For example, the elemental, structural, and molecular information of organic molecules have been obtained nearly simultaneously using glow discharge time-of-flight mass spectrometry (GDToFMS) coupled with gas chromatography. With careful control of operating parameters of GDToFMS, specific cluster ions or ion abundance ratios can be used for speciation of chromium oxides, manganese oxides, and iron oxides.;Chapters 3 and 4 focus on extending the application of pulsed glow discharge time-of-flight mass spectrometry for chemical speciation. Chapter 3 is the analysis of cysteine using pulsed glow discharge time-of-flight mass spectrometry. The characteristic fragment ion at m/z 76 is used for the quantitative analysis for cysteine. The calibration curve for cysteine standards obtained exhibits good linearity. In chapter 4, the application of pulsed glow discharge mass spectrometry is extended to direct NbxOy speciation. The effect of variations in temporal and spatial sampling along with variations in operating power on analyte ion signal distributions are studied and discussed. Under optimized conditions, the differentiation of three niobium oxides is achieved by comparison of ion abundance ratios