Nonlinear ion acceleration for improved space focusing in time-of-flight mass spectrometry

The initial spatial distribution of gas-phase ions is one of the primary factors limiting the achievable mass resolving power in time-of-flight mass spectrometry. While the effect of the spatial distribution is minimized along the flight path at a location known as the space-focus plane, the use of a single, linear acceleration field to generate this focus represents only a first-order approximation of ideal space focusing. Alternatively, ideal space focusing is possible through the use of a nonlinear ion acceleration field. A computational model delineates the requirements of the nonlinear potential profile, and suggests that substantial improvements to resolving power can be achieved when using an ion source configuration where a first field approximates the optimal nonlinear field gradient and a second, linear accelerating field imparts additional kinetic energy. Experimental results using a novel, static-field ion source geometry designed to allow selective position-specific ionization through photoinduced dissociation indicate that a 10× improvement in focusing can be achieved using this configuration.     

More efficient ion transport for inductively coupled plasma mass spectrometry

A modification of the ion-lens system in an inductively coupled plasma mass spectrometer has been studied in an attempt to improve the ion transport efficiency. The modification consists of the placement of a metal rod down the axis of the lens-stack. The rod is suspended in place via a ceramic insulator inserted through the photon stop. A negative potential is applied to the rod to help focus the positively charged ions into the quadrupole. Prior to the implementation of this modification, a simple model was developed to determine if the voltages required would be practical. The resulting simulated ion trajectories showed that focusing occurs with applied voltages on the order of −10 V, which was approximately the optimal voltage found experimentally. The experimental data presented show that, with the modification, the limits of detection for 15 elements (m/z ranging from 24 through 238) improved by factors ranging from two to ten times over those of the unmodified instrument     

Parameters contributing to efficient ion generation in aerosol MALDI mass spectrometry

The Bioaerosol Mass Spectrometry (BAMS) system was developed for the real-time detection and identification of biological aerosols using laser desorption ionization. Greater differentiation of particle types is desired; consequently MALDI techniques are being investigated. The small sample size ( approximately 1 microm3), lack of substrate, and ability to simultaneously monitor both positive and negative ions provide a unique opportunity to gain new insight into the MALDI process. Several parameters known to influence MALDI molecular ion yield and formation are investigated here in the single particle phase. A comparative study of five matrices (2,6-dihydroxyacetophenone, 2,5-dihydroxybenzoic acid, alpha-cyano-4-hydroxycinnamic acid, ferulic acid, and sinapinic acid) with a single analyte (angiotensin I) is presented and reveals effects of matrix selection, matrix-to-analyte molar ratio, and aerosol particle diameter. The strongest analyte ion signal is found at a matrix-to-analyte molar ratio of 100:1. At this ratio, the matrices yielding the least and greatest analyte molecular ion formation are ferulic acid and alpha-cyano-4-hydroxycinnamic acid, respectively. Additionally, a significant positive correlation is found between aerodynamic particle diameter and analyte molecular ion yield for all matrices. SEM imaging of select aerosol particle types reveals interesting surface morphology and structure.     

Imaging mass spectrometry using peptide isoelectric focusing

Imaging Mass Spectrometry (IMS) has emerged as a powerful technique in the field of proteomics. The use of Immobilized pH Gradient-IsoElectric Focusing (IPG-IEF) is also a new trend, as the first dimension of separation, in shotgun proteomics. We report a combination of these two outstanding technologies. This approach is based on the separation of shotgun-produced peptides by IPG-IEF. The peptides are then transferred by capillarity to a capture membrane, which is then scanned by the mass spectrometer to generate MS images. This high-throughput methodology allows a preview of the sample to be obtained in a single day. We report the application of this new pipeline for differential comparison of the membrane proteome of two different strains of Staphylococcus aureus bacteria in a proof-of-principle experiment.     

Reference standards for negative ion mass spectrometry

The negative ion mass spectra of phosphonitrile chlorides (PNCl₂)_n (n≥3) are studied. Since this series of compounds give very intense negative [M]^? and [M? Cl]^? ions, they can be used as good reference standards for negative ion mass spectrometry.     

A novel mass marker for metastable ion scans in a double focusing mass spectrometer

Characterization and assignment of the products of metastable decompositions which are transmitted by a double focusing mass spectrometer, is often complicated by the inability to measure precisely the parent/daughter ion mass values directly from the spectra which appear on chart paper. This is often because there are considerably fewer peaks in these types of spectra compared with a normal mass spectrum obtained from high energy reactions in the ion source. Electronic circuits are described which permit accurate parent/daughter ion mass values to be obtained when the products of metastable decompositions are transmitted through the ZAB-2F double focusing mass spectrometer. Digital logic circuitry is also described which is used to mark mass values during a scan by driving either a solenoid pen drive unit for a chart recorder or a galvanometer on a multi-range ultraviolet oscillograph recorder.     

High-pressure ion trap mass spectrometry

The effects of buffer gas pressure on ion trap stability, mass resolution/calibration, and choice of mass scanning are described. Pressure effects were treated phenomenologically by adding a drag term to the ion equations of motion. The resulting collisional damping enlarges the mass-dependent stability region but reduces the region in which mass-selective resonance ejection can be performed. The pressure effects can be reduced by increasing the frequency of the alternating quadrupole field.     

Three-dimensional field ion mass spectrometry

The three-dimensional atom probe (3 D-AP) is a new variant of the field ion microscope (FIM) combined with a time of flight mass spectrometer and single ion detection sensitivity (imaging atom probe). With the field ion microscope the topology of a surface, surface reactions and surface modifications can be studied in atomic detail. Using time of flight measurements, surface layers and interface layers can be chemically analyzed atom by atom and atomic layer by atomic layer. This three-dimensional atom probe permits the elemental reconstruction of a small volume of the specimen with near atomic resolution. This improvement is obtained by using the digitized video signal of the imaging atom probe detector and a separate time signal from the phosphor screen to achieve simultaneously the x and y position and the mass-to-charge ratio of individual ions striking the detector. Examples from a study on high speed steel are presented to demonstrate the usefulness of a recently built instrument.     

Negative ion liquid secondary ion mass spectrometry of carbamate-linked oligodeoxynucleosides. II

Carbamate-linked Oligodeoxynucleosides, in which the backbone consists of carbamate and N-methylcarbamate linkages, have been analyzed by negative ion liquid secondary ion mass spectrometry. Bidirectional sequence-determining fragmentations are postulated to occur from a common radical anion intermediate that is produced by capture of an ionizing electron by the neutral sample molecule. Fragmentation reactions appear to be related to whether a proton or methyl group is present on the amide nitrogen.     

Method for improved secondary ion yields in cluster secondary ion mass spectrometry

A method to increase useful yields of organic molecules is investigated by cluster secondary ion mass spectrometry (SIMS). Glycerol drops were deposited onto various inkjet‐printed arrays and the organic molecules in the film were rapidly incorporated into the drop. The resulting glycerol/analyte drops were then probed with fullerene primary ions under dynamic SIMS conditions. High primary ion beam currents were shown to aid in the mixing of the glycerol drop, thus replenishing the probed area and sustaining high secondary ion yields. Integrated secondary ion signals for tetrabutylammonium iodide and cocaine in the glycerol drops were enhanced by more than a factor of 100 compared with an analogous area on the surface, and a factor of 1000 over the lifetime of the glycerol drop. Once the analyte of interest is incorporated into the glycerol microdrop, the solution chemistry can be tailored for enhanced secondary ion yields, with examples shown for cyclotrimethylenetrinitramine (RDX) chloride adduct formation. In addition, depositing localized glycerol drops may enhance analyte secondary ion count rates to high enough levels to allow for site‐specific chemical maps of molecules in complex matrices such as biological tissues. Published in 2010 by John Wiley & Sons, Ltd.     

An ion optics for effective ion detection in single particle mass spectrometry

Recently, we reported that significant ion loss occurs prior to detection in conventional single particle mass spectrometry. A more serious type of loss is ion-kinetic-energy-dependent loss. This leads to significant errors in the measured chemical composition of nanoparticles, especially when they have a core-shell structure. In this paper, a novel ion optics for effective detection of ions generated from a single nanoparticle is designed. Using the commercial software SIMION, the trajectories of ions launched at different speeds inside a single particle mass spectrometer are simulated. The effects of changes are investigated with different repelling plates, Einzel lens additions, and substitutions of the tube electrode between extraction and acceleration grids on the ion flight. The best design was found when assembling the trials in the present condition. It was demonstrated experimentally that the new ion optics works well not only in theory, but also in practice.     

A novel approach for coincidence ion mass spectrometry

A novel approach is proposed for extracting a maximum of information from secondary ions ejected when surfaces are bombarded with keV mono or polyatomic ions. It is known that the event-by-event bombardment-detection mode allows identification of spatiotemporal relationships among individual secondary ions which in turn reveal surface composition within nanometric dimensions. We have devised a procedure for identifying spatiotemporal relationships among individual secondary ions without the requirement of pulsed sample interrogation (one single projectile at a time). The consequence of "mass separated time-of-flight mass spectrometry" is a much improved measurement duty cycle.