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.     

Mass Spectrometry,Review of the Basics: Electrospray,MALDI, and Commonly Used Mass Analyzers

Abstract

Mass spectrometry (MS) has progressed to become a powerful analytical tool for both quantitative and qualitative applications. The first mass spectrometer was constructed in 1912 and since then it has developed from only analyzing small inorganic molecules to biological macromolecules, practically with no mass limitations. Proteomics research, in particular, increasingly depends on MS technologies. The ability of mass spectrometry analyzing proteins and other biological extracts is due to the advances gained through the development of soft ionization techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) that can transform biomolecules into ions. ESI can efficiently be interfaced with separation techniques enhancing its role in the life and health sciences. MALDI, however, has the advantage of producing singly charges ions of peptides and proteins, minimizing spectral complexity. Regardless of the ionization source, the sensitivity of a mass spectrometer is related to the mass analyzer where ion separation occurs. Both quadrupole and time of flight (ToF) mass analyzers are commonly used and they can be configured together as QToF tandem mass spectrometric instruments. Tandem mass spectrometry (MS/MS), as the name indicates, is the result of performing two or more sequential separations of ions usually coupling two or more mass analyzers. Coupling a quadrupole and time of flight resulted in the production of high-resolution mass spectrometers (i.e., Q-ToF). This article will historically introduce mass spectrometry and summarizes the advantages and disadvantages of ESI and MALDI along with quadrupole and ToF mass analyzers, including the technical marriage between the two analyzers. This article is educational in nature and intended for graduate students and senior biochemistry students as well as chemists and biochemists who are not familiar with mass spectrometry and would like to learn the basics; it is not intended for mass spectrometry experts.     

Recent Developments in Ion Mobility Spectrometry

Abstract

The general principles and technical implementations of traditional time-of-flight ion mobility spectrometers and analyzers with field-dependent mobilities were reviewed in our last article in this journal. Recent advances in instrumentation and new applications since 2006 are highlighted in this review. In addition to traditional applications as military chemical-agent detectors, ion mobility techniques have become popular for different purposes. Though ion mobility spectrometry was solely used as vapor sensor in the past decades, further developments in ionization techniques (especially electrospray ionization) now permit its routine use for the analysis of liquid samples. The coupling of ion mobility spectrometry with selective sample preparation techniques such as molecular-imprinted polymers, coupling with chromatographic techniques, the use of dopants, and application of selective ionization sources has led to an expanded number of applications in industrial and environmental analysis with complex sample matrices due to an improved selectivity in comparison with traditional stand-alone spectrometers. Furthermore, new developments in hyphenated techniques, especially ion mobility–mass spectrometry couplings, has resulted in an increased number of new applications for the analysis of biomolecules and pharmaceutical samples and in clinical diagnostics.     

Mass Spectrometry,Review of the Basics: Ionization

Abstract: Mass spectrometry (MS) has become an integral tool in life sciences. The first step in MS analysis is ion formation (ionization). Many ionization methods currently exist; electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most commonly used. ESI relies on the formation of charged droplets releasing ions from the surface (ion evaporation model) or via complete solvent evaporation (charge residual model). MALDI ionization, however, is facilitated via laser energy and the use of a matrix. Despite wide use, ESI cannot efficiently ionize nonpolar compounds. Atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) are better suited for such tasks. APPI requires photon energy and a dopant, whereas APCI is similar to chemical ionization. In 2004, ambient MS was introduced in which ionization occurs at the sample in its native form. Desorption electrospray ionization (DESI) and direct analysis in real time (DART) are the most widely used methods. In this mini-review, we provide an overview of the main ionization methods and the mechanisms of ion formation. This article is educational and intended for students/researchers who are not very familiar with MS and would like to learn the basics; it is not for MS experts.     

Comparison of Mid-IR with NIR in Polymer Analysis

Abstract

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) represents a powerful technique for analyzing biological samples due to the ultrahigh resolving power, high mass accuracy, and multiple-stage tandem mass spectrometry (MSⁿ). With the advent of electrospray ionization (ESI), determinations of binding stoichiometry and binding sites for protein complexes are available. This review summarizes the recent FT-ICR MS applications in characterization of protein complexes, such as protein-peptide complexes, protein-protein complexes, and protein-nucleic acid complexes. Especially, combined with ECD and SWIFT techniques, FT-ICR MS has unique ion manipulation capabilities and plays a critical role in the analysis of protein complexes.     

The analysis of alkyl-capped alcohol ethoxylates and alcohol ethoxycarboxylates from alcohol ethoxylates by atmospheric pressure chemical ionization mass spectrometry

Alcohol ethoxylates (AEs) are nonionic surfactants. They are industrially important compounds that have historically been difficult to analyze, with the best results to date achieved through derivatization (e.g., silylation) followed by analysis by gas chromatography/mass spectrometry (GC/MS). Recently, mass spectrometric techniques such as field desorption (FD), time-of-flight secondary ion mass spectrometry (TOF-SIMS), fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) have been employed to analyze surfynol(R) 4xx. In an effort to produce low-cost alkyl-capped AEs and anionic detergents from AEs, a fast and reliable measure of the product yields and conversions from AEs is required in research. We found that the product yields and conversions from reactions of AEs, obtained by the employment of atmospheric pressure chemical ionization (APCI), were in good agreement with those obtained from proton nuclear magnetic resonance spectroscopy ((1)H-NMR). Therefore, APCI can be used as a validated tool for studying AE reactions. Mixtures that contain either silylated or unsilylated ethoxylates and/or carboxylates yield the same APCI mass spectra. Copyright -Copyright 1999 John Wiley & Sons, Ltd.     

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.     

Desorption of carbonium ions from zeolite surfaces; A comparison of reactive surface ionization with field ionization

In comparison with field ionization on platinum the thermal desorption of surface carbonium ions from zeolite surfaces has been investigated mass spectrometrically. CaY-zeolites — forming carbonium ions — were supported by Pt filaments or Pt emitter tips where hydrocarbons are chemisorbed in a non-ionic form. Thermal ion desorption at very low external fields has been observed with stable carbonium ions of triphenylmethyl compounds Ph₃ C-X. This ion desorption was achieved on CaY zeolite surfaces and occurred in the order X  Br > X  Cl > X  OH; however, no ions could be obtained with X  H and X  COOH up to 825°K. On Pt surfaces high fields were required for desorbing molecular ions.     

Field ionization mass spectrometry of organic compounds

A review is given on field ionization mass spectrometry of organic compounds. Four different subjects are treated and illustrated by means of significant examples: Experimental techniques, surface reactions induced by high electric fields, the kinetics of fast unimolecular decompositions of ions, and qualitative and quantitative analyses of organic compounds by field desorption methods.     

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     

Analysis of cancer cell lipids using matrix-assisted laser desorption/ionization 15-T Fourier transform ion cyclotron resonance mass spectrometry

A combination of methodologies using the extremely high mass accuracy and resolution of 15-T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) was introduced for the identification of intact cancer cell phospholipids. Lipids from a malignant glioma cell line were initially analyzed at a resolution of >200,000 and identified by setting the mass tolerance to ±1 mDa using matrix-assisted laser desorption/ionization (MALDI) 15-T FT-ICR MS in positive ion mode. In most cases, a database search of potential lipid candidates using the exact masses of the lipids yielded only one possible chemical composition. Extremely high mass accuracy (<0.1?ppm) was then attained by using previously identified lipids as internal standards. This, combined with an extremely high resolution (>800,000), yielded well-resolved isotopic fine structures allowing for the identification of lipids by MALDI 15-T FT-ICR MS without using tandem mass spectrometric (MS/MS) analysis. Using this method, a total of 38 unique lipids were successfully identified.     

Analysis of lipids with desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS) and desorption electrospray ionization-mass spectrometry (DESI-MS)

In this article, the effect of spray solvent on the analysis of selected lipids including fatty acids, fat-soluble vitamins, triacylglycerols, steroids, phospholipids, and sphingolipids has been studied by two different ambient mass spectrometry (MS) methods, desorption electrospray ionization-MS (DESI-MS) and desorption atmospheric pressure photoionization-MS (DAPPI-MS). The ionization of the lipids with DESI and DAPPI was strongly dependent on the spray solvent. In most cases, the lipids were detected as protonated or deprotonated molecules; however, other ions were also formed, such as adduct ions (in DESI), [M-H](+) ions (in DESI and DAPPI), radical ions (in DAPPI), and abundant oxidation products (in DESI and DAPPI). DAPPI provided efficient desorption and ionization for neutral and less polar as well as for ionic lipids but caused extensive fragmentation for larger and more labile compounds because of a thermal desorption process. DESI was more suitable for the analysis of the large and labile lipids, but the ionization efficiency for less polar lipids was poor. Both methods were successfully applied to the direct analysis of lipids from pharmaceutical and food products. Although DESI and DAPPI provide efficient analysis of lipids, the multiple and largely unpredictable ionization reactions may set challenges for routine lipid analysis with these methods.     

MATRIX-ASSISTED LASER DESORPTION MASS SPECTROMETRY

ABSTRACT

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is a technique that has attracted widespread interest since its introduction in 1988. It is primarily implemented using time-of-flight or trapped ion mass analyzers and greatly facilitates determination of molecular weights for biomolecules, polymers, and natural products. Numerous publications on these and related applications have appeared during the past ten years. The present brief MALDI review is intended to provide selected coverage of recent literature, with special emphasis on applications to proteomics, whole cells and tissues, polymers, organic and inorganic molecules. In view of the intense current interest, the choice of topics emphasizes biochemically-related applications.     

Laser Desorption Mass Spectrometry of Some Organic Acids

Abstract

Positive and negative ion laser desorption (LD) mass spectra of organic acids are characterized by the emission of the quasimolecular ions (M+H)⁺ and (M-H)^?. Generation of (M+H)⁺ ions is interpreted as evidence for the occurence of high pressure proton transfer reactions in LD. Fragment ions can be rationalized by loss of stable neutral molecules from quasimolecular ions although decomposition may also be occuring prior to ionization. Features unique to LD, including the detection of pyrolysis products along with ions characteristic of the sample, are discussed in terms of the internal energy distribution in the irradiated microvolume. Negative ion mass spectra of acids are dominated by (M-H)^?, while positive ion spectra contain abundant fragment ions, underlining the utility of detecting negative ions for acidic compounds.     

Investigation of interaction between lithium and tantalum under a silicon film coating by electron-stimulated desorption technique

The electron-stimulated desorption of Li⁺ ions from lithium layers adsorbed on the tantalum surface coated with a silicon film has been investigated. The measurements are performed using a static magnetic mass spectrometer equipped with an electric field-retarding energy analyzer. The threshold of the electron-stimulated desorption of lithium ions is close to the ionization energy of the Li 1s level. The secondary thresholds are observed at energies of about 130 and 150 eV. The threshold at an energy of 130 eV is approximately 30 eV higher than the ionization energy of the Si 2p level and can be associated with the double ionization. The threshold at 150 eV can be caused by the ionization of the Si 2s level. It is demonstrated that the yield of Li⁺ ions does not correlate with the silicon amount in near-the-surface region of the tantalum ribbon and drastically increases at high annealing temperatures. The dependence of the current of Li⁺ desorption on the lithium concentration upon annealing of the tantalum ribbon at T>1800 K exhibits two maxima. The ions desorbed by electrons with energies higher than 130 and 150 eV make the largest contribution to the current of lithium ions after the annealing. The yield of lithium ions upon ionization of the Li 1s level at an energy of 55 eV is considerably lesser, but it is observed at higher concentrations of deposited lithium. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model with allowance made for relaxation of the local surface field.     

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.     

Microplasma Technology and Its Applications in Analytical Chemistry

Abstract

This review article describes some existing microplasma sources and their applications in analytical chemistry. These microplasmas mainly include direct current glow discharge (DC), microhollow-cathode discharge (MHCD) or microstructure electrode (MSE), dielectric barrier discharge (DBD), capacitively coupled microplasmas (CCμPs), miniature inductively coupled plasmas (mICPs), and microwave-induced plasmas (MIPs). The historical development and recent advances in these microplasma techniques are presented. Fundamental properties of the microplasmas, the unique features of the reduced size and volume, as well as the advantageous device structures for chemical analysis are discussed in detail, with the emphasis toward detection of gaseous samples. The analytical figures of merit obtained using these microplasmas as molecular/elemental-selective detectors for emission spectrometry and as ionization sources for mass spectrometry are also given in this review article.     

Molecular beam surface ionization detection: II. Field reversal and surface ionization study of Na on Re with adsorbed oxygen

The desorption and surface ionization of Na on a polycrystalline Re surface with various amounts of adsorbed oxygen have been studied by field reversal, surface ionization and thermoelectronic emission methods. In this work the unique properties of the field reversal method are taken advantage of, i.e. that both neutral and ionic desorption rate constants can be determined simultaneously. Absolute ionization coefficients have been measured by field reversal and have been compared with values found by the “oxygen coverage” method and by static surface ionization. The application to beam flux density determinations is discussed. The simultaneous variation of the neutral and ionic desorption rate constants during oxygen adsorption and the temperature dependence of them have been studied. The Re surface in 2 × 10^?8 Torr of oxygen and at 1300–1500 K is shown to be very stable and to behave differently than in studies at higher temperatures. The very rapid change in both desorption rate constants at an effective work function Φ^e = 5.35 V is here correlated with the results of LEED experiments (Gorodetskii and Knysh) and is proposed to indicate a change from a stable Re oxide surface at low Φ^e (and oxygen coverage) to a different surface structure at higher Φ^e. Desorption energies have been determined at various values of Φ^e. The neutral desorption energy at low oxygen pressure is 2.70 ± 0.06 eV, which agress well with earlier, here corrected modulated beam results. The energy (Schottky) cycle for surface ionization is shown to be closed at low Φ^e, which has been difficult to show with other methods in any other case.     

Field ion and field desorption mass spectrometry of inorganic compounds

The review on recent developments in field ionization mass spectrometry of inorganic compounds concerns the different mechanisms of ion formation at surfaces and under the influence of extremely high electric fields, field dependent chemical reactivity at surfaces and ion desorption from surfaces for mass spectrometric analysis. Applications in various areas are discussed, where this method has been used to identify surface compounds or to study kinetic phenomena at interfaces.