CO2 laser ionization of acoustically levitated droplets

For many analytical purposes, direct laser ionization of liquids is desirable. Several studies on supported droplets, free liquid jets, and ballistically dispensed microdroplets have been conducted, yet detailed knowledge of the underlying mechanistics in ion formation is still missing. This contribution introduces a simple combination of IR-MALDI mass spectrometry and an acoustical levitation device for contactless confinement of the liquid sample. The homebuilt ultrasonic levitator supports droplets of several millimeters in diameter. These droplets are vaporized by a carbon dioxide laser in the vicinity of the atmospheric pressure interface of a time of flight mass spectrometer. The evaporation process is studied by high repetition rate shadowgraphy experiments elucidating the ballistic evaporation of the sample and revealing strong confinement of the vapor by the ultrasonic field of the trap. Finally, typical mass spectra for pure glycerol/water matrix and lysine as an analyte are presented with and without the addition of trifluoracetic acid, and the ionization mechanism is briefly discussed. The technique is a promising candidate for a reproducible mass spectrometric detection scheme for the field of microfluidics.

Development of liquid chromatography atmospheric pressure chemical ionization tandem mass spectrometry for analysis of halogenated flame retardants in wastewater

Until recently, atmospheric pressure photoionization (APPI) has typically been used for the determination of non-polar halogenated flame retardants (HFRs) by liquid chromatography (LC) tandem mass spectrometry. In this study, we demonstrated the feasibility of utilizing liquid chromatography atmospheric pressure chemical ionization (APCI) tandem mass spectrometry (LC-APCI-MS/MS) for analysis of 38 HFRs. This developed method offered three advantages: simplicity, rapidity, and high sensitivity. Compared with APPI, APCI does not require a UV lamp and a dopant reagent to assist atmospheric pressure ionization. All the isomers and the isobaric compounds were well resolved within 14-min LC separation time. Excellent instrument detection limits (6.1 pg on average with 2.0 μL injection) were observed. The APCI mechanism was also investigated. The method developed has been applied to the screening of wastewater samples for screening purpose, with concentrations determined by LC-APCI-MS/MS agreeing with data obtained via gas chromatography high resolution mass spectrometry.

Capillary Atmospheric Pressure Electron Capture Ionization (cAPECI): A Highly Efficient Ionization Method for Nitroaromatic Compounds

We report on a novel method for atmospheric pressure ionization of compounds with elevated electron affinity (e.g., nitroaromatic compounds) or gas phase acidity (e.g., phenols), respectively. The method is based on the generation of thermal electrons by the photo-electric effect, followed by electron capture of oxygen when air is the gas matrix yielding O₂ ^– or of the analyte directly with nitrogen as matrix. Charge transfer or proton abstraction by O₂ ^– leads to the ionization of the analytes. The interaction of UV-light with metals is a clean method for the generation of thermal electrons at atmospheric pressure. Furthermore, only negative ions are generated and neutral radical formation is minimized, in contrast to discharge- or dopant assisted methods. Ionization takes place inside the transfer capillary of the mass spectrometer leading to comparably short transfer times of ions to the high vacuum region of the mass spectrometer. This strongly reduces ion transformation processes, resulting in mass spectra that more closely relate to the neutral analyte distribution. cAPECI is thus a soft and selective ionization method with detection limits in the pptV range. In comparison to standard ionization methods (e.g., PTR), cAPECI is superior with respect to both selectivity and achievable detection limits. cAPECI demonstrates to be a promising ionization method for applications in relevant fields as, for example, explosives detection and atmospheric chemistry.

Soft Ionization of Saturated Hydrocarbons, Alcohols and Nonpolar Compounds by Negative-Ion Direct Analysis in Real-Time Mass Spectrometry

Large polarizable n-alkanes (approximately C18 and larger), alcohols, and other nonpolar compounds can be detected as negative ions when sample solutions are injected directly into the sampling orifice of the atmospheric pressure interface of the time-of-flight mass spectrometer with the direct analysis in real time (DART) ion source operating in negative-ion mode. The mass spectra are dominated by peaks corresponding to [M + O₂]￣^?. No fragmentation is observed, making this a very soft ionization technique for samples that are otherwise difficult to analyze by DART. Detection limits for cholesterol were determined to be in the low nanogram range.

High-resolution atmospheric pressure infrared laser desorption/ionization mass spectrometry imaging of biological tissue

An atmospheric pressure laser desorption/ionization mass spectrometry imaging ion source has been developed that combines high spatial resolution and high mass resolution for the in situ analysis of biological tissue. The system is based on an infrared laser system working at 2.94 to 3.10 μm wavelength, employing a Nd:YAG laser-pumped optical parametrical oscillator. A Raman-shifted Nd:YAG laser system was also tested as an alternative irradiation source. A dedicated optical setup was used to focus the laser beam, coaxially with the ion optical axis and normal to the sample surface, to a spot size of 30 μm in diameter. No additional matrix was needed for laser desorption/ionization. A cooling stage was developed to reduce evaporation of physiological cell water. Ions were formed under atmospheric pressure and transferred by an extended heated capillary into the atmospheric pressure inlet of an orbital trapping mass spectrometer. Various phospholipid compounds were detected, identified, and imaged at a pixel resolution of up to 25 μm from mouse brain tissue sections. Mass accuracies of better than 2 ppm and a mass resolution of 30,000 at m/z?=?400 were achieved for these measurements.

Determination of rhenium and osmium complexes by surface-assisted laser desorption/ionization coupled to Orbitrap mass analyzer

A surface-assisted laser desorption/ionization (SALDI) source is coupled to the Orbitrap mass analyzer; the instrumental approach is tested for the analysis of rhenium (Re) and osmium (Os) complexes with 8-mercaptoquinoline. Silicon (Si) material obtained by laser treatment of monocrystalline Si is used as SALDI substrate. All studied complexes are detected as radical cations, with no protonated molecules. The comparison of SALDI, matrix-assisted laser desorption/ionization (MALDI), and direct laser desorption/ionization (LDI) on metal plates in the same instrumental setup demonstrated that the detection of the studied complexes using SALDI provides the highest sensitivity. The ability to analyze samples rapidly, high purity of spectra, and good analytical parameters make SALDI coupled to the Orbitrap mass analyzer a potentially powerful tool for the detection of Re and Os complexes and related organic, UV-absorbing compounds.

Resonance-enhanced multiphoton ionization with circularly polarized light: chiral carbonyls

An introduction to the principle and possibilities of the new method of circular dichroism laser mass spectrometry is given and its state of development is reviewed. This method allows enantiosensitive, mass-selective probing of chiral molecules. It is based on the combination of resonance-enhanced multiphoton ionization with circularly polarized light and specially modified time-of-flight mass spectrometry. As an example, application to carbonyls is presented.

Simultaneous sampling of volatile and non-volatile analytes in beer for fast fingerprinting by extractive electrospray ionization mass spectrometry

By gently bubbling nitrogen gas through beer, an effervescent beverage, both volatile and non-volatile compounds can be simultaneously sampled in the form of aerosol. This allows for fast (within seconds) fingerprinting by extractive electrospray ionization mass spectrometry (EESI-MS) in both negative and positive ion mode, without the need for any sample pre-treatment such as degassing and dilution. Trace analytes such as volatile esters (e.g., ethyl acetate and isoamyl acetate), free fatty acids (e.g., caproic acid, caprylic acid, and capric acid), semi/non-volatile organic/inorganic acids (e.g., lactic acid), and various amino acids, commonly present in beer at the low parts per million or at sub-ppm levels, were detected and identified based on tandem MS data. Furthermore, the appearance of solvent cluster ions in the mass spectra gives insight into the sampling and ionization mechanisms: aerosol droplets containing semi/non-volatile substances are thought to be generated via bubble bursting at the surface of the liquid; these neutral aerosol droplets then collide with the charged primary electrospray ionization droplets, followed by analyte extraction, desolvation, ionization, and MS detection. With principal component analysis, several beer samples were successfully differentiated. Therefore, the present study successfully extends the applicability of EESI-MS to the direct analysis of complex liquid samples with high gas content.

Rapid comprehensive characterization of crude oils by thermogravimetry coupled to fast modulated gas chromatography–single photon ionization time-of-flight mass spectrometry

Comprehensive multi-dimensional hyphenation of a thermogravimetry device (i.e. a thermobalance) to gas chromatography and single photon ionization–time-of-flight mass spectrometry (TG–GC×SPI–MS) has been used to investigate two crude oil samples of different geographical origin. The source of the applied vacuum ultraviolet radiation is an electron beam pumped rare gas excimer lamp (EBEL). The soft photoionization favors the formation of molecular ions. Introduction of a fast, rapidly modulated gas chromatographic separation step in comparison with solely TG–SPI–MS enables strongly enhanced detection especially with such highly complex organic matrices as crude oil. In contrast with former TG–SPI–MS measurements, separation and identification of overlying substances is possible because of different GC retention times. The specific contribution of isobaric compounds to one mass signal is determined for alkanes, naphthalenes, alkylated benzenes, and other compounds.

A New Matrix Assisted Ionization Method for the Analysis of Volatile and Nonvolatile Compounds by Atmospheric Probe Mass Spectrometry

Matrix assisted ionization of nonvolatile compounds is shown not to be limited to vacuum conditions and does not require a laser. Simply placing a solution of analyte dissolved with a suitable matrix such as 3-nitrobenzonitrile (3-NBN) or 2,5-dihydroxyacetophenone on a melting point tube and gently heating the dried sample near the ion entrance aperture of a mass spectrometer using a flow of gas produces abundant ions of peptides, small proteins, drugs, and polar lipids. Fundamental studies point to matrix-mediated ionization occurring prior to the entrance aperture of the mass spectrometer. The method is analytically useful, producing peptide mass fingerprints of bovine serum albumin tryptic digest consuming sub-picomoles of sample. Application of 100 fmol of angiotensin I in 3-NBN matrix produces the doubly and triply protonated molecular ions as the most abundant peaks in the mass spectrum. No carryover is observed for samples containing up to 100 pmol of this peptide. A commercial atmospheric samples analysis probe provides a simple method for sample introduction to an atmospheric pressure ion source for analysis of volatile and nonvolatile compounds without using the corona discharge but using sample preparation similar to matrix-assisted laser desorption/ionization.

Analysis of Biomolecules by Atmospheric Pressure Visible-Wavelength MALDI-Ion Trap-MS in Transmission Geometry

We report the development of a new AP visible-wavelength MALDI-ion trap-MS instrument with significantly improved performance over our previously reported system (Int. J. Mass Spectrom. 315, 66–73 (2012)). A Nd:YAG pulsed laser emitting light at 532 nm was used to desorb and ionize oligosaccharides and peptides in transmission geometry through a glass slide. Limits of detection (LODs) achieved in MS mode correspond to picomole quantities of oligosaccharides and femtomole quantities of peptides. Tandem MS (MS/MS) experiments enabled identification of enzymatically digested proteins and oligosaccharides by comparison of MS/MS spectra with data found in protein and glycan databases. Moreover, the softness of ionization, LODs, and fragmentation spectra of biomolecules by AP visible-wavelength MALDI-MS were compared to those obtained by AP UV MALDI-MS using a Nd:YAG laser emitting light at 355 nm. AP visible-wavelength MALDI appears to be a softer ionization technique then AP UV MALDI for the analysis of sulfated peptides, while visible-wavelength MALDI-MS, MS/MS, and MS/MS/MS spectra of other biomolecules analyzed were mostly similar to those obtained by AP UV MALDI-MS. Therefore, the methodology presented will be useful for MS and MSⁿ analyses of biomolecules at atmospheric pressure. Additionally, the AP visible-wavelength MALDI developed can be readily used for soft ionization of analytes on various mass spectrometers.

Atmospheric Pressure Photo Ionization Hydrogen/Deuterium Exchange Mass Spectrometry—a Method to Differentiate Isomers by Mass Spectrometry

In this report, a method for in-source hydrogen/deuterium (H/D) exchange at atmospheric pressure is reported. The method was named atmospheric pressure photo ionization hydrogen/deuterium exchange mass spectrometry (APPI HDX MS). H/D exchange was performed by mixing samples dissolved in toluene with CH₃OD solvent and analyzing the mixture using atmospheric pressure photo ionization mass spectrometry (APPI-MS). The APPI HDX spectra obtained with contact times between the analyte solution and methanol-OD (CH₃OD) of?<?0.5 s or 1 h showed the same pattern of H/D exchange. Therefore, it was concluded that APPI HDX occurred in the source but not in the solution. The proposed method does not require a specific type of mass spectrometer and can be performed at atmospheric pressure. H/D exchange can be performed in any laboratory with a mass spectrometer and a commercial APPI source. Using this method, multiple H/D exchanges of aromatic hydrogen and/or H/D exchange of active hydrogen were observed. These results demonstrated that H/D exchange can be used to distinguish between isomers containing primary, secondary, and tertiary amines, as well as pyridine and pyrrole functional groups.

Laser desorption VUV postionization MS imaging of a cocultured biofilm

Laser desorption postionization mass spectrometry (LDPI-MS) imaging is demonstrated with a 10.5 eV photon energy source for analysis and imaging of small endogenous molecules within intact biofilms. Biofilm consortia comprised of a synthetic Escherichia coli K12 coculture engineered for syntrophic metabolite exchange are grown on membranes and then used to test LDPI-MS analysis and imaging. Both E. coli strains displayed many similar peaks in LDPI-MS up to m/z 650, although some observed differences in peak intensities were consistent with the appearance of byproducts preferentially expressed by one strain. The relatively low mass resolution and accuracy of this specific LDPI-MS instrument prevented definitive assignment of species to peaks, but strategies are discussed to overcome this shortcoming. The results are also discussed in terms of desorption and ionization issues related to the use of 10.5 eV single-photon ionization, with control experiments providing additional mechanistic information. Finally, 10.5 eV LDPI-MS was able to collect ion images from intact, electrically insulating biofilms at ～100 μm spatial resolution. Spatial resolution of ～20 μm was possible, although a relatively long acquisition time resulted from the 10 Hz repetition rate of the single-photon ionization source.

Are Clusters Important in Understanding the Mechanisms in Atmospheric Pressure Ionization? Part 1: Reagent Ion Generation and Chemical Control of Ion Populations

It is well documented since the early days of the development of atmospheric pressure ionization methods, which operate in the gas phase, that cluster ions are ubiquitous. This holds true for atmospheric pressure chemical ionization, as well as for more recent techniques, such as atmospheric pressure photoionization, direct analysis in real time, and many more. In fact, it is well established that cluster ions are the primary carriers of the net charge generated. Nevertheless, cluster ion chemistry has only been sporadically included in the numerous proposed ionization mechanisms leading to charged target analytes, which are often protonated molecules. This paper series, consisting of two parts, attempts to highlight the role of cluster ion chemistry with regard to the generation of analyte ions. In addition, the impact of the changing reaction matrix and the non-thermal collisions of ions en route from the atmospheric pressure ion source to the high vacuum analyzer region are discussed. This work addresses such issues as extent of protonation versus deuteration, the extent of analyte fragmentation, as well as highly variable ionization efficiencies, among others. In Part 1, the nature of the reagent ion generation is examined, as well as the extent of thermodynamic versus kinetic control of the resulting ion population entering the analyzer region.

Direct probe atmospheric pressure photoionization/atmospheric pressure chemical ionization high-resolution mass spectrometry for fast screening of flame retardants and plasticizers in products and waste

In this study, we develop fast screening methods for flame retardants and plasticizers in products and waste based on direct probe (DP) atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization (APCI) coupled to a high-resolution (HR) time-of-flight mass spectrometer. DP-APPI is reported for the first time in this study, and DP-APCI that has been scarcely exploited is optimized for comparison. DP-APPI was more selective than DP-APCI and also more sensitive for the most hydrophobic compounds. No sample treatment was necessary, and only a minimal amount of sample (few milligrams) was used for analysis that was performed within a few minutes. Both methods were applied to the analysis of plastic products, electronic waste, and car interiors. Polybrominated diphenylethers, new brominated flame retardants, and organophosphorus flame retardants were present in most of the samples. The combination of DP with HR mass spectra and data processing based on mass accuracy and isotopic patterns allowed the unambiguous identification of chemicals at low levels of about 0.025 % (w/w). Under untargeted screening, resorcinol bis(biphenylphosphate) and bisphenol A bis(bisphenylphosphate) were identified in many of the consumer products of which literature data are still very limited.

Characterization of an Electron Ionization Source Trap Operating in the Presence of a Magnetic Field Through Computer Simulation

We explore the feasibility of conducting electron ionization (EI) in a radio-frequency (rf) ion source trap for mass spectrometry applications. Electrons are radially injected into a compact linear ion trap in the presence of a magnetic field used essentially to lengthen the path of the electrons in the trap. The device can either be used as a stand-alone mass spectrometer or can be coupled to a mass analyzer. The applied parallel magnetic field and the oscillating rf electric field produced by the trap give rise to a set of coupled Mathieu equations of motion. Via numerical simulations, electron trajectories are studied under varying intensities of the magnetic field in order to determine the conditions that enhance ion production. Likewise, the dynamic behavior of the ions are investigated in the proposed EI source trap and the fast Fourier transform FFT formalism is used to obtain the frequency spectrum from the numerical simulations to study the motional frequencies of the ions which include combinations of the low-frequency secular and the high-frequency micromotion with magnetron and cyclotron frequencies. The dependence of these motional frequencies on the trapping conditions is examined and particularly, the limits of applying a radial magnetic field to the EI ion trap are characterized.

Gas chromatography/multiphoton ionization/time-of-flight mass spectrometry using a femtosecond laser

A laser can be used for the selective excitation and subsequent ionization of a molecule with an absorption band at the laser wavelength. This technique of multiphoton ionization (MPI), when combined with time-of-flight mass spectrometry (TOF-MS), permits the efficient detection of induced ions in mass analysis. This combination of MPI/TOF-MS can be coupled with gas chromatography (GC) to achieve even more enhanced selectivity. Thus, GC/MPI/TOF-MS can be employed for trace analysis of samples containing numerous chemical species. A variety of laser sources have been used for this purpose. Since molecules that are classified as environmental pollutants frequently contain chlorine and bromine atoms, the lifetime of the excited state can be decreased by energy transfer from the singlet to triplet levels by spin–orbit interaction. A high-power femtosecond laser with a pulse width shorter than the lifetime of the analyte molecule provides femtogram or even subfemtogram detection limits, which have not yet been achieved using the most sensitive high-resolution double-focus sector-type mass spectrometers. Numerous environmental pollutants such as dioxins in soils and pesticides in foods have been successfully quantified using GC/MPI/TOF-MS, and this technique has proven itself to be a useful and practical method for trace analysis.

Impact of homogeneous and filamentary discharge modes on the efficiency of dielectric barrier discharge ionization mass spectrometry

The present study contributes to the evaluation of dielectric barrier discharge-based ambient ionization for mass spectrometric analysis (DBDI-MS) by providing a further step towards an understanding of underlying ionization processes. This examination highlights the effect of physical discharge modes on the ionization efficiency of the DBDI source. A distinction is made between the homogeneous and filamentary discharge mode due to different plasma gases in barrier configurations. Therefore, we first report on discharge modes of DBDI by demonstrating a universally applicable method to classify the predominant modes. Then, the ionization efficiency of these two modes is evaluated by a laser desorption-DBDI-MS with different molecular analytes. Here, the laser desorption is used to deliver neutral analytes which will be ionized by the plasma jet applied as dielectric barrier discharge ionization. With a clear increase of signal intensities in the homogeneous mode in contrast to the filamentary one, the present study indicates a pronounced dependence of the ionization efficiency on the discharge mode allowing further insight into the mechanisms of the ionization process.

Quantification of Saquinavir from Lysates of Peripheral Blood Mononuclear Cells Using Microarrays and Standard MALDI-TOF-MS

Drug monitoring is usually performed by liquid chromatography coupled with optical detection or electrospray ionization mass spectrometry. More recently, matrix-assisted laser desorption/ionization (MALDI) in combination with triple quadrupole or Fourier-transform (FT) mass analyzers has also been reported to allow accurate quantification. Here, we present a strategy that employs standard MALDI time-of-flight (TOF) mass spectrometry (MS) for the sensitive and accurate quantification of saquinavir from an extract of blood peripheral mononuclear cells. Unambiguous identification of saquinavir in the mass spectra was possible because of using internal mass calibration and by an overall low chemical noise in the low mass range. Exact mass determination of the constant background peaks of the cell extract, which were used for recalibration, was performed by an initial MALDI-FT-MS analysis. Fast and multiplexed sample analysis was enabled by microarray technology, which provided 10 replicates in the lower nL range for each sample in parallel lanes on a chip. In order to validate the method, we employed various statistical tests, such as confidence intervals for linear regressions, three quality control samples, and inverse confidence limits of the estimated concentration ratios.

Sensitive and simplified analysis of natural and synthetic steroids in water and solids using on-line solid-phase extraction and microwave-assisted solvent extraction coupled to liquid chromatography tandem mass spectrometry atmospheric pressure photoionization

Analytical methods for the determination of several natural and synthetic steroids in water and solid samples were developed using liquid chromatography tandem mass spectrometry. Atmospheric pressure photoionization (APPI) in positive mode was used as the source which appeared to reduce background and interferences as compared to electrospray ionization. Toluene was identified as the most appropriate dopant and its concentration optimized to enhance ionization efficiency. Method detection limits were in the low-nanogram-per-liter range for water samples using on-line solid-phase extraction and near 0.2 ng/g using microwave-assisted solvent extraction for solid samples. This on-line method, using APPI as a source and toluene as a dopant, is beneficial as it saves time due to on-line extraction and reduces interference and matrix effects using gas-phase photoionization. Groundwater, wastewater, runoff, and soil samples were collected and analyzed using the method indicating that traces of several androgens, estrogens, and other steroid-like compounds do occur in samples impacted by animal waste.