Charge assisted laser desorption/ionization mass spectrometry of droplets

We propose and evaluate a new mechanism to account for analyte ion signal enhancement in ultraviolet-laser desorption mass spectrometry of droplets in the presence of corona ions. Our new insights are based on timing control of corona ion production, laser desorption, and peptide ion extraction achieved by a novel pulsed corona apparatus. We demonstrate that droplet charging rather than gas-phase ion-neutral reactions is the major contributor to analyte ion generation from an electrically isolated droplet. Implications of the new mechanism, termed charge assisted laser desorption/ionization (CALDI), are discussed and contrasted with those of the laser desorption atmospheric pressure chemical ionization method (LD-APCI). It is also demonstrated that analyte ion generation in CALDI occurs with external electric fields about one order of magnitude lower than those needed for atmospheric pressure matrix assisted laser desorption/ionization or electrospray ionization of droplets.     

Technological Advancements in Mass Spectrometry and Its Impact on Proteomics

Amalgamation of mass spectrometry (MS) and proteomics has led to the most awaited technological inventions such as discovery of clinically potential biomarkers and generation of effective drugs. This review focuses on the synergistic growth in MS instrumentation, proteomics and its impact on biomedical sciences. Novel ionization methods: surface enhanced laser desorption ionization, electrospray assisted laser desorption ionization, desorption electrospray ionization, laser diode thermal desorption are discussed. Different mass analyzers: ion trap, time-of-flight, Fourier transform ion cyclotron resonance and their applications are outlined. New ion fragmentation techniques: electron capture dissociation, electron transfer dissociation, infrared multiphoton dissociation and their attributes are described.     

Laser desorption lamp ionization source for ion trap mass spectrometry

A two‐step laser desorption lamp ionization source coupled to an ion trap mass spectrometer (LDLI‐ITMS) has been constructed and characterized. The pulsed infrared (IR) output of an Nd:YAG laser (1064 nm) is directed to a target inside a chamber evacuated to ~15 Pa causing desorption of molecules from the target's surface. The desorbed molecules are ionized by a vacuum ultraviolet (VUV) lamp (filled with xenon, major wavelength at 148 nm). The resulting ions are stored and detected in a three‐dimensional quadrupole ion trap modified from a Finnigan Mat LCQ mass spectrometer operated at a pressure of ≥ 0.004 Pa. The limit of detection for desorbed coronene molecules is 1.5 pmol, which is about two orders of magnitude more sensitive than laser desorption laser ionization mass spectrometry using a fluorine excimer laser (157 nm) as the ionization source. The mass spectrum of four standard aromatic compounds (pyrene, coronene, rubrene and 1,4,8,11,15,18,22,25‐octabutoxy‐29H,31H‐phthalocyanine (OPC)) shows that parent ions dominate. By increasing the infrared laser power, this instrument is capable of detecting inorganic compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

Pulse timing and optical interface between a neodymium: yttrium aluminum garnet laser and a Fourier transform ion cyclotron resonance mass spectrometer.

Laser desorption/ionization combined with pulsed (time-of-flight or Fourier transform ion cyclotron resonance) mass spectrometric detection is a powerful technique for analysis of involatile compounds and mixtures. Such experiments were originally conducted with pulsed CO2 lasers. Although a pulsed CO2 laser can be operated in single-shot mode, Nd: YAG lasers perform best with multiple flashes for warm-up before the final Q-switch output light pulse, thus creating the need to synchronize the desired final laser-output pulse with the event sequence for mass spectrometric analysis. In this paper, we describe a new and simple interface (both optical and electronic components) between a Continuum (formerly Quantel) Model YG 660A Nd:YAG laser and an Extrel FTMS-2000 mass spectrometer. The optics are modified from a prior pulsed CO2 laser interface from Extrel. Synchronization between the Nd:YAG laser and the mass spectrometer event sequence is achieved by means of a simple timing circuit that uses an inexpensive pulsing device and is triggered by pulses generated directly from the Extrel 1280 data system and cell controller, in contrast to the only prior published method that required an auxiliary microcomputer. The present interface method is highly flexible, and makes possible complex sequence events involving laser pulses for e.g.: desorption/ionization of solids; photoionization of gaseous neutrals; and photodissociation and photodetachment of gaseous ions.     

Analysis of native milk oligosaccharides directly from thin-layer chromatography plates by matrix-assisted laser desorption/ionization orthogonal-time-of-flight mass spectrometry with a glycerol matrix

We have recently presented a new method for direct coupling of high-performance thin-layer chromatography (HPTLC) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), illustrated by the analysis of a complex ganglioside mixture. In the current communication, an adaptation of this procedure to mixtures of native oligosaccharides from human and from elephant milk is described. The key features in this method are (1) glycerol as a liquid matrix, to provide a homogeneous wetting of the silica gel and a simple and fast MALDI preparation protocol, (2) an infrared (IR) laser for volume material ablation and particular soft desorption/ionization conditions, and (3) an orthogonal time-of-flight mass spectrometer for a high mass accuracy, independent of any irregularity of the silica gel surface. Chromatographic "mobility profiles" were determined by scanning the laser beam across the analyte bands. The current limit of detection for the MS analysis was determined to approximately 10 pmol of individual oligosaccharides spotted for chromatography. A liquid composite matrix, containing glycerol and the ultraviolet (UV-)MALDI matrix alpha-cyano-4-hydroxycinnamic acid, allows a direct HPTLC-MALDI-MS analysis with a 337 nm-UV laser as well. Compared to the IR-MALDI mode, the analytical sensitivity in UV-MALDI was found to be lower by one order of magnitude, whereas unspecific analyte ion fragmentation as well as adduct formation was found to be more extensive.     

The use of desorption/ionization on porous silicon mass spectrometry for the detection of negative ions for fatty acids

The study of low molecular weight compounds by matrix-assisted laser desorption/ionization (MALDI) is difficult because of the presence of ions originating from the matrix in the low-m/z range. In order to resolve these problems, new matrix-free approaches were developed based on laser desorption/ionization from the surface of various materials such as graphite and porous silicon. Our work involves the use of 'desorption ionization on porous silicon mass spectrometry' (DIOS-MS) in the negative ion mode to study fatty acid compounds. The potential of the DIOS-MS technique is shown and an insight into the ionization mechanism provided.     

A high voltage RF oscillator for driving multipole ion guides

A high voltage RF oscillator circuit has been designed and constructed for driving multipole ion guides. The circuit is tunable from 500 kHz to 1.5 MHz by changing a capacitor and provides 0–1000 V_p-p that is controlled by a 0–10 V input using a negative feedback circuit. This inexpensive circuit uses a set of high voltage transistors oscillating in tandem and does not require tuning of the resonance drive frequency as the oscillator automatically resonates at the (LC)^−1/2 frequency. Matrix-assisted laser desorption/ionization-Fourier transform mass spectrometry (MALDI-FTMS) mass spectra were acquired using this tunable RF oscillator circuit to allow transmission of protein ions in the 8.5–39 kDa range through the quadrupole ion guide from the ion source to the mass analyzer.     

基质与分析物的摩尔比和激光能量对分析物离子信号的影响

A method of aerosol introduction for matrix-assisted laser desorption/ionization (MALDI) is described. The aerosol particles containing matrix and analyte enter directly into the aerosol time-of-flight mass spectrometer (ATOFMS) at atmospheric pressure. The scattered light signals from the aerosol particles are collected by a photomultiplier tube (PMT) and are passed on to an external electronic timing circuit, which determines particle size and is used to trigger a 266 nm pulsed Nd:YAG laser. The aerosol MALDI mass spectra and aerodynamic diameter of single particles can be obtained in real-time. Compared with other methods of liquid sample introduction, this method realizes detection of single particles and, more importantly, the sample consumption is lower. The effects of matrix-to-analyte ratio and laser pulse energy on analyte ion yield are examined. The optimal matrix-to-analyte ratio and laser energy are 50-110:1 and 200-400 μJ respectively.     

Liquid matrix deposition on conductive hydrophobic surfaces for tuning and quantitation in UV-MALDI mass spectrometry

With its highly fluctuating ion production matrix-assisted laser desorption/ionization (MALDI) poses many practical challenges for its application in mass spectrometry. Instrument tuning and quantitative ion abundance measurements using ion signal alone depend on a stable ion beam. Liquid MALDI matrices have been shown to be a promising alternative to the commonly used solid matrices. Their application in areas where a stable ion current is essential has been discussed but only limited data have been provided to demonstrate their practical use and advantages in the formation of stable MALDI ion beams. In this article we present experimental data showing high MALDI ion beam stability over more than two orders of magnitude at high analytical sensitivity (low femtomole amount prepared) for quantitative peptide abundance measurements and instrument tuning in a MALDI Q-TOF mass spectrometer. Samples were deposited on an inexpensive conductive hydrophobic surface and shrunk to droplets <10 nL in size. By using a sample droplet <10 nL it was possible to acquire data from a single irradiated spot for roughly 10,000 shots with little variation in ion signal intensity at a laser repetition rate of 5-20 Hz.     

Combined chemical and topographic imaging at atmospheric pressure via microprobe laser desorption/ionization mass spectrometry–atomic force microscopy

The operational characteristics and imaging performance are described for a new instrument comprising an atomic force microscope coupled with a pulsed laser and a linear ion trap mass spectrometer. The operating mode of the atomic force microscope is used to produce topographic surface images having sub‐micrometer spatial and height resolution. Spatially resolved mass spectra of ions, produced from the same surface via microprobe‐mode laser desorption/ionization at atmospheric pressure, are also used to create a 100 × 100 µm chemical image. The effective spatial resolution of the image (～2 µm) was constrained by the limit of detection (estimated to be 10⁹–10¹⁰ molecules) rather than by the diameter of the focused laser spot or the step size of the sample stage. The instrument has the potential to be particularly useful for surface analysis scenarios in which chemical analysis of targeted topographic features is desired; consequently, it should have extensive application in a number of scientific areas. Because the number density of desorbed neutral species in laser desorption/ionization is known to be orders‐of‐magnitude greater than that of ions, it is expected that improvements in imaging performance can be realized by implementation of post‐ionization methods. Published in 2009 by John Wiley & Sons, Ltd.     

Visible-laser desorption/ionization on gold nanostructures

In this report, we describe the visible-laser desorption/ionization of biomolecules deposited on gold-coated porous silicon and gold nanorod arrays. The porous silicon made by electrochemical etching was coated with gold using argon ion sputtering. The gold nanorod arrays were fabricated by electrodepositing gold onto a porous alumina template, and the subsequent partial removal of the alumina template. A frequency-doubled/tripled Nd : YAG laser was used to irradiate the gold nanostructured substrate, and the desorbed molecular ions were mass-analyzed by a time-of-flight mass spectrometer. The desorption/ionization of biomolecules for both substrates was favored by the use of the 532-nm visible-laser, which is in the range of the localized surface plasmon resonance of the gold nanostructure. The present technique offers a potential analytical method for low-molecular-weight analytes that are rather difficult to handle in the conventional matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.     

Laser spray: electric field-assisted matrix-assisted laser desorption/ionization

A new liquid chromatography/mass spectrometry interface, the laser spray, has been developed. Explosive vaporization and mist formation occur when an aqueous solution effusing out from the tip of the stainless-steel capillary is irradiated from the opposite side of the capillary by a 10.6 microm infrared laser. Weak ion signals could be detected when the plume was sampled through the ion sampling orifice. When a high voltage (3-4 kV) was applied to the stainless-steel capillary, strong ion signals appeared. The ion abundances were found to be orders of magnitude greater than those obtained by conventional electrospray ionization in the case of aqueous solutions. The present method is regarded as an electric-field assisted form of matrix-assisted laser desorption/ionization in which the liquid chromatographic solvent (water, etc.) acts as a liquid matrix. Laser spray ionization is expected to become a versatile method for biological mass spectrometry because this method is compatible with the natural solvent, water.     

Rapid analysis of polychlorinated biphenyls in the gas phase with resonance-enhanced two-photon ionization: optimal injection of ions into the ion-trap storage/time-of-flight mass spectrometer.

LI-IT-TOFMS (laser ionization/ion-trap storage/time-of-flight mass spectrometry) is expected to be a powerful tool for environmental monitoring. In the research reported here, real-time LI-IT-TOFMS measurements were carried out on gaseous 2-4 chlorinated PCBs in order to evaluate the applicability of an environmental monitoring method. With respect to ion-trap storage for PCBs, we found that the effect was due to the driving RF voltage on the ring electrode in the ion trap. For PCBs ions produced by laser irradiation, we observed that it was more efficient to reach the center of the ion trap by using a gated RF voltage rather than by using a continuous RF voltage. The ion trajectories in the ion trap were simulated by SIMION 7.0. We found that the voltage of the exit end cap electrode affected both the number of ions trapped and the orbit of ions inside the trap cell. Optimization of this parameter was performed using both simulated and experimental results. The achievable PCBs sensitivity for real-time (1 min) measurement using the LI-IT-TOFMS method was found to be in the pptV range (<0.01 mg/m3N) by means of a comparison with the conventional gas sampling/GS-MS method. A satisfactory proportional relationship was confirmed between the laser-based and conventional results.     

Benefits of 2.94 micron infrared matrix-assisted laser desorption/ionization for analysis of labile molecules by Fourier transform mass spectrometry

A 2.94 microm Er:YAG laser was used together with a commercial Fourier transform mass spectrometer to study labile biomolecules. The combination has shown superior performance over conventional 337 nm ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) Fourier transform mass spectrometry (FTMS), especially for the analysis of peptides with post-translational modifications. With succinic acid as a matrix, the sensitivity of the single-shot analysis was increased by an order of magnitude to the low femtomole level, with significantly less fragmentation observed. Intact molecular ions of a range of O-glycosylated and sulfated peptides were detected. Urea was found to induce even less fragmentation, although at the expense of the total ion yield. Molecular ions of a noncovalent complex (vancomycin + diacetyl-L-Lys-D-Ala-D-Ala) have been observed for the first time in MALDI-FTMS. 2.94 microm infrared (IR) MALDI also produced abundant molecular ions of a range of nonbiological samples, including C60 and C70 fullerenes as well as dimetal coordination complexes.     

Coupling laser ablation/desorption electrospray ionization to atmospheric pressure drift tube ion mobility spectrometry for the screening of antimalarial drug quality

Significant developments in the field of ambient desorption/ionization mass spectrometry (MS) have led to high-throughput direct analysis and imaging capabilities. However, advances in coupling ambient ionization techniques with standalone drift tube ion mobility spectrometry (DTIMS) have been comparatively slower, despite the attractive ruggedness and simplicity of IMS. In this study, we have developed and characterized a laser ablation/desorption electrospray ionization (LADESI) DTIMS platform, and applied it to the detection of active pharmaceutical ingredients (APIs) in antimalarial tablets collected in developing countries. The overarching goal of this work was to perform an initial evaluation of LADESI DTIMS as a technique with the potential for constituting the core of a portable drug quality-testing platform. The set-up consisted of an IR laser for desorption and an electrospray ionizer for capturing the ablated plume coupled to a high-resolution monolithic resistive glass drift tube ion mobility spectrometer. For more confident API identification, tablet extracts were also investigated via electrospray IM MS to correlate LADESI DTIMS reduced mobility (K(0)) values to m/z values. Overall, it was found that the IR LADESI DTIMS platform provided distinct ion mobility spectral fingerprints that could be used to detect the presence of the expected APIs, helping to distinguish counterfeit drugs from their genuine counterparts.     

Attachment of alkali cations on beta-D-glucopyranose: matrix-assisted laser desorption/ionization time-of-flight studies and ab initio calculations

In matrix-assisted laser desorption/ionization mass spectrometry, carbohydrates ionize by attachment of an alkali cation, and the ion yield varies with the nature of the cation. In an attempt to contribute to the understanding of the mechanisms involved, we have conducted matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) experiments on the simple glucose molecule with the alkali cations Li+, Na+ and K+, and have also performed ab initio calculations. The calculations show that, for the most stable carbohydrate-cation geometry, the carbohydrate ring is twisted and the cation is coordinated to four oxygen atoms. Calculations also show that in these complexes the positions of the three cations are very similar, and the smaller the cation, the closer it is to the oxygen atoms. Finally, the calculated formation enthalpies of the different complexes agree with the experimental results obtained for the order of stability of these complexes.     

High-resolution mass spectrometry of large molecules in a linear time-of-flight mass spectrometer

This report describes a new high-resolution linear time-of-flight mass spectrometer that has been constructed at this institute. The instrument is used for investigations of both direct and matrix-assisted laser desorption/ionization of large molecules. A unique feature of this new instrument is the incorporation of a 10-cm postsource pulse-focusing region for enhancing the resolution of the detected ion signals. This technique can correct for both high initial ion translational energies and long durations of ion formation and is expected to be particularly well suited for laser desorption/ionization applications. Results of calculations are presented to illustrate the gains in mass resolution that may be expected for a variety of ion formation conditions. In addition, initial experimental results are presented that demonstrate the capability of this new instrument to produce high-resolution ion signals. Signals with mass resolutions as high as 2750 (full width at half maximum) have been obtained for both direct and matrix-assisted laser desorption/ionization signals.     

Covariance mapping in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A novel method for acquisition and numerical analysis of matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectral data is described. The digitized ion current transient from each consecutive laser shot is first acquired and stored independently. Subsequently, statistical correlation parameters between all stored transients are computed. We illustrate the uses of this event-by-event analysis method for studies of sample surface heterogeneity as well as for elucidating the mechanisms of ion formation in MALDI. Other potential applications of the method are also outlined.     

Multiphoton ionization of molecules: A comparison between femtosecond and nanosecond laser pulse ionization efficiency

Multiphoton ionization mass spectra of nonvolatile molecules laser desorbed into a supersonic beam are recorded. It is shown by indirect measurements that the laser desorption of neutrals is not mass limited, but lead to the formation of neutrals with intesities large enough for intense signals. To investigate the efficiency of the multiphoton ionization process with varying laser pulse durations, simultaneous laser pulses of 500 fs and 5 ns or 100 fs and 5 ns have been applied to the neutral beam. The energies of both femtosecond and nanosecond laser pulses are held in a comparable magnitude, and thus produce, in the resulting ion intensity, very large differences up to 4 orders of magnitude. For larger evaporated molecules (> 500 u) the ionization efficiency from nanosecond laser pulses drops significantly in comparison to femtosecond laser pulse excitation. A variety of possible reasons for the different ionization and dissociation behavior in femtosecond and nanosecond laser pulse excitations are discussed in this paper. It is rationalized that even with very short laser pulses and large molecules the “ladder switching model” for ionization and fragmentation is valid.     

Two-step laser mass spectrometry of asphaltenes

Defined by their solubility in toluene and insolubility in n-heptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and most polar fraction of crude oil. Although asphaltenes are critically important to the exploitation of conventional oil and are poised to rise in significance along with the exploitation of heavy oil, even as fundamental a quantity as their molecular weight distribution is unknown to within an order of magnitude. Laser desorption/ionization (LDI) mass spectra vary greatly with experimental parameters so are difficult to interpret: some groups favor high laser pulse energy measurements (yielding heavy molecular weights), arguing that high pulse energy is required to detect the heaviest components of this mixture; other groups favor low pulse energy measurements (yielding light molecular weights), arguing that low pulse energy is required to avoid aggregation in the plasma plume. Here we report asphaltene mass spectra recorded with two-step laser mass spectrometry (L2MS), in which desorption and ionization are decoupled and no plasma is produced. L2MS mass spectra of asphaltenes are insensitive to laser pulse energy and other parameters, demonstrating that the asphaltene molecular weight distribution can be measured without limitation from insufficient laser pulse energy or plasma-phase aggregation. These data resolve the controversy from LDI, showing that the asphaltene molecular weight distribution peaks near 600 Da and previous measurements reporting much heavier species suffered from aggregation effects.