Matrix-assisted nanoelectrospray mass spectrometry for soft ionization of metal(i)-protein complexes.

Metal ions play significant roles in biological processes, and investigation of metal-protein interactions provides a basis to understand the functions of metal ions in such systems. In the current study, a novel matrix-assisted nanoelectrospray ionization mass spectrometry (MANESI-MS) method was developed for investigating the interactions between metal ions (i.e., Cu+) and protein molecules (i.e., myoglobin) using Cu nanoparticles as the matrix. The results demonstrated that the present method not only was an efficient strategy for the generation of various complexes with monovalent metal ions, such as Cu+, in which no redox transitions between Cu+ and Cu2+ were observed, but also allowed a softer ionization of the generated Cu+-myoglobin complexes compared to that of myoglobin molecules with conventional nanoESI. Several parameters (i.e., the mixing mode of the myoglobin sample and Cu nanoparticle solution, size of the Cu particle, oxidation state of the Cu species, and acidity of the myoglobin solution) were found to be crucial in determining the ionization efficiency of the MANESI method. First loading a Cu nanoparticle solution into the electrospray tip followed by a myoglobin solution resulted in a favorable interaction between the generated Cu+ ions and myoglobin molecules, in which a smaller size of the Cu particle and a lower oxidation state of the metal species (Cu > Cu2O > CuO) gave a lower average charge state and hence a softer ionization of the resulting Cu+-myoglobin complexes, possibly due to the reduced denaturing effects of the Cu+ complex. The MANESI method has also been successfully used to ionize the complexes between Cu+ and other biological molecules such as cytochrome c and angiotension II, although an exception was found for lysozymes, which show an increase in the charge state. Analogous to the study with Cu, a variety of other metal nanoparticles (Ni, Fe, W, Ag, Al, Zn and Co) were explored to study their interactions with myoglobin, but only Zn and Co could produce monovalent metal ions (i.e., Zn+ and Co+) followed by a favorable interaction with myoglobin, and a soft ionization of the resulting complexes.

Correction: Metal salt assisted electrospray ionization mass spectrometry for the soft ionization of GAP polymers in negative ion mode.

Correction for 'Metal salt assisted electrospray ionization mass spectrometry for the soft ionization of GAP polymers in negative ion mode' by Theoneste Muyizere et al., Analyst, 2020, 145, 34-45.

Metal salt assisted electrospray ionization mass spectrometry for the soft ionization of GAP polymers in negative ion mode.

Glycidyl azide polymers (GAP) are one of the most important energetic polymers, but it is still a challenge to elucidate their structures using mass spectrometry due to their fragility upon ionization. Herein we developed a soft metal salt assisted electrospray ionization (MSAESI) to characterize directly GAP polymers using mass spectrometry. This technique combines paper spray ionization and the complexing effect of anions from metal salts with GAP in the negative ion mode to softly ionize GAP polymers prior to mass spectrometry analysis. The effects of experimental parameters (e.g., ion mode, applied voltage, and type and concentration of metal salts) have been investigated in detail. In contrast to the positive ion mode, a softer ionization was observed for GAP polymers when the negative ion mode was applied. The radius and average charge of cations and anions in metal salts were found to play crucial roles in determining the performance of the MSAESI analysis of GAP. For a given charge number, a smaller radius of cations favored the soft ionization of GAP polymers (e.g., Na+ > K+ > Rb+), whereas a larger radius of anions led to a preferred performance (e.g., F- < Cl- < Br- < I-) due to variation in dissolution ability. For anions with multiple charges, the ones with fewer charges gave a more favorable ionization to the GAP sample because of their better complexing to GAP molecules than those with more charges in the structure of anions (e.g., NO3- > SO42- > PO43-). According to the experimental observation and evidence from mass spectrometry, we proposed the plausible electrospray mechanisms of MSAESI for GAP analysis with the involvement of metal salts. Moreover, the developed protocol has been applied successfully to the analysis of various GAP samples, and works for other types of sources such as nanoelectrospray ionization.

Elucidating the Reaction Mechanisms between Triazine and Hydrogen Sulfide with pH Variation Using Mass Spectrometry.

Triazine is one of the most economical and effective scavengers for hydrogen sulfide (H2S) removal, but the reaction mechanisms between triazine and H2S with pH variation in solution are still poorly understood. Herein, we show that the reaction process can be directly probed by means of paper spray mass spectrometry, in which an aprotic solvent (e.g., acetonitrile) is more favorable to the observation of reaction intermediates than a protic solvent (e.g., methanol), because of hydrogen bond interaction. Varying the pH of the reaction leads to completely different reaction pathways. With the pH in the range of 5.58 to 7.73, the major product was thiadiazine. With a pH of 3.02-3.69, thiadiazine is converted to 2-(5-(2-hydroxyethyl)-1,3,5-thiadiazinan-3-yl)acetaldehyde, which differs from the traditional pathway of analogous reactions. However, as ammonia was added into the reaction and the pH was adjusted to the range 8.45-9.43, triazine readily undergoes hydrolysis, and the formed intermediate reacts with ammonia and formaldehyde generated in situ from triazine to produce 1-(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo [3.3.1.13,7]decane (HTAD). Further increasing the pH up to 10.27-11.21 leads to the decomposition of HTAD. Based on the experimental observation and evidence from high-resolution and tandem mass spectrometry, we propose the plausible reaction mechanisms between triazine and H2S, as well as the derived reaction from triazine under different pH conditions.

Optimal fabrication methods for miniature coplanar ion traps.

RATIONALE: Ion trap mass spectrometers are beneficial due to their intrinsic sensitivity and specificity. Therefore, a portable version for in situ analysis of various compounds is very attractive. Miniaturization of ion traps is paramount for the portability of such mass spectrometers. METHODS: We developed an optimized design for a planar linear ion trap mass spectrometer, consisting of two trapping plates with photolithographically patterned electrodes. Each plate is constructed using a machined glass substrate and standard microfabrication procedures. The plates are attached to a patterned circuit board via wire bonds then positioned approximately 5 mm apart. RESULTS: Trapped ions are detected by ejecting them through tapered slits, which alleviate charge buildup. Mass analysis can be performed through either boundary or resonant ion ejection. Better than unit mass resolution is demonstrated with resonant ejection. CONCLUSIONS: The optimized planar linear ion trap provides good resolution and the potential for further miniaturization. This was accomplished by vigorously testing variables associated with ion trap design including electrical connections, substrate materials, and electrode designs.

Double resonance ejection using novel radiofrequency phase tracking circuitry in a miniaturized planar linear ion trap mass spectrometer.

RATIONALE: Ion trap mass spectrometers are attractive due to their inherent sensitivity and specificity. Miniaturization increases trap portability for in situ mass analysis by relaxing vacuum and voltage requirements but decreases the trapping volume. To overcome signal/resolution loss from miniaturization, double resonance ejection using phase tracking circuitry was investigated. METHODS: Phase tracking circuitry was developed to induce double resonance ejection in a planar linear ion trap using the ß 2/3 hexapole resonance line. RESULTS: Double resonance was observed using phase tracking circuitry. Resolution of 0.5 m/z units and improved signal-to-noise ratio (SNR) compared with AC resonant ejection were achieved. CONCLUSIONS: The phase tracking circuitry proved effective despite deviations from a true phase locked condition. Double resonance ejection is a means to increase signal intensity in a miniaturized planar ion trap.

Sub-ppt Mass Spectrometric Detection of Therapeutic Drugs in Complex Biological Matrixes Using Polystyrene-Microsphere-Coated Paper Spray.

Polystyrene (PS) is a class of polymer materials that offers great potential for various applications. However, the applications of PS microspheres in paper spray mass spectrometry are largely underexplored. Herein we prepared a series of PS microspheres via a simple dispersion polymerization and then used them as coating materials for paper spray mass spectrometry (MS) in high-sensitivity analysis of various therapeutic drugs in complex biological matrixes. In the preparation of PS-coated papers, the coating method was found playing a key role in determining the performance of the resulting paper substrate in addition to other parameters (e.g., starch type and amount, PS coating amount, and spray solvent). We also found that as a solvent was applied on PS-coated paper for paper spray, the analytes of interest would be first extracted out and then moved to the tip of paper triangle for spray along with the applied solvent. In the process, the surface energy of PS particles had a strong impact on the desorption performance of analytes from PS-coated paper substrate, and the PS with a high surface energy favored the elution of analytes to allow a high MS sensitivity. When the prepared PS coated paper was used as a substrate for paper spray, it gave high sensitivity in analysis of therapeutic drugs in various biological matrixes such as whole blood, serum, and urine with excellent repeatability and reproducibility. In contrast to uncoated filter paper, an improvement of 10-546-fold in sensitivity was achieved using PS-coated paper for paper spray, and an estimated lower limit of quantitation (LLOQs) in the range of 0.004-0.084 ng mL-1 was obtained. The present study is significant in exploring the potential of PS for high-sensitivity MS analysis, and it provides a promising platform in the translation of the MS technique to clinical applications.

Dynamics of rebounding Bacillus subtilis spores determined using image-charge detection.

A novel image-charge detection technique was used to investigate the mechanical elasticity of bare bacterial spores during high-velocity impact. Spores of Bacillus subtilis introduced to vacuum using electrospray and aerodynamic acceleration impacted and rebounded off of a glass plate. A dual-stage, asymmetric image-charge detector measured the velocity and direction of each spore both before and after impact with the glass surface. Two ranges of impact velocity were investigated, with average initial velocities of 197 ± 17 and 145 ± 12 m/s. Impacts were strongly inelastic, with most of the translational kinetic energy lost upon impact, similar to polystyrene particles of similar size under similar impact velocities. Specifically, 69% (± 16%) and 74% (± 11%) of initial kinetic energy was lost in impacts at the two velocity ranges, respectively. The average coefficients of restitution for the two velocity regimes were 0.53 ± 0.15 and 0.49 ± 0.12. There was no statistically significant difference in the fractional kinetic energy loss between these two populations. The variance of these results is much larger than experiments using polystyrene spheres of comparable size. These results imply significant plastic deformation of the spore-a striking result given that spores of this strain of B. subtilis are known to survive impacts on glass at these velocities. Triboelectric charge transfer during impact was also observed. Although much is known about spore elasticity from static measurements, this is the first study to investigate the elastic properties of bacterial spores in a dynamic scenario, as well as the first demonstration of an image charge detector for measurements of rebounding particles.

Miniaturized Linear Wire Ion Trap Mass Analyzer.

We report a linear ion trap (LIT) in which the electric field is formed by fine wires held under tension and accurately positioned using holes drilled in two end plates made of plastic. The coordinates of the hole positions were optimized in simulation. The stability diagram and mass spectra using boundary ejection were compared between simulation and experiment and good agreement was found. The mass spectra from experiments show peak widths (fwhm) in units of mass-to-charge of around 0.38 Th using a scan rate of 3830 Th/s. The limits of detection are 137 ppbv and 401 ppbv for benzene and toluene, respectively. Different sizes of the wire ion trap can be easily fabricated by drilling holes in scaled positions. Other distinguishing features, such as high ion and photon transmission, low capacitance, high tolerance to mechanical and assembly error, and low weight, are discussed.

Miniaturization of a planar-electrode linear ion trap mass spectrometer.

RATIONALE: We describe the miniaturization of a linear-type ion trap mass spectrometer for possible applications in portable chemical analysis. This work demonstrates the potential and the advantages of using lithographically patterned electrode plates in realizing an ion trap with dimension y0 less than 1 mm. The focus of this work was to demonstrate the viability and flexibility of the patterned electrode approach to trap miniaturization, and also to discover potential obstacles to its use. METHODS: Planar, low-capacitance ceramic substrates were patterned with metal electrodes using photolithography. Plates that were originally used in a linear trap with a half-spacing (y0 ) of 2.19 mm were positioned much closer together such that y0 = 0.95 mm. A capacitive voltage divider provided different radiofrequency (RF) amplitudes to each of 10 electrode elements (5 on each side of the ejection slit), and the capacitor values were adjusted to provide the correct electric field at this closer spacing. The length of the trapping region, 45 mm, is unchanged from the previous device. RESULTS: Electron ionization mass spectra of toluene and dichloromethane demonstrate instrument performance, with better than unit mass resolution for the molecular ion and fragment ion peaks of toluene. Compared with the larger plate spacing, the signal is reduced, corresponding to the reduced trapping capacity of the smaller device. However, the mass resolution of the larger device is retained. CONCLUSIONS: Lithographically patterned substrates are a viable pathway to fabricating highly miniaturized ion traps for mass spectrometry. These results also demonstrate the possibility of significant reduction of the ion trap volume without physical modification of the electrodes. These experiments show promise for further miniaturization using assemblies of patterned ceramic plates. Copyright © 2014 John Wiley & Sons, Ltd.

Flow-through charge detection mass spectrometry to categorize micron-sized particles.

This paper outlines the use of charge detection mass spectrometry to simultaneously measure the charge and mass of micron-sized particles. In a flow-through instrument, the detection of charge was achieved through charge induction onto cylindrical electrodes that connect to a differential amplifier. Mass was determined by particle acceleration under the influence of an electric field. Particles ranging from 30 to 400 fg (3 to 7 µm diameter) were tested. The detector design can measure particle mass within 10% accuracy for particles up to 620 fg with total charge ranging from 500e- to 56 ke-. This charge and mass range are expected to be relevant for dust on Mars.

High-speed rotor for microparticle impact studies.

We report on the design, construction, and testing of a high-speed rotor intended for use in hypervelocity microparticle impact studies. The rotor is based on a four-wing design to provide rotational stability and includes flat "paddle" impact surfaces of ∼0.5 cm2 at the tips of each wing. The profile of each wing minimizes the variation in tensile forces at any given rotational speed. The rotor was machined using titanium (grade 5) and operated in high vacuum using magnetically levitated bearings. Initial experiments were run at several speeds up to 100 000 rpm (revolutions per minute), corresponding to a tip speed of 670 m/s. Elongation at the wing tips as a function of rotational speed was measured with a precision of several micrometers using a focused diode laser and found to agree with an elastic modulus of 1.16 GPa for the rotor material. Applications to microparticle impact experiments are discussed.

Simple, inexpensive mass spectrometric analyzer for thermogravimetry.

A low-cost mass spectrometer attachment for thermogravimetric analysis has been constructed from readily available commercial instruments and components. The benefits of this set-up include excellent mass-flow repeatability, simple design, and significantly lower adoption cost as opposed to ready-built commercial solutions. The inclusion of an open source software package allows semi-automated, highly simplified data analysis. The results from the instrument show excellent sensitivity for small volumes of evolved gas, as well as highly reproducible signal strengths. The GUI-based software package provides data analysis in a way that is very intuitive and that can be easily modified to work with a broad range of TG instruments.

Coaxial ion trap mass spectrometer: concentric toroidal and quadrupolar trapping regions.

We present the design and results for a new radio-frequency ion trap mass analyzer, the coaxial ion trap, in which both toroidal and quadrupolar trapping regions are created simultaneously. The device is composed of two parallel ceramic plates, the facing surfaces of which are lithographically patterned with concentric metal rings and covered with a thin film of germanium. Experiments demonstrate that ions can be trapped in either region, transferred from the toroidal to the quadrupolar region, and mass-selectively ejected from the quadrupolar region to a detector. Ions trapped in the toroidal region can be transferred to the quadrupole region using an applied ac signal in the radial direction, although it appears that the mechanism of this transfer does not involve resonance with the ion secular frequency, and the process is not mass selective. Ions in the quadrupole trapping region are mass analyzed using dipole resonant ejection. Multiple transfer steps and mass analysis scans are possible on a single population of ions, as from a single ionization/trapping event. The device demonstrates better mass resolving power than the radially ejecting halo ion trap and better sensitivity than the planar quadrupole ion trap.

Parkinsonism in patients with a history of amphetamine exposure.

We recently found a higher rate of prolonged amphetamine exposure in patients diagnosed with Parkinson's disease (PD) than in spouse/caregiver controls. Since distinguishing features have been described in some patients with parkinsonism due to environment exposures (e.g., manganese), we sought to compare the clinical features of patients with PD with prolonged amphetamine exposure with unexposed patients with PD. Prolonged exposure was defined as a minimum of twice a week for >or=3 months, or weekly use >or=1 year. We reviewed the clinical records of patients with PD who had participated in a telephone survey of drug and environmental exposures and compared the clinical features of patients with a history of prolonged amphetamine exposure to patients who had no such exposure. Records were available for 16 of 17 (94%) patients with prior amphetamine exposure and 127 of 137 (92%) of those unexposed. Age at diagnosis was younger in the amphetamine-exposed group (49.8 +/- 8.2 years vs. 53.1 +/- 7.4 years; P < 0.05), but other features, including presenting symptoms, initial and later treatments, development of motor fluctuations, and MRI findings were similar between these groups. Because we did not detect clinical features that differentiate parkinsonism in patients with prolonged amphetamine exposure, research to determine whether amphetamine exposure is a risk factor for parkinsonism will require detailed histories of medication and recreational drug use.

Computational study of isoprene hydroxyalkyl peroxy radical-water complexes (C5H8(OH)O2-H2O).

Herein we report an extensive ab initio study on the existence of eight beta-hydroxy isoprene peroxy radical-water complexes. Binding energies calculated at the MP2(full)/6-311++G(2d,2p)//CCSD(T)/6-311++G(d,p) level of theory range between 3.85 and 5.66 kcal mol(-1). The results of natural bond orbital calculations are presented to help rationalize complex formation. Atmospheric lifetimes, equilibrium constants, heats of formation, and the relative abundance of complexed to uncomplexed peroxy radicals are also reported and discussed.

Mechanistic Investigation of Charge Separation in Electrospray Ionization using Microparticles to Record Droplet Charge State.

The mechanism of charge separation and droplet breakup during electrospray has been investigated using both biological and nonbiological microparticles as probes to preserve and record the charge state of droplets in the micron size range. Charged droplets and/or microparticles were observed using two-stage image charge detection after entering a differentially pumped, atmospheric pressure inlet. Microparticle probes allowed observation of not only droplets that did not fully evaporate but also product droplets small enough to fully evaporate, as well as the ability to distinguish between these two types of droplets. In both positive- and negative-mode electrospray ionization of aqueous suspensions, we observed roughly 20% of particles carrying charges opposite to the biased voltage on the capillary. Both positive and negative modes yielded distributions of charge states with maxima in the expected polarity but with tails extending into the opposite polarity of charge. Both polarities produced similar fractions of particles with opposite charge, with no preference for either polarity. This result is consistent with electric-field-induced charge separation and droplet breakup within the high-field region between the capillary and the counter electrode. These results suggest that electric-field-induced charge separation may be the dominant mechanism, at least in the micron size range and under the present experimental conditions, in which primary charged droplets of either polarity split into smaller progeny with a range of charges spanning both polarities. Implications for electrospray ionization mass spectrometry are discussed.

Accurately Mapping Image Charge and Calibrating Ion Velocity in Charge Detection Mass Spectrometry.

Image charge detection is the foundation of charge detection mass spectrometry (CDMS). The mass-to-charge ratio, m/z, of a highly charged ion or particle is determined by measuring the particle's charge and velocity. Charge is typically determined from a calibrated image charge signal, and the particle velocity is calculated using the peaks from the shaped signal as they relate to the particle position and time-of-flight through a detector of known length. Although much has been done to improve the charge accuracy in CDMS, little has been done to address the inconsistencies in the particle velocity measurements and the interpretation of peak position and effective electrode length. In this work, we combine SIMION ion trajectory software and the Shockley-Ramo theorem to accurately determine the effective electrode length, peak position, and shape of the signal peaks. Six model charge detector geometries were examined with this method and evaluated in laboratory experiments. Experimental results in all cases agreed with the simulations. Using a charge detector with multiple, 12.7 mm-long cylindrical electrodes, experimental velocities across and between electrodes agreed within 0.25% relative standard deviation (RSD) when this method was used to correct for effective electrode lengths, corresponding to an uncertainty in the effective electrode length of only 40 µm. For a detector with multiple electrodes and varied electrode spacing, experiments showed that the peak amplitude and shape vary with the geometry and with the particle path through the detector, whereas all peak areas agreed to within 2.3% RSD. For a charge detector made of two printed circuit boards, the velocities agreed within 0.44% RSD using the calculated effective electrode length.

Simulation and measurement of image charge detection with printed-circuit-board detector and differential amplifier.

We present a novel and thorough simulation technique to understand image charge generated from charged particles on a printed-circuit-board detector. We also describe a custom differential amplifier to exploit the near-differential input to improve the signal-to-noise-ratio of the measured image charge. The simulation technique analyzes how different parameters such as the position, velocity, and charge magnitude of a particle affect the image charge and the amplifier output. It also enables the designer to directly import signals into circuit simulation software to analyze the full signal conversion process from the image charge to the amplifier output. A novel measurement setup using a Venturi vacuum system injects single charged particles (with diameters in the 100 s of microns range) through a PCB detector containing patterned electrodes to verify our simulation technique and amplifier performance. The measured differential amplifier presented here exhibits a gain of 7.96 µV/e- and a single-pass noise floor of 1030 e-, which is about 13× lower than that of the referenced commercial amplifier. The amplifier also has the capability to reach a single-pass noise floor lower than 140 e-, which has been shown in Cadence simulation.

Paul trap mass analyzer consisting of opposing microfabricated electrode plates.

We report the design and performance of a novel radio-frequency (RF) ion-trap mass analyzer, the planar Paul trap, in which a quadrupolar potential distribution is made between two electrode plates. Each plate consists of a series of concentric, lithographically deposited 100-microm-wide metal rings, overlaid with a thin resistive layer. A different RF amplitude is applied to each ring, such that the trapping field produced is similar to that of the conventional Paul trap. The accuracy and shape of the electric fields in this trap are not limited by electrode geometry nor machining precision, as is the case in traps made with metal electrodes. The use of two microfabricated plates for ion trap construction presents a lower-cost alternative to conventional ion traps, with additional advantages in electrode alignment, electric field optimization, and ion-trap miniaturization. Experiments demonstrate the effects of ion ejection mode and scan rate on mass resolution for several small organic compounds. The current instrument has a mass range up to approximately 180 Thomsons (Th), with better than unit mass resolution over the entire range.