International Journal of Mass Spectrometry最新文献

Crosslinked materials have long been utilized to form films of enhanced physical strength and chemical resistance. Unfortunately, their complex structures and insolubility obstruct their characterization by most analytical techniques. Atmospheric solids analysis probe mass spectrometry (ASAP-MS) offers a solution by applying heated nitrogen gas to mildly pyrolyze and desorb material from the polymer film. This results in the observation of oligomeric species that are large enough to reveal the connectivity of the microstructure. The system studied was comprised of a neopentyl glycol terephthalate base polymer crosslinked by a tetrafunctional β-hydroxyalkyl amide crosslinking agent. Base polymer and crosslinking agent were mixed at different weight ratios and cured onto glass capillaries to form networks with varied degrees of crosslinking. ASAP-MS analysis of the individual reactants and crosslinked products led to the observation of signature ions from both the raw materials as well as the resulting network. The relative abundances of these ions provided a full quantitative analysis of the crosslinker loadings, while the time delay between ASAP onset and ion formation permitted estimation of the cure temperature and thermal history. On the other hand, tandem mass spectrometry (MS/MS) experiments on the ions diagnostic of the crosslinked network allowed for identification of the crosslinking bonds formed upon curing. Such ability to reliably evaluate the extent of crosslinker loading, curing conditions, and types of bonds in a network has a significant impact on understanding film performance and defects in industrial coatings.

In this paper, 70 % methanol was used as solvent to extract the principal components from Moringa oleifera (Lam) seed with the ratio (w/v, 1:10) by ultrasonic-assisted extraction. The extract in Lam seed was detected by high resolution mass spectrometry and MS/MS spectra. The result suggests that there are three principal components including malic acid, d-trehalose and 4-(α-L-rhamnopyranosyloxy) benzyl glucosinolate in this extract. Based on this result, the spatial distribution of these components in Lam seed was visually obtained by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). There is much malic acid in the center of endosperm in Lam kernel and little in the shell of Lam seed. d-trehalose is mainly distributed in the seed coat of Lam seed, and almost no d-trehalose is distributed in the kernel of Lam seed. 4-(α-L-rhamnopyranosyloxy) benzyl glucosinolate is distributed in the seed coat and endosperm of Lam seed. The relative content of this compound in the seed coat of Lam seed is much higher than that in the endosperm. The result further suggests that the different component in Lam seed is distributed in different part. This study can provide some valuable reference for efficient extracting active components from Lam.

A comparative study on direct current (DC) and alternating current (AC) corona discharge ionization for positive and negative mode of operation was performed by mass spectrometry. The needle, positioned 3 mm in front of the inlet of a mass spectrometer, was moved upward and intensities of ions were measured as a function of vertical distance d. For DC corona discharge, the unipolar plasma sizes for positive and negative ions were found to be nearly the same with d ≈ ∼5 mm. In contrast, for AC corona with 15 kHz, while positive ions were found to be much more confined in a narrow region with d ≈ ∼2 mm, negative ions were detected with much larger d ≈ ∼10 mm. This marked difference of unipolar plasma sizes of positive and negative ions for AC corona was ascribed to the much larger diffusion lengths of electrons than those of positive ions. In the negative mode of DC and AC corona discharge ionization, the intensity of CH₃COO⁻ (m/z 59) originating from an acetic acid impurity was found to increase with d. This peculiar behavior was attributed to the proton transfer reaction of O₂⁻ with acetic acid vapor taking place distant from the tip of the electrode needle. The ionization efficiency of AC corona is higher than that of DC corona for both positive and negative modes of operation. The higher ionization efficiencies for AC corona are attributed to the generation of remnant electrons and positive ions for positive and negative phase AC corona, respectively. These remnant charges play crucial roles as the triggers for plasma breakdown in every cycle of RF frequency. The ion intensities measured by proximity corona discharge ionization are almost proportional to the analyte concentrations when they are less than 1 ppm.

Portable ion trap mass spectrometers, especially for those with continuous atmospheric pressure interfaces, usually implement vacuum pumps with limited power for miniaturization, which leads to high vacuum pressures and reduced resolutions. In this study, two three-stage high-resolution ion isolation methods of SWIFT - AC frequency sweep - SWIFT and SWIFT - RF voltage ramp - SWIFT were proposed. Compared to the 2.8 m/z isolation resolution of the traditional SWIFT ion isolation method, higher isolation resolutions of 2.2 m/z and 1.5 m/z have been achieved by SWIFT - AC frequency sweep - SWIFT and SWIFT - RF voltage ramp - SWIFT ion isolation methods at an isolation efficiency of 40 %. In addition, our detailed experiments showed that all ion isolation methods can obtain higher isolation resolution under the conditions of lower vacuum pressure, higher isolation q value, and stronger axial confinement potential barrier, like which the mass resolution behaves. In order to inspect the effectiveness of the high-resolution isolation method, the control experiments of hair samples spiked with drugs had been conducted. The signal-to-noise ratio of O⁶-monoacetylmorphine can be improved by a factor of 2.4 using the SWIFT - RF voltage ramp - SWIFT ion isolation method. Our research results indicate that the performance of ion trap tandem mass spectrometry can be effectively improved by using the three-stage high-resolution ion isolation methods.

The work presents studies of the cationization agent effect on the ionization efficiency, and on the stability of secondary molecules, which are desorbed from the surface as a result of bombardment by MeV primary ions. Different organic molecule standards were mixed with various trifluoroacetate (TFA) salts, namely AgTFA, LiTFA, NaTFA and KTFA, with Ag, Li, Na and K being common cationization agents of secondary ions besides hydrogen. Results show modest differences in ionization efficiency, and significant variation of secondary ions’ stability when different cationization agents are attached to the molecule. For all reference samples, lithium provided the most stable secondary ions with almost none detected fragmentations after the desorption, while secondary ions with attached potassium were measured as much less stable, and the ratio between neutrals (fragments) and normal positive ions is estimated to be more than 0.2. However, even potassium – ionized molecules were recognized as more stable than protonated molecules.

Gabapentin (GBP) is an anticonvulsant agent and its determination using a fast and simple method is challenging. In this research, a novel sensitive, and economical method for the fast determination of GBP in biological samples was developed. This method benefits from the advantages of ion mobility spectrometry (IMS) as a fast detection and separation system and dispersive liquid-liquid microextraction (DLLME) for the preconcentration/extraction of GBP. In the developed method (DLLME-IMS), Ag nanoparticles serve as modifiers. Moreover, the quantum theory of atoms in molecules (QTAIM) analysis was employed to investigate the interaction mechanism between GBP and Ag nanoparticles. Also, a delay time after the injection of the sample was applied to improve the method sensitivity. Under the optimum condition, the developed method showed a linear response over the concentration range of 0.3–12 μg mL⁻¹ and a detection limit of 0.12 μg mL⁻¹. Eventually, DLLME-IMS was successfully employed in plasma sample, and was validated against the HPLC method with UV detection as a reference method. The developed method will open up a new avenue for the development of effective, rapid, and simple analytical methods.

Many powerful methods in mass spectrometry rely on activation of ions by high-energy collisions with gas particles. For example, multiple Collision Induced Dissociation (CID) has been used for many years to determine structural information for ions ranging from small organics to large, native-like protein complexes. More recently, Collision Induced Unfolding (CIU) has proved to be a very powerful method for understanding high-order protein structure and detecting differences between similar proteins. Quantifying the thermochemistry underlying dissociation/unfolding in these experiments can be quite challenging without reliable models of ion heating and cooling. Established physical models of CID are valuable in predicting ion heating but do not explicitly include mechanisms for cooling, which may play a large part in CID/CIU in modern instruments. Ab initio and Molecular Dynamics methods are extremely computationally expensive for modeling CID/CIU of large analytes such as biomolecular ions. In this tutorial perspective, limiting behaviors of ion kinetic energy damping, heating, and cooling set by “extreme” cases are explored, and an Improved Impulsive Collision Theory and associated software (“Ion Simulations of the Physics of Activation”, IonSPA) are introduced that can model all of these for partially inelastic collisions. Finally, examples of modeled collisional activation of native-like protein ions under realistic experimental conditions are discussed, with an outlook toward the use of IonSPA in accessing the thermochemical information hidden in CID breakdown curves and CIU fingerprints.

In the present study, we have examined the ionization of a set of small atmospheric acid–base clusters. The calculated ionization energy (IE) ranges from ∼660 kJ mol⁻¹ to ∼1120 kJ mol⁻¹, with clusters consisting of just the bases yielding the smallest IEs, while those of the acids yield the largest. For neutral clusters that form most readily, the IEs range from ∼790 kJ mol⁻¹ to ∼880 kJ mol⁻¹. These values are generally smaller than those of other chemicals with significant atmospheric presence. Thus, the radical cations of the acid–base clusters are not likely to ionize other atmospheric species. Charged atmospheric species have been hypothesized to facilitate the growth of large clusters. We have carried out a case study of the sequential hydration of an acid–base cluster and compared our results with previous studies of forming pure water clusters. The neutral acid–base cluster binds water more strongly than pure water clusters. Further strengthening of the binding can be seen for the radical cationic acid–base cluster. The resulting acid–base–water clusters may further represent a source of H₃O⁺, which may proceed to facilitate cloud formation.

Single-frequency ion parking, a useful technique in electrospray mass spectrometry (ESI-MS), involves gas-phase charge-reduction ion/ion reactions in an electrodynamic ion trap in conjunction with the application of a supplementary oscillatory voltage to selectively inhibit the reaction rate of an ion of interest. The ion parking process provides a means for limiting the extent of charge reduction in a controlled fashion and allows for ions distributed over a range of charge states to be concentrated into fewer charge states (a single charge state under optimal conditions). As charge reduction inherently leads to an increase in the mass-to-charge (m/z) ratio of the ions, it is important that the means for storing and analyzing ions be able to accommodate ions of high m/z ratios. The so-called ‘digital ion trap’ (DIT), which uses a digital waveform as the trapping RF, has been demonstrated to be well-suited for the analysis of high m/z ions by taking advantage of its ability to manipulate the waveform frequency. In this study, the feasibility of ion parking in a 3D quadrupole ion trap operated as a DIT using a slow-amplitude single-frequency sine-wave for selective inhibition of an ion/ion reaction is demonstrated. A recently described model that describes ion parking has been adjusted for the DIT case and is used to interpret experimental data for proteins ranging in mass from 8600 Da to 467,000 Da.