Description of gas-phase ion/neutral interactions in differential ion mobility spectrometry: CV prediction using calibration runs

CaptiveSpray Differential Ion Mobility Spectrometry Device with Enhanced Ion Transmission and Improved Resolving Power

A performance enhanced CaptiveSpray differential ion mobility device was designed and constructed by incorporating a circular channel and a gas flow homogenizing channel (GFHC) between the CaptiveSpray ion source and planar differential ion mobility spectrometry (DMS). The GFHC was used to reduce gas flow heterogeneity prior to the entrance of the DMS device. The optimal flared entrance greatly reduces gas flow velocity at the inlet region owing to its relatively large gas inlet interface, which assists in reducing disparities between the minimum and maximum gas velocity along the x-axis. The circular electrode was machined with channels along the x- and y-axis for the passage of auxiliary gas and was applied with a potential to focus the incoming ions from the CaptiveSpray source into the DMS channel. Using reserpine as a reference standard, substantial signal enhancement was achieved with a concomitant reduction of the peak width in the ionogram.

Clustering and Nonclustering Modifier Mixtures in Differential Mobility Spectrometry for Multidimensional Liquid Chromatography Ion Mobility-Mass Spectrometry Analysis

Modifiers provide fast and reliable tuning of separation in differential mobility spectrometry (DMS). DMS selectivity for separating isomeric molecules depends on the clustering modifier concentration, which is typically 1.5-3 mol % ratio of isopropanol or ethanol in nitrogen. Low concentrations (0.1%) of isopropanol were found to improve resolution and sensitivity but at the cost of practicality and robustness. Replacing the single-channel DMS pump with a binary high-performance liquid chromatography (HPLC) pump enabled the generation of modifier mixtures at a constant flow rate using an isocratic or gradient mode, and the analytical benefits of the system were investigated considering cyclohexane, n-hexane, or n-octane as nonclustering modifiers and isopropanol or ethanol as clustering modifiers. It was found that clustering and nonclustering modifier mixtures enable optimization of selectivity, resolution, and sensitivity for different positional isomers and diastereoisomers. Data further suggested different ion separation mechanisms depending on the modifier ratios. For 85 analytes, the absolute difference in compensation voltages (CoVs) between pure nitrogen and cyclohexane at 1.5 mol % ratio was below 4 V, demonstrating its potential as a nonclustering modifier. Cyclohexane's nonclustering behavior was further supported by molecular modeling using density functional theory (DFT) and calculated cluster binding energies, showing positive ΔG values. The ability to control analyte CoVs by adjusting modifier concentrations in isocratic and gradient modes is beneficial for optimizing multidimensional LCxDMS-MS. It is fast and effective for manipulating the DMS scanning window size to realize shorter mass spectrometry (MS) acquisition cycle times while maintaining a sufficient number of CoV steps and without compromising DMS separation performance.

Charge Retention/Charge Depletion in ESI-MS: Experimental Evidence

The effects of liquid and gas phase additives (chemical modifiers) on the ion signal distribution for Substance P (SP), recorded with a nanoelectrospray setup, are evaluated. Depletion of the higher charge state of Substance P ([SP+3H]³⁺) is observed with polar protic gas phase modifiers. This is attributed to their ability to form larger hydrogen-bonded clusters, whose proton affinity increases with cluster size. These clusters are able to deprotonate the higher charge state. "Supercharging agents" (SCAs) as well as aprotic polar gas phase modifiers, which promote the retention of the higher charge state of Substance P, do not form such large clusters under the given conditions and are therefore not able to deprotonate Substance P. Both SCAs and aprotic modifiers form clusters with the higher charge state, leading to stabilization of the charge. Whereas supercharging agents have low vapor pressures and are therefore enriched in late-stage electrospray droplets, the gas phase modifiers are volatile organic solvents. Collision induced dissociation experiments revealed that the addition of a modifier significantly delays the droplet evaporation and ion release process. This indicates that the droplet takes up the gas phase modifier to a certain extent (accommodation). Depending on the modifier's properties either charge depletion or retention may eventually be promoted.

Performance Enhancements in Differential Ion Mobility Spectrometry-Mass Spectrometry (DMS-MS) by Using a Modified CaptiveSpray Source

Differential ion mobility spectrometry (DMS) spatially separates ions in the gas phase using the mobility differences of the ions under applied low and high electric fields. The use of DMS as an ion filter (or ion selector) prior to mass spectrometry analysis has been compromised by the limited ion transmission efficiency. This paper reports enhancement of the DMS-MS sensitivity and signal stability using a modified CaptiveSpray™ source. In terms of the ion sampling and transmission efficiency, the modified CaptiveSpray source swept ~ 89% of the ions generated by the tapered capillary through the DMS device (compared to ~ 10% with a conventional microspray source). The signal fluctuation improved from 11.7% (relative standard deviation, RSD) with microspray DMS-MS to 3.6% using CaptiveSpray-DMS-MS. Coupling of LC to DMS-MS via the modified CaptiveSpray source was simple and robust. Using DMS as a noise-filtering device, LC-DMS-MS performed better than conventional LC-MS for analyzing a BSA digest standard. Although LC-DMS-MS had a lower sequence coverage (55%), a higher Mascot score (283) was obtained compared to those of LC-MS (sequence coverage 65%; Mascot score 192) under the same elution conditions. The improvement in the confidence of the search result was attributed to the preferential elimination of noise ions. Graphical Abstract ᅟ.

Differential mobility spectrometry/mass spectrometry history, theory, design optimization, simulations, and applications

This review of differential mobility spectrometry focuses primarily on mass spectrometry coupling, starting with the history of the development of this technique in the Soviet Union. Fundamental principles of the separation process are covered, in addition to efforts related to design optimization and advancements in computer simulations. The flexibility of differential mobility spectrometry design features is explored in detail, particularly with regards to separation capability, speed, and ion transmission. 2015 Wiley Periodicals, Inc. Mass Spec Rev 35:687-737, 2016.

Shift reagents in ion mobility spectrometry: the effect of the number of interaction sites, size and interaction energies on the mobilities of valinol and ethanolamine

Overlapping peaks interfere in ion mobility spectrometry (IMS), but they are separated introducing mobility shift reagents (SR) in the buffer gas forming adducts with different collision cross-sections (size). IMS separations using SR depend on the ion mobility shifts which are governed by adduct's size and interaction energies (stabilities). Mobility shifts of valinol and ethanolamine ions were measured by electrospray-ionization ion mobility-mass spectrometry (MS). Methyl-chloro propionate (M) was used as SR; 2-butanol (B) and nitrobenzene (N) were used for comparison. Density functional theory was used for calculations. B produced the smallest mobility shifts because of its small size. M and N have two strong interaction sites (oxygen atoms) and similar molecular mass, and they should produce similar shifts. For both ethanolamine and valinol ions, stabilities were larger for N adducts than those of M. With ethanolamine, M produced a 68% shift, large compared to that using N, 61%, because M has a third weak interaction site on the chlorine atom and, therefore, M has more interaction possibilities than N. This third site overrode the oxygen atoms' interaction energy that favored the adduction of ethanolamine with N over that with M. On the contrary, with valinol mobility shifts were larger with N than with M (21 vs 18%) because interaction energy favored even more adduction of valinol with N than with M; that is, the interaction energy difference between adducts of valinol with M and N was larger than that between those adducts with ethanolamine, and the third M interaction could not override this larger difference. Mobility shifts were explained based on the number of SR's interaction sites, size of ions and SR, and SR-ion interaction energies. This is the first time that the number of interaction sites is used to explain mobility shifts in SR-assisted IMS. Copyright © 2016 John Wiley & Sons, Ltd.

Determining the Binding Sites of β-Cyclodextrin and Peptides by Electron-Capture Dissociation High Resolution Tandem Mass Spectrometry

Cyclodextrins (CDs) are a group of cyclic oligosaccharides, which readily form inclusion complexes with hydrophobic compounds to increase bioavailability, thus making CDs ideal drug excipients. Recent studies have also shown that CDs exhibit a wide range of protective effects, preventing proteins from aggregation, degradation, and folding. These effects strongly depend on the binding sites on the protein surface. CDs only exhibit weak interactions with amino acids, however; conventional analytical techniques therefore usually fail to reveal the exact location of the binding sites. Moreover, some studies even suggest that CD inclusion complexes are merely electrostatic adducts. Here, electron capture dissociation (ECD) was applied in this proof-of-concept study to examine the exact nature of the CD/peptide complexes, and CD binding sites were unambiguously located for the first time via Fourier-transform ion cyclotron resonance (FTICR) tandem mass spectrometry.

On the isobaric space of 25-hydroxyvitamin D in human serum: potential for interferences in liquid chromatography/tandem mass spectrometry, systematic errors and accuracy issues

RATIONALE

Isobaric interferences in human serum can potentially influence the measured concentration levels of 25-hydroxyvitamin D [25(OH)D], when low resolving power liquid chromatography/tandem mass spectrometry (LC/MS/MS) instruments and non-specific MS/MS product ions are employed for analysis. In this study, we provide a detailed characterization of these interferences and a technical solution to reduce the associated systematic errors.

METHODS

Detailed electrospray ionization Fourier transform ion cyclotron resonance (FTICR) high-resolution mass spectrometry (HRMS) experiments were used to characterize co-extracted isobaric components of 25(OH)D from human serum. Differential ion mobility spectrometry (DMS), as a gas-phase ion filter, was implemented on a triple quadrupole mass spectrometer for separation of the isobars.

RESULTS

HRMS revealed the presence of multiple isobaric compounds in extracts of human serum for different sample preparation methods. Several of these isobars had the potential to increase the peak areas measured for 25(OH)D on low-resolution MS instruments. A major isobaric component was identified as pentaerythritol oleate, a technical lubricant, which was probably an artifact from the analytical instrumentation. DMS was able to remove several of these isobars prior to MS/MS, when implemented on the low-resolution triple quadrupole mass spectrometer.

CONCLUSIONS

It was shown in this proof-of-concept study that DMS-MS has the potential to significantly decrease systematic errors, and thus improve accuracy of vitamin D measurements using LC/MS/MS.

Using differential mobility spectrometry to measure ion solvation: an examination of the roles of solvents and ionic structures in separating quinoline-based drugs

Understanding the mechanisms and energetics of ion solvation using differential mobility spectrometry.