Pulsed Nano-ESI Atmospheric-Pressure Ion Mobility Mass Spectrometry with Enhanced Ion Sampling

Surface charge-induced electrospray for high-throughput analysis of complex samples and electrochemical reaction intermediates using mass spectrometry

Electrospray-related ion sources are promising for direct mass spectrometric analysis of complex samples, but current protocols suffer from complicated components and low analytical sensitivity. Here, we propose a surface charge-induced electrospray ionization (SCIESI) inspired by flashover on an insulator surface under high voltage. This protocol not only effectively avoids contact between the sample solution and metal electrode, but also allows completion of the entire analytical process in less than 40 seconds and limits of detection in the pictogram per milliliter range. SCIESI coupled to mass spectrometry can also be used to monitor electro-chemical processes, and a number of oxidation and reduction reactions have been studied, demonstrating that it is a powerful tool for understanding electrochemical reaction mechanisms.

Proteomics mining of cancer hallmarks on a single-cell resolution

Dysregulated proteome is an essential contributor in carcinogenesis. Protein fluctuations fuel the progression of malignant transformation, such as uncontrolled proliferation, metastasis, and chemo/radiotherapy resistance, which severely impair therapeutic effectiveness and cause disease recurrence and eventually mortality among cancer patients. Cellular heterogeneity is widely observed in cancer and numerous cell subtypes have been characterized that greatly influence cancer progression. Population-averaged research may not fully reveal the heterogeneity, leading to inaccurate conclusions. Thus, deep mining of the multiplex proteome at the single-cell resolution will provide new insights into cancer biology, to develop prognostic biomarkers and treatments. Considering the recent advances in single-cell proteomics, herein we review several novel technologies with particular focus on single-cell mass spectrometry analysis, and summarize their advantages and practical applications in the diagnosis and treatment for cancer. Technological development in single-cell proteomics will bring a paradigm shift in cancer detection, intervention, and therapy.

Rapid detection of sulfur mustard hydrolysis products based on microextraction by packed sorbent combined with nano-electrospray ionization mass spectrometry

RATIONALE

Sulfur mustard is a blister agent prohibited by the Chemical Weapons Convention, and the detection of its hydrolysis product, thiodiglycol (TDG), is an important indicator of blister agent contamination. Due to the poor volatility and low extraction efficiency of TDG, derivatization gas chromatography or liquid chromatography is required for conventional methods, and the detection process is cumbersome and time-consuming.

METHODS

A microextraction by packed sorbent (MEPS) device and a nano-electrospray ionization (nano-ESI) device were used. The central composite design (CCD) model of Response Surface Methodology was used to optimize the elution procedure; the variance analysis under equal repeated trials with multiple factors was used to quantitatively analyze the significance of the impact of related factors on the nano-ESI efficiency. The MEPS-nano-ESI-MS experimental conditions were optimized.

RESULTS

A new detection method of sulfur mustard hydrolysis products in water based on MEPS-nano-ESI-MS was established; the detection limit was 1 ng/mL and was linear between 5 ng/mL and 100 ng/mL (R²  = 0.9911) with a precision of ≤7.2%, and the recovery rate was 107.89% when the sample concentration was 40 ng/mL.

CONCLUSIONS

The experimental results showed that the proposed method could quickly detect the contaminated water samples without chromatographic separation and derivatization, thereby verifying the contamination of sulfur mustard on site.

Optical and mass spectral characterization of the electrospray ionization/corona discharge ionization interface

The interchange between electrospray ionization (ESI) and corona discharge ionization (CDI) with respect to applied bias on the needle is customarily placed at the point where light production begins at the tip of the needle. If a liquid sample is flowing through a needle that is observed to produce light, the ionization process is assumed to be harsher and the term coronaspray ionization has been coined to describe this hybrid ionization mechanism. In this work, the transition between ESI and CDI is investigated with respect to applied bias through optical and mass spectrometric measurements. As a function of applied bias potential, the optical signal at the tip of the needle was recorded simultaneously with the resultant ionization products. In this effort, the production of ions from an electrospray ionization needle has been demonstrated to produce light regardless of bias if ions are also formed. With this understanding, an ESI/CDI needle was designed to allow the bias to be temporarily pulsed over the 'onset' voltage necessary for ionization and the rise and decay of the optical signal was measured. Positive mode CDI onset to a stable discharge state within 0.05 ms, while positive ESI required 1.9 ms to reach a stable condition. In the negative mode, the stability of the ionization process was highly variable in both ESI and CDI modes, though CDI was generally faster to reach the stable mode of operation. When the resultant ions were investigated, the effect of increased bias on an ESI needle was found to be species-dependent. Recognizing that the range of compounds probed was limited, for those examined, it appears that stable, non-labile species may be investigated via ESI under extremely high biases while labile species demonstrate a narrow range of stable biases before significant fragmentation occurs.

Perturbation-induced high-frequency pulsing of nano-ESI with facile ion selection at atmospheric pressure

Nano-ESI is a commonly used ionization technique with continually expanding analytical advantages. Here, we report a facile way for high-frequency (500-3800 Hz) pulsing of nano-ESI, providing a high flux of mobility-selected ions. The pulsing is accomplished using a relatively low-voltage modulation (80 V peak-to-peak) of an electrode placed <1 cm downstream of a nano-ESI emitter biased to a constant potential. Configuring the electrode as an ion gate enables mobility-based ion selection by scanning the modulation frequency. Our investigations indicate that the electrode modulation perturbs continuous nano-ESI, resulting in solution accumulation at the emitter tip between spray pulses. Selective transmission of ions occurs at frequencies corresponding to harmonics of a fundamental frequency determined by the travel time of each ion from the emitter to the ion gate (pulsing electrode). Remarkably, the intensities of ions selected in this fashion are similar across the harmonics, suggesting that the ionization efficiencies of analytes have minimal dependence on the accumulated volume at the emitter tip. Moreover, intensities of ion-mobility-selected analytes using this technique reach >50% of those in continuous nano-ESI without ion selection, underscoring efficient ion generation via high-frequency pulsing. These findings indicate the potential of the pulsed nano-ESI for enhanced analytical utility, such as a high-flux selected-reagent-ion supplier at atmospheric pressure, and chart new avenues to further enhance the analytical performance of nano-ESI.

Pulsed Nano-ESI: Application in Ion Mobility-MS and Insights into Spray Dynamics

We have previously shown that pulsed nano-ESI offers direct ion introduction into an AP-IM cell in the absence of conventional gates and desolvation. Here, we further characterize this ion injection method and utilize it to gain insights into nano-ESI pulsed spray dynamics. We demonstrate that a pulsed nano-ESI operated at 20 Hz with ion generation pulses of 170-510 μs offers reproducible ion arrival times (0.09-0.21% RSD). Arrival times are then translated to effective collision cross sections (CCSs) using tetraalkylammonium ions as CCS internal standards. For ions with low solvent affinity, effective CCS values match those reported for fully desolvated ions. For amino acids and a series of alkylamine homologues, the effective CCS values are higher than those for fully desolvated ions and correlate with solvent affinity, suggesting that ions with high hydration affinities traverse the mobility cell as hydrated ions. Notably, hydrates are not observed in the MS spectra due to ion activation during the transport into vacuum. Using these observations as a framework to interpret effective CCS values, we investigate the impact of nano-ESI pulse duration on ion properties. We observe that longer pulse durations lead to the enhancement of ion abundance for low-ionization-efficiency analytes and a reduction in clustering. However, effective CCSs are not significantly altered by spray pulse duration, implying that similar ion structures emerge rapidly at all investigated pulse durations. Ion abundance results suggest a temporal evolution of droplets in pulsed nano-ESI where droplets emitted later in the spray formation appear to be smaller, providing enhanced ionization.