Recent developments in ionization techniques for single-cell mass spectrometry

The variation among individual cells plays a significant role in many biological functions. Single-cell analysis is advantageous for gaining insight into intricate biochemical mechanisms rarely accessible when studying tissues as a whole. However, measurement on a unicellular scale is still challenging due to unicellular complex composition, minute substance quantities, and considerable differences in compound concentrations. Mass spectrometry has recently gained extensive attention in unicellular analytical fields due to its exceptional sensitivity, throughput, and compound identification abilities. At present, single-cell mass spectrometry primarily concentrates on the enhancement of ionization methods. The principal ionization approaches encompass nanoelectrospray ionization (nano-ESI), laser desorption ionization (LDI), secondary ion mass spectrometry (SIMS), and inductively coupled plasma (ICP). This article summarizes the most recent advancements in ionization techniques and explores their potential directions within the field of single-cell mass spectrometry.

FACS-assisted single-cell lipidome analysis of phosphatidylcholines and sphingomyelins in cells of different lineages

Recent advances in single-cell genomics and transcriptomics technologies have transformed our understanding of cellular heterogeneity in growth, development, ageing, and disease; however, methods for single-cell lipidomics have comparatively lagged behind in development. We have developed a method for the detection and quantification of a wide range of phosphatidylcholine and sphingomyelin species from single cells that combines fluorescence-assisted cell sorting with automated chip-based nanoESI and shotgun lipidomics. We show herein that our method is capable of quantifying more than 50 different phosphatidylcholine and sphingomyelin species from single cells and can easily distinguish between cells of different lineages or cells treated with exogenous fatty acids. Moreover, our method can detect more subtle differences in the lipidome between cell lines of the same cancer type. Our approach can be run in parallel with other single-cell technologies to deliver near-complete, high-throughput multi-omics data on cells with a similar phenotype and has the capacity to significantly advance our current knowledge on cellular heterogeneity.

Pd Reaction Intermediates in Suzuki-Miyaura Cross-Coupling Characterized by Mass Spectrometry

Palladium-catalyzed Suzuki-Miyaura (SM) coupling is widely utilized in the construction of carbon-carbon bonds. In this study, nanoelectrospray ionization mass spectrometry (nanoESI-MS) is applied to simultaneously monitor precatalysts, catalytic intermediates, reagents, and products of the SM cross-coupling reaction of 3-Br-5-Ph-pyridine and phenylboronic acid. A set of Pd cluster ions related to the monoligated Pd (0) active catalyst is detected, and its deconvoluted isotopic distribution reveals contributions from two neutral molecules. One is assigned to the generally accepted Pd(0) active catalyst, seen in MS as the protonated molecule, while the other is tentatively assigned to an oxidized catalyst which was found to increase as the reaction proceeds. Oxidative stress testing of a synthetic model catalyst 1,5-cyclooctadiene Pd XPhos (COD-Pd-XPhos) performed using FeCl3 supported this assignment. The formation and conversion of the oxidative addition intermediate during the catalytic cycle was monitored to provide information on the progress of the transmetalation step.

High-resolution mass spectrometry reveals a complex ganglioside pattern and novel polysialylated structures associated with the human motor cortex

We have developed here a superior methodology based on high-resolution mass spectrometry for screening and fragmentation analysis of gangliosides extracted and purified from the human motor cortex . The experiments, conducted on a nanoelectrospray Orbitrap mass spectroscope in the negative ion mode, allowed the discrimination in the native mixture extracted from human motor cortex of no less than 83 different gangliosides, which represents the highest number of structures identified so far in this brain region. The spectral data, acquired in high-resolution mass spectrometry mode with a remarkable sensitivity and an average mass accuracy of 4.48 ppm, also show that the gangliosidome of motor cortex is generally characterized by species exhibiting a much higher degree of sialylation than previously known. Motor cortex was found dominated by complex structures with a sialylation degree ≥3, exhibiting long saccharide chains, in the G1 class. Fucogangliosides and species with the glycan chain elongated by either O-acetylation and/or acetate anion attachments were also detected; the later modification was for the first time discovered in this brain region. Of major significance is the identification of hepta and octasialylated species of GS1 and GO1 type, which are among the structures with the longest oligosaccharide chain discovered so far in the human brain. In the last stage of research, tandem mass spectrometry performed by higher energy collision dissociation provided structural data documenting the occurrence of GT1b (d18:1/20:0) isomer in the human motor cortex.

Electric modeling and characterization of pulsed high-voltage nanoelectrospray ionization sources by a miniature ion trap mass spectrometer

A better understanding of nanoelectrospray ionization (nano-ESI) would be beneficial in further improving the performances of nano-ESI. In this work, the pulsed high-voltage (HV) nano-ESI has been electrically modeled and then systematically characterized by both voltage-current and mass spectrometry measurements. First, the equivalent resistance of a nano-ESI source changes with respect to both emitter tip diameter and the HV applied. Increased voltage could improve both spray current and ionization efficiency of the pulsed HV nano-ESI. Compared with conventional DC HV method, a pulsed HV has less heating effect on the capillary tip and thus allowing the application of a much higher voltage onto a nano-ESI source. As a result, a pulsed HV nano-ESI could further boost the ionization efficiency of nano-ESI by employing even higher voltages than conventional DC nano-ESI sources.

Mass spectrometric analysis of carisoprodol and meprobamate in rat brain microdialysates

We report the evaluation of several mass spectrometry-based methods for the determination of carisoprodol and meprobamate in samples obtained from the rat brain by in vivo intracranial microdialyis. Among the techniques that aspire to perform analyses without chromatographic separation and thereby increase throughput, chip-based nanoelectrospray ionization and the use of an atmospheric pressure solids analysis probe fell short of requirements because of insufficient detection sensitivity and hard ionization, respectively. Although direct analysis in real time provided the required soft ionization, shortcomings of a tandem mass spectrometry-based assay also included inadequate detection sensitivity and, in addition, poor quantitative reproducibility. Therefore, liquid chromatography coupled with atmospheric pressure chemical ionization tandem mass spectrometry was developed to determine carisoprodol and meprobamate from artificial cerebrospinal fluid as the medium. No desalting and/or extraction of the samples was necessary. The assay, combined with in vivo sampling via intracranial microdialyis, afforded time-resolved concentration profiles for the drug and its major metabolite from the nucleus accumbens region of the brain in rats after systemic administration of carisoprodol. Copyright © 2016 John Wiley & Sons, Ltd.

Chip-based nanoelectrospray ionization with Fourier transform mass spectrometric detection to screen for local anesthetics intended to mask limb sore in walking horses

We report a high-throughput chip-based nanoelectrospray ionization method coupled with Fourier transform mass spectrometry to screen for local anesthetics in samples collected by swabbing. These drugs have been used to mask pain on the limbs of walking horses after forbidden practices of soring or physical abuse. Optimized for lidocaine, the method afforded sub-ppm mass accuracy for nine local anesthetics included in the study. From doped cotton swabs, two third and all of the analytes were detected after adding 10 ng and 100 ng of each drug, respectively. Benzocaine and/or lidocaine were found on positive swab samples collected during walking horse competitions.