LC-Swan Probe: An Integrated In Situ Sampling Interface for Liquid Chromatography Separation and Mass Spectrometry Analysis

Falcon Probe: A High-Pressure and Robust Sampling Interface for Coupling Lossless Liquid Chromatography Injection with In Situ Nanoliter-Scale Sample Pretreatment

With the increasing demand for trace sample analysis, injecting trace samples into liquid chromatography-mass spectrometry (LC-MS) systems with minimal loss has become a major challenge. Herein, we describe an in situ LC-MS analytical probe, the Falcon probe, which integrates multiple functions of high-pressure sample injection without sample loss, high-efficiency LC separation, and electrospray. The main body of the Falcon probe is made of stainless steel and fabricated by the computer numerical control (CNC) technique, which has ultrahigh mechanical strength. By coupling a nanoliter-scale droplet reactor made of polyether ether ketone (PEEK) material, the Falcon probe-based LC-MS system was capable of operating at mobile-phase pressures up to 800 bar, which is comparable to those of conventional ultraperformance liquid chromatography (UPLC) systems. Using the probe pressing microamount in situ (PPMI) injection approach, the Falcon probe-based LC-MS system showed high separation efficiency and good repeatability with relative standard deviations (RSDs) of retention time and peak area of 1.8% and 9.9%, respectively, in peptide mixture analysis (n = 6). We applied this system to the analysis of a trace amount of 200 pg of HeLa protein digest and successfully identified an average of 766 protein groups (n = 5). By combining in situ sample pretreatment at the nanoliter range, we further applied the present system in single-cell proteomic analysis, and 241 protein groups were identified in single 293 cells, which preliminarily demonstrated its potential in the analysis of trace amounts of samples with complex compositions.

Microfluidic Sampling of Undissolved Components from Subcellular Regions of Living Single Cells for Mass Spectrometry Analysis

Precise sampling of undissolved chemical components from subcellular regions of living single cells is a prerequisite for their in-depth analysis, which could promote understanding of subtle early stage physiological or pathological processes. Here we report a microfluidic method to extract undissolved components from subcellular regions for MS analysis. The target single cell was isolated by the microchamber beneath the microfluidic probe and washed by the injected biocompatible isotonic glucose aqueous solution (IGAS). Then, the sampling solvent was injected to extract undissolved components from the expected subcellular region of the living single cell, where the position and size of the sampling region could be controlled. The components immobilized by undissolved cellular structures were proven to be successfully extracted. Since unextracted subcellular regions were protected by IGAS, the single cell could survive after a tiny part was extracted, providing the possibility of repetitive sampling of the same living cell. Phospholipids extracted from the subcellular regions were successfully identified. The results demonstrated the feasibility of our method for subcellular sampling and analysis.

Emerging open-channel droplet arrays for biosensing

Open-channel droplet arrays have attracted much attention in the fields of biochemical analysis, biofluid monitoring, biomarker recognition and cell interactions, as they have advantages with regard to miniaturization, parallelization, high-throughput, simplicity and accessibility. Such droplet arrays not only improve the sensitivity and accuracy of a biosensor, but also do not require sophisticated equipment or tedious processes, showing great potential in next-generation miniaturized sensing platforms. This review summarizes typical examples of open-channel microdroplet arrays and focuses on diversified biosensing integrated with multiple signal-output approaches (fluorescence, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical, etc.). The limitations and development prospects of open-channel droplet arrays in biosensing are also discussed with regard to the increasing demand for biosensors.

Microfluidic Mixer for In Situ Ammonia Analysis of Single Cells in Mass Spectrometry

Mass spectrometry (MS) is a powerful tool for exploring single-cell heterogeneity. However, due to the ultralow absolute content of most substances in a single cell, existing methods can only analyze high-content substances; conventional methods are incompetent for quantitative analysis of important trace-amount small-molecule metabolites such as ammonia and sulfide. Herein, a method integrating single-cell extraction, online derivatization, and MS for multifunctional and more accurate MS analysis is reported. For application, ammonia content in a single cell was analyzed, where the cellular heterogeneity in ammonia metabolism was revealed. First, the extraction room of a microfluidic probe was covered on the target single cell, and the extraction fluid was allowed to flow through a single cell and dissolve cellular ammonia. Then, the ammonia was mixed and reacted with the pretreatment reagent input from another inlet to achieve the derivatization and signal amplification, enhancing the analysis sensitivity on MS. Finally, the sample was sent to MS, and the ammonia content was successfully quantitatively evaluated by analyzing its derivative urotropine, demonstrating the potential of this method to advance single-cell mass spectrometry analysis to higher sensitivity.

Coupling handheld liquid microjunction-surface sampling probe (hLMJ-SSP) to the miniature mass spectrometer for automated and in-situ surface analysis

In-situ surface analysis has always been a significant but challenging issue in many analytical applications. To enable surface sampling and MS analysis on the field, a handheld liquid microjunction-surface sampling probe (hLMJ-SSP) was coupled to a miniature mass spectrometer in this study. The hLMJ-SSP was connected to the miniature mass spectrometer with a 50 cm long flexible tube and controlled by an integrated microcontroller unit (MCU). By simply pressing a button on the probe, automated surface analysis could be carried out and completed within ∼2 min, in which solvent delivery, sample extraction, extract aspiration, injection and MS analysis were performed in sequence. A limit of detection (LOD) as low as 5 pg was achieved. The capability of in-situ surface analysis was also demonstrated by directly analyzing illicit drug residues on fingers and medicines in biological fluids. With the merits of automated operation, high sensitivity and nondestructive analysis, this LMJ-SSP-mini MS system would be a promising tool for various in-situ analytical applications.

Droplet-assisted electrospray phase separation using an integrated silicon microfluidic platform

We report a silicon microfluidic platform that enables monolithic integration of transparent micron-scale microfluidic channels, an on-chip segmentation of analyte flows into picoliter-volume droplets, and a nano-electrospray ionization emitter that enables spatial and temporal separation of oil and aqueous phases during electro-spray for subsequent mass spectrometry analysis.

Peptide and protein assays using customizable bio-affinity arrays combined with ambient ionization mass spectrometry

High-throughput identification and quantification of protein/peptide biomarkers from biofluids in a label-free manner is achieved by interfacing bio-affinity arrays (BAAs) with nano-electrospray desorption electrospray ionization mass spectrometry (nano-DESI-MS). A wide spectrum of proteins and peptides ranging from phosphopeptides to cis-diol biomolecules as well as thrombin can be rapidly extracted via arbitrarily predefined affinity interactions including coordination chemistry, covalent bonding, and biological recognition. An integrated MS platform allows continuous interrogation. Profiling and quantitation of dysregulated phosphopeptides from small-volume (∼5 μL) serum samples has been successfully demonstrated. As a front-end device adapted to any mass spectrometer, this MS platform might hold much promise in protein/peptide analysis in point-of-care (POC) diagnostics and clinical applications.

Customizable bio-affinity arrays were interfaced with ambient ionization mass spectrometry for high-throughput assays of protein/peptide biomarkers in biofluids.

Petrel Probe: An Integrated In Situ Sampling and Injection Interface for Fast, High-Efficiency Liquid Chromatography-Mass Spectrometry Analysis

Herein, we describe an in situ analysis probe, Petrel probe, highly integrating multiple functions of in situ sampling, in situ sample injection, high-performance liquid chromatography (HPLC) separation, and mass spectrometry (MS) electrospray. The Petrel probe was fabricated based on a single capillary, which consists of a micrometer-sized hole for sampling, a packed column for LC separation, and a tapered tip for MS electrospray. The design of the Petrel probe was optimized to obtain higher structural strength, and a polytetrafluoroethylene (PTFE) chip was used for sealing the probe-sampling hole to meet the high-pressure (∼30 MPa) requirement of LC manifold. On the basis of the Petrel probe, we developed a novel valveless LC injection method, that is, the probe pressing microamount in situ (PPMI) injection method, which performs sample injection by pressing the probe-sampling hole on the PTFE chip, using the mobile phase to dissolve the sample dry spot in the sampling area on the chip, and injecting it into the LC column under high-pressure conditions for separation and subsequent MS analysis. The LC-MS system with the PPMI injection method exhibits rapid injection and separation speed, as well as minimum injection dead volume. It can yield a high separation efficiency comparable to those of conventional HPLC systems. The present system was optimized using standard peptide samples, and four peptides were separated within 11 min in a probe with an effective column length of 5 cm, achieving the highest theoretical plate number up to ∼5,500,000/m. The system was also applied in the separation of cytochrome C digest to demonstrate its separation ability for complex samples, and 21 peptides were detected in 8 min with an amino-acid coverage of 83%.

Spatially resolved absolute quantitation in thin tissue by mass spectrometry

Mass spectrometry (MS) has become the de facto tool for routine quantitative analysis of biomolecules. MS is increasingly being used to reveal the spatial distribution of proteins, metabolites, and pharmaceuticals in tissue and interest in this area has led to a number of novel spatially resolved MS technologies. Most spatially resolved MS measurements are qualitative in nature due to a myriad of potential biases, such as sample heterogeneity, sampling artifacts, and ionization effects. As applications of spatially resolved MS in the pharmacological and clinical fields increase, demand has become high for quantitative MS imaging and profiling data. As a result, several varied technologies now exist that provide differing levels of spatial and quantitative information. This review provides an overview of MS profiling and imaging technologies that have demonstrated quantitative analysis from tissue. Focus is given on the fundamental processes affecting quantitative analysis in an array of MS imaging and profiling technologies and methods to address these biases.Graphical abstract.

h-FBN assisted negative ion paper spray for the sensitive detection of small molecules

Negative ion mode paper spray mass spectrometry (PS-MS) suffers from intense background noise and unstable MS signal. For the first time, we reported fluorinated boron nitride nanosheet (h-FBN) assisted negative ion PS-MS for the detection of a series of molecules. We demonstrated that the introduction of h-FBN can greatly improve the detection sensitivity and signal stability in the negative ion mode.