Development of a miniature protein mass spectrometer capable of analyzing native proteins

SAM-SFM: High-Efficiency and High-Resolution Tandem Mass Spectrometry Enabled by Sinusoidal Amplitude Modulation of Multiple Sinusoidal Frequency-Modulated Waveforms

In miniature ion trap mass spectrometry, achieving a balance between isolation resolution and efficiency is a formidable challenge. The presence of absorption curves causes target ions to inadvertently absorb energy from AC signal components near their resonant frequencies. To mitigate this issue, SAM-SFM waveforms introduce a parameter known as the decreasing factor. Unlike SWIFT waveforms, SAM-SFM's spectral profile intentionally departs from a rectangular window, adopting an arch-shaped excitation window to minimize the impact on target ions and improve ion isolation efficiency. SAM-SFM waveforms have the advantage of low computational complexity, enabling real-time computation using an embedded FPGA technology. Regardless of any parameter changes, the FPGA can consistently guarantee waveform output within 1 μs. This not only enhances throughput but also eliminates the need for a PC in miniature mass spectrometry devices. The performance of SAM-SFM has been validated on an improved "Brick" miniature ion trap mass spectrometer. Comparative experiments with SWIFT waveforms confirm the lossless unit-mass isolation capabilities of SAM-SFM. This waveform has the capability to simultaneously isolate multiple target ions, even allowing for the lossless isolation of ions with lower abundance within isotopic clusters, albeit at the cost of requiring extended isolation durations.

Capillary self-aspirating electrospray ionization (CSESI) for convenient and versatile mass spectrometry analysis

Electrospray ionization (ESI) is one of the most widely used ionization techniques, and its simplification can benefit many interested users. In this study, sample introduction by capillary action was studied and used to develop a simple ESI source called capillary self-aspirating electrospray ionization (CSESI). A conventional CSESI source requires only a common capillary of appropriate diameter in addition to the support of high voltage (HV). No pumps and sample loading is needed because the solution can spontaneously climb across the capillary by capillary action. With the proper modification of the glass capillary, the operation of CSESI can be further simplified and efficient. Specifically, cold plasma processing of the capillary creates a more hydrophilic surface that can facilitate sample introduction. Moreover, sputtering a thin platinum layer on the capillary tip makes the application of HV more convenient, and it also eliminates the influence of air bubbles in the capillary to ensure a sustained and stable electrospray. Overall, CSESI exhibits multiple desirable features such as simple structure, self-aspiration ability, low sample consumption, and inherent physical filtration capability. Apart from the routine ESI-MS analysis, it has also been applied in real-time monitoring of the oxidative dimerization of 8-methyl-1,2,3,4-tetrahydroquinoline, as well as direct analysis of muddy soil solutions without pretreatment.

Development of a High-Field "Brick" Mass Spectrometer with Extended Mass Range and Capable of Characterizing Native Proteins

One of the main challenges of analyzing intact proteins on an ion trap mass spectrometer is the mass range limitation, especially for miniature mass spectrometers. In this study, a high-field frequency scanning ion trap miniature mass spectrometer, namely the high-field "Brick" mass spectrometer, was developed to analyze intact proteins. A high-voltage broadband radio frequency (rf) amplifier was designed with a maximum output of 900 Vp-p over a frequency range of 130-700 kHz. Compared to the 600 Vp-p rf amplifier equipped in the conventional "Brick" mass spectrometer, the mass range of the instrument could be extended from 2000 to over 8000 Th. Sensitivity and mass resolution for native protein analyses were also evaluated and compared. Various proteins as well as their mixtures were analyzed on the high-field "Brick" mass spectrometer. The noncovalent interaction between protein-ligand complexes, lysozyme with triN-acetylchitotriose, was also analyzed. In addition, a hybrid ion scan mode was explored to further expand the mass range of the instrument at both low- and high-mass ends. In the hybrid ion scan mode, both rf frequency and amplitude were tuned, and a mass range from 100 to 12,000 Th was realized. As a result, both small drugs and proteins could be analyzed in a single mass scan. As proof-of-concept demonstrations, a mixture of atenolol and bovine serum albuminand oligomers of transferrin were analyzed.

Miniature mass spectrometers and their potential for clinical point-of-care analysis

Mass spectrometry (MS) has become a powerful technique for clinical applications with high sensitivity and specificity. Different from conventional MS diagnosis in laboratory, point-of-care (POC) analyses in clinics require mass spectrometers and analytical procedures to be friendly for novice users and applicable for on-site clinical diagnosis. The recent decades have seen the progress in the development of miniature mass spectrometers, providing a promising solution for clinical POC applications. In this review, we report recent advances of miniature mass spectrometers and their exploration in clinical applications, mainly including the rapid analysis of illegal drugs, on-site monitoring of therapeutic drugs, and detection of biomarkers. With improved analytical performance, miniature mass spectrometers are also expected to apply to more and more clinical applications. Some promising POC analyses that can be performed by miniature mass spectrometers in the future are discussed. Lastly, we also provide our perspectives on the challenges in technical development of miniature mass spectrometers for clinical POC analysis.

Development of a linear ion trap mass spectrometer capable of analyzing megadalton MALDI ions

Detecting megadalton matrix-assisted laser desorption/ionization (MALDI) ions in an ion trap mass spectrometer is a technical challenge. In this study, megadalton protein and polymer ions were successfully measured by MALDI linear ion trap mass spectrometer (LIT-MS) for the first time. The LIT-MS is comprised of a Thermo linear ion trap mass analyzer and a highly sensitive charge-sensing particle detector (CSPD). A newly designed radio frequency (rf) scan mode with dipolar resonance ejection techniques is proposed to extend the mass range of LIT-MS up to one million Thomson (Th). We analyze high mass ions with mass-to charge (m/z) ratios ranging from 100 kTh to 1 MTh, including thyroglobulin, alpha-2-macroglobulin, immunoglobulins (e.g., IgG and IgM), and polymer (∼ 940 kTh) ions. Besides, it is also very challenging for ion trap mass spectrometry to detect megadalton ions at low concentrations. By adopting high affinity carboxylated/oxidized detonation nanodiamonds (oxDNDs) to enrich IgM molecules and form antibody-nanodiamond conjugates, we have successfully reached ∼ 5 nM (5 μg/mL) concentration which is better than that by the other techniques.

Development and application of a miniature mass spectrometer with continuous sub-atmospheric pressure interface and integrated ionization source

For the miniature mass spectrometer (MS) with a continuous atmospheric pressure interface (CAPI), the gas in the multi-stage chambers directly affects the performance of the instrument. In this study, a sealed ionization chamber is designed to couple with a conventional mini CAPI-MS. In this configuration, the gas environment in the first ionization chamber can be flexibly changed to regulate the gas conditions throughout the entire instrument. By studying the effect of gas pressure on the performance of the instrument, we found that the instrument shows some unique advantages when the first ionization chamber is under sub-atmospheric pressure (SAP) conditions, such as reducing the load of the vacuum pump by 40%, achieving pump-free injection for gas and liquid samples, and improving the resolution by a factor of 2 without loss of detection sensitivity. Therefore, we propose a new integrated interface called continuous sub-atmospheric pressure interface (CSAPI) for building a miniature ion trap mass spectrometer. The CSAPI specially integrates sample introduction, gas/ions interface, and ionizations, including electrospray ionization (ESI) and secondary electrospray ionization (SESI), making this system more convenient for non-professional handlers to rapidly identify or monitor target analytes in gaseous- and solution-phase samples. We also use this system to study gas composition to further improve performance, being able to achieve a 5-fold sensitivity and 2-fold resolution improvement. At last, some custom applications of the current CSAPI-MS platform are explored and demonstrated, including real-time monitoring of chemical reactions in solution and long-distance sampling and analysis of dried Chinese herbs. In conclusion, this study provides a new approach to constructing a complete, versatile and practical miniature MS instrument.

Native top-down mass spectrometry for higher-order structural characterization of proteins and complexes

Progress in structural biology research has led to a high demand for powerful and yet complementary analytical tools for structural characterization of proteins and protein complexes. This demand has significantly increased interest in native mass spectrometry (nMS), particularly native top-down mass spectrometry (nTDMS) in the past decade. This review highlights recent advances in nTDMS for structural research of biological assemblies, with a particular focus on the extra multi-layers of information enabled by TDMS. We include a short introduction of sample preparation and ionization to nMS, tandem fragmentation techniques as well as mass analyzers and software/analysis pipelines used for nTDMS. We highlight unique structural information offered by nTDMS and examples of its broad range of applications in proteins, protein-ligand interactions (metal, cofactor/drug, DNA/RNA, and protein), therapeutic antibodies and antigen-antibody complexes, membrane proteins, macromolecular machineries (ribosome, nucleosome, proteosome, and viruses), to endogenous protein complexes. The challenges, potential, along with perspectives of nTDMS methods for the analysis of proteins and protein assemblies in recombinant and biological samples are discussed.