Discontinuous Subatmospheric Pressure Interface Reduces the Gas Flow Effects on Miniature CAPI Mass Spectrometer

Detecting the therapeutic drugs in blood samples through PDMS-printed paper spray mass spectrometry

In this paper, paper microfluidic channel fabricated by directly screen-printing of polydimethylsiloxane (PDMS) is proposed for paper spray mass spectrometry analysis of therapeutic drugs in the blood samples. Compared with traditional paper spray, PDMS-printed paper spray (PP-PS) allows fluid to flow to the tip of paper with less sample loss which significantly improved the signal intensity of target compounds in blood samples. As paper can reduce the matrix effect, PP-PS also has a greater advantage than electro-spray Ionization (ESI) when directly analyzing complex biological sample in terms of the detection efficiency. Linearity and limits of detection (LOD) were evaluated for five psychotropic drugs: olanzapine, quetiapine, 9-hydroxyrisperidone, clozapine, risperidone. As a result, PP-PS improved the signal intensity of the psychotropic drugs at a concentration of 250 ng/ml in blood samples by a factor of 2-5 times and lowered the relative standard deviation (RSD) by a factor of 2-5.6 times compared with traditional paper spray. And PP-PS also improved signal intensity by a factor of 9-33 times compared with ESI. Quantitative experiments of PP-PS mass spectrometry indicated that the linear range was 5-500 ng/ml and the LOD were improved by a factor of 5-71 times for all these drugs compared with traditional paper spray. In addition, PP-PS was applied to the home-made miniaturized mass spectrometer and the precursor ions of all five psychotropic drugs (250 ng/ml) in the mass spectrometry results were obtained as well. These could prove that PP-PS has the potential to analyze complex biological samples in application on the miniaturized mass spectrometer which can be used outside the laboratory.

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.

A simple numerical simulation model can elucidate the key factors for designing a miniaturized ion trap mass spectrometer

BACKGROUND

Miniature ion trap mass spectrometer enables mass-to-charge ratio analysis of ions via quadrupole field in a low vacuum environment. It plays an important role in on-site detection due to its portability and specificity. In order to gain a deeper understanding of the analysis mechanism of miniature ion trap mass spectrometers, a quadrupole MS ion trajectory numerical simulation model (QITNS) is established in this paper for ions trajectory calculation under the action of quadrupole field, exciting field and neutral gas molecule collision. Compared with the existing methods, the model in this paper is simpler and more direct, which effectively explored the effects of dipole excitation and quadrupole excitation on ion manipulation under high background pressure.

RESULTS

The simulation results demonstrate that high RF amplitude, low auxiliary AC amplitude and quadrupole excitation can effectively improve the isolation resolution. Besides, it clarified the difference between the analysis mechanism of ion trap mass spectrometers under high background pressure (above 13.332 Pa) and absolute vacuum conditions. The relevant results are consistent with the conclusions of previous experiments and other theories, proving the applicability and accuracy of the proposed calculation model and solution method.

SIGNIFICANCE

This research bears the guiding significance for further understanding the mechanism of quadrupole mass spectrometry as well as designing and developing miniature mass spectrometers.

Development and characterization of discontinuous atmospheric pressure interface - Dual pressure chamber miniature mass spectrometer

Miniaturized mass spectrometers have become increasingly prevalent for real-time detection and analysis, owing to their compact size and portability. The pursuit of performance enhancement in these instruments is a pivotal objective within the domain of mass spectrometry miniaturization. This study introduces a novel miniature mass spectrometer featuring a discontinuous atmospheric pressure interface and a dual pressure chamber. Compared to conventional single-chamber, discontinuous sampling interface mass spectrometers, the newly developed instrument demonstrates a more than tenfold improvement in detection efficiency. This significant enhancement is achieved without the need for complex control of switch coupling time series, thereby streamlining the circuit design and improving the instrument's fault tolerance. Furthermore, by capitalizing on the benefits of discontinuous sampling, the instrument reduces the operational pressure relative to traditional continuous sampling in differential pressure vacuum chambers. It accommodates larger inlet capillary (0.38 mm) and skimmer (0.5 mm) diameters, leading to a ninefold increase in response strength for risperidone and lowering the detection limit to 0.5 ppb. The instrument's capacity for rapid drug detection, along with enhanced resolution and detection limits, underscores its potential utility. Additionally, it facilitates the use of smaller mechanical pumps, significantly diminishing both the instrument's volume and power consumption. This presents a promising avenue for further miniaturization of mass spectrometers.

A Miniature Orthogonal Injection Ion Funnel (MO-IF) Providing Enhanced Performance for the Miniature Mass Spectrometer

The sensitivity of the miniature mass spectrometer (mini-MS) is largely restricted by the ion transmission in rough vacuum region. Even though various "in-line" ion transfer devices have improved mini-MS sensitivity, the severe dynamic gas is still weakening the efficiency of ion transmission in this region. Inspired by the "off-axis" ion funnel design in the lab-scale mass spectrometers, a miniature orthogonal injection ion funnel (MO-IF) was developed in this study for the mini-MS with a continuous atmospheric pressure interface. Capable of directing injected ions by 90° and then transport them forward to the downstream skimmer, the MO-IF enabled the separation of ions from the dynamic gas flow jetted out of the inlet capillary. The key factors were optimized for the MO-IF, including the effects of RF amplitude, DC electric fields, and the position of the repeller. Under optimized conditions, the MO-IF minimized the negative effects of dynamic gas and improved the ion transmission efficiency by ∼2-fold in comparison with the in-line injection ion funnel. As a result, a lower limit of detection of 0.5 ng/mL were obtained with good linearity for hypaconitine. Additionally, the MO-IF further decreased the buffer gas pressure in the second vacuum chamber and improved the mass resolution by 1.1-1.5 times at different scan rates.

Structural annotation of unknown molecules in a miniaturized mass spectrometer based on a transformer enabled fragment tree method

Structural annotation of small molecules in tandem mass spectrometry has always been a central challenge in mass spectrometry analysis, especially using a miniaturized mass spectrometer for on-site testing. Here, we propose the Transformer enabled Fragment Tree (TeFT) method, which combines various types of fragmentation tree models and a deep learning Transformer module. It is aimed to generate the specific structure of molecules de novo solely from mass spectrometry spectra. The evaluation results on different open-source databases indicated that the proposed model achieved remarkable results in that the majority of molecular structures of compounds in the test can be successfully recognized. Also, the TeFT has been validated on a miniaturized mass spectrometer with low-resolution spectra for 16 flavonoid alcohols, achieving complete structure prediction for 8 substances. Finally, TeFT confirmed the structure of the compound contained in a Chinese medicine substance called the Anweiyang capsule. These results indicate that the TeFT method is suitable for annotating fragmentation peaks with clear fragmentation rules, particularly when applied to on-site mass spectrometry with lower mass resolution.

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.

SR-Unet: A Super-Resolution Algorithm for Ion Trap Mass Spectrometers Based on the Deep Neural Network

The mass spectrometer is an important tool for modern chemical analysis and detection. Especially, the emergence of miniature mass spectrometers has provided new tools for field analysis and detection. The resolution of a mass spectrometer reflects the ability of the instrument to discriminate between adjacent mass-to-charge ratio ions, and the higher the resolution, the better the discrimination of complex mixtures. Quadrupole ion traps are generally considered as a low-resolution mass spectrometry method, but they have gained wide attention and development in recent years because of their suitability for miniaturization and high qualitative capability. For an ion trap mass spectrometer, the mass sensitivity and resolution can be mutually constrained and need to be balanced by setting an appropriate scanning speed. In this study, a super-resolution U-net algorithm (SR-Unet) is proposed for ion trap mass spectrometry, which can estimate the possible ions from the overlapping ion peaks of low-resolution spectra and improve the equivalent resolution while ensuring sufficient sensitivity and analysis speed of the instrument. By determining the mass spectra of a linear ion trap mass spectrometer (LTQ XL) in Turbo and Normal scan modes, the same unit mass resolution as that at a scan speed of 16,667 Da/s was successfully obtained at 125,000 Da/s. Also, the experiments demonstrated that the algorithm is capable of the mass-to-charge ratio and instrument migration. SR-Unet can be migrated and applied to a miniature mass spectrometer for cruise detection of volatile organic compounds (VOCs), and the identification of VOC species in Photochemical Assessment Monitoring Stations (PAMS) was improved from 31 to 50 species with the same monitoring and analysis speed requirement. Further, super-unit mass resolution peptide detection was achieved on a miniature mass spectrometer with the help of the SR-Unet algorithm, which reduced the full width at half-maxima (FWHM) of bradykinin divalent ions (m/z 531) from 0.35 to 0.15 Da at a scan speed of 375 Da/s and improved the equivalent resolution to 3540. The proposed method provides a new idea to enhance the field mixture detection capability of miniature ion trap mass spectrometers.

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.

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.

SWIFTSIN: A High-Resolution Ion Isolation Waveform for the Miniaturized Linear Ion Trap Mass Spectrometer by Coarse to Fine Excitation

To figure out the reason for the drawback of the stored waveform inverse Fourier transform (SWIFT) waveform and realize the high-resolution ion isolation on the miniaturized linear ion trap mass spectrometer, we studied the efficiency that ions can be excited under different excitation durations and amplitudes at different frequencies and compared the overlap ratios of the effective excitation frequency bandwidths of the adjacent ions. According to this, we proposed a new coarse-to-fine isolation waveform named SWIFTSIN. By superposing one or more sinusoidal waveforms on the SWIFT waveform and modulating the phases of the superposed sinusoidal waveforms, the generated SWIFTSIN waveform can achieve unit mass isolation on the miniaturized linear ion trap mass spectrometer without reducing the intensity of the target ion. The isolation ability of the SWIFTSIN waveform was verified by isolating a single isotope peak in the mixed samples.

Field-Gradient-Focusing Ion Guide for Enhanced Transfer Efficiency of Low-Mass Ions

Efficient transmission of low-mass ions in a rough vacuum pressure region has always been a challenging issue in mass spectrometry (MS). In this study, a novel ion guide, namely, field-gradient-focusing ion guide (FGF-IG), was proposed to improve the transfer efficiency of ions, especially low-mass ions in a rough vacuum region. The FGF-IG has 12 electrodes whose surfaces gradually narrowed and tilted inward, and its electric field gradually varies from dodecapole (or multipole) to quadrupole along the ion transfer route. The field radius was gradually decreased from 6 to 2 mm in the multipole region (65 mm in length) and finally remained unchanged as 2 mm in the quadrupole region (20 mm in length). By integrating into a miniature mass spectrometer (mini-MS) with a continuous atmospheric pressure interface, this ion guide was optimized in terms of inlet capillary position, radio frequency amplitude, and direct current voltage applied on it. Results showed that a reduced low-mass discrimination effect and improved efficiency of simultaneously transferring mid and low m/z ions were achieved for FGF-IG compared with a conventional ion funnel. Under optimized conditions, a limit of detection of 1 ng/mL was obtained for both reserpine (m/z 609) and arginine (m/z 175) ions by integrating FGF-IG into the mini-MS. The sensitivity of smaller arginine ions using FGF-IG was enhanced by ∼10 times than that obtained using the conventional ion funnel (10 ng/mL) in comparative experiments. The idea of smooth transfer from dodecapole to quadrupole fields could be extended to other multipole fields, as well as in lab-scale MS instruments.

Hexapole-Assisted Continuous Atmospheric Pressure Interface for a High-Pressure Photoionization Miniature Ion Trap Mass Spectrometer

Miniature mass spectrometers are powerful tools for on-site chemical analysis in the fields of homeland security, personal healthcare, and environmental monitoring. This study presents a novel hexapole-assisted continuous atmospheric pressure interface for a high-pressure photoionization miniature ion trap mass spectrometer (HA-HPPI-IT). Efficient ion transmission was achieved by combining radial focusing by an RF electric field and axial driving by gas flow, which was demonstrated by SIMION simulation and experimental verification. The pressure in the ionization-transmission chamber and the inner diameter of the skimmer were optimized, which helped in determining the number density of product ions and affected the ion transmission in the hexapole, respectively. After systematic optimizations, about 16-fold increase in signal intensity was achieved as the RF amplitude was varied from 140 to 400 V_pp, and a limit of detection of 1 ppbv was obtained. In addition, the HA-HPPI-IT exhibited high stability and the relative standard deviation was as low as 5.47%. Finally, the apparatus was applied for discovering the simulated spot for illicit drug synthesis by detecting toluene and propiophenone released to air and monitoring the evolutions of perchloroethylene residues from dry-cleaned clothes.

Portable mass spectrometry system: instrumentation, applications, and path to 'omics analysis

Mass spectrometry (MS) is an information rich analytical technique and plays a key role in various 'omics studies. Standard mass spectrometers are bulky and operate at high vacuum, which hinder their adoption by the broader community and utility in field applications. Developing portable mass spectrometers can significantly expand the application scope and user groups of MS analysis. This review discusses the basics and recent advancements in the development of key components of portable mass spectrometers including ionization source, mass analyzer, detector, and vacuum system. Further, major areas where portable mass spectrometers are applied are also discussed. Finally, a perspective on the further development of portable mass spectrometers including the potential benefits for 'omics analysis is provided.

Rapid screening of illegally added drugs in functional food using a miniature ion trap mass spectrometer

With the expansion of the functional food market, the qualification assessment of these products has become a major challenge, and efficient analytical tools are urgently needed. Here, a miniature mass spectrometer (MS) with self-aspiration capillary electrospray ionization (SACESI) source and ion trap analyzer was developed for rapid screening of various illegally added drugs in functional foods. No chromatographic separation was required, but a simplified two-step pretreatment method was developed to reduce the operational procedures and time consumption of the entire analysis. SACESI source uses capillary action to drive solution injection, which utilizes a simple structure and convenient operation to constitute a kind of disposable MS detection solution. To achieve accurate and automatic identification, an intelligent recognition algorithm with steps of spectrum preprocessing, characteristic peak matching, and support vector machine learning was constructed. The relative accuracy of rapid screening of 31 suspicious drugs in various samples is up to 99.78%. It achieves 100% correct identification for the 55 batches of actual samples captured by on-site inspection, which demonstrates the feasibility of the proposed analytical system and strategy in food safety applications.

Handheld Mass Spectrometer with Intelligent Adaptability for On-Site and Point-of-Care Analysis

The development of miniature mass spectrometry (MS) systems with simple analysis procedures is important for the transition of applying MS analysis outside traditional analytical laboratories. Here, we present Mini 14, a handheld MS instrument with disposable sample cartridges designed based on the ambient ionization concept for intrasurgical tissue analysis and surface analysis. The instrumentation architecture consists of a single-stage vacuum chamber with a discontinuous atmospheric interface and a linear ion trap. A major effort in this study for technical advancement is on making handheld MS systems capable of automatically adapting to complex conditions for in-field analysis. Machine learning is used to establish the model for autocorrecting the mass offsets in the mass scale due to temperature variations and a new strategy is developed to extend the dynamic concentration range for analysis. Mini 14 weighs 12 kg and can operate on battery power for more than 3 h. The mass range exceeds m/z 2000, and the full peak width at half-maximum is Δm/z 0.4 at a scanning speed of 700 Th/s. The direct analysis of human brain tissue for identifying glioma associated with isocitrate dehydrogenase mutations has been achieved and a limit of detection of 5 ng/mL has been obtained for analyzing illicit drugs in blood.

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

Current miniature mass spectrometers were usually designed for the detection of small and medium size molecules, including volatile (semi-volatile) compounds, drugs and lipids. In this study, a miniature protein mass spectrometer was developed in this work, which could serve as a biosensor for the rapid identification of proteins as well as their conformations. A linear ion trap with a field radius of 2.5 mm was designed to extend mass range of the instrument to over 6500 Th. Mass resolution and sensitivity of the instrument were also optimized for protein ions by increasing the buffer gas pressure and using a high-gain Faraday detector. It is then demonstrated that the mass spectra of native proteins, such as IgG1, could be acquired by coupling the instrument with a soft electrospray ionization source. As a proof-of-concept demonstration, results suggest that the current instrument could be used to identify target proteins and probe/distinguish their conformations in solutions.

Data-driven and coarse-to-fine baseline correction for signals of analytical instruments

Baseline correction is an indispensable step in the signal processing of chemical analysis instruments. With the increasing demand for on-site applications, a variety of analytical instruments require a more friendly, rapid and adaptive baseline correction method. In this paper, a data-driven and coarse-to-fine (DD-CF) baseline correction scheme mainly based on the empirical mode decomposition (EMD) algorithm is proposed. For eliminating the mode-mixing effect of the original EMD, the proposed method firstly obtains a coarse baseline estimation using automatic peak detection, elimination and interpolation; and the EMD is applied on the coarse baseline to get a fine baseline finally. We have compared this method with the adaptive iteratively reweighted Penalized Least Squares algorithm (airPLS) and the sparse representation baseline correction methods using simulated signals and experimental signals from different analytical instruments. Results indicate that the proposed DD-CF scheme can effectively estimate the baseline more accurate than the comparing methods for varies of analytical signals such as mass spectrometer, ion mobility spectrometer, gas chromatograph, etc. Furthermore, with signals of different length, different peak distributions and even from totally different instruments, the proposed method requires minimal user intervention, in which the parameters of the comparing methods should be adjusted for a wide range.

Development of membrane inlet photoionization ion trap mass spectrometer for trace VOCs analysis

With the development of instrumental miniaturization, the portable mass spectrometer is becoming a new tool for on-site rapid analysis of environmental samples. Membrane inlet (MI) and photoionization (PI) are two commonly used sampling and ionization techniques, respectively, as they both exhibit detection selectivity for volatile organic compounds (VOCs). In this paper, a membrane inlet photoionization ion trap mass spectrometer was developed for the direct analysis of VOCs in gaseous samples. With the new structure and timing design, various operation modes were proposed and tested. In particular, the use of pulse carrier gas can integrate the appropriate pressure conditions required by each module, thus improving the efficiency of analyte transport, ionization, and mass analysis. The detection limit of sub-ppb was obtained, and the response time can be greatly reduced by increasing the sample flow rate. Furthermore, the capability of selective enrichment for organic analytes was also realized by using a special accumulation mode with a modified sequence, which is easy to operate because no additional devices are needed.