Synchronized Discharge Ionization for Analysis of Volatile Organic Compounds Using a Hand-Held Ion Trap Mass Spectrometer

Portable particle mass spectrometer

In situ and real-time analysis of airborne particulate matter mass distributions using portable particle mass spectrometer.

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 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.

Fast protein analysis enabled by high-temperature hydrolysis

While the bottom-up protein analysis serves as a mainstream method for biological studies, its efficiency is limited by the time-consuming process for enzymatic digestion or hydrolysis as well as the post-digestion treatment prior to mass spectrometry analysis. In this work, we developed an enzyme-free microreaction system for fast and selective hydrolysis of proteins, and a direct analysis of the protein digests was achieved by nanoESI (electrospray ionization) mass spectrometry. Using the microreactor, proteins in aqueous solution could be selectively hydrolyzed at the aspartyl sites within 2 min at high temperatures (∼150 °C). Being free of salts, the protein digest solution could be directly analyzed using a mass spectrometer with nanoESI without further purification or post-digestion treatment. This method has been validated for the analysis of a variety of proteins with molecular weights ranging from 8.5 to 67 kDa. With introduction of a reducing agent into the protein solutions, fast cleavage of disulfide bonds was also achieved along with high-temperature hydrolysis, allowing for fast analysis of large proteins such as bovine serum albumin. The high-temperature microreaction system was also used with a miniature mass spectrometer for the determination of highly specific peptides from Mycobacterium tuberculosis antigens, showing its potential for point-of-care analysis of protein biomarkers.

A "Brick" Mass Spectrometer with Photoionization for Direct Analysis of Trace Volatile Compounds

With high portability and favorable performance, miniature mass spectrometers have become one of the most attractive tools for on-site analysis of trace volatile compounds. Based on the "Brick" mass spectrometer (BMS) developed previously, a hand-held BMS integrated with a photoionization source (PI-BMS) was developed in this study for volatile compound analysis. With compact dimensions of 30 cm × 18.5 cm × 27.6 cm (length × width × height), the PI-BMS was equipped with a 10.6 eV UV lamp and capable of generating molecular ions. The capabilities of qualitative and quantitative analyses for different volatile samples were demonstrated and characterized. Under optimized conditions, high detection sensitivity in open air was obtained for the PI-BMS with a limit of detection (LOD) of ∼10 ppbv. As demonstrations of mixture analysis, four different fresh fruits were directly analyzed using PI-BMS, observing characteristic mass spectra for each type of fruit.

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

In the range of miniature mass spectrometers, the miniature ion trap mass spectrometer with continuous atmospheric pressure interface (CAPI) shows good performance potential and advantages due to its excellent sensitivity and analysis speed. However, in previous cases, placing the ion trap directly near the skimmer aperture means it will suffer high gas shock, which may affect performance. In this study, an improved miniature CAPI ion trap mass spectrometer was developed by gas flow optimization. According to the experimental results, excessive gas flow affects stability and resolution. The impact of the gas flow can be effectively reduced by reducing the inner diameter of the skimmer and adding an additional lens element to move the ion trap away from the skimmer aperture. However, this method will affect the sensitivity of the instrument to some extent, so a discontinuous subatmospheric pressure interface (DSPI) was developed to reduce the gas flow effects and improve the comprehensive performance. When using the DSPI system with a 0.4 mm skimmer and entrance lens, the resolution for roxithromycin was up to 2800 at a scanning speed of 1015 Th/s, which was 3.4-fold higher that without DSPI. The dynamic range of concentration reached 4 orders of magnitude and the detection limit for repaglinide was as low as 1 ng/mL. This study offers a new approach to develop better miniature ion trap mass spectrometers and to extend their practical application.

Automatic Analyte-Ion Recognition and Background Removal for Ambient Mass-Spectrometric Data Based on Cross-Correlation

Ambient mass spectrometry is a powerful approach for rapid, high-throughput, and direct sample analysis. Due to the open-air desorption and ionization processes, random fluctuations of ambient conditions can lead to large variances in mass-spectral signals over time. The mass-spectral data also can be further complicated due to multiple analytes present in the sample, background-ion signals stemming from the desorption/ionization source itself, and other laboratory-specific conditions (e.g., ambient laboratory air, nearby hardware). Thus, background removal and analyte-ion recognition can be quite difficult, particularly in non-targeted analyses. Here, we demonstrate the use of a cross-correlation-based approach to exploit chemical information encoded in the time domain to group/categorize mass-spectral peaks from a single analysis dataset. Ions that originate from ambient (or other) background species were readily flagged and removed from spectra; the result was a decrease in mass-spectral complexity by over 70% due to the removal of these background ions. Meanwhile, analyte ions were differentiated and categorized based on their time-domain profiles. With sufficient mass resolving-power and mass-spectral acquisition rate, isolated mass spectra containing ions from the same species in a sample could be extracted, leading to a reduction in mass-spectral complexity by more than 98% in some cases. The cross-correlation approach was tested with different ionization sources as well as reproducible and irreproducible sample introduction. Software built in-house enabled fully automated data processing, which can be performed within a few seconds. Ultimately, this approach provides an additional dimension of analyte separation in ambient mass-spectrometric analyses with information that is already recorded throughout the analysis.

Direct sampling mass spectrometry for clinical analysis

Direct sampling mass spectrometry (MS) has been advancing aggressively, showing immense potential in translating MS into the clinical field. Unlike traditional MS analysis involving extensive sample preparation and chromatographic separation, quick and simple procedures with minimal sample pretreatment or purification became available with direct sampling. An overview of the development in this field is provided, including some representative ambient ionization and fast extraction methods. Quantitative applications of these methods are emphasized and their efficacy are highlighted from a clinical aspect; non-quantitative applications in clinical analysis are also discussed. This review also discusses the integration of direct sampling MS with miniature mass spectrometers and its future outlook as an emerging clinical tool for point-of-care analysis.

Pulsed capillary introduction applied to a miniature mass spectrometer for efficient liquid analysis

RATIONALE

Capillary sampling of liquids for direct mass spectrometry (MS) analysis is introduced. The low transfer rate of the solution in the capillary will affect the analytical sensitivity and the response time; hence a pulsed capillary introduction (PCI) method was proposed and characterized.

METHODS

The experiments were carried out using a miniature quadrupole mass spectrometer, and liquid can be spontaneously drawn into the vacuum chamber for subsequent ionization and detection. A simple up-and-down motor platform was used to control the brief contact of the capillary inlet with the liquid sample and implement pulsed injection. The pulsed sampling parameters were optimized based on the characterization and dynamic study of liquid transfer in capillaries.

RESULTS

Compared with continuous capillary introduction (CCI), PCI can reduce the response time of MS analysis from more than half a minute to a few seconds. In addition, it provides better detection sensitivity as the ion signals of all solution components are enhanced and the acquired limit of detection (LOD) of toluene is about eight times lower than CCI analysis. For each analysis, the consumed sample volume is only a few nanoliters and the absolute consumption of the analyte can reach the femtogram level.

CONCLUSIONS

The proposed PCI method is proved to be successful in improving the sampling efficiency when performing direct liquid analysis without increasing the vacuum load. A miniature MS instrument with a proper capillary inlet can possess flexible operation modes to meet different application demands.

Ambient Ionization and Miniature Mass Spectrometry Systems for Disease Diagnosis and Therapeutic Monitoring

Mass spectrometry has become a powerful tool in the field of biomedicine. The combination of ambient ionization and miniature mass spectrometry systems could most likely fulfill a significant need in medical diagnostics, providing highly specific molecular information in real time for clinical and even point-of-care analysis. In this review, we discuss the recent development of ambient ionization and miniature mass spectrometers as well as their potential in disease diagnosis and therapeutic monitoring, with an emphasis on their capability in analysis of biofluids and tissues. We also speculate the future development of the integrated, miniature MS systems and provide our perspectives on the challenges in technical development as well as possible solutions for path forward.

A Miniaturized Linear Wire Ion Trap with Electron Ionization and Single Photon Ionization Sources

A linear wire ion trap (LWIT) with both electron ionization (EI) and single photon ionization (SPI) sources was built. The SPI was provided by a vacuum ultraviolet (VUV) lamp with the ability to softly ionize organic compounds. The VUV lamp was driven by a pulse amplifier, which was controlled by a pulse generator, to avoid the detection of photons during ion detection. Sample gas was introduced through a leak valve, and the pressure in the system is shown to affect the signal-to-noise ratio and resolving power. Under optimized conditions, the limit of detection (LOD) for benzene was 80 ppbv using SPI, better than the LOD using EI (137 ppbv). System performance was demonstrated by distinguishing compounds in different classes from gasoline. Graphical Abstract ᅟ.

Microplasma Ionization of Volatile Organics for Improving Air/Water Monitoring Systems On-Board the International Space Station

Low molecular weight polar organics are commonly observed in spacecraft environments. Increasing concentrations of one or more of these contaminants can negatively impact Environmental Control and Life Support (ECLS) systems and/or the health of crew members, posing potential risks to the success of manned space missions. Ambient plasma ionization mass spectrometry (MS) is finding effective use as part of the analytical methodologies being tested for next-generation space module environmental analysis. However, ambient ionization methods employing atmospheric plasmas typically require relatively high operation voltages and power, thus limiting their applicability in combination with fieldable mass spectrometers. In this work, we investigate the use of a low power microplasma device in the microhollow cathode discharge (MHCD) configuration for the analysis of polar organics encountered in space missions. A metal-insulator-metal (MIM) structure with molybdenum foil disc electrodes and a mica insulator was used to form a 300 μm diameter plasma discharge cavity. We demonstrate the application of these MIM microplasmas as part of a versatile miniature ion source for the analysis of typical volatile contaminants found in the International Space Station (ISS) environment, highlighting their advantages as low cost and simple analytical devices. Graphical Abstract ᅟ.

An aerodynamic assisted miniature mass spectrometer for enhanced volatile sample analysis

Low ppb-level VOC detection sensitivity was achieved by integrating an in-vacuum plasma ionization source into the continuous atmospheric pressure interfaced miniature mass spectrometer.

A hand-portable digital linear ion trap mass spectrometer

A hand-portable digital linear ion trap mass spectrometer (DLIT-MS) has been developed for VOC analysis.

Design and performance evaluation of a linear ion trap mass analyzer featuring half round rod electrodes

A novel linear ion trap mass analyzer featuring half round rod electrodes (HreLIT) has been built. It is mainly composed of two pairs of stainless steel electrodes which have a cross-section of half round rod and a pair of end electrodes. The HreLIT has a simple structure and so it could be assembled by hand with relatively high mechanical accuracy. The external dimension of HreLIT is 50 mm × 29.5 mm × 28 mm (length × width × height) and its internal volume is about 3.8 cm(3). A home-made HreLIT mass spectrometer with three-stage vacuum system was built and the performance of HreLIT was characterized using reserpine solution and PPG standard solution. When the scan rate was 254 u/s, mass peak with FWHM of 0.14 u was achieved for ions with m/z 609, which corresponds to a mass resolution of 4350. The HreLIT was also operated at a low q value of 0.28 to extend its mass range. The experiment result showed a mass range of over 2800 u and the amplitude of radio frequency (rf) signal was only 1560 V (0-p). Three-stage tandem mass spectrometry was successfully performed in the HreLIT, and the collision-induced dissociation (CID) efficiencies of MS(2) (CID of ions with m/z 609) and MS(3) (CID of ions with m/z 448) were 78% and 59%, respectively.

A microelectromechanical systems-enabled, miniature triple quadrupole mass spectrometer

Miniaturized mass spectrometers are becoming increasingly capable, enabling the development of many novel field and laboratory applications. However, to date, triple quadrupole tandem mass spectrometers, the workhorses of quantitative analysis, have not been significantly reduced in size. Here, the basis of a field-deployable triple quadrupole is described. The key development is a highly miniaturized ion optical assembly in which a sequence of six microengineered components is employed to generate ions at atmospheric pressure, provide a vacuum interface, effect ion guiding, and perform fragmentation and mass analysis. Despite its small dimensions, the collision cell efficiently fragments precursor ions and yields product ion spectra that are very similar to those recorded using conventional instruments. The miniature triple quadrupole has been used to detect thiabendazole, a common pesticide, in apples at a level of 10 ng/g.

Development of a miniature mass spectrometer with continuous atmospheric pressure interface

The demand for on-the-spot analysis is met by a miniature mass spectrometer which is preferred to be robust, stable, as small as possible and capable of analyzing different samples by coupling with various ionization methods.

Characterization of a DAPI-RIT-DAPI system for gas-phase ion/molecule and ion/ion reactions

The discontinuous atmospheric pressure interface (DAPI) has been developed as a facile means for efficiently introducing ions generated at atmospheric pressure to an ion trap in vacuum [e.g., a rectilinear ion trap (RIT)] for mass analysis. Introduction of multiple beams of ions or neutral species through two DAPIs into a single RIT has been previously demonstrated. In this study, a home-built instrument with a DAPI-RIT-DAPI configuration has been characterized for the study of gas-phase ion/molecule and ion/ion reactions. The reaction species, including ions or neutrals, can be introduced from both ends of the RIT through the two DAPIs without complicated ion optics or differential pumping stages. The primary reactant ions were isolated prior to reaction and the product ions were mass analyzed after controlled reaction time period. Ion/molecule reactions involving peptide radical ions and proton-transfer ion/ion reactions have been carried out using this instrument. The gas dynamic effect due to the DAPI operation on internal energy deposition and the reactivity of peptide radical ions has been characterized. The DAPI-RIT-DAPI system also has a unique feature for allowing the ion reactions to be carried out at significantly elevated pressures (in 10(-1) Torr range), which has been found to be helpful to speed up the reactions. The viability and flexibility of the DAPI-RIT-DAPI system for the study of gas-phase ion reactions have been demonstrated.

Sensitive low-pressure dielectric barrier discharge ion source

RATIONALE

We developed a novel highly sensitive soft ionization method: a low-pressure dielectric barrier discharge ionization (LP-DBDI) source. In this configuration, samples pass through the inside of a dielectric barrier discharge (DBD). Since samples pass through a DBD and its plasma jet, high ionization efficiency is expected. Furthermore, high transmission efficiency from the ion source to the mass spectrometer is also expected since the ion source is placed in a vacuum.

METHODS

Mass spectrometric detection was carried out in positive ion mode using an ion trap mass spectrometer. The LP-DBDI source or a conventional atmospheric pressure chemical ionization (APCI) source was attached to the mass spectrometer. Samples were vaporized and sent to ion sources with air flowing at a constant flow rate of 1.5 L/min. The LP-DBDI source was compared with a conventional APCI source.

RESULTS

Mass spectra of methyl salicylate, 2-undecanone and methamphetamine were acquired using the LP-DBDI source. Protonated molecules were mainly observed in the mass spectra. The sensitivities for methyl salicylate and 2-undecanone obtained using the LP-DBDI source were 44 times and 39 times higher, respectively, than those obtained using an APCI source.

CONCLUSIONS

LP-DBDI is a soft ionization method characterized by only minor fragmentation, similar to APCI. The sensitivity of the LP-DBDI source was found to be about 40 times higher than that of the conventional APCI source.