Continuous Atmospheric Pressure Interface Research Articles

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.

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 Vpp, 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.

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.

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.

Ambient ionization coupled with a miniature mass spectrometer for rapid identification of unauthorized adulterants in food

Considerably simplified workflows based on a miniature mass spectrometer combined with versatile ambient ionization methods were developed for rapid, on-site identification of illicit adulterants in select food (medicinal liquor, chili powder, hotpot seasoning and watermelon). The labor- and time-intensive procedures of sample pretreatment and chromatographic separation, traditionally required for mass spectrometry analysis, have been bypassed using capillary nano-electrospray ionization, paper spray ionization, or syringe spray ionization. A miniature ion trap mass spectrometer with a continuous atmospheric pressure interface and tandem mass spectrometry capability was employed for the analysis of adulterated additives. Key experimental parameters were carefully optimized in order to achieve the optimal performance of the integrated system. Maintaining the ease of analytical protocol, limits of detection as low as 30–40 μg/L for sex stimulants in medicinal liquor, 20–50 μg/kg for illegal dyes in chili powder and hotpot seasoning, and 15 μg/kg for plant growth regulator in watermelon were obtained. The capability of conducting quantitative analysis with an isotopic internal standard was demonstrated. The developed methodologies coupling ambient ionization and miniature mass spectrometry represent promising potential for fast and in situ screening of food adulteration.

Boosting the Sensitivity and Selectivity of a Miniature Mass Spectrometer Using a Hybrid Ion Funnel.

Besides high portability, high analytical performances are also crucial concerns for a miniature mass spectrometer to meet the demands in in situ analysis. As a continuous effort in improving analytical performances of the miniature mass spectrometer with continuous atmospheric pressure interface, a hybrid ion funnel was developed and coupled into the system in this study. The hybrid ion funnel consisted of a rectangular ion funnel region and a planar quadrupole field region, which were fabricated by the printed circuit board technology. After systematic optimization, a limit of detection of 1 ng/mL was obtained, which was improved by 10 folds relative to that of 10 ng/mL previously reported for the miniature mass spectrometer. Besides improved ion transmission efficiency, this hybrid ion funnel was also capable of filtering ions according to their mobilities, thus improving the system selectivity. This capability was demonstrated by separation and selective transmission of protein ions at different charge states, reserpine in PEG background and isobaric peptide ions. Resolution of this system was also tested by analyzing isotopic peaks of reserpine. The ppb-level detection sensitivity and isotope resolving capability achieved in this work would greatly expand the application range of miniature mass spectrometers.

Portable linear ion trap mass spectrometer with compact multistage vacuum system and continuous atmospheric pressure interface.

A portable linear ion trap mass spectrometer featuring a compact three-stage vacuum system, a continuous atmospheric pressure interface (CAPI), and a miniature ion funnel was developed and characterized. The dimensions and weight of the instrument were 38 × 26 × 23 cm3 and ∼20 kg, respectively. The combination of a three-stage vacuum system and CAPI reduced the pressure smoothly from atmospheric to ∼5 × 10-4 Torr, ensuring that the miniature ion funnel, quadrupole ion guide, and linear ion trap operated in a suitable and stable vacuum environment. The analytical performance of the instrument was evaluated with a nano-electron spray ionization source and a reserpine sample solution. A satisfactory mass resolution up to 4060 (m/Δm, FWHM) was achieved at m/z 609 when the mass scan rate was 495 Da s-1. Unit mass resolution was achieved at a mass scan rate of 6000 Da s-1. In addition, a limit of detection of 5 ng mL-1 was achieved and tandem mass spectrometry (MS3) was successfully performed with the instrument. Furthermore, the measurements showed high repeatability and stability (RSD < 6%). This portable mass spectrometer shows great potential for practical applications in on-site analyses, such as those required for food safety, drug analysis, environmental protection, forensic investigations, and homeland security.

Improving the sensitivity of miniature linear ion trap mass spectrometer by a DC voltage applied on the eject electrodes.

A miniaturized linear ion trap mass spectrometer with continuous atmospheric pressure interface has been built in our lab. Significant extension in mass range and reduction in power consumption have been realized by the supplemental alternating current frequency scan mode. However, relatively poor sensitivity has been witnessed, which is directly dominated by the detection efficiency of the ion detector. Theoretical analysis has been implemented to find ways to improve the detection efficiency. The results show that enhanced sensitivity can be obtained by applying a direct current voltage on the pair of electrodes in eject direction. Experiments show that the sensitivity has been improved by more than one time due to the application of direct current voltage. With this design, this homemade miniature linear ion trap mass spectrometer can be used to analyze more rarefied samples, especially to on-site chemical analysis and space application.

Mini 2000: A Robust Miniature Mass Spectrometer with Continuous Atmospheric Pressure Interface

A miniature mass spectrometer with continuous atmospheric pressure interface (CAPI) developed previously in our lab has proved to have high stability and rapid analysis speed. With the aim of achieving smaller size, better performance and easier maintenance, in this study, an upgraded miniature mass spectrometer with CAPI was developed, in which all components were optimized and redesigned into a packaged unit. Using a more powerful pumping system, better analytical performances were obtained for this system. The miniature mass spectrometer has the capability to perform tandem mass spectrometry, and could be coupled with ambient ionization sources for analysis of different samples. Good stability (signal relative standard deviation, RSD &lt; 5%), high sensitivity (limit of detection, LOD 10 ng/mL), better than unit mass resolution, and a broad mass range (from 150 Da to 2000 Da) were obtained. Integrated with a tablet computer for system control, the miniature mass spectrometer has dimensions of 38 cm × 23 cm × 34 cm (length × width × height), and is 13 kg in total weight. The whole system is powered by an adapter with a power consumption of 200 watts in total.

Recent developments of miniature ion trap mass spectrometers

With outstanding analytical performance and portability, miniature mass spectrometer is one of the most powerful tools for in-situ analysis. The miniaturization of mass spectrometers has lasted for more than ten years, during which a number of miniature mass spectrometers employing different techniques have been developed. Small-in-size, working at relatively high pressure region and capable of performing tandem mass spectrometry, ion trap is the most widely used mass analyzer in miniature mass spectrometer systems. The recent development of miniature ion trap mass spectrometer systems in the last ten years was reviewed in this paper. These instruments adopt different atmospheric pressure interfaces (APIs), which are membrane inlets (MIs), discontinuous atmospheric pressure interface (DAPI) and continuous atmospheric pressure interface (CAPI). This review emphasizes on the mini mass spectrometry (MS) system that can be handheld by one person, but not the field-able ones that are too large to be hand-portable.

Development of a low power miniature linear ion trap mass spectrometer with extended mass range

A miniaturized ion trap mass spectrometer with continuous atmospheric pressure interface was built, which could be used in conjunction with internal ionization (in-vacuum plasma ionization) and external ionization (electrospray ionization). To improve its mass range, a supplemental AC signal frequency scan mode was performed for this miniature mass spectrometer, in which the fundamental RF signal was remained constant with an amplitude as low as several hundreds volts. Experiments showed that the upper limit of the mass range can be extended to 2500 Da for the miniature mass spectrometer developed in this work, and the power consumption can be reduced by 2/5, while maintaining good balance with performance parameters such as stability, sensitivity, and resolution. Due to the improvement in mass range and significant reduction in energy consumption compared with the instrument using the traditional resonant ejection mode, the scan method developed in this work is helpful to promote the miniature design of mass spectrometers for field analysis and space exploration.

A dual-source miniature mass spectrometer with improved sensitivity

Previously, we have reported the development of a miniature mass spectrometer with a continuous atmospheric pressure interface, whose sensitivity was sacrificed due to the limited gas flow rate. In this study, a circular lens was designed and integrated on the ion transfer pathway between the two pumping stages, and a ten-fold sensitivity improvement could be achieved when coupling with external ionization sources. Besides the capability of working with ionization sources in the atmosphere, there is also an in-vacuum plasma ionization source, so that both volatile and non-volatile samples could be analyzed. A 10 ppbv detection sensitivity was achieved for dimethyl methyl phosphonate (DMMP) in air using the plasma ionization source. Furthermore, the miniature mass spectrometer was also demonstrated for the direct analyses of progesterone in drinking water and atrazine in soil samples using paper spray and syringe spray, respectively.

Mini Mass Spectrometer Integrated with a Miniature Ion Funnel.

Previously, a continuous atmospheric pressure interfaced miniature mass spectrometer was developed in our lab. The continuous atmospheric pressure interface improves system robustness, stability, and scan speed, but it also results in limited ion transfer efficiency and reduced mass resolution. To solve these problems, a miniature ion funnel was designed and integrated into the miniature mass spectrometer for the first time. Besides ion transfer efficiency, dimension and power consumption of the ion funnel also need to be considered throughout the design process. After a systematic optimization, the designed miniature ion funnel could increase ion transfer efficiency by more than 10 times, while lowering the background pressure of ion trap by ∼2 times. As a result, sensitivity and mass resolution of the second generation miniature mass spectrometer were improved by 20 times and ∼2 times, respectively, while maintaining its high scan speed and stability. A sensitive and robust mini-MS, capable of coupling with ambient ionization sources would meet the needs of many on-site chemical analysis applications, such as in food, drug, and agricultural administrations, forensic science, homeland security, and etc.

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. However, largely constrained by the atmospheric pressure interface (API), these aspects are difficult to be realized in one system. Herein, we describe the development of a new miniature mass spectrometer with balanced performance. The miniature mass spectrometer is small in size (30 × 30 × 18 cm) but has a continuous API, which was achieved by high-pressure ion trap operation and maximized ion transfer efficiency with the utilization of a differential pumping system. The miniature mass spectrometer was characterized and optimized in terms of stability, sensitivity, mass range, mass resolution and scan speed. Rapid analysis of mixtures was demonstrated by coupling the miniature mass spectrometer with the ambient ionization technique of paper spray. This is the smallest miniature mass spectrometer to date, which has a continuous API.

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

The sensitivity of mass analysis is highly dependent on the efficiencies of the sample introduction, analyte ionization, and subsequent ion transfer to the mass analyzer. For miniature mass spectrometry (MS) systems, it is highly desirable to maximize the amount of the samples that can be introduced at the pumping capability highly compromised and to have the MS analysis performed with simple electronics and robust in-vacuum components. In this study, a discharge ionization source synchronized with the sample introduction has been developed for hand-held mass spectrometers with discontinuous atmospheric pressure interfaces (DAPI). The flow rate of the sample introduced with a DAPI can be much higher than that allowed with a conventional continuous atmospheric pressure interface. The pressure variation associated with the DAPI operation was used to turn on and off the synchronized discharge ionization (SDI). For analysis of volatile organic compounds (VOCs) in air samples, the SDI has been shown to be soft and molecular ions of the VOCs have been observed as the major species. Since the ions could be transferred directly to the ion trap mass analyzer without a barrier for maintaining pressure differences, high sensitivity in chemical analysis was also achieved, with limits of detection (LODs) better than 1 ppbv obtained for PAHs (polycyclic aromatic hydrocarbons) in air.