Direct determination of trace amounts of acetic acid using a novel ambient glow discharge ion source

<正>A novel ambient glow discharge ion source with improved line-cylinder electrodes is put forward and designed in this paper.The diameters of inner and outer electrodes are 0.16 and 4 mm respectively.With a special assembly method,a perfect coaxiality of the two electrodes is obtained.From the gas discharge experiment,it can be seen that the discharge can stably work in normal glow discharge mode.The operating currents of the ion source are in an order of milliamperes and can generate a much larger number and wider variety of reagent ions.The MS experiment shows that the ion source has higher detection sensitivity.     

一种用于高场非对称波形离子迁移谱系统的新型敞开式化学离子源

根据高场非对称波形离子迁移谱(FAIMS)系统的要求, 本文提出了一种新型敞开式直流电晕放电化学离子源. 该离子源主要由内线电极、外筒电极和牵引电极组成, 内、外电极半径分别是0.08、2 mm. 筒壁电极上开有对称的4个槽, 用于通入样品和牵引离子. 质谱实验结果表明, 该离子源能够在敞开环境下很好地离子化丙酮、乙醇、苯胺、N,N-二甲基甲酰胺、甲基磷酸二甲酯(DMMP)、乙酸乙酯、甲酸、乙酸、苯酚等正、负电性物质. 静电计测试实验结果说明该离子源能够稳定地产生离子电流. 通过分析不同时刻的谱图发现, 在不同时间点上产生的主要离子相同, 具有很好的稳定性. 利用感应耦合等离子体(ICP)工艺在硅片上加工了该离子源, 从而验证了该结构可以由微机电系统(MEMS)加工技术实现. 该离子源具有体积小、结构简单、无辐射、工作稳定等特点, 不仅可以满足FAIMS系统的要求, 还可用于敞开式质谱、微型质谱仪、离子迁移谱(IMS)等仪器.     

Super‐atmospheric pressure chemical ionization mass spectrometry

Super‐atmospheric pressure chemical ionization (APCI) mass spectrometry was performed using a commercial mass spectrometer by pressurizing the ion source with compressed air up to 7 atm. Similar to typical APCI source, reactant ions in the experiment were generated with corona discharge using a needle electrode. Although a higher needle potential was necessary to initiate the corona discharge, discharge current and detected ion signal were stable at all tested pressures. A Roots booster pump with variable pumping speed was installed between the evacuation port of the mass spectrometer and the original rough pumps to maintain a same pressure in the first pumping stage of the mass spectrometer regardless of ion source pressure. Measurement of gaseous methamphetamine and research department explosive showed an increase in ion intensity with the ion source pressure until an optimum pressure at around 4–5 atm. Beyond 5 atm, the ion intensity decreased with further increase of pressure, likely due to greater ion losses inside the ion transport capillary. For benzene, it was found that besides molecular ion and protonated species, ion due to [M + 2H]⁺ which was not so common in APCI, was also observed with high ion abundance under super‐atmospheric pressure condition. Copyright © 2013 John Wiley & Sons, Ltd.     

单向气流负电晕放电离子迁移谱的研究及其在爆炸物检测中的应用

基于离子迁移谱的爆炸物探测仪多采用放射性电离源,发展非放射性电离源一直是该技术的研究热点。本研究基于电晕放电原理设计了一种新型负电晕放电电离源结构,结合自行研制的离子迁移谱仪,应用于痕量爆炸物的快速、高灵敏检测。单向气流模式下,对此电离源的气流、放电电压等运行参数进行了系统优化,得到最佳实验条件为：电晕放电电离源结构的电极环孔直径为3 mm,针-环距离为2 mm,放电电压为2400 V,漂气流速为1200 mL/min。在此条件下,避免了放电副产物氮氧化物和臭氧等引发的一系列复杂反应,得到了单一的反应试剂离子O-2(H2O)n。将其应用于爆炸物,如2,4,6-三硝基甲苯(TNT)、硝酸铵(AN)、硝化甘油( NG)、太安( PETN)、黑索金( RDX)等的高灵敏快速直接检测,对TNT的检测限达到200 pg/μL。结果表明,此负电晕放电电离源具有灵敏度高、结构简单、无辐射性、反应试剂离子单一等优点,在爆炸物快速高灵敏检测、公共安全保障等方面具有广阔的应用前景。     

电晕放电二氧化碳冷等离子体转化特性研究

在常压、室温条件下利用电晕放电使二氧化碳通过冷等离子体反应分解为一氧化碳和氧气,由四极质谱在线定量在分析产物组成。考察反应条件（电晕类型、能量密度、气体流量等）对反应转化率的影响,分析了该反应的能量效率。当放电功率为40W、CO2流量为30mL.min^-1时,正电晕等离子体CO2分解反应的转化率为15．2％;CO2,专座经率随体系能量密度的增加上升,随反应时间的增加而增大,当CO2流量为90mL.min^-1、正电晕放电功率为37．6W时,反应体系的能量效率为5．89％。实验发现,正电晕放电时CO2的转化率高负电晕的转化率。     

Mechanisms for negative reactant ion formation in an atmospheric pressure corona discharge

In an effort to better understand the formation of negative reactant ions in air produced by an atmospheric pressure corona discharge source, the neutral vapors generated by the corona were introduced in varying amounts into the ionization region of an ion mobility spectrometer/mass spectrometer containing a ⁶³Ni ionization source. With no discharge gas the predominant ions were O₂ ⁻, however, upon the introduction of low levels of discharge gas the NO₂ ⁻ ion quickly became the dominant species. As the amount of discharge gas increased the appearance of CO₃ ⁻ was observed followed by the appearance of NO₃ ⁻. At very high levels, NO₃ ⁻ species became effectively the only ion present and appeared as two peaks in the IMS spectrum, NO₃ ⁻ and the NO₃ ⁻·HNO₃ adduct, with separate mobilities. Since explosive compounds typically ionize in the presence of negative reactant ions, the ionization of an explosive, RDX, was examined in order to investigate the ionization properties with these three primary ions. It was found that RDX forms a strong adduct with both NO₂ ⁻ and NO₃ ⁻ with reduced mobility values of 1.49 and 1.44 cm²V⁻¹ s⁻¹, respectively. No adduct was observed for RDX with CO₃ ⁻ although this adduct has been observed with a corona discharge mass spectrometer. It is believed that this adduct, although formed, does not have a sufficiently long lifetime (greater than 10 ms) to be observed in an ion mobility spectrometer.     

Atmospheric pressure laser desorption/chemical ionization mass spectrometry: a new ionization method based on existing themes

We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI).     

A pulsed corona discharge switchable high resolution ion mobility spectrometer-mass spectrometer

A pulsed corona discharge ionisation source, a candidate replacement for 63Ni ionisation sources for ion mobility spectrometry, is described along with a new design of ion mobility spectrometer-mass spectrometer. Preliminary research on the characterisation of the reactant ion peaks associated with the use of this ionisation source was undertaken by assembling a pulsed corona discharge ionisation switchable high-resolution ion mobility spectrometer-mass spectrometer to enable the mobility spectra, atmospheric chemical ionisation mass spectra and selected-mass mobility spectra to be obtained. With ammonia doping at 2.39 mg m(-3) in air and a water content of approximately 80 mg m(-3) in the positive mode the observed response was attributable to the formation of 1(H2O)(n)NH4]+ and [(H2O)n(NH3)NH4]+ in the reaction region. The observed responses in the negative mode were more complex with evidence for the formation [(H2O)(n)O2]-, [(H2O)(n)CO3]-, [(H2O)(n)HCO3]-, [(H2O)(n)CO4]- and [(H2O)(n)NO3]-. The responses due to these species were clearly discernible in the resultant mobility spectra, with enough oxygen-based species formed to support analytically useful responses.     

Development of a dielectric barrier discharge ion source for ambient mass spectrometry

A new ion source based on dielectric barrier discharge was developed as an alternative ionization source for ambient mass spectrometry. The dielectric barrier discharge ionization source, termed as DBDI herein, was composed of a copper sheet electrode, a discharge electrode, and a piece of glass slide in between as dielectric barrier as well as sample plate. Stable low-temperature plasma was formed between the tip of the discharge electrode and the surface of glass slide when an alternating voltage was applied between the electrodes. Analytes deposited on the surface of the glass slide were desorbed and ionized by the plasma and the ions were introduced to the mass spectrometer for mass analysis. The capability of this new ambient ion source was demonstrated with the analysis of 20 amino acids, which were deposited on the glass slide separately. Protonated molecular ions of [M + H](+) were observed for all the amino acids except for L-arginine. This ion source was also used for a rapid discrimination of L-valine, L-proline, L-serine and L-alanine from their mixture. The limit of detection was 3.5 pmol for L-alanine using single-ion-monitoring (SIM). Relative standard deviation (RSD) was 5.78% for 17.5 nmol of L-alanine (n = 5). With the advantages of small size, simple configuration and ease operation at ambient conditions, the dielectric barrier discharge ion source would potentially be coupled to portable mass spectrometers.     

Corona Discharge Ionization Source for a Planar High-Field Asymmetric Waveform Ion Mobility Spectrometer

A corona discharge (CD) ionization source was prepared for a planar high-field asymmetric waveform ion mobility spectrometer (FAIMS). The effects of discharge current and discharge distance on ionization efficiency were investigated; and the electric field dependence of the ion injection in the reaction region was studied. The results showed that the discharge current of CD source had good linearity with the intensity of reactant ion peak (RIP), and the RIP intensity increased to a stable level at the discharge distance of >5 mm. An injection electrode was introduced to improve the ionization efficiency. A square-wave voltage applied to the electrode was found to provide optimal performance of ion injection and utilization. The operating parameters of the CD-FAIMS were optimized to achieve trace level detection of dimethyl methylphosphonate (DMMP) sample. The detection limit for DMMP was 0.5 µg/m^3.     

Coupling electrospray corona discharge, charge reduction and ion mobility mass spectrometry: From peptides to large macromolecular protein complexes

We present the design and implementation of a home-built point-to-plane corona discharge probe, which rapidly and efficiently charge reduces biological ions generated by electrospray ionization (ESI). The molecules analysed ranged from small peptides such as Glu-fibrinopeptide B (1.5 kDa), small proteins such as myoglobin (16.9 kDa), polymers such as polyethylene glycol (PEG 10 k) which all showed intense singly charged ions; to large native multiprotein complexes such as GroEL (802 kDa) which show a broad range of charge-reduced species. The corona discharge probe operates at atmospheric pressure and was directly interfaced with a standard-ESI or nanoflow-ESI source of quadrupole ion mobility time-of-flight mass spectrometer. The corona discharge probe is completely modular and could potentially be mounted to any commercial or research grade mass spectrometer with an ESI source. The level of charge reduction is precisely controlled by the applied voltage and/or probe gas flow rate and when in operation, results in approximately a 50 % reduction in total ion current. We also present the combination of corona discharge and travelling wave ion mobility and assign helium collision cross-section values (Ω_He) to the charge reduced species of the native protein complex pyruvate kinase. It would appear that the Ω_He of the +20 charge state for pyruvate kinase is approximately 20 % smaller than the +35 charge state. Finally, we discuss the potential benefits and concerns of utilising charge reduced protein species as a means of extending the travelling wave collision cross-section calibration range over that which is already published.     

Development of ambient sampling chemi/chemical ion source with dielectric barrier discharge

The development of a new configuration of chemical ionization (CI)‐based ion source is presented. The ambient air containing the gaseous sample is sniffed into an enclosed ionization chamber which is of sub‐ambient pressure, and is subsequently mixed with metastable species in front of the ion inlet of the mass spectrometer. Metastable helium atoms (He*) are used in this study as the primary ionizing agents and are generated from a dielectric barrier discharge (DBD) source. The DBD is powered by an AC high‐voltage supply and the configuration of the electrodes is in such a way that the generated plasma is confined within the discharge tube and is not extended into the ionization chamber. The construction of the ion source is simple, and volatile compounds released from the bulky sample can also be analyzed directly by approaching the sample to the sampling nozzle. When combined with heated nitrogen or other desorption methods, its application can also be extended to non‐volatile compounds, and the consumption for helium can be kept minimum solely for maintaining the stable discharge and gas phase ionization. Applications to non‐proximate sample analysis, direct determination of active ingredients in drug tablets and the detection of trace explosive such as hexamethylene triperoxide diamine are demonstrated. Copyright © 2010 John Wiley & Sons, Ltd.     

Detection of explosives on solid surfaces by thermal desorption and ambient ion/molecule reactions

A simple, fast and direct method is presented for detecting traces of solid explosives on cotton swabs or in particulate samples: ions are transferred into a mass spectrometer after thermal desorption and corona discharge chemical ionization in ambient air; specificity is enhanced using ambient ion/molecule reactions or by conventional tandem mass spectrometry.     

Characterization of a distributed plasma ionization source (DPIS) for ion mobility spectrometry and mass spectrometry

A recently developed atmospheric pressure ionization source, a distributed plasma ionization source (DPIS), was characterized and compared to commonly used atmospheric pressure ionization sources with both mass spectrometry (MS) and ion mobility spectrometry (IMS). The source consisted of two electrodes of different sizes separated by a thin dielectric. Application of a high RF voltage across the electrodes generated plasma in air yielding both positive and negative ions. These reactant ions subsequently ionized the analyte vapors. The reactant ions generated were similar to those created in a conventional point-to-plane corona discharge ion source. The positive reactant ions generated by the source were mass identified as being solvated protons of general formula (H₂O)_nH⁺ with (H₂O)₂H⁺ as the most abundant reactant ion. The negative reactant ions produced were mass identified primarily as CO₃⁻, NO₃⁻, NO₂⁻, O₃⁻ and O₂⁻ of various relative intensities. The predominant ion and relative ion ratios varied depending upon source construction and supporting gas flow rates. A few compounds including drugs, explosives and amines were selected to evaluate the new ionization source. The source was operated continuously for 3 months and although surface deterioration was observed visually, the source continued to produce ions at a rate similar that of the initial conditions.     

On-line characterization of gaseous and particulate organic analytes using atmospheric pressure chemical ionization mass spectrometry

A modified atmospheric pressure chemical ionization ion source is applied for direct analysis of volatile or low volatile organic compounds in air. The method is based on the direct introduction of the analytes in the gas phase and/or particle phase into the ion source of a commercial ion-trap mass spectrometer. Two methods are employed for the production of primary ions at atmospheric pressure, photoionization and corona discharge. It is shown that in the presence of a dopant, photoionization can be a highly efficient ionization method also for real-time analysis with detection limits for selected analytes in the lower ppt-range. Using corona discharge for the production of primary ions, which is instrumentally easier since no additional chemicals have to be added to the sample flow, we demonstrate the analytical potential of on-line atmospheric pressure chemical ionization mass spectrometry for reaction monitoring experiments. To do so, an atmospherically relevant gas phase reaction is carried out in a 500 l reaction chamber and gaseous and particulate compounds are monitored in the positive and negative ion mode of the mass spectrometer.     

Programmable gate delayed ion mobility spectrometry-mass spectrometry: a study with low concentrations of dipropylene-glycol-monomethyl-ether in air

The concept of using a short ionisation event, in this case a pulsed corona discharge, in conjunction with programmed gate delay is described. This technique is proposed for the selective study of different ionisation processes within the reaction region of an ion mobility spectrometer. The utility of such an approach was tested in a study of the ionisation of dipropylene-glycol-monomethyl-ether (DPM); a compound commonly used to test the operation of ion mobility spectrometers. Dipropylene-glycol-monomethyl-ether at a concentration of 113 microg m(-3) in air, with a water level of 75 mg m(-3) in air, was analysed using a switchable, high resolution ion mobility spectrometer, operating in the positive mode at 40 degrees C at ambient pressure. The ion mobility spectrometer was fitted with a pulsed corona discharge ionisation source, doped with ammonia at a concentration of 1.3 mg m(-3) in the reaction region, and interfaced to a mass spectrometer. Synchronisation of the ionisation event to the operation of the shutter grids for the drift region enabled different parts of the product ion population to be injected into the drift tube, and programming the gate delays produced a map of the gate delay verses drift time response surface. Ammonium bound dipropylene-glycol-monomethyl-ether was observed, [(DPM)NH4]+ (m/z 166) as well as the ammonium bound dimer [(DPM)2NH4]+ (m/z 314), the same as those observed with a 63Ni source. Two other species were also observed, but their molecular identity was not elucidated. One of them m/z 146, also observed with 63Ni, formed ammonium bound ions [(m/z 146)NH4]+ (K0= 1.49 cm2 V(-1) s(-1)), ammonium bound dimer ions [(m/z 146)2NH4]+(K0= 1.18 cm2 V(-1) s(-1)) and a mixed cluster ion with DPM [(m/z 146)(DPM)NH4]+(K0= 1.18 cm2 V(-1) s(-1)); while the other, m/z 88 a decomposition product, formed ammonium bound monomer [(m/z 88)NH4]+(K0= 1.68 cm2 V(-1) s(-1)), dimer ions [(m/z 88)2NH4]+(K0= 1.40 cm2 V(-1) s(-1)) and a mixed cluster ion containing DPM and ammonium, [(DPM)(m/z 88)2NH4]+(K0= 1.40 cm2 V(-1) s(-1)). The assignment of responses to these ions required the additional dimensionality in the data provided from the gate delay studies. The relationships evident in the programmable gate delay data enabled these ions to be differentiated from alternative assignments of possible nitrogen clusters, formed at the interface of the mass spectrometer.     

等离子体常压解吸离子源质谱

近年来,常压开源质谱解吸/离子化(ambient desorption/ionization mass spectrometry)技术的开发促进了质谱分析的蓬勃发展.作为常压开源质谱技术的一大重要分支,基于等离子体的常压质谱解吸/离子化技术因无需样品预处理、无溶剂化过程、高灵敏度、高通量、能实时在线检测样品等优点,受到了学术界及仪器制造、化学和生物分析等相关产业界的广泛关注.本文从离子源的种类、原理及应用等角度对目前我国在开源质谱领域中自主研发的基于等离子体的常压离子源的研究进展进行了总结,并对此类等离子体常压离子源的发展进行了展望,重点涉及微波等离子体源、常压微辉光放电等离子体源和解吸电晕束离子源.     

Atmospheric pressure matrix-assisted laser desorption/ionization with analysis by ion mobility time-of-flight mass spectrometry

The use of an atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) source was employed with an atmospheric pressure ion mobility spectrometer (APIMS) and an orthogonal acceleration reflector time-of-flight mass spectrometer (TOFMS) to analyze dipeptide and biogenic amine mixtures from a liquid glycerol 2,5-dihydroxybenzoic acid (DHB) matrix. Improved sensitivities were obtained by the addition of a localized electrical (corona) discharge in conjunction with the AP-MALDI source. Enhanced sample ionization efficiency created by this combination provided an overall elevation in signal intensity of approximately 1.3 orders in magnitude. Combinations of three dipeptides (Gly-Lys, Ala-Lys, and Val-Lys) and nine biogenic amines (dopamine, serotonin, B-phenylethylamine, tyramine, octopamine, histamine, tryptamine, spermidine, and spermine) were resolved in less than 18 ms. In many cases, reduced mobility constants (K(o)) were determined for these analytes for the first time. Ion mobility drift times, flight times, arbitrary signal intensities, and collision-induced dissociation (CID) fragmentation product signatures are reported for each of the samples.     

High Pressure (>1 atm) Electrospray Ionization Mass Spectrometry

High pressure electrospray ionization mass spectrometry has been performed by pressurizing a custom made ion source chamber with compressed air to a pressure higher than the atmospheric pressure. The ion source was coupled to a commercial time-of-flight mass spectrometer using a nozzle-skimmer arrangement. The onset voltage for the electrospray of aqueous solution was found to be independent on the operating pressure. The onset voltage for the corona discharge, however, increased with the rise of pressure following the Paschen’s law. Thus, besides having more working gas for the desolvation process, gaseous breakdown could also be avoided by pressurizing the ESI ion source with air to an appropriate level. Stable electrospray ionization has been achieved for the sample solution with high surface tension such as pure water in both positive and negative ion modes. Fragmentation of labile compounds during the ionization process could also be reduced by optimizing the operating pressure of the ion source.     

大气采样辉光放电质谱仪的设计与应用

针对某些弱极性类物质难以通过大气压离子源直接电离的问题,提出基于大气采样辉光放电电离方式实现弱极性物质在大气压下直接进样、电离和质谱分析的方法.通过在大气压接口-四极质谱仪的第一级真空中的离子透镜上施加交流高压产生放电,简化了辉光放电离子源的设计,能直接离子化大气压接口吸人的物质,离子在离子透镜的传输下进入四极杆质量分析器实现质谱分析.实验表明,本方法能电离电喷雾电离离子源和大气压化学电离离子源未能电离的弱极性物质——艾试剂,并且负离子工作模式比正离子工作模式的信号至少强40倍.