Isotopic metrology of carbon dioxide. II. Effects of ion source materials,conductance, emission,and accelerating voltage on dual-inlet cross contamination

We report high-precision isotopic carbon dioxide measurements, made before and after ion source modification to gas isotope ratio mass spectrometry (IRMS) instruments. Measurement protocols were designed to explore the effects of ion source material substitution, source conductance, inlet pressure, electron emission, acceleration potential, and inlet changeover equilibration time. After modification of the IRMS instruments at the National Institute of Standards and Technology (NIST) and the Max-Planck-Institute for Chemistry (MPI-Mainz), immediate changes were observed. At NIST, measurements were no longer sensitive to inlet equilibration times greater than 15 s, and different settings of ion source conductance resulted in delta(13)C shifts of about 0.04 per thousand per 10 per thousand measurement difference between sample and reference, a five-fold improvement. No significant changes in machine performance were observed after a month of use. After a year, performance had degraded slightly, but was controlled by ion source cleaning and the use of low-energy ion acceleration to minimize sputtering. At MPI-Mainz, results were very similar. We report cross-contamination coefficients measured since 1996, and discuss the role of adsorption, ion implantation, and sputtering on cross contamination in mass spectrometry systems. We recommend that users of high-precision IRMS instruments test for and minimize the effects described.     

Dynamic calibration and dissolved gas analysis using membrane inlet mass spectrometry for the quantification of cell respiration

A membrane inlet mass spectrometer connected to a miniaturized reactor was applied for dynamic dissolved gas analysis. Cell samples were taken from 7 mL shake flask cultures of Corynebacterium glutamicum ATCC 13032, and transferred to the 12 mL miniaturized reactor. There, oxygen uptake and carbon dioxide and its mass isotopomer production rates were determined using a new experimental procedure and applying nonlinear model equations. A novel dynamic method for the calibration of the membrane inlet mass spectrometer using first-order dynamics was developed. To derive total dissolved concentration of all carbon dioxide species (C(T)) from dissolved carbon dioxide concentration ([CO(2)](aq)), the ratio of C(T) to [CO(2)](aq) was determined by nonlinear parameter estimation, whereas the mass transfer coefficient of CO(2) was determined by the Wilke-Chang correlation. Subsequently, the suitability of the model equations for respiration measurements was examined using residual analysis and the Jarque-Bera hypothesis test. The resulting residuals were found to be random with normal distribution, which proved the adequacy of the application of the model for cell respiration analysis. Hence, dynamic changes in respiration activities could be accurately analyzed using membrane inlet mass spectrometry with the novel calibration method.     

The importance of both charge exchange and proton transfer in the analysis of polycyclic aromatic compounds using atmospheric pressure chemical ionization mass spectrometry

The response of atmospheric pressure chemical ionization (APCI) mass spectrometry to selected polycyclic aromatic compounds (PACs) was examined in a Micromass Quattro atmospheric pressure ion source as a function of both solvents and source gases. Typical PACs found in petroleum samples were represented by mixtures of naphthalene, fluorene, phenanthrene, pyrene, fluoranthene, chrysene, triphenylene, perylene, carbazole, dibenzothiophene, and 9-phenanthrol. A large range of different gases in the APCI source was studied, with emphasis on nitrogen, air, and carbon dioxide. Solvents used included water-acetonitrile, acetonitrile, dichloromethane, and hexanes. The signal responses were dependent on both the gases and solvents used, as was the ionization mechanism, as indicated by the degree of protonation with respect to the level of charge exchange. The combination of carbon dioxide in the nebulizer gas stream with nitrogen in the other streams gave a three- to fourfold better sensitivity than using nitrogen alone for both test mixtures and for complex samples.     

Initial implementation of an electrodynamic ion funnel with fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has become a widely used method to study biopolymers. The method, in combination with an electrospray ionization (ESI) source has demonstrated the highest resolution and accuracy yet achieved for characterization of biomolecules and their noncovalent complexes. The most common design for the ESI interface includes a heated capillary inlet followed by a skimmer having a small orifice to limit gas conductance between a higher pressure (1 to 5 torr) source region and the lower pressure ion guide. The ion losses in the capillary-skimmer interface are large (estimated to be more than 90%) and thus reduce achievable sensitivity. In this work, we report on the initial implementation of a newly developed electrodynamic ion funnel in a 3.5 tesla ESI-FTICR mass spectrometer. The initial results show dramatically improved ion transmission as compared to the conventional capillary-skimmer arrangement. An estimated detection limit of 30 zeptomoles (approximately 18,000 molecules) has been achieved for the analysis of the proteins with molecular weights ranging from 8 to 20 kDa.     

Origin of adduct ions in the electron-capture mass spectrum of tetracyanoethylene

The high-pressure electron-capture (HPEC) mass spectrum of tetracyanoethylene (TCNE) is dominated by unexpected hydrogen atom and hydrocarbon radical adduct ions when methane is used as the buffer gas. The origin of these unexpected ions was investigated by three separate mass spectrometric experiments: the electron-capture (EC) rate constant of TCNE was determined and integrated into a previously developed kinetic model of HPEC ion source events; electron impact mass spectra of TCNE were obtained following exposure of the ion source surfaces to irradiated methane and irradiated carbon dioxide; and TCNE was determined by gas chromatographic introduction into the HPEC ion source with multiple ion monitoring. All of these experiments suggest that the reactions leading to the major adduct ions observed in the HPEC mass spectrum of TCNE are initiated by alteration of TCNE on the walls of the ion source rather than in the gas phase.     

Improving mass spectrometer sensitivity using a high-pressure electrodynamic ion funnel interface

We report on a new electrodynamic ion funnel that operates at a pressure of 30 torr with no loss of ion transmission. The enhanced performance compared with previous ion funnel designs optimized for pressures of <5 torr was achieved by reducing the ion funnel capacitance and increasing the RF drive frequency (1.7 MHz) and amplitude (100-170 V peak-to-peak). No degradation of ion transmission was observed for pressures from 2 to 30 torr. The ability to operate at higher pressure enabled a new tandem ion funnel mass spectrometer interface design that can accommodate a greater gas load (e.g., from an ESI source). When combined with a multicapillary inlet, the interface provided more efficient introduction of ions, resulting in a significant enhancement in mass spectrometer sensitivity and detection limits.     

Studies on the positive-ion mass spectra from atmospheric pressure chemical ionization of gases and solvents used in liquid chromatography and direct liquid injection

Detailed studies have been made using different source gases and solvents in a Micromass Quattro mass spectrometer under positive ion atmospheric pressure chemical ionization conditions. The major background ions from nitrogen, air, or carbon dioxide were investigated by tandem mass spectrometry, followed by similar studies on solvents commonly employed in normal- and reversed-phase high-performance liquid chromatography, namely, water-acetonitrile, acetonitrile, and dichloromethane, with nitrogen, air, or carbon dioxide; hydrocarbon solvents were studied using nitrogen. Spectra were interpreted in terms of the gases, solvents, and their impurities. The acetonitrile spectra provided clear evidence for both charge exchange and proton transfer, the former being facilitated by the introduction of some air into a flow of nitrogen. Radical cations of acetonitrile dimers, trimers, and tetramers were observed, as were protonated dimer and trimer species. Examination of the analytical response of four polycyclic aromatic hydrocarbons in various hydrocarbon solvents, with nitrogen gas, showed that the sensitivity of detection for an analyte and its ionization mechanism are dependent on both the analyte structure and the solvent, with pyrene showing the highest sensitivity, phenanthrene and fluorene being intermediate, and naphthalene having the lowest sensitivity. The degree of protonation followed the same trend. Signal intensity and degree of protonation were dependent on the alkane solvent used, with isooctane providing the best overall sensitivity for the sum of protonated molecules and molecular ions. The ions observed in these studies appeared to be the most stable ions formed under equilibrium conditions in the source.     

Incorporation of a flared inlet capillary tube on a Fourier transform ion cyclotron resonance mass spectrometer

Flared inlet capillary tubes have been coupled with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to help the ion transmission from the atmospheric pressure to the first vacuum region. We investigated different types of atmospheric pressure ionization methods using flared inlet tubes. For most of the ionization methods, such as ESI and DESI, increased ion current transmitted from the atmospheric pressure ion source to the first stage vacuum system was observed with the use of our enhanced ion inlet designs. The corresponding ion intensity detected on a FT-ICR mass spectrometer was also observed to increase two- to fivefold using ESI or DESI with the flared tube inlet. Moreover, increased spray tip positional tolerance was observed with implementation of the flared inlet tube. We also include our preliminary results obtained by coupling AP-MALDI with flared inlet tube in this paper. For AP-MALDI, the measured ion current transferred through the flared inlet tube was about 2 to 3 times larger than the ion current through the control non-flared inlet tube.     

Isotopic metrology of carbon dioxide. I. Interlaboratory comparison and empirical modeling of inlet equilibration time,inlet pressure,and ion source conductance

We report a pilot study of high-precision differential isotope ratio measurements made on replicate samples of pure carbon dioxide using three instruments of identical manufacture. Measurement protocols were designed to explore the effects of sample size, ion source conductance, and inlet changeover equilibration time on the raw measurements. Our goal was better understanding of factors that influence these measurements in order to establish procedures for highly reproducible and accurate determinations of Reference Material (RM) isotopic compositions. Evaluation and modeling of reported data illuminated effects consistent with two instrumental memory sources--one short-lived (t((1/2)) approximately 10 s) and the other long-lived (t((1/2)) approximately 6-10 min), uncompensated by normal background measurements--that can significantly influence measurements made by the dual inlet method. These biases, proportional to the difference in isotopic compositions between the measured sample and reference gases, decrease in magnitude with increasing sample size, source conductance, and equilibration time. We observed biases as high as 0.1 per thousand per 10 per thousand difference between sample and reference gases. These memory sources may be responsible for measured delta(13)C values of RMs generally being highly reproducible within any single laboratory but less reproducible among independent laboratories. The magnitude of the bias is consistent with the ranges of delta(13)C values reported in prior laboratory intercomparisons. Uncertainties are most likely due to high and variable long-lived memory among the instruments tested.     

Determination of organophosphorus pesticides by gas chromatography with mass spectrometry using a large‐volume injection technique after magnetic extraction

A fast and efficient method was developed for the extraction and determination of organophosphorus pesticides in water samples. Organophosphorus pesticides were extracted by solid‐phase extraction using magnetic multi‐walled carbon nanotubes and determined by gas chromatography with ion‐trap mass spectrometry. Parameters affecting the extraction were investigated. Under optimum conditions of the method, 10 mg magnetic multi‐walled carbon nanotubes were added into 10 mL sample. After 2 min, adsorbent particles settled at the bottom of test tube with a magnet. After removing aqueous supernatant, the analytes were desorbed with acetonitrile. Then, 70 μL of acetonitrile phase was injected into the gas chromatography and mass spectrometry system that had an ion‐trap analyzer. To achieve high sensitivity, the large‐volume‐injection technique was used with a programmed temperature vaporization inlet, and the ion‐trap mass spectrometer was operated in single ion storage mode. Under the best conditions, the enrichment factors and extraction recoveries were in the range of 113–124 and 74–103%, respectively. The limits of detection were between 3 and 15 ng/L, and the relative standard deviations were < 10%. This method was successfully used for the determination of organophosphorus pesticides in dam water, lagoon water, and river water samples with good reproducibility and recovery.     

Determination of 2-methylisoborneol and geosmin in aqueous samples by static headspace-gas chromatography-mass spectrometry with ramped inlet pressure

A method for determining the earthy and musty odors 2-methylisoborneol (2-MIB) and geosmin in drinking water using static headspace-GC-MS is described. To achieve lower detection limits, split ratio was optimized with ramped inlet pressure for large headspace sampling volume. The ramped inlet pressure, which held higher pressure (higher column flow rate) only during injection, allowed us to inject 3-mL volume to GC with very low split ratio (2:1). Although sequential analysis with a stainless steel ion source often changed the mass spectrum of 2-MIB, this spectral change was eliminated by using an inert ion source with a 6 mm drawout plate. The detection limits of this method were 0.36 and 0.14 ng/L, respectively, for 2-MIB and geosmin. The repeatabilities (n = 30) were 6.6 and 4.8%, respectively, at 1 ng/L for 2-MIB and geosmin.     

CO(2) concentration measurements on air samples by mass spectrometry

A new technique for measuring CO(2) concentration in air samples, based on mass spectrometry, is described as an alternative to the common gas chromatographic method. Using a dual inlet isotope ratio mass spectrometer (IRMS), the ratio of the abundances of the m/z peaks 44 and 28 is determined. The precision of measurements (standard deviation <3 ppmv) is generally as good as the analysis with gas chromatography for small air samples (<1 ml STP of air). A major advantage of this new method is the possibility of parallel elemental and isotopic measurements of many air components. The technique is further improved by new wide mass range mass spectrometers allowing simultaneous intensity measurements of several m/z values between 28 and 44, resulting in an uncertainty of <0.5 ppm. The precision is somewhat limited by the production of N(2)O and NO(2) from N(2) and O(2) in the ion source, which accounts for about half of the signal strength at m/z 44. Copyright 2000 John Wiley & Sons, Ltd.     

Conversion of Methane and Carbon Dioxide in a DBD Reactor: Influence of Oxygen

A continuous plug flow reactor supported by a dielectric barrier discharge (DBD) is used to study the conversion of methane, carbon dioxide, and oxygen at different compositions. The three studied gases were diluted with helium to 3 % with an overall flow rate of 200 sccm. The 13.56 MHz plasma was ignited at atmospheric pressure. The product stream and the inlet flow were analyzed by a FTIR spectrometer equipped with a White-cell and by a quadrupole mass spectrometer. The DBD reactor generates hydrogen, carbon monoxide, ethane, ethene, acetylene, formaldehyde, and methanol. Additional oxygen in the feed has positive effects on the yield of methanol, formaldehyde and carbon monoxide and reduces the total consumed energy. The hydrogen yield reaches its maximum at medium amounts of oxygen in the inlet flow. The conversion of methane increases to a limiting value of about 35 %. Methane rich feeds increase the yield of hydrogen, ethane and methanol. On the other hand, additional oxygen has a negative influence on the produced amount of C2 hydrocarbons. The conversion of methane and carbon dioxide as well as the yield of synthesis gas components and C2 hydrocarbons increases by changing the plasma power to higher values.     

Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.     

Calibrant delivery for mass spectrometry

This article describes a means of sampling ions that are created at a location remote from the primary ion source used for mass spectral analysis. Such a source can be used for delivery of calibrant ions on demand. Calibrant ions are sprayed into an atmospheric pressure chamber, at a position substantially removed from the sampling inlet. A gas flow sweeps the calibrants towards the sampling inlet, and a new means for toggling the second ion beam into the instrument can be achieved with the use of a repelling field established by an electrode in front of the sampling inlet. The physical separation of two or more sources of ions eliminates detrimental interactions due to gas flows or fields. When using a nanoflow electrospray tip as the primary ion source, the potential applied to the tip completely repels calibrant ions and there is no compromise in terms of electrospray performance. When calibrant ions are desired, the potential applied to the nanoflow electrospray tip is lowered for a short period of time to allow calibrant ions to be sampled into the instrument, thus providing a means for external calibration that avoids the typical complications and compromises associated with dual spray sources. It is also possible to simultaneously sample ions from multiple ion beams if necessary for internal mass calibration purposes. This method of transporting additional ion beams to a sampling inlet can also be used with different types of atmospheric pressure sources such as AP MALDI, as well as sources configured to deliver ions of different polarity.     

Enhancement of negative ion currents by argon/CO2 mixtures in an electron impact ion source

Argon and mixtures of argon with carbon dioxide in the range of 1–50% have been used as moderator gases for negative ion production in an electron impact ion source of a VG ZAB-2F mass spectrometer. The enhancement of quasi-molecular anion ([M – H]^?) currents of oxygen-containing samples has been studied. It is a function of the composition of moderator gas mixture, moderator gas pressure, and sample pressure in the ion source. Most intensive quasi-molecular anion currents have been observed at a pressure in the ion source of c. 2.5 × 10^?5 Torr (measured by the ion gauge remote from the ion source of the ZAB-2F) using a moderator gas mixture of 10% CO₂ in argon. Fragmentation of the samples may be increased by increasing the concentration of CO₂ in the moderator gas.     

基于敞开式直接电离质谱技术的尿液中毒品检测北大核心CSCD

尿液作为一种易于获取的体内毒品检材,在吸毒人员快速筛查中被广泛应用。针对传统快速筛查技术存在假阳性率高、定量能力不足以及实验室质谱技术在快速检测中存在前处理复杂、检测耗时长、使用环境苛刻等问题,该文提出了一种基于敞开式直接电离质谱技术的生物样本快速检测方法。该研究采用探针式电喷雾离子源与便携式质谱仪联用快速检测平台,优化了喷雾电压和质谱入口毛细管温度,开发了高效快速的前处理技术。基于该平台和前处理技术,5种常规毒品(甲基苯丙胺、氯胺酮、可卡因、O^(6)-单乙酰吗啡和3,4-亚甲双氧甲基苯丙胺)的尿液加标溶液的检出限为0.5~30 ng/mL,且其中4种毒品定量检测的线性相关系数大于0.99。除此之外,5种常规毒品在3个不同水平下的加标回收率为56.1%~103.7%,多次检测结果的相对标准偏差为9.0%~27.8%,说明联用检测平台与前处理方法结合可以达到良好的准确度。为了进一步检验该联用仪器的实战能力,测试了某社区戒毒康复中心40份阳性和110份阴性实际尿液样本,总体检测的准确率接近99%,且通过一次进样在20 s内可同时检测多种毒品。该研究成果有利于推动快速检测技术的发展,促进敞开式直接电离质谱仪技术的推广应用,提升一线执法服务水平。     

A Simple Method for Estimating Effective Ion Source Residence Time

A method is presented that uses the well-understood O₂/Ax ion-molecule reaction system to determine the effective ion source residence time of a chemical ionization source. The process consists of: (1) defining the kinetic system in terms of reactions, reaction rates, and ionization cross sections; (2) solving the differential equations that describe the time evolu-tion of the kinetic system, and (3) comparing the calculated results to experimentally measured relative ion intensities. These steps are repeated for a variety of 0₂/Ar sample ratios and inlet pressures. The method leads to a simple relationship between inlet pressure and effective ion source residence time, independent of the 0₂/Ar sample ratio.     

射频放电等离子体中CO2及CO2-H2混合气转化反应的原位研究

Currently, worldwide attention is focused on controlling the continually increasing emissions of greenhouse gases, especially carbon dioxide. To this end, a number of investigations have been carried out to convert the carbon dioxide molecules into value-added chemicals. As carbon dioxide is thermodynamically stable, it is necessary to develop an efficient carbon dioxide utilization method for future scaled-up applications. Recently, several approaches, such as electrocatalysis, thermolysis, and non-thermal plasma, have been utilized to achieve carbon dioxide conversion. Among them, non-thermal plasma, which contains chemically active species such as high-energy electrons, ions, atoms, and excited gas molecules, has the potential to achieve high energy efficiency without catalysts near room temperature. Here, we used radio-frequency (RF) discharge plasma, which exhibits the non-thermal feature, to explore the decomposition behavior of carbon dioxide in non-thermal plasma. We studied the ionization and decomposition behaviors of CO₂ and CO₂-H₂ mixtures in plasma at low gas pressure. The non-thermal plasma was realized by our custom-made inductively coupled RF plasma research system. The reaction products were analyzed by on-line quadrupole mass spectrometry (differentially pumped), while the plasma status was monitored using an in situ real-time optical emission spectrometer. Plasma parameters (such as the electron temperature and ion density), which can be tuned by utilizing different discharge conditions, played significant roles in the carbon dioxide dissociation process in non-thermal plasma. In this study, the conversion ratio and energy efficiency of pure carbon dioxide plasma were investigated at different values of power supply and gas flow. Subsequently, the effect of H₂ on CO₂ decomposition was studied with varying H₂ contents. Results showed that the carbon dioxide molecules were rapidly ionized and partially decomposed into CO and oxygen in the RF field. With increasing RF power, the conversion ratio of carbon dioxide increased, while the energy efficiency decreased. A maximum conversion ratio of 77.6% was achieved. It was found that the addition of hydrogen could substantially reduce the time required to attain the equilibrium of the carbon dioxide decomposition reaction. With increasing H₂ content, the conversion ratio of CO₂ decreased initially and then increased. The ionization state of H₂ and the consumption of oxygen owing to CO₂ decomposition were the main reasons for the V-shape plot of the CO₂ conversion ratio. In summary, this study investigates the influence of power supply, feed gas flow, and added hydrogen gas content, on the carbon dioxide decomposition behavior in non-thermal RF discharge plasma.     

Desorption electrospray ionization of aerosol particles

We have applied desorption electrospray ionization to aerosol particles. Ions were formed from aerosols by merging suspended dry particles with an electrospray of solvent in a modified ion trap mass spectrometer. Dry aerosol particles were generated using a fluidized bed powder disperser and directed toward the inlet of the mass spectrometer. A nanospray source was used to create a spray of solvent droplets directed at the inlet and at a right angle with respect to the aerosol. Ions generated by the interaction of the particles and electrospray were transferred into the ion trap mass spectrometer. Using this method, pure samples of caffeine and erythromycin A were analyzed. In addition, commonly available food and drug powders including instant cocoa powder, artificial sweetener and ibuprofen were analyzed.