Probing the Effects of Reagent Gas Pressure and Ion Source Temperature for Dimethyl Ether Chemical Ionization of Tricyclic Antidepressants in an External Source Ion Trap Mass Spectrometer

This study examines the reagent gas pressure and ion source temperature dependence for dimethyl ether chemical ionization (DME CI) mass spectra recorded with an external source ion trap mass spectrometer (ITMS). Information for better controls of the reagent gas pressure in order to obtain fair CI spectra is provided. The origin of M^+? ions observed in DME CIMS is discussed in detail. Furthermore, the ion source temperature effect on the DME CI is also investigated.     

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.     

Studies of gas-jet assisted RF-glow discharge atomic absorption spectrometry

The gas jet assisted glow discharge source has been employed for the RF-glow discharge atomic absorption spectrometry (RF-GDAAS). Data are described to illustrate the role of discharge power, pressure as well as gas flow rate on the sample loss rate, absorbance and self-bias potential. Results show that the optimum discharge conditions depend not only on the pressure and discharge power but also on the gas flow rate. The absorbance increases as much as 1 order of magnitude as the gas flow rate increases from 50 mL/min to 500 mL/min at a pressure of 3 mbar. The use of a gas-jet source allows the direct analysis of solids by RF-GDAAS.     

Studies of gas-jet assisted RF-glow discharge atomic absorption spectrometry

The gas jet assisted glow discharge source has been employed for the RF-glow discharge atomic absorption spectrometry (RF-GDAAS). Data are described to illustrate the role of discharge power, pressure as well as gas flow rate on the sample loss rate, absorbance and self-bias potential. Results show that the optimum discharge conditions depend not only on the pressure and discharge power but also on the gas flow rate. The absorbance increases as much as 1 order of magnitude as the gas flow rate increases from 50 mL/min to 500 mL/min at a pressure of 3 mbar. The use of a gas-jet source allows the direct analysis of solids by RF-GDAAS.     

Experimental study of gas mixtures in strong non-uniform electric fields

Argon-based gas mixtures of various concentration ratios at low pressures were studied in a gas proportional counter. Over-exponential increase of gas gain in mixtures with low partial pressure of any admixture was observed. The best energy resolution was measured at a gas gain of several hundreds for lower admixture concentrations, but at high gas gains R degraded very fast. The first Townsend ionization coefficient /P in low-pressure gases depended also on the gas pressure. The single-electron spectra had a peaked shape in mixtures with relatively high content of the molecular gas. In mixtures with lower molecular gas amount they changed the shape toward the over-exponential distribution. A partial pressure of 10 kPa was determined as a requirement for good quenching property of the mixture.     

Producing and detecting very large clusters

The cluster source we use, a low pressure, rare gas condensation cell, is capable of producing clusters containing more than 45 000 atoms or having masses exceeding 2 500 000 amu. Details of this source and the dependence of the cluster size distribution on adjustable working parameters (oven temperature, inert gas pressure, inert gas type) are discussed in this report. Measurements of the mass-dependent velocity distributions of the clusters emitted by the source are presented and compared to a simple model calculation. The clusters are mass-analyzed with a time-of-flight mass spectrometer and detected by a multi-channel plate. The dependence of the detectability of large clusters on the acceleration voltage is investigated.     

An investigation of peak-broadening effects arising when combining CE with MS

In this study, peak-broadening effects caused by nebulizing gas flow and lack of temperature control have been investigated for separation capillaries with three different inner diameters. The study was performed with serial UV/ESI-MS detection in an effort to distinguish between peak broadening arising in the separation and peak broadening arising in the ion source. The nebulizing gas was found to significantly affect both migration time and separation efficiency when using capillaries with 50 and 75 microm id. If the nebulizing gas is on during injection, the injection volume increases to such an extent that significant peak broadening is induced. Reducing the id to 25 microm minimizes the parabolic flow induced by the nebulizing gas. Results indicate that the nebulizing gas pressure can be optimized to minimize peak broadening in the ion source. A decrease in detection sensitivity, possibly related to the orthogonal design of the interface, was observed when the nebulizing gas pressure was increased. A tapered capillary tip was found to provide superior separation efficiency as well as sensitivity.     

High sensitivity in gas analysis with photoacoustic detection

Introduction of a new type of pressure sensor has been shown to improve orders of magnitude the sensitivity of a photoacoustic measurement system using a black body radiation source. A new pressure sensor was developed to overcome the limitations in the capacitive microphone technology and to obtain ultimate sensitivity in photoacoustic gas detection when using low modulation frequency below 500 Hz. The pressure sensor is a cantilever-type microphone with interferometric measurement of the sensor displacement. By using conventional filter-type photoacoustic setup with the cantilever microphone and a black body radiation source, we obtained a detection limit in the sub-ppb range for methane gas with 100 s measurement time.     

Pressurized laboratory experiments show no stable carbon isotope fractionation of methane during gas hydrate dissolution and dissociation

The stable carbon isotopic ratio of methane (δ(13)C-CH(4)) recovered from marine sediments containing gas hydrate is often used to infer the gas source and associated microbial processes. This is a powerful approach because of distinct isotopic fractionation patterns associated with methane production by biogenic and thermogenic pathways and microbial oxidation. However, isotope fractionations due to physical processes, such as hydrate dissolution, have not been fully evaluated. We have conducted experiments to determine if hydrate dissolution or dissociation (two distinct physical processes) results in isotopic fractionation. In a pressure chamber, hydrate was formed from a methane gas source at 2.5 MPa and 4 °C, well within the hydrate stability field. Following formation, the methane source was removed while maintaining the hydrate at the same pressure and temperature which stimulated hydrate dissolution. Over the duration of two dissolution experiments (each ~20-30 days), water and headspace samples were periodically collected and measured for methane concentrations and δ(13)C-CH(4) while the hydrate dissolved. For both experiments, the methane concentrations in the pressure chamber water and headspace increased over time, indicating that the hydrate was dissolving, but the δ(13)C-CH(4) values showed no significant trend and remained constant, within 0.5‰. This lack of isotope change over time indicates that there is no fractionation during hydrate dissolution. We also investigated previous findings that little isotopic fractionation occurs when the gas hydrate dissociates into gas bubbles and water due to the release of pressure. Over a 2.5 MPa pressure drop, the difference in the δ(13)C-CH(4) was <0.3‰. We have therefore confirmed that there is no isotope fractionation when the gas hydrate dissociates and demonstrated that there is no fractionation when the hydrate dissolves. Therefore, measured δ(13)C-CH(4) values near gas hydrates are not affected by physical processes, and can thus be interpreted to result from either the gas source or associated microbial processes.     

A functional on-chip pressure generator using solid chemical propellant for disposable lab-on-a-chip

This paper presents a functional on-chip pressure generator that utilizes chemical energy from a solid chemical propellant to perform fluidic delivery in applications of plastic-based disposable biochips or lab-on-a-chip systems. In this functional on-chip pressure generator, azobis-isobutyronitrile (AIBN) as the solid chemical propellant is deposited on a microheater using a screen-printing technique, which can heat the AIBN at 70 degrees C to produce nitrogen gas. The output pressure of nitrogen gas, generated from the solid chemical propellant, is adjustable to a desired pressure by controlling the input power of the heater. Using this chemical energy source, the generated pressure depends on the deposited amount of the solid chemical propellant and the temperature of the microheater. Experimental measurements show that this functional on-chip pressure generator can achieve around 3 000 Pa pressure when 189 mJ of energy is applied to heat the 100 microg of AIBN. This pressure can drive 50 nl of water through a microfluidic channel of 70 mm and cross-sectional area of 100 microm x 50 microm. Due to its compact size, ease of fabrication and integration, high reliability (no moving parts), biologically inert gas output along with functionality of gas generation, this pressure generator will be an excellent pressure source for handling the fluids of disposable lab-on-a-chip, biochemical analysis systems or drug delivery systems.     

Development of an on-line low gas pressure cell for laser ablation-ICP-mass spectrometry.

An on-line low gas pressure cell device has been developed for elemental analysis using laser ablation-ICP-mass spectrometry (LA-ICPMS). Ambient gas in the sample cell was evacuated by a constant-flow diaphragm pump, and the pressure of the sample cell was controlled by changing the flow rate of He-inlet gas. The degree of sample re-deposition around the ablation pit could be reduced when the pressure of the ambient gas was lower than 50 kPa. Produced sample aerosol was drawn and taken from the outlet of the diaphragm pump, and directly introduced into the ICP ion source. The flow rate of He gas controls not only the gas pressure in the sample cell, but also the transport efficiency of the sample particles from the cell to the ICP, and the gas flow rate must be optimized to maximize the signal intensity of the analytes. The flow rates of the He carrier and Ar makeup gas were tuned to maximize the signal intensity of the analytes, and in the case of (238)U from the NIST SRM610 glass material, the signal intensity could be maximized with gas flow rates of 0.4 L/min for He and 1.2 L/min for Ar. The resulting gas pressure in the cell was 30 - 35 kPa. Using the low gas pressure cell device, the stability in the signal intensities and the resulting precision in isotopic ratio measurements were evaluated. The signal intensity profile of (63)Cu obtained by laser ablation from a metallic sample (NIST SRM976) demonstrated that typical spikes in the transient signal, which can become a large source of analytical error, were no longer found. The resulting precision in the (65)Cu/(63)Cu ratio measurements was 2 - 3% (n = 10, 2SD), which was half of the level obtained by laser ablation under atmospheric pressure (6 - 10%). The newly developed low-pressure cell device provides easier optimization of the operational conditions, together with smaller degrees of sample re-deposition and better stability in the signal intensity, even from a metallic sample.     

Optimized splitless injection of hydroxylated PCBs by pressure pulse programming

Hydroxylated polychlorobiphenyls (polyhydroxy-PCBs) are subjected to severe discrimination effect when they are introduced into a capillary gas chromatograph by splitless injection. This effect can be considerably reduced by using a pulsed pressure injection to favor sample transfer and by further programming the eluent gas pressure during the GC run. By multistep gas pressure programming, splitless injection provides analytical performances and chromatograms for (poly)hydroxy-PCBs closely resembling those obtained by on-column injection, whose use is generally discouraged with real environmental samples. Among the six model compounds studies, the dihydroxy congeners exhibit the best results.     

Inline pneumatically assisted atmospheric pressure matrix‐assisted laser desorption/ionization ion trap mass spectrometry

Atmospheric pressure (AP) matrix‐assisted laser desorption/ionization (MALDI) is known to suffer from poor ion transfer efficiencies as compared to conventional vacuum MALDI (vMALDI). To mitigate these issues, a new AP‐MALDI ion source utilizing a coaxial gas flow was developed. Nitrogen, helium, and sulfur hexafluoride were tested for their abilities as ion carriers for a standard peptide and small drug molecules. Nitrogen showed the best ion transport efficiency, with sensitivity gains of up to 1900% and 20% for a peptide standard when the target plate voltage was either continuous or pulsed, respectively. The addition of carrier gas not only entrained the ions efficiently but also deflected background species and declustered analyte–matrix adducts, resulting in higher absolute analyte signal intensities and greater signal‐to‐noise (S/N) ratios. With the increased sensitivity of pneumatically assisted (PA) AP‐MALDI, the limits of detection of angiotensin I were 20 or 3 fmols for continuous or pulsed target plate voltage, respectively. For analyzing low‐mass analytes, it was found that very low gas flow rates (0.3–0.6 l min^?1) were preferable owing to increased fragmentation at higher gas flows. The analyte lability, type of gas, and nature of the extraction field between the target plate and mass spectrometer inlet were observed to be the most important factors affecting the performance of the in‐line PA‐AP‐MALDI ion source. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A high-performance matrix-assisted laser desorption/ionization orthogonal time-of-flight mass spectrometer with collisional cooling

A high-performance orthogonal time-of-flight (TOF) mass spectrometer was developed specifically for use in combination with a matrix-assisted laser desorption/ionization (MALDI) source. The MALDI source features an ionization region containing a buffer gas with variable pressure. The source is interfaced to the TOF section via a collisional focusing ion guide. The pressure in the source influences the rate of cooling and allows control of ion fragmentation. The instrument provides uniform resolution up to 18,000 FWHM (full width at half maximum). Mass accuracy routinely achieved with a single-point internal recalibration is below 2 ppm for protein digest samples. The instrument is also capable of recording spectra of samples containing compounds with a broad range of masses while using one set of experimental conditions and without compromising resolution or mass accuracy.     

Evaluation and prediction of the shape of gas chromatographic peaks

The evaluation and prediction of the shape of asymmetric gas chromatographic peaks is important as the knowledge of the amount of tailing permits to foresee the resolution between closely eluting peaks and to select the best analytical conditions for an efficient and rapid separation. A model function was tested in order to approximate the true peak shape obtained on non-polar column by injecting different compounds. The trend of the parameters involved in the used equation has been investigated as a function of column temperature and inlet pressure. The reproduction of the symmetrical or asymmetrical shape of gas chromatographic peaks is satisfactory and the method also permits to predict the shape of peaks obtained in different conditions of temperature and pressure.     

Prediction of the plate height of capillary columns operated at any inlet pressure of the carrier gas by using few retention data measured under isobaric conditions

Programming inlet pressure in gas chromatography permits to decrease the analysis time without changing the elution order of compounds of different polarity whose relative retention changes with changing temperature. The choice of the best values of the inlet pressure and flow-rate of the carrier gas often requires many preliminary analyses with different parameters to be carried out. A method for the prediction of the separation by starting from few experimental data measured in isothermal and isobaric conditions decreases the time required for the optimisation of the analysis. The efficiency of the separation depends on the change of the theoretical plate height at various pressures and temperatures, due to pressure drop along the column. By calculation of the diffusion coefficients of the analysed compounds into the mobile and stationary phase it is possible to evaluate the column efficiency and predict the number of theoretical plates at any inlet pressure. A procedure for the prediction of the plate height of a capillary column at any inlet pressure of the carrier gas and column temperature by using retention data of polar and non-polar compounds (1-alcohols and linear alkanes) obtained in few isobaric runs is described.     

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.     

氮化镓生长系统中源区反应的热力学分析和实验研究

本文报道化学气相淀积法生长GaN薄膜材料的Ga-HCl-NH₃载气体系的源区反应热力学分析和实验研究结果.     

Examination of the best pressure range for ion/molecule reactions of anthraquinones in an external source ion trap mass spectrometer.

This study outlines some observations of the pressure effect for gas phase ion-molecule reactions of anthraquinone derivatives with dimethyl ether in an external source ion trap mass spectrometer. At the reagent pressure of 7.998 x 10(-2) Pa, formation of the protonated ions, [M + 13]+, [M + 15]+, and [M + 45]+ ions, of anthraquinones can be observed. However, at the pressure of 1.066 x 10(-2) Pa, formation of molecular ions and many fragment ions of the M+. or [M + H]+ ions have been observed. Since the pressure effect is notable within a small range of pressures for many compounds, it is important to draw attention to the use of the ion trap with an external source where other factors such as ion source residence time may play a role. This can also provide some information for better and more careful controls of the reagent pressure in order to obtain fair CI spectra in an external source ion trap mass spectrometer.