Downstream Mass Research Articles

Selected Ion Flow Tube Mass Spectrometry for On-Line Trace Gas Analysis in Biology and Medicine

Selected ion flow tube mass spectrometry, (SIFT-MS), is a technique for simultaneous real-time quantification of several trace gases in air and exhaled breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas, using H(3)O(+), NO(+) and O(2)(+) reagent (precursor ions). Reactions between the precursor ions and the trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to parts-per-billion (ppb) levels, obviating sample collection into bags or onto traps. Calibration using chemical standards is not required, as the concentrations are calculated using the known reaction rate constants and measured flow rates and pressures. SIFT-MS has been used for many pilot investigations in several areas of research, especially as a non-invasive breath analysis tool to investigate physiological processes in humans and animals, for clinical diagnosis and for therapeutic monitoring. Examples of the results obtained from several such studies are outlined to demonstrate the potential of SIFT-MS for trace gas analysis of air, exhaled breath and the headspace above liquids.

Real-time observation and optimization of tungsten atomic layer deposition process cycle

In the search for a chemical sensing strategy to monitor atomic layer deposition (ALD) processes suitable for real-time application in wafer manufacturing, we have applied downstream mass spectrometry sampling to study process dynamics during ALD cycles for tungsten deposition from WF6 and SiH4. The ALD reactor has UHV cleanliness conditions and incorporated a minireactor chamber to simulate the small reaction volume anticipated for manufacturing tools to achieve adequate throughput. Mass spectrometry revealed essential surface reaction dynamics through real-time signals associated with by-product generation as well as reactant introduction and depletion for each ALD half-cycle. These were then used to optimize process cycle time and to study the effect of process recipe changes on film growth. The reaction by-products were clearly observed as H2 from SiH4 exposure and SiF4 from WF6 exposure. For each of the two half-cycles, rapid increase of by-product leds to steady-state adsorption/reaction conditions, following by by-product decrease and complementary reactant increase as surface saturation was achieved, indicating self-limiting surface reaction. From this direct observation of the surface reactions, exposure times could be minimized without sacrificing ALD growth rate per cycle, as verified experimentally. With gas flow parallel to the wafer surface in the minireactor, deviations from across-wafer uniformity were small when sufficient reactant doses were applied, but uniformity suffered markedly when doses were insufficient for surface saturation. Increasing WF6 concentration accelerated surface saturation as expected. Growth rates per cycle showed a notable temperature dependence, with small but noticeable activation energies (∼3kcal∕mol) consistent with previous reports. The effect of varying gas doses of one reactant while keeping the other constant suggests a complex interdependence between the half-cycles, in which the reactivity in one half-cycle is influenced by the prior dose achieved in the previous half-cycle.

Selected ion flow tube mass spectrometry (SIFT‐MS) for on‐line trace gas analysis

Selected ion flow tube mass spectrometry (SIFT-MS) is a new analytical technique for the real-time quantification of several trace gases simultaneously in air and breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas using H(3)O(+), NO(+), and O(2) (+.) precursor ions. Reactions between the precursor ions and trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer, thus effecting quantification. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to ppb levels, obviating sample collection and calibration. Illustrative examples of SIFT-MS studies include (i) analysis of gases from combustion engines, animals and their waste, and food; (ii) breath and urinary headspace studies of metabolites, ethanol metabolism, elevated acetone during ovulation, and exogenous compounds; and (iii) urinary infection and the presence of tumors, the influence of dialysis on breath ammonia, acetone, and isoprene, and acetaldehyde released by cancer cells in vitro. Flowing afterglow mass spectrometry (FA-MS) is briefly described, which allows on-line quantification of deuterium in breath water vapor.

NUMERICAL SIMULATION AND EXPERIMENTAL VALIDATION OF THE SWIRLING TURBULENT AIR FLOW AND MIXING PROCESSES

The work reported in this article investigates the influences of inlet boundary conditions on the precombustion chamber internal flow patterns, validated by experimental data. An axial fixed-vane swirling diffuser with 12 vanes of declining angle 20° is used for the primary air flow. For the swirling air flow inlet boundary condition specifications, two methods are compared employing the standard κ–ϵ turbulence model. The conventional method is to specify the inlet velocities based on totally constant axial and tangential momentum fluxes. For the new method, the whole simulation domain is extended to the supply duct, and detailed air flows between the supply swirling diffuser and precombustion chamber are linked using the multigrid technique. Comparisons with experimental data reveal that the new method can more accurately predict turbulent air flows and mixing processes near the swirling diffuser, and consequently more accurately predict the size of the recirculation zone, and farther downstream velocities and air jet mass fractions.

Dispersion of a particle-laden air jet in a confined rectangular crossflow

Dispersion of a particle-laden air jet issuing into a confined rectangular crossflow has been studied experimentally by means of a planar light-scattering technique. Experiments were run for a jet injected at 24° to the crossflow, using three jet-to-crossflow velocity ratios ( V r=0.5, 1.0 and 1.5), five downstream measurement locations ( x/ D=7.5, 12, 16, 20, 25) and mass fractions ( σ) of particles-to-jet air ranging from 3.5×10 −2 to 1.8×10 −1. Spherical particles with a mean diameter of d p=130–200 μm and material density ρ p=1050 kg/m 3 and nonspherical particles with a mean diameter of d p=200 μm and ρ p=1200 kg/m 3 were studied. The distribution of the 200-μm spherical particles downstream of the jet-crossflow intersection is more uniform than that of nonspherical particles, indicating that particle shape affects dispersion. The dispersion of both types of 200-μm particles in the crossflow is generally greater with increasing downstream distance. However, dispersion of 130-μm spherical particles becomes non-uniform with increasing downstream distance due to the action of a large-scale counter-rotating vortex pair. Dispersion of the particle-laden air jet in the crossflow is not greatly dependent on loading rate over the range of loadings studied. For both spherical and nonspherical particles, the penetration of particle-laden air jet issuing into the crossflow is greater than that of a single-phase air jet. Measurements of static pressure along the top of the main duct (crossflow) indicate that the effect of particles on the air-phase static pressure downstream of the jet-crossflow intersection is significant only for the higher particle loading rates.

Quantitative selected ion flow tube mass spectrometry: the influence of ionic diffusion and mass discrimination

Selected ion flow tube mass spectrometry, (SIFT-MS), involves the partial conversion of mass-selected precursor ions to product ions in their reactions with the trace gases in an air sample that is introduced into helium carrier gas in a flow tube. The precursor and product ions are then detected and counted by a downstream quadrupole mass spectrometer. Quantification of particular trace gases is thus achieved from the ratio of the total count rate of the product ions to that for the precursor ions. However, it is important to appreciate that in this ion chemistry the light precursor ions (usually H 3O + ions) are invariably converted to heavier product ions. Hence, the product ions diffuse to the flow tube walls more slowly and thus they are more efficiently transported to the downstream mass spectrometer sampling orifice. This phenomenon we refer to as diffusion enhancement. Further, it is a well-known fact that discrimination can occur against ions of large mass-to-charge ratio, ( m/z), in quadrupole mass spectrometers. If not accounted for, diffusion enhancement usually results in erroneously high trace gas concentrations and mass discrimination results in erroneously low concentrations. In this experimental investigation, we show how both these counteracting effects can be accounted for to increase the accuracy of SIFT-MS quantification. This is achieved by relating the currents of ions of various m/z that arrive at the downstream mass spectrometer sampling orifice disc to their count rates at the ion detector after mass analysis. Thus, both diffusion enhancement and mass discrimination are parameterized as a function of m/z and these are combined to provide an overall discrimination factor for the particular analytical instrument.

Effect of collisional damping and reactions in a dynamic reaction cell on the precision of isotope ratio measurements

Collisional damping of ion density fluctuations of an ion beam extracted from an ICP in a linear rf quadrupole cell [dynamic reaction cell (DRC)] is studied. Target gas pressure, parameter q, entrance and exit lens potentials and DRC rod offset were optimized to broaden the 107Ag+ ion packets of 0.2 ms duration to 2–4 ms (FWHM). It is shown that operating the DRC under such optimized conditions reduces the RSD of isotope ratio measurements obtained with a downstream quadrupole mass analyzer to the theoretical limit defined by the counting statistics error. For Ag and Pb samples at 40 ng ml−1, measured with the cell pressurized by Ne, isotope ratio RSDs of 0.02–0.03% at a counting statistics error of 0.01–0.02% are shown. All 4 isotopes of Pb were measured in the same acquisition. For NIST 982 Isotope Standard at a concentration of 10 ng ml-1, 204Pb/206Pb, 207Pb/206Pb and 208Pb/206Pb isotope ratios were determined with an external precision (n = 7) of 0.056–0.117%, 0.022–0.034% and 0.014–0.028% RSD, and with an average inaccuracy of 340 ppm, 781 ppm and 180 ppm, respectively. NIST 981 at 10 ng ml−1 was used as an external standard. Reactive elimination of isobaric interferences in the DRC is shown to allow precise isotope ratio measurement of normally interfered isotopes of Fe by a quadrupole ICP-MS. ArN+ and ArO+ interferences on Fe+ were removed by ion–molecule reactions with NH3. Collisional damping of the ion beam fluctuations was sustained and internal precision is shown to be limited only by counting statistics. For Fe at 50 ng ml−1, the average internal precision for 19 samples (9 replicates per sample) of 54Fe/56Fe and 57Fe/56Fe ratios was 0.068% and 0.1% RSD, with a corresponding average counting statistics error of 0.053% and 0.082% and external precision of 0.055% and 0.056% RSD, respectively.

Budget and sources of suspended sediment transported in the St. Lawrence River, Canada

A mass balance budget of the suspended sediment in the St. Lawrence River was established for the sector stretching from Cornwall, Ontario, to Quebec City, Quebec, for the period 1989–1993. The approach consisted of analysing the amount of sediment contributed by the different tributaries, on a watershed-by-watershed basis, through ‘sediment concentration–discharge’ models incorporating more than 4000 data points collected since 1983. Lake Ontario contributes less than 3% of the particulate load at Quebec City, while St. Lawrence tributaries on the south and north shores contribute 19% and 13%, respectively, of the sediment load. Our findings indicate that nearly 65% of the suspended sediments come from erosion of the bed and banks of the St. Lawrence River. This finding is broadly supported by numerous geomorphological and sedimentological observations and is consistent with the geological history of the river and the structures built on its banks in recent decades. Upstream–downstream mass balance studies conducted on individual river sectors indicate that the sources of erosion are located mainly in the Beauharnois Canal region, between Montreal and Les Grèves, and further downstream, between the outlet of Lake Saint-Pierre and Portneuf. Copyright © 2000 John Wiley & Sons, Ltd.

Budget and sources of suspended sediment transported in the St. Lawrence River, Canada

A mass balance budget of the suspended sediment in the St. Lawrence River was established for the sector stretching from Cornwall, Ontario, to Quebec City, Quebec, for the period 1989–1993. The approach consisted of analysing the amount of sediment contributed by the different tributaries, on a watershed-by-watershed basis, through ‘sediment concentration–discharge’ models incorporating more than 4000 data points collected since 1983. Lake Ontario contributes less than 3% of the particulate load at Quebec City, while St. Lawrence tributaries on the south and north shores contribute 19% and 13%, respectively, of the sediment load. Our findings indicate that nearly 65% of the suspended sediments come from erosion of the bed and banks of the St. Lawrence River. This finding is broadly supported by numerous geomorphological and sedimentological observations and is consistent with the geological history of the river and the structures built on its banks in recent decades. Upstream–downstream mass balance studies conducted on individual river sectors indicate that the sources of erosion are located mainly in the Beauharnois Canal region, between Montreal and Les Grèves, and further downstream, between the outlet of Lake Saint-Pierre and Portneuf. Copyright © 2000 John Wiley & Sons, Ltd.

Mass spectrometric measurements on inductively coupled fluorocarbon plasmas: Positive ions, radicals and endpoint detection

Positive ions and radicals in C2F6 and CHF3 high density discharges were measured using a direct-line-of-sight mass spectrometer. The ion energy distributions of the dominant ions were measured as a function of process conditions. Appearance potential mass spectrometry was performed to measure trends of the radical densities. For C2F6 plasmas CF3 and CF3+are the most abundant neutral and ionic species, respectively. CF3 is the most abundant neutral species for a CHF3 plasma, whereas CHF2+ and CF+ are the most abundant ionic species at 600–1000 and 1400 W, respectively. Erosion of the quartz coupling window is an important contaminant source for our inductively coupled plasma system. For comparison, downstream mass spectrometry was also applied using a closed ion source system since this approach is of interest for real-time monitoring and control. Endpoint detection for Si and SiO2 film etching in a CHF3 plasma was investigated using the downstream mass spectrometer system and compared with data obtained simultaneously using the direct-line-of-sight mass spectrometer and optical emission spectroscopy. It was found that the downstream mass spectrometer system can be used for endpoint detection during SiO2 over Si selective etching. The signal changes of different species measured by these techniques for different SiO2 and Si etching processes as a function of time are reported and compared.

Process sensing and metrology in gate oxide growth by rapid thermal chemical vapor deposition from SiH4 and N2O

Active sampling mass spectrometry has been used for process sensing in gate oxide growth by rapid thermal chemical vapor deposition from SiH4 and N2O. Equipment and process behavior throughout the short process cycle were revealed in the detailed time-dependent changes of downstream mass spectroscopic signals. A H2 reaction product was clearly identified during SiO2 deposition for SiH4/N2O ratios of 0.5%–2.0% at 5 Torr total pressure and in the temperature range 750–850 °C. No H2O product was observed, suggesting that the process is dominated by a two-step reaction involving SiH4 pyrolysis and subsequent N2O oxidation of the deposited Si to form SiO2. The evolution of the H2 product signal during a process was then used as a process indicator. The integrated H2 signal was found linearly proportional to the deposited oxide thickness, providing the basis for real-time, noninvasive thickness metrology applications. This work demonstrates that properly configured real-time mass spectrometry is capable of providing not only time-dependent chemical information about system behavior, but also quantitative metrology for the film deposition process.

Electrospray Ionization High-Resolution Ion Mobility Spectrometry−Mass Spectrometry

A hybrid atmospheric pressure ion mobility spectrometer is described which exhibits resolving power approaching the diffusion limit for singly and multiply charged ions (over 200 for the most favorable case). Using an electrospray ionization source and a downstream quadrupole mass spectrometer with electron multiplier as detector, this ESI-IMS-MS instrument demonstrates the potential of IMS for rapid analytical separations with a resolving power similar to liquid chromatography. The first measurements of gas-phase mobility spectra of mass-identified multiply charged ions migrating at atmospheric pressure are reported. These spectra confirm that collision cross sections are strongly affected by charge state. Baseline separations of multiply charged states of cytochrome c and ubiquitin demonstrate the improved resolving power of this instrument compared with previous atmospheric pressure ion mobility spectrometers. The effects of electric potential, initial pulse duration, ion-molecule reactions, ion desolvation, Coulombic repulsion, electric field homogeneity, ion collection, and charge on the resolving power of this ion mobility spectrometer are discussed.

Dynamics of Zebra Mussel Oxygen Demand in Seneca River, New York

The magnitude and dynamics of oxygen depletion from zebra mussel metabolism for a severely infested 1.4 km section of the Seneca River, NY, is documented for a 3-month period of 1994, based on hourly measurements of dissolved oxygen (DO) concentrations at an upstream site and a downstream site and mass balance calculations. The average areal zebra mussel oxygen demand (ZOD) in the study section during a 30-day low-flow period (population density ∼40 000 individuals m-2) was 39.4 g m-2 d-1. The average value of ZOD for the same 30-day period, normalized for zebra mussel biomass mass (42.4 mg of O2 (g of zebra mussel)-1 d-1), compares favorably to recently reported oxygen consumption rates from laboratory experiments, sup porting its application elsewhere to accommodate the effects on oxygen resources of invaded systems. Values of ZOD varied greatly during the study reflecting responses of the invader to changes in ambient conditions, including flow and a major decrease in the population.

Monitoring InP and GaAs etched in Cl2/Ar using optical emission spectroscopy and mass spectrometry

Optical emission spectroscopy (OES) and mass spectrometry (MS) have been used to monitor the etching of InP and GaAs in a Cl2/Ar plasma. The plasma was generated by an electron cyclotron resonance source. Emission from atomic In and Ga has been detected with the strongest signal to background measured at 410.2 nm for In and 417.2 nm for Ga. The optical emission signals have been studied for varying etch conditions to determine their effectiveness for real-time control applications. It was found that, by increasing the total gas flow rate, the etch rate remained unchanged but the detected etch product signals decreased due to reduced residue in the plasma system. The residence time of the etch products causes the optical emission signals to be dependent upon previous etch conditions. The In and Ga emission signals have been correlated to the etch rate for various etch conditions. A downstream mass spectrometer was able to detect P and As etch products. The strongest signal for InP etching was 101PCl+2 and the strongest signal for GaAs etching was 145AsCl+2. The ability of the mass spectrometer to detect the group V etch products coupled with the ability of OES to detect the group III elements allows for noninvasive study of a variety of III–V material systems. Smooth, vertical etching of InP via holes at 2.7 μm/min for GaInAs/AlInAs-based power devices has been monitored. Optical emission measurements showed that the surface oxide removal time for the via hole etching can be decreased from 95 s to an immediate removal by increasing the self-induced dc bias (‖Vdc‖) from 50 to 300 V, in agreement with MS results. End point detection for via hole etching of GaInAs/AlInAs power devices to Ti/Au contacts on InP substrate was studied using 145AsCl+2 and 155TiCl+3 signals. The thickness of Ti removed can be limited to 18 nm. The optical emission from Ga and Al (at 396.1 nm) have been used to monitor etching of GaInAs/AlInAs heterojunction bipolar transistors. Less than 10 nm of the GaInAs base material is removed at the end of the emitter etch.

Modeling of multicomponent pervaporation for removal of volatile organic compounds from water

A resistance-in-series model was used to study the pervaporation of multiple volatile organic compounds (VOCs)-water mixtures. Permeation experiments were carried out for four membranes: poly(dimethylsiloxane) (PDMS), polyether-block-polyamides (PEBA), polyurethane (PUR) and silicone-polycarbonate copolymer (SPC) membranes. Three VOCs, i.e., toluene, 1,1,1-trichloroethane and methylene chloride were studied. Both organic and water permeabilities of the PEBA membrane for 1 VOC-water, 2 VOCs-water and 3 VOCs-water mixtures were found to be comparable with each other. Coupling effects for trace organic transfer through the membrane were not observed when the downstream pressure was close to zero. However, at high downstream pressure, if the downstream side mass transfer resistance dominated the overall mass transport, coupling effects might occur within the vapor phase. The downstream pressure effect for the PDMS membrane was determined. The experimental results were correlated very well by a simple mass transfer equation. The downstream pressure may have positive or negative effects on the separation factor, depending on the ratio of overall organic permeability over water permeability, β perm. The value of β perm is a function of the intrinsic organic and water permeabilities, liquid boundary layer mass transfer coefficient as well as membrane thickness. The vapor phase mass transfer resistance was found to be negligible at low downstream pressure (<15 mmHg). It was clearly shown in this work that the resistance-in-series model could be effectively to describe the pervaporation of dilute multiple VOCs-water mixtures through polymeric membranes.

Gas‐Phase Precursors to Diamond and the sp2 Carbon Impurity

Diamond film deposition studies were carried out in a microwave plasma reactor using feed gases containing methane and acetylene with one reagent labeled with carbon‐13. Stable gas‐phase species and their carbon‐13 content were characterized by downstream mass spectrometry. Raman spectroscopy along with curve‐fitting analysis was used to determine the carbon‐13 content of diamond and the sp2 carbon impurity. Diamond and the sp2 carbon impurity originated predominantly from methane and/or methyl radical, as opposed to acetylene. Thus, it appears that the differentiation between diamond and sp2 carbon formation occurs on the growth surface, and not in the gas phase.

Diagnostics and modeling of RF discharge dissociation in N/sub 2/O

Measurements were made of the RF discharge dissociation of N/sub 2/O in a parallel-plate reactor by downstream mass spectrometry using a wide range of gas flows and powers at 10 kHz and 13.56 MHz. The results show that the mass 44 signal (N/sub 2/O/sup +/), which is a measure of the amount of undissociated N/sub 2/O, is a function of the discharge input energy per N/sub 2/O molecule (eV/N/sub 2/O). A plug flow, rate equation model of the discharge was used to predict the experimental dissociation rates. A DC Monte Carlo simulation was used to calculate rate coefficients for electron-impact neutral dissociation, ionization, and dissociative ionization. The rate equation model also includes reactions among the dissociation products of N/sub 2/O and species which are synthesized in the discharge, as well as neural and electron-ion recombination at the electrodes. The model predictions identify the major reaction pathways and the sensitivities of the results to the rate coefficient values used. >

Mass transfer in a laminar-flow parallel plate electrolytic cell with simultaneous development of velocity and concentration boundary layers

Ionic mass transfer in a parallel plate electrochemical cell having a flow entrance representative of practical industrial cells has been studied under laminar flow condition using the ferri- and ferrocyanide redox couple. The working electrode was segmented so that local mass transfer coefficient could be measured. The mass transfer coefficients, measured for the entrance region of the cell, which is defined as the region where both hydrodynamic and concentration boundary layers are developing simultaneously, are correlated and compared with results from other studies. For the electrode section beyond the entrance region, where the flow is fully developed but the concentration boundary layer continues to grow, a correction term is added to the electrode dimension parameter in the Graetz number. In this way the Leveque equation can still be used to correlate the local mass transfer coefficient. The value of the correction term is calculated by matching the downstream mass transfer boundary layer thickness to that of a Leveque-type cell. This scheme improves substantially the convergence of the experimental data correlation and results in good agreement with the Leveque equation.

Electron and chemical kinetics in methane rf glow-discharge deposition plasmas

Experimental measurements and theoretical modeling of methane deposition plasmas have led to the identification of the most likely homogeneous and heterogeneous reaction paths leading to the deposition of amorphous carbon thin films. Experimental measurements of the voltage, current waveforms, mass flow rates, and pressure are used as inputs to the model. The magnitude and flow-rate dependence of the discharge luminosity, film deposition rates, and downstream mass spectra are compared with the model predictions and used to identify the dominant reaction paths. The model uses Monte Carlo simulation of the electron kinetics to predict the electron impact dissociation and ionization rates. These rates provide input for a plug flow chemical kinetics model.

Activities of benthic nitrifiers in streams and their role in oxygen consumption

The in situ rates of oxygen consumption by benthic nitrifiers were estimated at 11 study sites in 4 streams. Two methods were used: an in situ respiration chamber method and a method involving conversion of nitrifying potential measurements to in situ rates. Estimates of benthic nitrogenous oxygen consumption (BNOC) rate ranged from 0-380 mmol of O2 m(-2)·day(-1), and BNOC contributed between 0-85% of the total benthic oxygen consumption rate. The activity of nitrifiers residing in the sediments was influenced by O2 availability, temperature, pH, and substrate. Depending upon site, nitrification could approximate either first-order or zero-order kinetics with respect to ammonium concentration. The source of ammonium for benthic nitrifiers could be either totally from within the sediment or totally from the overlying water. Nitrate produced in the sediments could flux to the water above or be lost within the sediment. The sediments could act as a source (positive flux) or sink (negative flux) for both ammonium (-185 mmol·m(-2)·day(-1) to +195 mmol·m(-2)·day(-1)) and nitrate (-135 mmol·m(-2)·day(-1) to +185 mmol·m(-2)·day(-1)).This study provides evidence to suggest that measurements of down-stream mass flow changes in inorganic nitrogen forms may give poor estimates of in situ rates of nitrification in flowing waters.