Partial pressure mass spectrometer

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The central transport chamber is an 80-cm-diameter stainless steel vessel, and is pumped by a 1000-1/s turbomolecular pump, which is backed by a small (501/s) turbomolecular pump to increase the compression ratio for hydrogen, and by a 16-m /h rotating-vane pump. UHV is obtained after a bake-out at temperatures above 100°C (measured with thermocouples at the outside surface) of the whole system for about a week. A pressure in the low 10 " -mbar range is then obtained. With a residual gas analyzer (quadrupole mass spectrometer, QMS) the partial pressures of various gases can be measured. During use of the system, the pressure in the central chamber is in the low 10 -mbar range due to loading of samples. Water vapor then is the most abundant species in the chamber. 

The partial pressures of the stable neutral molecules in the discharge (silane, hydrogen, disilane, trisilane) can be measured by a quadrupole mass spectrometer (QMS). The QMS usually is mounted in a differentially pumped chamber, which is connected to the reactor via a small extraction port [286]. In the ASTER system a QMS is mounted on the reactor that is used for intrinsic material deposition. The QMS background pressure (after proper bake-out) is between 10 and 10 mbar. The controllable diameter in the extraction port is adjusted so that during discharge operation the background pressure never exceeds 10"" mbar. 

Partial gas- or vapor pressures during freeze drying can also be measured by a mass spectrometer, and water vapor pressures by hygrometers, sensitive only for water vapor. Both systems are necessary for development- and analytical work, but in production plants they need only to be used to check or identify process data. 

Mass spectrometry techniques are the most usual and versatile methods for analysis of the gas [90], Here the effusing vapour is ionized by an ionization source and the product analyzed with a mass spectrometer. The different vapour species are identified and the partial pressures of all species determined. The partial pressure of species i of a compound or a solution with a specific composition is at a specific temperature ... 

Total pressure, required for detailed interpretation of the mass spectra, is determined with an ionization gauge (S). The gas inlet system (A, B, C) is used for calibration purposes. The relation between measured total pressure and the ion current of an injected specific gas permits calibration of the mass spectrometer in absolute partial pressure units or amps/torr. 

Partial pressure analysis using a mass spectrometer or the pressure rise method may be used to differentiate between these two causes. Since the pressure rise method will only prove the presence of a leak without Indicating Its location In the apparatus. It Is advisable to use a helium leak detector with which leaks can, in general, also be located much more quickly. 

In this case the gas quantity per unit of time Is the leak rate being sought the total pressure may not be used, but only the share for helium or the partial pressure for helium. This signal Is delivered by the mass spectrometer when It Is set for atomic number 4 (helium). The value for Is a constant for every series of leak detectors, making It possible to use a... 

N cuum technology mass spectrometer partial pressure gauges, definitions, characteristic variables, operating conditions 11/86... 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

It is not enough merely to reach a low pressure it must also be possible to measure it. Ionization gauges are almost always used for this purpose. In these the residual gas is ionized, collected at an electrode, and the resulting current measured. The current varies linearly with the gas pressure down to about 10 11 torr. If the ions are separated by mass —making the gauge a mass spectrometer —then the partial pressures of various gases in the vacuum chamber can be determined. 

Figure 7 shows the CTL response and partial pressure of the desorbed gases during heating (1 °C/s) from the y-AkOs catalyst which pre-adsorbed ethanol vapor at room temperature. In Fig. 7a, the mass spectrometer measurements show the desorptions of water (m/z 18), the physisorbed ethanol (m/z 46), diethylether (m/z 59), and ethylene (m/z 25) for the catalyst heated in Ar, but the CTL emission is not observed. In Fig. 7b, the desorptions of ethanol (80 °C), diethylether (230 °C) and ethylene (250 °C) are observed above the same temperature as in Ar, in the course of heating in a mixed gas of 21% O2 and 79% Ar. The desorption of ethylene in the atmosphere containing oxygen, however, begins to decrease at a lower temperature (320 °C) in Fig. 7b than the peak temperature (340 °C) of ethylene desorption in Fig. 7a. Desorption peaks of CO2 (m/z 44) and water appear at 340 °C, and the CTL peak is observed at the same temperature in Fig. 7c.

The greatest challenge in membrane extraction with a GC interface has been the slow permeation through the polymeric membrane and the aqueous boundary layer. The problem is much less in MIMS, where the vacuum in the mass spectrometer provides a high partial pressure gradient for mass transfer. The time required to complete permeation is referred to as lag time. In membrane extraction, the lag time can be significantly longer than the sample residence time in the membrane. An important reason is the bound-... 

Residual gas analysis with quadrupole mass spectrometers is being used increasingly in many applications for characterisation and problem diagnosis. Typical data were shown (Examples 5.16 and 5.17) and the manipulation of such data to yield partial pressures performed in Examples 5.18 and 5.19. 

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