Quadrupole mass spectrometer QMS

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. 

Table 6.30 lists the main characteristics of quadrupole mass spectrometers. QMS is a relatively simple and robust analyser which does not need such a high vacuum as a sector instrument. The maximum admissible pressure at the source of the spectrometer is 10 6mbar in continuous regime and 10-5—10 4 mbar during short time intervals. Quadrupole technology assures reproducible and accurate molecular weight measurements day in and day out. For figures of merit, see Table 6.27. 

The CO-covered Pt(l 1 0) surface was then exposed to a mixture of CO/02 gases, with the ratio of CO/02 adjusted by flow meters. The pressure changes of CO, 02, and the reaction product, C02, were monitored by leaking the gases from the flow reactor to a quadrupole mass spectrometer (QMS) attached to the flow-reactor STM. The surface structure and reactivity of Pt(l 1 0) could be measured simultaneously with the combination of STM and QMS. 

Fig. 1. Photo and illustration of the HRTEM allowing acquisition of images of catalysts under working conditions (4). The microscope is equipped with an FEG, a quadrupole mass spectrometer (QMS), a Gatan image filter (GIF), and a Tietz F144 CCD for data acquisition. The differential pumping system consists of IGPs, turbo molecular pump units (TMP, MDP), and an oil diffusion pump (ODP). The differential pumping stages are set up by apertures inside the TEM column (denoted by black bars) at the objective lens (OL), the first (Cl) condenser aperture, the second (C2) condenser aperture, and the selected area aperture (SA).

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

The identifications of atomic and molecular species is undertaken with a variety of mass spectroscopies. Time-of-flight (TOF) mass spectroscopy is of value for very short lived or highly peaked emissions. More sustained emissions are more readily studied with a quadrupole mass spectrometer (QMS), which can be tuned to a single mass peak. The time evolution (on a microsecond time scale) of a particular mass emission can be determined from the observed signals. Under the appropriate conditions, both these tools can be applied to studies of neutral emission (with ionizer) and positive or negative ion emission (without ionizer). 

The experiments were performed in two different ultra high vacuum (UHV) chambers using two different Pt(lll) single crystals. The X-ray photoelectron spectra were obtained in a chamber with a base pressure of lxlO" Torr. The system has been described in detail elsewhere. In brief, the UHV chamber is equipped with low energy electron diffraction (LEED), an X-ray photoelectron spectrometer (XPS), a quadrupole mass spectrometer (QMS) for temperature programmed desorption (TPD), and a Fourier transform infrared spectrometer (FTIR) for reflection absorption infrared spectroscopy (RAIRS). All RAIRS and TPD experiments were performed in a second chamber with a base pressure of 2 X 10 ° Torr. The system has been described in detail elsewhere. In brief, the UHV chamber is equipped for LEED, Auger electron spectroscopy (AES) and TPD experiments with a QMS. The chamber is coupled to a commercial FTIR spectrometer, a Bruker IFS 66v/S. To achieve maximum sensitivity, an... 

The TPD experiments are carried out using a differentially pumped quadrupole mass spectrometer (QMS), connected to the UHV apparatus employed for the MIES/UPS studies. The ramping time in TPD is considerably shorter (IK/s) than in MIES and UPS (IK/min). For this reason, the maximum desorption rate in MIES/UPS occurs about 15K earlier than in TPD. 

In a typical CMB experiment, beams of atoms and molecules with narrow angular and velocity spread are crossed in a vacuum chamber and the angular and time-of-flight (TOF) distributions of the products are recorded after well defined collisional events take place. The detector is an electron-impact ionizer followed by a quadrupole mass spectrometer (QMS) filter the whole detector unit can be rotated in the collision plane around the axis passing through the collision center (Figure 14.1). The crossed beam machine used in the present experiments has been described in detail elsewhere [67, 79,80]. Briefly, it consists of two source chambers (10 mbar), a stainless-steel scattering chamber (10 mbar), and a rotatable, differentially pumped quadrupole mass spectrometric detector ( <8 X 10" mbar). 

Measurement of the evolved He is made by peak height comparison with standard gases on sector-type mass spectrometers such as the MAP 215-50 and VG-3600 (e.g.. Wolf et al. 1996a, Warnock et al. 1997), or by He isotope dilution (ID) on a quadrupole mass spectrometer (QMS). We find that the precision and sensitivity of the ID-QMS technique are superior to those of the sector MS-peak height method. Reproducibility of gas standards suggests that for typical amounts of He evolved from a sample (e.g., of order 1 x 10 cc STP), the ID-QMS technique has a precision of -0.5% (la). The accuracy of this measurement depends on the accuracy of the standard used for calibration, which is probably better than 1% when capacitance manometry is used. 

Figure 30. Cross-sectional view of the molecular-beam VUV photoionization apparatus. (1) Nozzle (2) skimmer (3) reaction gas cell (4) vertical quadrupole mass spectrometer (QMS) (5) steradiancy electron energy analyzer (6) spherical sector electron energy analyzer (7) channeltron electron detector (8) horizontal QMS. Taken from ref. 113.

Figure 12. Basic configuration used for the mode that assays a nonvolatile substrate concentration, C by means of a volatile product. M is the semipermeable membrane, with an enzyme layer of thickness Xg and an assumed aqueous unstirred layer of thickness X on the left. The vacuum of the mass spectrometer (here a quadrupole mass spectrometer (QMS) shown with its ionizer) is to the right of the membrane. J, is the steady-state substrate flux to the enzyme layer, while J ,r and ] ,i are the steady-state right- and left-going volatile product fluxes (40).

Two almost simultaneous papers reported on the feasibility of rapid-scanning quadrupole mass spectrometers (qMS) with an electron-capture negative ion (ECNI) option as detector for GCxGC analysis of PCBs [26] and PCDD/Fs [25]. In both studies, the instrument was operated in the single ion monitoring (SIM) mode. The limitation in the number of scanned ions resulted in the desired 33-50 Hz acquisition rate. However, the selected mass range should still ensure that... 

FIGURE 9.13 Schematic of ESI-FAIMS instrument interfaced to quadrupole mass spectrometer (QMS). (From Purves and Guevremont, Electrospray ionization high-field asymmetric waveform ion mobility spectrometry-mass spectrometry. Anal. Chem. 1999, 71(13) 2346-2357. With permission.)... 

Fig. 2.17. Side elevation of the vacuum setup of the molecular beam machine. Chi vacuum chamber to generate the molecular beam by adiabatic expansion. The oven shown in Fig. 2.19 is inside. Ch2 vacuum chamber where the molecules interact with the laser pulses and are detected by a quadrupole mass spectrometer (QMS) or a Langmuir-Taylor detector (Fig. 2.22). Chi is pumped by an oil diffusion pump (DP) with a cold trap (CT), Ch2 by a turbomolecular pump (TP). Prevacuum is provided by a Roots (RVP) and a rotary valve vacuum pump (RVVP)...

The Molecular Beam Machine and the Detection System. The second component of the real-time MPI experiments is a molecular supersonic beam machine [116] with a quadrupole mass spectrometer (QMS), allowing the detection of ionized molecules and clusters with high sensitivity. A side elevation is shown in Fig. 2.17. The production of the molecular beam and the interaction of the laser pulse trains with the molecular beam are performed in a differentially pumped vacuum apparatus consisting of two separate chambers, which are briefly described in the following two paragraphs. A more detailed sketch of the two-chamber system is presented in Fig. 2.18. The production sub-chamber (oven chamber) is pumped by a 3000 /s oil diffusion pump (Balzers) with a baffle at the flange to the oven chamber to allow a pressure in the chamber of less then 10 mbar. During the experiments the pressure is typically 5 x 10 to 3 x 10 bar. In the second chamber a maximum pressure of 10 mbar is established by a 2200 /s turbomolecular pump (Balzers). 

---