Source drift region

Figure 4.1 Schematic of PTR-MS system. HC hollow cathode SD source drift region VI Venturi-type inlet (Hansel et a ., 1998).

From the hollow-cathode source, ions are extracted into a short source drift region filled with water vapor. After passing this small drift section, the H30" ions reach a reaction region which is in the form of a drift section of about 20 cm length and 5 cm inner diameter, filled with the air (pressure... 

The above ions are injected into a short source drift region and further react with H2O ultimately leading to the formation of via ion-molecule reactions ... 

Unfortunately, the water vapor in the source drift region inevitably can form a few of cluster ions H30 (H20) via the three-body combination process... 

Fig. 83 A schematic diagram of a PTR-MS instrument showing the hollow cathode (HC) ion source, the source drift region (SD), the Venturi inlet (VI) and the drift tube...

Fig. 7.8. PTR mass spectrometer. The ion source of the lonicon PTR-TOF 2000 is divided into a compartment for reactant ion generation from H2O by (1) a hollow cathode and (2) a source drift region in front of the sample inlet, and (3) the drift tube for analyte ion formation. The TOF analyzer has its own turbomolecular pump to maintain high vacuum not shown). By courtesy of lonicon Analytik GmbH, Innsbrack, Austria...

Figure 1.6 Schematic of a PTR-MS instrument, where in this case the mass spectrometer is a quadrupoie device. HC signifies a hoiiow cathode discharge ion source and SD is known as the source drift region. These, along with other components of this instrument, are discussed in more detail in Chapter 3.

The large rate coefficients shown above are all indicative of ion-molecule reactions that occur at essentially a collision-limiting rate. To allow these reactions to fully take place, so that a high yield of H3O+ is attained, a so-called source drift region may be added between the HCD and the drift tube. 

To complete the calculation of the concentration ratio [H3O (H20)]/[H30+], the partial pressure of water vapour in the drift tube is required. One source of water in the drift tube is the water deliberately added to the ion source. This is minimized in some instruments by adding a source drift region and pumping this to reduce the quantity of water vapour that can exit into the drift tube. A second source of water is the analyte gas. To keep the calculation simple, we will assume that the water in the drift tube originates from the analyte gas only, and this is assumed to enter with 100% relative humidity. The saturated vapour pressure of water at 25°C is 31.7 mbar and we can equate this to the partial pressure of water, pu o, in the expression below ... 

In this mode, ions are formed continuously in the ion source (a), but the electrostatic accelerating potential is applied in pulses (b). Thus, a sample of ions is drawn into the drift region (c) with more ions formed in the source. As shown in Figure 26.1, the ions separate according to m/z values (d) and arrive at the detector (e), the ions of largest m/z arriving last. 

The apparatuses described thus far were all designed to detect the ionic products from ion-neutral interactions, whereas the TOF apparatus described by Moran and co-workers9911,146,147 has been employed to detect the neutral products from these processes. In the latter device the ions formed by electron impact are accelerated out of the source by an extraction pulse and are further accelerated by a series of grids. The primary beam is then focused by deflection and enters the drift region or flight tube, where mass separation of the different velocity groups occurs. 

Figure 8. Schematic diagram of typical ion-cyclotron resonance (ICR) cell used for ion-molecule reaction studies. Regions A, B, and C designate ion source, analyzer, and ion collector regions, respectively. Electrodes 2 and 4 are used to apply trapping potential, 1 and 3 for source drift potential, 5 and 6 for analyzer drift and RF fields, and 7 to 10 for total ion collection.148...

Fig. 2. Schematic representation of a mobility spectrometer. Ions created in the ion source are separated in the drift region based on their mobility. The ion swarms reach the detector where their drift times are recorded and plotted in the form of a mobility spectrum (Originally published in the article of Buryakov et al. [9]).

Fig. 5. Tandem quadrupole acceleration-deceleration mass spectrometer for NRMS studies. A - ion source B - quadrupole mass analyzer C - ion acceleration lens D - neutralization cell E - neutral drift region F - reionization cell G - ion deceleration lens H - energy filter I - quadrupole mass analyzer J - off-axis ion detector K - laser optics L - Ar-ion laser. Inset shows the drift region where residual precursor ions are reflected and the neutral beam overlaps with the laser beam...

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