PTR-MS Drift Tube

An important parameter in PTR-MS is the energy provided by collisions between the ions and the neutral gas in the drift tube. This energy depends on the applied electric field, E, and on the neutral gas density, N (number of molecules per cubic centimeter), in the drift tube. The ratio E/N is called the reduced electric field. For a typical drift tube at a pressme of 1.5 Torr, the gas density is 4.4 x 10 mol/cm. The air flow into the instrument (that may also contain the sample in some cases), at a pressure of about 1.5 Torr is aroimd 20 cm min at standard temperature and pressure (STP 273 K, 1 atmosphere=760 Torr). The electric field strength is 500 V in 10 cm. 

From which the reduced electric field may be calculated 

To reduce the difficulty of woridng with such small numbers, another unit has been defined called the Townsend (Td), where 

In the PTR-MS drift tube described here, the reduced electric field strength 

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Although there are variations, as already pointed out in the previous section, the typical pressure inside a PTR-MS drift tube is in the region of 1-2 mbar. Analyte gas is added to... 

Table 8.2 Type of drug, molecular formula and structure, and product ions and their branching ratios (percentages) recorded with a PTR-MS drift tube operating at a reduced electric field of 120 Td... 

Fig. 15.15 The PTR-MS apparatus. It consists of a series of three main chambers. In the first chamber, H2O is introduced and protonated in an electrical discharge to form H3O. These ions are then driven by a small field through an orifice into the drift tube (chemical ionisation chamber). Coaxial to this orifice, neutral volatile organic compounds (VOCs) are introduced into the drift tube and collide at thermal energies with H3O. VOCs with proton affinities exceeding 166.5 kcal/mol are ionised by proton transfer from H3O and are accelerated out of the drift tube into the quadrupole mass filter and onto the detector. (Adaptedfrom [190])...

PTR-MS combines the concept of Cl with the swarm technique of the flow tube and flow-drift-tube mentioned above. In a PTR-MS instrument, we apply a Cl system which is based on proton-transfer reactions, and preferentially use HsO" " as the primary reactant ion. As discussed earlier, HsO" " is a most suitable primary reactant ion when air samples containing a wide variety of trace gases or VOCs are to be analyzed. HsO" " ions do not react with any of the natural components of air, as these have proton affinities lower than that of H2O molecules this is illustrated in Table 1. This table also shows that common VOCs containing a polar functional group or unsaturated bonds (e.g. alkenes, arenes) have proton affinities larger than that of H2O and therefore proton transfer occurs between H30" and any of these compounds (see Equation 4). The measured thermal rate constants for proton transfer to VOCs are nearly identical to calculated thermal, collisional limiting values (Table 1), illustrating that proton transfer occurs on every collision. 

Proton transfer reaction mass spectrometry (PTR-MS) was first developed at the Institute of Ion Physics of Innsbruck University in the 1990s. Nowadays, PTR-MS is a well-developed and commercially available technique for the on-line monitoring of trace volatile organic compounds (VOCs) down to parts per trillion by volume (ppt) level. PTR-MS has some advantages such as rapid response, soft chemical ionization (Cl), absolute quantification, and high sensitivity. In general, a standard PTR-MS instrument consists of external ion source, drift tube, and mass analysis detection system. Figure... 

Thus, the ambient air can be directly introduced to achieve an on-line measurement in the PTR-MS operation. Due to the presence of electric field, in the reaction region, the ion energy is closely related to the reduced-field E/N, where E is the electric field and N is the number density of gas in the drift tube. In a typical PTR-MS measurement, E/N is required to set to an appropriate value normally in the range of 120-160 Td (1 Td = 10 Vcm /molecule), which may restrain the formation of the water cluster ions H30 (H20) n = 1-3) to avoid the ligand switch reaction with analyte M [6] ... 

Normally, PTR-MS can determine the absolute concentrations of trace VOCs according to well-established ion-molecular reaction kinetics. K trace analyte M reacts with then the signal does not decline significantly and can be deemed to be a constant. Thus, the density of product ions [Mff ] at the end of the drift tube is given in Equation 28.13 [6] ... 

A variation of reduced-field E/N in PTR-MS is quite useful for the identification of compounds, which was first discussed by Hansel et al. [4,6]. An increase in reduced-field E/N leads to higher collision energies between the ions and neutral gas molecule in the drift tube and a detailed description about the relationship of reduced-field E/N with the ionic kinetic energy was given in a review by Lindinger et al. [6]. When E/N increases, due to CID, the ions with the same mass but... 

Our laboratory has also explored changing E/N in PTR-MS by a voltage adjustment that crosses the whole drift tube [28], The product ion distributions were studied for n-propanol/iso-propanol/acetic acid, propanal/acetone, and four isomeric butyl alcohols, and propanal and acetone, with different concentration... 

Proton Transfer Reaction Ion Trap Mass Spectrometer (PTR-IT-MS) The IT system used in PTR-MS was firstly reported by Prazeller and coworkers [17], and its cross-sectional view of a prototype instrument is shown in Figure 28.4. The PTR-IT-MS consists of a hollow cathode ion source, a drift tube that... 

FIGURE 28.5 Schematic drawing of the PTR-TOF-MS system with a hollow cathode ion source. SD, source drift tube FDT, flow drift tube TO, transfer optics MCP, microchannel plate. Reprinted with permission from Reference [15]. Copyright 2005 Elsevier. 

Keck, L., Oeh, U., Hoeschen, C. (2007) Corrected equation for the concentrations in the drift tube of a proton transfer reaction-mass spectrometer (PTR-MS). International Journal of Mass Spectrometry, 264, 92-95. 

Shen, C.Y, Li, J.Q., Wang, Y.J., Wang, H.M., Han, H.Y, Chu, YN. (2010) Discrimination of isomers and isobars by varying the reduced-field across drift tube in proton-transfer-reaction mass spectrometry (PTR-MS). Inter-... 

These clusters complicate the interpretation of the mass spectra. Depending on the pressure and the E/N value, the (H20) clusters can be present in the drift tube and react with the trace gas compounds. Since the PA of the clusters is higher than the PA of water, the PTR with a water cluster is more selective. This reaction can be equally efficient as the PTR, depending on the dipole moment of the neutral R. For nonpolar molecules like benzene, cluster reactions will not take place. Therefore, the sensitivity or detection efficiency of a molecule like benzene can be humidity dependent, since the amount of water clusters depends on humidity. The formation of these clusters with PTR-MS techniques can be limited and controlled by increasing the electric field applied over the reaction region or lowering the pressure. 

This fragmentation may be dependent not only on the structure of the molecule itself, but also on the drift tube E/N value that controls the kinetic energy of the molecules. Therefore, it is important to know, either via direct measurements of pure compounds or literature values, the behavior of the trace gas compounds under study. Even though there are libraries available with fragmentation patterns obtained with El, these spectra cannot be used as reference for PTR-MS because of the completely different nature of ionization. 

A PTR-MS (see Figure 52.1) normally consists of an ion source (1) in which H30" ions are produced by a discharge in water vapor a drift tube (2) where the trace gases from the gas sample are ionized by PTRs with H30" ions a quadrupole mass filter (3) where the ions are mass filtered based on its m/z and a secondary electron multiplier (4) that counts the ions. 

FIGURE 52.1 PTR-MS scheme with (1) an ion source, (2) the drift tube, (3) a quadrupole mass selector, and (4) the secondary electron multipher. TMP, turbo molecular pump. 

A veiy good summary of PTR-MS has been presented in a recent book by Ellis and May hew 2014 [50]. The PTR-MS instrument consists of the hollow cathode (HC) ion source, a drift tube, a mass spectrometer, and an ion detector. There are several instrument manufacturers producing instruments that can be broadly classed as PTR-MS units. Those discussed here have generally been manufactured by lo-nicon in Austria. 

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...

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