Collision chamber

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

Two main types of experimental equipment which can be distinguished are (a) the perpendicular type in which in the collision chamber the incident positive ions and the product ions move in paths perpendicular to each other (b) the longitudinal type in which the paths are parallel. 

The ions A move in mass spectrometer A in a vertical plane and then cross the collision chamber. The reaction products (ithe ions B) are extracted from the collision chamber at right angles to the direction of the ion beam A and are analyzed by mass spectrometer B, which is placed in a horizontal plane. 

Radius of mass spectrometer A is only 2.5 cm so that very low velocities of the ions (A) can be used. In the collision chamber, k, reaction products (ions B) are formed from the gas by charge exchange or ion-molecule reactions. All ions move in the same direction in the collision chamber and are accelerated by special electrostatic lenses hence, they all reach the slit r of mass spectrometer B (not shown) independently of their initial velocities in the collision chamber. The discrimination in mass spectrometer B can therefore be considered negligible... 

Ion-molecule reactions can be investigated in a double mass spectrometer in two ways (a) In the collision between the incident ion and the gas molecule, transfer of part of one of these structures can take place. The pressure in the collision chamber must be low (b) The pressure in the collision chamber is increased. The slow incident ions ionize the gas molecules by charge exchange. Then ion-molecule reactions take place between the ionized gas molecules or their fragment ions and other gas molecules. 

In the first investigation 20), ethylene in the collision chamber was bombarded with positive ions, and the intensities of the fragment ions, obtained after the charge exchange, were recorded. The mass spectra were thus not normalized. At low pressure only primary ions were observed that were formed from ethylene in the charge exchange, but at higher pressures also secondary and tertiary ions were obtained as a result of ion-molecule reactions between the primary ions and the ethylene molecules in the collision chamber. 

In Figure 4 the logarithm of the observed ion intensities was plotted as a function of the logarithm of the pressure in the collision chamber. As the intensity of a product ion of a certain order increases proportionally to the same power of the pressure, the curves in the diagram corresponding to primary, secondary, and tertiary ions are represented by straight lines of slopes equal to 1, 2, and 3, respectively. Measurements were performed with 11 incident ions with different recombina-... 

Figure 9. Apparatus for the determination of CID thresholds. Electrospray ions are produced as in Figures 4 and 7, however pressure reduction capillary CAP leading to 10 torr low pressure chamber LPS is coaxial with orifices leading to triple quadrupole. The IT chamber at 10 torr is used for ion thermalization. Collision chamber at Q2 is usually used with collision gas Ar or Xe. The last quadrupole Q3 is not shown on Figure.

The tandem mass spectrometer consists of a mass spectrometer followed by a field-free collision chamber, followed by a second mass spectrometer. Specific ions... 

Electron ionization sources produce constant ion beams of about 10 8 A with low initial energy spread. The ion current measured depends strongly on the ionization degree of the gas analyzed (type of atoms and molecules). Positive ions and electrons are formed by the interaction of electrons of sufficient energy with gas atoms or molecules. The ion current /+ is proportional to the pressure (p) of the gaseous sample, to the electron current /e, the length (/) of the collision chamber and the differential ionization (s) of elements as a function of the ionization energy ... 

Figure 16.18—FAB and MALDI techniques, a) Principle of fast-atom generation using xenon b) formation of a fast-atom beam of argon in a collision chamber and bombardment of the sample (using a FAB gun) c) MALDI. The impact of a photon leads to a similar result as with a fast atom. The mechanism for desorption ionisation is not entirely known. These ionisation modes are particularly well suited for the study of medium to high molecular weight species. They are mostly used in biomedical sciences but not for routine determinations.

Figure 16.23—Triple quadrupote MS MS instrument. In the triple qnadrupole arrangement, the middle quadrupole is used as a collision chamber. It is operated in the radiofrequency voltage mode only, where it will transmit all masses. A gas pressure introduced in the second quadrupole is responsible for collision activation. Triple quadrupole instruments can conduct all three types of MS — MS analysis described above.

Collision region. The neutral reactant can be introduced into the collision region in the form of a neutral beam, having well-defined velocity and orientation (e.g., produced by a nozzle), or neutrals may be admitted to a collision chamber, where their orientation is random and the velocity is characterized by a Boltzmann distribution. 

Another commonly used type of mass spectrometer is the tandem mass unit, also referred to as an MS/MS (Fig. 15.4) or a triple quad mass spectrometer. Originally, this was made up of two or three mass spectrometers used in series. One MS is used to separated ions, the middle unit is used as a collision chamber in which selected ions are allowed to impact heavy gas molecules and fragment, and the last MS is used to separate and measure the fragment ions. In one... 

In addition, Ponikarov et al. [112] studied more special impinging stream absorption equipment. It employs the flow configuration of rotating impinging streams and the impingement occurs in a collision chamber of half-circle form. This equipment appears to be of less practical interest and so will not be discussed further here. 

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