Neutral gas

A non-tliennal, non-equilibrium plasma is characterized by an electron temperature much larger tlian tire ion temperature and tire neutral gas temperature (T T. Typical non-tliennal, non-equilibrium plasmas... 

Figure C2.13.5. Schematic illustrations of isotropic etching by a neutral gas and anisotropic plasma etching.

Also in general terms, the TOF part of the hybrid is used mostly for MS/MS studies in which ions produced in the magnetic sector are collided with neutral gas molecules to induce decomposition (see Chapter 23). In this mode the instrument produces more highly resolved product ion spectra than can be attained in simple magnetic-sector instruments. 

Alternatively, ions of any one selected m/z value can be chosen by holding the magnetic field steady at the correct strength required to pass only the desired ions any other ions are lost to the walls of the instrument. The selected ions pass through the gas cell and are detected in the singlepoint ion collector. If there is a pressure of a neutral gas such as argon or helium in the gas cell, then ion-molecule collisions occur, with decomposition of some of the selected incident ions. This is the MS/MS mode. However, without the orthogonal TOF section, since there is no further separation by m/z value, the new ions produced in the gas cell would not be separated into individual m/z values before they reached the detector. Before the MS/MS mode can be used, the instrument must be operated in its hybrid state, as discussed below. 

Typical MS/MS configuration. Ions produced from a source (e.g., dynamic FAB) are analyzed by MS(1). Molecular ions (M or [M + H]+ or [M - H]", etc.) are selected in MS(1) and passed through a collision cell (CC), where they are activated by collision with a neutral gas. The activation causes some of the molecular ions to break up, and the resulting fragment ions provide evidence of the original molecular structure. The spectrum of fragment ions is mass analyzed in the second mass spectrometer, MS(2). 

The total collisional energy between an ion and a neutral gas atom can be calculated from Equation 49.2, in which m, m are the masses of the colliding neutral gas molecule and the ion, respectively. Elab is ths energy imparted to the ion, and Ecm is the collisional energy referred to the center-of-mass of the ion and molecule. 

The density of ions and electrons increases quickly in the argon gas, at the same time increasing their kinetic energies as they are pulled back and forth in the applied electromagnetic field and undergo frequent collisions with neutral gas atoms. Some recombination of ions and electrons also occurs to form neutrals. 

The ion guides are also used as gas collision cells. When ions collide with neutral gas atoms in such a cell, it is important that ion losses due to deflection or collision should be minimized. Ion guides perform this task. 

If charge exchange occurs when the incident positive ion passes the neutral gas molecule with a certain velocity, transfer of translational energy will usually take [place. This transfer of translational energy... 

The uncured compacts are presintered in vacuum or in a stream of neutral gas (Ar, H2) at 800-1400°C. Presintering ensures the removal of the organic binder to avoid later contamination of the sintering furnace by pyrolysis of the by-products. It also facilitates machining and finishing, which are difficult and expensive after final sintering because of the hardness of borides. 

The application of high tension (e.g. 20 kV, 0.5 MHz) in an evacuated system (0.2. .. 8 torr) causes the residual gas to form a highly ionized mixture of positive and negative ions, electrons, photons and neutral gas molecules. In the presence of active sorbents this plasma reacts with the chromatographically separated substances to eld reactive ions and radicals. 

An electric potential drop across the boundary between two dissimiliar phases as well as at their surfaces exposed to a neutral gas phase is the most characteristic feature of every interface and surface electrified due to ion separation and dipole orientation. This charge separation is usually described as an ionic double layer. 

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