Collisional ionisation

This is a similar value to the temperature of the Sun derived optically or from A.max in the black body spectrum. In a colder star the Balmer series is weaker still, but in a hotter star the Balmer series lines are stronger. In very hot stars the Balmer series may become weaker again due to collisional ionisation of H atoms, removing the electrons from the atoms completely. 

The principle of MS/MS for direct analysis of a multicomponent system is shown in Figure 6.18, in which the first mass spectrometer (MS I) operates with soft ionisation (FI, FD, Cl, LD), and thus produces an ensemble of molecular ions (M + H+, M — H+, or adducts). For identification of molecule ABC only ABC+ is allowed to enter an interface or fragmentation zone for excitation by collisional activation, laser radiation or surface-induced dissociation. Within the time of one vibration (10-13s), ABC+ dissociates into fragments characterising the original molecule. They are separated and detected by MS II [226]. Soft ionisation with FI/FD produces low ion yields, which may be insufficient for MS/MS LVEI (typically at 20 V) can be an alternative. Complete analysis of a multicomponent system is carried out in some 20 min. 

PCI, positive chemical ionisation El, electron impact CID, collisionally induced dissociation. 

When formed from thermal electrons at room temperature its lifetime with respect to auto-ionisation is about 25 psec170. Consequently few of these ions could escape collisional stabilization... 

Easily ionisable elements, such as alkali and alkaline earth metal elements, can alter the emission intensity and may cause an enhancement or a depression. This problem is more serious with DCPs, MIPs, and CMPs, while less important with ICPs. Several mechanisms have been suggested to explain these changes, for example lateral diffusion, changes in thermal conductivity, an altered volatihsation rate, ambipolar diffusion, or shifts in the ionisation equilibrium and the collisional processes. 

The electrospray ionisation mass spectrometric analyses were performed using a Finnigan LCQ ion trap mass spectrometer. The samples were dissolved in methanol or chloroform methanol system (10 1 v/v) and such solutions were introduced to the ESI source by continuous infosion by means of the instrument syringe pmnp with the rate of 3 iL/min. The ESI source was operated at 4.25 kV and the capillary heater was set to 200°C. For ESI-MS" experiments mass selected mono-isotopic parent ions were isolated in the trap and collisionally activated with 33% ejection RF-amplitude at standard He pressure. The experiments were performed in the positive and negative-ion mode. 

Loo, J.A. Edmonds, C.G. Udseth, H.R. Smith, R.D. Collisional activation and dissociation of large multiply charged proteins produced by electrospray ionisation. Anal. Chim. Acta. 1990, 241, 167-173. 

It would be desirable to ionise all samples by both hard and soft techniques, thereby gaining the maximum information. This is not always possible as certain ionisation techniques may be precluded by the nature of the sample, for example most cationic and anionic surfactants cannot be ionised directly by EL Where species can only be ionised by soft techniques, structural information may be obtained by collisional activation of the molecular ion (or quasimolecular ion, depending on the ionisation mode) to induce fragmentation. This type of technique is denoted as CID in Table 12.1, where ionisation modes appropriate to applications are summarised. 

The products of reaction (2) are critical to the model because the degree of ionisation of the gas within the shock is affected. When the correct photodissociation products (C and ff ") are adopted, n H ) > n(c ) in the vicinity of the maximum of T , whereas n(H ) n(C ) when the incorrect products (C and H) are assumed. The increase in n(H" ) leads to enhanced collisional coupling between the ionised and neutral fluids, a higher maximum of T and a reduced shock width, these changes being reflected in the computed column densities, as may be seen in Table 2. Unfortunately, the "correct" results (row (b) of Tcible 2) are not in good agreement with the observed column densities. 

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