Ion source pressures

The relative abundances of these product ions change dramatically as the ion source pressure increases from El conditions to 25 Pa. Above 100 Pa, the relative concentrations stabilize at the levels represented by the Cl spectrum of methane reagent gas (Pig. 7.3). [4,19] Portunately, the ion source pressure of some 10 Pa in Cl practice is in the plateau region of Pig. 7.3, thereby ensuring reproducible Cl conditions. The influence of the ion source temperature is more pronounced than in PI because the high collision rate rapidly effects a thermal equilibrium. 

Fig. 7.2. Comparison of the methane spectrum upon electron ionization at different ion source pressures (a) approx. 10 Pa, (b) approx. 10 Pa. The latter represents the typical methane reagent gas spectmm in positive-ion CL...

Hancock, R.A. Hodges, M.G. A Simple Kinetic Method for Determining Ion-Source Pressures for Ammonia CI-MS. Int. J. Mass Spectrom. Ion Phys. 1983, 46, 329-332. 

Figure 13. Plot of reciprocal bimolecular clustering rate constant vs. reciprocal ion source pressure from which the transient Sn2 lifetime may be derived.

With respect to the practical considerations of gas flow and vacuum requirements, the PHPMS experiment might, upon cursory consideration, appear to be easily extended into the VHP region. That is, several MS-based analysis techniques routinely use ion source pressures of 1 atm. However, when an attempt to increase the pressure within a PHPMS ion source is made, the factors that do become problematic are those related to the subtle principles on which the method is based. Most importantly, the PHPMS method requires that the fundamental mode of diffusion be quickly established within the ion source after each e-beam pulse, so that all ions are transported to the walls in accordance with a simple first-order rate law while the IM reactions of interest are occurring. This ensures that a constant relationship exists between the ion density in the cell and the detected ion signal. The rates of the IM reactions can then be quantitatively determined from the observed time dependencies of the reactant ion signal because the contribution of diffusion to the time dependencies are well known and easily accounted for. 

As the ion source pressure rises, ion-molecule reactions become possible, sample ions reacting with sample molecules. In the case of exact mass measurement, reaction can occur with the PFK mass reference 126). The observed reactions in the mass spectrum of ruthen-nium porphyrincarbonyl, yielding ions of the type [M-CO -h C, F2n] n = 1-4), illustrate this problem. Similarly, in the spectrum of Ni(PF3)4 ion-molecule reactions result in species such as Ni2(PF3) + n = 2-5) and Ni2(PF)2(PF3)m (m = 2-4) and, in the (CO)5CrC(CH3)OCH3 system, reactions of the following type are observed 127). 

Increased ion-source pressures and/or introduction of paramagnetic molecules, such as NO, cause deactivating collisions that convert excited ions into ground-state species13 61,63,64-66 (see Table I). The fractional abundance of excited states in an ion beam may thus be lowered without a concomitant reduction in total ion intensity, which would occur if the ionizing electron energy were simply lowered. 

Figure 30, Relative cross section as function of nominal kinetic energy (center of mass), E0, for collisional dissociation of He on He. Data are presented for He formed at two different ion-source pressures, as indicated.97...

It is useful to examine the consequences of a closed ion source on kinetics measurements. We approach this with a simple mathematical model from which it is possible to make quantitative estimates of the distortion of concentration-time curves due to the ion source residence time. The ion source pressure is normally low enough that flow through it is in the Knudsen regime where all collisions are with the walls, backmixing is complete, and the source can be treated as a continuous stirred tank reactor (CSTR). The isothermal mole balance with a first-order reaction occurring in the source can be written as... 

The chemical ionisation (Cl) mass spectrum Fig. 3, was recorded on a Finnigan 4000 Mass Spectrometer with ion source pressure 0.3 Torr, ion source temperature 150°C, emission current 300 yA, electron energy 100 eV using methane as a reagent gas. The electron impact (El) mass spectrum Fig. 4, was recorded on Varian MAT 311 Spectrometer, with an ion source pressure 10 6 Torr, ion source temperature 180, emission current 300 yA and electron energy of 70 eV. 

The equilibrium constants for numerous reactions of the general type described by equation 34, where AH is a Bronsted acid and X is a halogen, are available from a series of investigations which utilized high ion source-pressure and FT-ICR mass spectrometers. 

The reaction of H atoms with O2 molecules, the classical method for HO2 production and the reaction used in the mass spectrometric detection of the HO2 radical (7), does not yield high concentrations of HO2 radicals. This is partly due to the requirement of a third-body M to remove excess energy, which makes the reaction inefficient at low pressures. However, the introduction of stabilizing gas molecules into the reaction has the undesirable effect of reducing the sensitivity of the mass spectrometer, because the ion source pressure is approximately the same for the various experiments and, therefore, the detectability of a free radical is essentially determined by its mole fraction in the gas rather than by its absolute concentration. Furthermore, H atoms can react with HO2 radicals, and attempts to increase the HO2 concentration by increasing the H atom input were unsuccessful, probably for this reason. Typically, the concentration of HO2 obtained by the reaction... 

That is, a fast ion, formed by electron impact and accelerated toward the plasma surface, excites the H ion to form H , and then the H ion reacts via a third body electron capture process to form the ion inferred from the experiments. Only the ions formed at the surface of the plasma could be accelerated through the same potential drop as the primary H ions. Further, Schnitzer and Anbar argue that since there is no significant change in the Ha ion current as the ion source pressure is varied, the formation of does not occur within the... 

Medium-pressure Cl at ion-source pressures between 1 and 2000 Pa is widely used. In LC-MS, it is important in particle-beam and thermospray interfacing. Either an externally-added reagent gas like methane, isobutane, or ammonia is... 

The mass spectra (10) of salicylic acid was obtained by electron impact ionization (Fig. 7) and was recored on Finigen MAT 90 mass spectrometer. The spectrum was scanned from 50-250 a.m.a. Electron energy was 70 ev. Emission current 1 mA and ion source pressure 10 6 torr. The base peak is with a relative intensity 100%. Table 5 shows the most prominent fragments and their relative intensities. 

Figure 2. Dependence of the relative intensities of fragment ions and certain product ions in ethyl chloride on ion source pressure...

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