Sources ionizing

Spark (source) ionization. Occurs when a solid sample is vaporized and partially ionized by an intermittent electric discharge. Further ionization occurs in the discharge when gaseous atoms and small molecular moieties interact with energetic electrons in the intermittent discharge. It is recommended that the word source be dropped from this term. 

Liquid chromatography, coupled to the different ionization sources, is generally the technique most used to characterize the phenolic profile in fruit and vegetable products. With regard to the source ionization, it seems that ESI is used more frequently than other sources, such as APCI or APPI. Another important aspect of this technique is the ionization of phenolic compounds. Negative ionization seems to be more suitable... 

Watson et al. (1985) performed analysis of conjugated steroids by coupling EG to MS by means of a thermospray (Th)-LC/MS interface. In their studies, negative ion Th-mass spectra were recorded for several steroid conjugates. This interface vaporizes the sample before it passes through a small orifice at the tip of a heated stainless steel tube in the ion source. Ionization occurs at the same time, usually promoted by a volatile buffer, such as ammonium acetate. 

In order for mass spectrometry to be used as an effective tool in LC, the analyte must be ionized in the interface region (source) since MS can only detect charged species. Consequently, the role of the ionization source is twofold (1) it is responsible for interfacing the chromatographic instrument with the detector and (2) it provides an effective and efficient means for sample ionization. It should be pointed out that source ionization efficiency plays a significant role in sample sensitivity for LC-MS analysis. The application of LC is ideally... 

Ion source. Ionization and formation of excited ionic states can be accomplished by any of the methods described earlier in this section (e.g., electron impact). 

External Source Ionization region which is physically... 

Such sources ionize the sample at atmospheric pressure and then transfer the ions into the mass spectrometer. An atmospheric pressure interface is then used to transfer ions into the high vacuum of the mass analyser. The problem lies in coupling an atmospheric pressure source compartment with an analyser compartment that must be kept at a very low pressure or at a very high vacuum (10 5 Torr). 

The Pd clusters have been produced by a recently developed high-frequency laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [16-18]. The monodispersed clusters have been deposited with low kinetic energy (0.1-2 eV) onto a MgO thin film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [30]. 

Sources Ionization energies cited in this chapter are from C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, National Standard Reference Data Series, U.S. National Bureau of Standards, NSRDS-NBS 34, Washington, DC, 1970, unless noted otherwise. Electron affinity values listed in this chapter are from H. Hotop and W. C. Lineberger, J. Phys. Chem. Ref. Data, 1985,14,731. Standard electrode potentials listed in this chapter are from A. J, Bard, R. Parsons, and... 

A weak radioactive source such as americium is used in some smoke detectors. Radiation from the source ionizes the air to produce a weak current. Smoke particles interrupt the current flow by attracting the ions. This decrease in current triggers the alarm. 

It is important to be aware that equation (1.12) shows that at a given distance from the source, ionization drops when the density is increased locally (like in the case of a density clump). On the other hand, of two nebulae with uniform density and ionized by the same star, the highest average ionization will occur for the densest one. 

Contact angle data, and the surface tensions of the test liquids, were taken from the work of Zisman et aL [4, 5, 8,9,14,15,22]. Data for the computation of 4>, to use in Equation 11, were from the following sources Ionization energies were from Field and Franklin [ll] and (for fluorocarbons) from Reed [28]. Dipole moments were taken from Sm3dli [29] and from the compilation of Wesson [30]. Polarizabilities were computed from refractive indices by the formula... 

The dynamic range of photographic detection is smaller in comparison with electric registration. However, the values for precision and accuracy found are more suitable than those noted for single ion monitoring (p. 27) and are comparable to results from spark source ionization. If a comparator, a data system and the suitable... 

The sensitivity of ESI-MS is largely determined by the efficiency of gas-phase ion production in the ESI source (ionization efficiency) and the effective sampling of the analyte ions from the atmospheric pressure ESI source to the high vacuum MS analyzer (ion transmission... 

Which situation will produce more positive-ion current in a thermal ionization source ionization of sodium salt on (a) rhenium (( ) = 4.98 eV) filament heated to 2200°C or (b) tungsten (( ) = 4.58 eV) filament heated to 2100°C ... 

Bradley then constructed an apparatus for measuring the force required to separate two silica spheres from adhesive contact (see Fig. 4.12). The rig could be evacuated to remove moisture and other contamination. Heaters were used to bake the glass and a radioactive source ionized the gas to leak away any stray charges. The silica balls were heated to incandescence immediately before the measurements. A deflection was applied to the bottom sphere until it detached from the upper sphere, giving the results shown in Fig. 4.13. 

The extent of ionization depends on the element, the temperature, and the free electron density. It varies from source to source. Ionization can be written as an equilibrium process ... 

Excessive ionization can reduce the atomic spectroscopy signal from the neutral atom, although accurate quantitation can still be conducted as long as the degree of ionization remains constant for all samples and standards. In some high-temperature sources, ionization is sufficiently large for ions to be... 

---