Field ionization FI

In addition to the above methods utilizing conventional ionization modes, the field ionization technique has appeared [75]. The very intense electric field (about 1 V/A), produced by an electrode, results in the ionization of molecules in the gas phase. This soft ionization technique is often used competitively with Cl, since it does not pollute the source and may yield sufficiently reproducible results. The transit time of ions in the source is on the order of 10 to 10 second. The radical molecular ions (M ) produced are characterized by a low internal energy, and thus can be detected easily. As a result of dispersion within the source, however, sensitivity is about two orders of magnitude lower than that of El. As in the case of El, the fragments produced by FI can furnish interesting structural data on carbohydrates, amino acids, peptides and cardenolides [76], 

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The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

The study of metastable ions concerns substances that have been ionized by electrons and have undergone fragmentation. The stable molecular ions that are formed by soft ionization methods (chemical ionization. Cl field ionization, FI) need a boost of extra energy to make them fragment, but in such cases other methods of investigation than linked scanning are generally used. 

The distortion caused by the field allows an electron to pass from the molecule to the tip if the applied potential is positive or from the tip to the molecule if the potential is negative. This is called field ionization (FI), and the electron transfer occurs through quantum tunneling. Little or no vibrational excitation occurs, and the ionization is described as mild or soft. 

Volatile or volatilizable compounds may be introduced into the spectrometer via a pinhole aperture or molecular leak which allows a steady stream of sample molecules into the ionization area. Non-volatile or thermally labile samples are introduced directly by means of an electrically heated probe inserted through a vacuum lock. Numerous methods of sample ionization are available of which the most important are electron impact (El), chemical ionization (CY), field ionization (FI), field desorption (FD), fast atom bombardment (FAB), and radio-frequency spark discharge. 

Field ionization FI Ionization by strong electric field Volatile molecular ions Molecular compounds... 

Molecules can lose an electron when subjected to a high electric potential resulting in field ionization (FI) [366,534,535]. High fields can be created in an ion source by applying a high voltage between a cathode and an anode called a field emitter. A field emitter consists of a wire covered with microscopic carbon dendrites, which greatly amplify the effective field at the carbon points. 

Fig. 5.12. DIP of a JEOL JMS-700 sector instrament for use with El, chemical ionization (Cl) and field ionization (FI). The copper probe tip holds the glass sample vial and is fitted to a temperature-controlled heater (left). The heater, a thermocouple, and circulation water cooling are provided inside. The (white) ceramics insulator protects the operator from the high voltage of the ion source.

Analytical pyrolysis is defined as the characterization of a material or a chemical process by the instrumental analysis of its pyrolysis products (Ericsson and Lattimer, 1989). The most important analytical pyrolysis methods widely applied to environmental samples are Curie-point (flash) pyrolysis combined with electron impact (El) ionization gas chromatography/mass spectrometry (Cp Py-GC/MS) and pyrolysis-field ionization mass spectrometry (Py-FIMS). In contrast to the fragmenting El ionization, soft ionization methods, such as field ionization (FI) and field desorption (FD) each in combination with MS, result in the formation of molecule ions either without, or with only very low, fragmentation (Lehmann and Schulten, 1976 Schulten, 1987 Schulten and Leinweber, 1996 Schulten et al., 1998). The molecule ions are potentially similar to the original sample, which makes these methods particularly suitable to the investigation of complex environmental samples of unknown composition. 

GC-MS and Electron and Chemical Ionization (EI/CD-MS rely on the ability of organic species to survive volatilization prior to ionization. In many cases, this requires a degree of heating which often leads to decomposition. In desorption chemical ionization (DCI), field ionization (FI), thermospray (TSP) or fast atom bombardment (FAB) ionization occurs before volatilization, and measurement by the mass spectrometer often occurs before decomposition can result. These techniques have allowed determination of many high molecular weight and polar species, which could not previously be analyzed. 

INTERNAL ENERGIES OF MOLECULAR IONS FORMED BY FIELD IONIZATION (FI)... 

Field ionization (FI) is a method that uses very strong electric fields to produce ions from gas-phase molecules. Its use as a soft ionization method in organic mass spectrometry is principally due to Beckey [8], Like El or Cl, FI is only suitable for gas-phase ionization. Therefore, the sample is introduced into the FI source by the same techniques that are commonly used in El and Cl sources, for example using a direct probe that can be heated or the eluent from a gas chromatograph. 

In order to obtain even simpler MS spectra that will counteract a number of drawbacks in the common El ionization procedures applied in Py-EI MS, several special ionization techniques were applied. One of them is Cl ionization, but also field ionization (FI), field desorption (FD), and photoionization (PI) were utilized to obtain simplified mass spectra for pyrolysates. Also, MS/MS techniques were utilized for the analysis in attempts to substitute at least in part for the need of a separation. 

Field ionization (FI) and field desorption (FD) techniques used in Py-MS. 

A possible solution to the above problems would be the triple-dimensional analysis by using GC x GC coupled to TOFMS. Mass spectrometric techniques improve component identification and sensitivity, especially for the limited spectral fragmentation produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a unique identity for overlapping components in the chromatogram makes identification much easier. Thus MS is the most recognized spectroscopic tool for identification of GC X GC-separated components. However, quadru-pole conventional mass spectrometers are unable to reach the resolution levels required for such separations. Only TOFMS possess the necessary speed of spectral acquisition to give more than 50 spectra/sec. This area of recent development is one of the most important and promising methods to improve the analysis of essential oil components. 

The resolution of this latter problem passed through the development of the so-called soft ionization methods, in which ions are directly produced from the solid or liquid state. Field ionization (FI) and field desorption (FD) were two of the first alternative ionization methods. A few years later, other techniques were developed. Examples of these include desorption/chemical ionization (D/CI), secondary ion mass spectrometry (SI-MS), and fast atom bombardment (FAB). 

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