Ionization, field

Field ionization occurs when gas-phase sample molecules are inteijected in a strong electrical field that is on the order of 10 Vcm The field distorts the electron cloud around the sample molecule and lowers the barrier for the removal of an electron. The quantum mechanical tunneling of this electron from the molecule to the conduction bands of the emitter produces M+ ions [10]. The heart of the FI ion source is an emitter electrode made fi om a sharp metal object such as a razor blade or thin wire. The emitter electrode is placed approximately 1 mm away from the cathode. The field is produced by applying a high potential (10 to 20 kV) to the tip of the emitter electrode. FI is a very soft ionization technique that produces primarily a molecular ion signal. It is applicable to volatile samples only. 

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. 

Comparison of El (top) and FI (bottom) spectra of methyl stearate. Field ionization yields simple spectrum that shows intense molecular ion detected at m/z 298 without fragmentation. Reproduced, with permission from Micromass documentation. 

Field ionization (FI), which is an outgrowth of field ion microscopy, produces ions in the high electric field (on the order of 10 V/cm) developed around a small tip or blade held at high potential. The molecular ions formed in this way have little excess energy and often give rise to the most intense peak in the spectrum, even when no molecular ion is seen in the spectrum produced by electron bombardment. 

This phenomenon is of particular importance in the analysis of complex mixtnres of organic componnds (snch as those obtained from petroleum distillates) the absence of fragment ion peaks makes identification and measurement of molecnlar ion abnndances easier and, perhaps of greater significance, more reliable. The advantages of an nnambignons identification of the molecnlar ion in structure elucidations have already been discnssed. 

a high-potential electric field is applied to an anitter with a sharp surface, snch as a razor blade or a filament from which tiny whiskers have formed. This results in a very high electric field, which can result in ionization of gaseous molecules of the analyte. Mass spectra produced by FI have little or no fragmentation. They are dominated by molecular radical cations M or, less often, by protonated molecules [M + Hf . 

Field Emission and Field Ionization, Harvard University Press, 

Binnig, H. Rhorer, Ch. Gerber E. Weibel, Phys. Rev. Lett., 50, 120 

The critical distance for field ionization is related to I and j by16 

The probability of ionization during a time interval from 0 to t is given by 

If this probability is to be calculated for a given hopping path between xa and xb, then 

If the excited level is not too far from the ionization limit, the molecule may also be ionized by thermal collisions with other atoms or molecules. If Ejj lies above the ionization limit of the collision partners A, Penning ionization [6.57] becomes an efficient process which proceeds as 

If the excited level Ej lies closely below the ionization limit, the molecule M (Ej ) can be ionized by an external electric DC field (Fig.6.20a). This method is particularly efficient if the excited level is a long-lived highly excited Rydberg state. The required minimum electric field can readily be estimated from Bohr s atomic model, which gives a good approximation for atomic levels with large principal quantum number n. The ionization potential for the outer electron at the mean radius r from the nucleus is determined by the Coulomb field of the nucleus shielded by the inner electron core. 

Techniques, where the laser excites the atoms, but the ionizing step is performed by field ionization, have found increasing applications in the detection of Rydberg atoms in molecular beams (Sect. 14.6), in analytical chemistry for trace elements or small concentrations of pollutants [6.58]. 

For levels 10 meV below the ionization limit, (6.32) gives Eq 1.7-10 V/m for the ionizing external field. However, because of the quantum-mechanical tunnel effect the fields required for complete ionization are even lower. 

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. 

Unless extremely high potentials are to be used, the intense electric fields must be formed by making the radius of curvature of the needle tip as small as possible. Field strength (F) is given by Equation 5.1 in which r is the radius of curvature and k is a geometrical factor for a sphere, k = 1, but for other shapes, k 1. Thus, if V = 5000 V and r = 10 m, then, for a sphere, F = 5 x 10 V/m with a larger curvature of, say, Iff m (0.1 mm), a potential of 500,000 V would have to be applied to generate the same field. In practice, it is easier to produce and apply 5000 V rather than 500,000 V. 

An electric potential placed across a needle and a flat (plate) electrode. The lines of equipotential in the resulting electric field are focused around the tip of the needle, where the electric field becomes very large. 

The electrical reverse of the above arrangement produces negative ions. Thus, a negative needle tip places an electron on the molecule (M) to give a negative ion (M -), which is repelled toward a positive counter electrode. 

In the preceding discussions of optical and SFI detection we have assumed that the radiofrequency field was always on. Here we describe a 

While this technique has found its widest application in conjunction with field ionization detection, it is worth noting that it is equally useful in optical detection. In Eq. (23) we noted that with a cw rf source the optical signal corresponding to radiofrequency transition depends upon the relative radiative decay rates of the two levels, and the sensitivity decreases as the radiative decay rate of the final state. If the final state decays more slowly than the initial state, the signal is nearly proportional to the final state decay rate. In fact, if the pulsed laser excitation is used, and the rf is turned off 

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R. Gomer, Field Emission and Field Ionization, Harvard University Press, Cambridge, MA, 1961. 

Gomer R 1994 Field emission, field ionization, and field desorption Surf. Sc/299/300 129-52... 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

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. 

The process of field ionization presupposes that the substance under investigation has been volatilized by heat, so some molecules of vapor settle onto the tips held at high potential. In such circumstances, thermally labile substances still cannot be examined, even though the ionization process itself is mild. To get around this difficulty, a solution of the substance under investigation can be placed on the wire and the solvent allowed to evaporate. When an electric potential is applied, positive or negative ions are produced, but no heating is necessary to volatilize the substance. This technique is called field desorption (FD) ionization. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

Field desorption. The formation of ions in the gas phase from a material deposited on a solid surface (known as an emitter) that is placed in a high electrical field. Field desorption is an ambiguous term because it implies that the electric field desorbs a material as an ion from some kind of emitter on which the material is deposited. There is growing evidence that some of the ions formed are due to thermal ionization and some to field ionization of material... 

Field ionization. The removal of electrons from any species by interaction with a high electrical field. 

The mass spectra of free carbohydrates and their glycosides, obtained by ionization upon electron impact, are limited in their usefulness for structural studies. Peaks corresponding to molecular ions are generally not observed due to extensive fragmentation to ions of low m/e (4,9,11, 24, 26). In contrast, positive ions produced by field ionization do not give fragment spectra as characteristic as do those produced by electron impact, but the molecular ion peaks are intense, often the most intense in the spectra (3). 

Inghram and Corner showed that the mass spectra of molecules were much simpler using a field ionization source than with an electron bombardment ion source. Mainly parent ions are formed, unlike under electron impact which gives rise to considerable fragmentation. The simplicity of the mass spectra offers obvious applications in analysis of complex organic mixtures and their use is likely to become widespread... 

Anbar, Determination of Subprogram Amounts of Chemical Agents in the Atmosphere , Edge-woodArs Contract Rept EC-CR-74028, SRI Proj 3122 (1974) ( A method of mass spectroscopy, employing a silicone membrane and field ionization, which involves other new techniques, is presented which is sensitive to picogram amts of chemical agents in the atm)... 

Mass Spectra. Obtained by Gillis et al (Ref 104). Field ionization and electron impact ionization mass spectra are given by Brunee et al (Ref 54) Mechanical Properties < Sound Velocity. Hoge (Ref 77) obtained the following ultimate stress as a function of strain rate for machined discs (1.77g/cc) of PETN (all failures were brittle fractures)... 

Accelerated electrons in the applied electric field ionize gas molecules, and in these ionization processes extra electrons are created. In the steady state the loss of charged particles is balanced by their production. Due to their much lower mass, electrons move much faster than ions. As a result, charge separation creates... 

H.D. Beckey, Principles of Field Ionization and Field Desorption Mass Spectrometry, Pergamon Press, London (1977). 

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