Ionization, electron

Electron ionization (El) is one of the oldest modes of ionization, first used by Dempster in 1918 [1], It is the most popular means of ionization for organic compounds with molecular mass less than 600 Da. Several other classes of compounds can also be analyzed conveniently by EI-MS. It is, however, restricted to thermally stable and relatively volatile compounds. Many solids and liquids are quite volatile at the prevailing vacuum of the instrument, whereas others must first be vaporized at elevated temperatures. 

In the El process, the vaporized sample molecules are bombarded with a beam of energetic electrons at low pressure (ca. 10 to 10 torr). An electron from the target molecule (M) is expelled during this colhsion process to convert the molecule to a positive ion with an odd number of electrons. This positive ion, called a molecular ion or radical cation, is represented by the symbol M+  

In an attempt to achieve higher ionization efficiency, filament current, emission current, and ionizing current must be optimized. The filament current is the current supplied to the filament to heat it to incandescent. The emission current, a measure of the rate of electron emission from the filament, is the current measured between the filament and the electron entry slit. The ionizing current, the rate of electron arrival at the trap, is a direct measure of the number of electrons in the chamber that are available for ionization. 

Solution Ion-source block = 8000 V electron filament (cathode) = 7930 V electron trap = about 8100 V ion repeller = about 8020 V and the exit slit at ground potential (i.e., at 0 V). 

The sample ion current /+, which is a measure of the ionization rate, can be enhanced by manipulation of the ion extraction efficiency P, the total ionizing cross section Qj, the effective ionizing path length L, the concentration of the sample molecules [77], and the ionizing current /  

Electron ionization (El) was introduced in 1921 by Dempster, who used it to measure lithium and magnesium isotopes [31]. Modern El sources are, however, based on the design by Bleakney [32] and Nier [33, 34], who both worked in Prof. J. T. Tate s laboratory. In El ions are produced by directing an electron beam into a low pressure vapor of analyte molecules. 

a value that is quite close to the maximum cross section for most molecules [35], The molecular ion intensity will be close to maximum and at the same time fairly intense fragment ions are obtained that can give structural information. 

It should be noted that the time spent on ionization contributes to temporal resolution of TRMS this intrinsic dead time of the analytical procedure will be discussed separately for different ionization technoques. Although the time spent in the ion source is quite short (sub-microseconds to sub-milliseconds), it may in some cases affect the observation of the reaction of interest. Thus, it is worthwhile knowing the time spans characterizing different ionization techniques. 

It should be noted that the time required to produce ionic species is related to the speed of the energetic electron beam and the size of the analyte molecules. When the electron beam is supplied at 70 eV, it is estimated that the ionization time in El is within the sub-femtosecond to low femtosecond range [13, 14]. However, the residence time of the 

the description of El is restricted to what is essential for understanding the ionization process itself [18,19] and the consequences for the fate of the ions created. The practical aspects of El and the interpretation of El mass spectra are discussed later (Chaps. 5, 6). 

When a neutral is hit by an energetic electron carrying several tens of electron-volts (eV) of kinetic energy, some of the energy of the electron is transferred to the neutral. If the electron, in terms of energy transfer, collides very effectively with the neutral, the amount of energy transferred can effect ionization by ejection of one electron out of the neutral, thus making it a positive radical ion  

El predominantly creates singly charged ions from the precursor neutral. If the neutral was a molecule as in most cases, it started as having an even number of electrons, i.e., it was an even-electron (closed-shelT) molecule. The molecular ion formed must then be a radical cation or an odd-electron open-shell) ion as these species are termed, e.g., for methane we obtain  

In the rare case the neutral was a radical, the ion created by electron ionization would be even-electron, e.g., for nitric oxide  

Depending on the analyte and on the energy of the primary electrons, doubly and even triply charged ions can also be observed. [20] In general, these are of low abundance. 

Collisions between ions and molecules in the source can result in the formation of ions with higher m/z values than the molecular ion. A common ion-molecule reaction is that between a proton, H+, and an analyte molecule, M, to give a protonated molecule, MH+ or (M h- H)+. Such a species has a H-1 charge and a mass that is 1 u greater than that of the molecule and is called a proton adduct, often represented as (M + 1). In LC-MS experiments, sodium salts from the LC buffer solution often produce a mass (M + 23) Na-adduct ion. One reason for keeping the sample pressure low in the El ionization source is to prevent reactions between ions and molecules that would complicate interpretation of the mass spectrum. 

In the ion sources, the analysed samples are ionized prior to analysis in the mass spectrometer. A variety of ionization techniques are used for mass spectrometry. The most important considerations are the internal energy transferred during the ionization process and the physico-chemical properties of the analyte that can be ionized. Some ionization techniques are very energetic and cause extensive fragmentation. Other techniques are softer and only produce ions of the molecular species. Electron ionization, chemical ionization and field ionization are only suitable for gas-phase ionization and thus their use is limited to compounds sufficiently volatile and thermally stable. However, a large number of compounds are thermally labile or do not have sufficient vapour pressure. Molecules of these compounds must be directly extracted from the condensed to the gas phase. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

The ion sources produce ions mainly by ionizing a neutral molecule in the gas phase through electron ejection, electron capture, protonation, deprotonation, adduct formation or by the transfer of a charged species from a condensed phase to the gas phase. Ion production often implies gas-phase ion-molecule reactions. A brief description of such reactions is given at the end of the chapter. 

The electron ionization (El) source, formerly called electron impact, was devised by Dempster and improved by Bleakney [1] and Nier [2], It is widely used in organic mass spectrometry. This ionization technique works well for many gas-phase molecules but induces extensive fragmentation so that the molecular ions are not always observed. 

Mass Spectrometry Principles and Applications, Third Edition Edmond de Hoffmann and Vincent Stroobant Copyright 2007, John Wiley Sons Ltd 

The generation of El mass spectra is well understood with a rich documentation of the fragmentation processes. But in contrast to NMR, mass spectra still cannot be calculated from a theoretical standpoint. A general method to compute El mass spectra based on a combination of fast quantum chemical methods, molecular dynamics and stochastic preparation of hot ionized species was published by Grimme. It provides mass spectra that compare well with their experimental counterparts, even in subtle details (Grimme, 2013). 

The energy necessary for ionization of organic molecules is lower than the effective applied energy of 70eV and is usually less than 15eV (Table 2.40). The El operation of all MS instruments at the high ionization energy of 70 eV was 

20-dimethylamino-5a-pregnane. The a-cleavage of the amino group dominates in the spectrum information on the structure of the sterane unit is completely absent  

Further increases in the signal intensity are, therefore, not achieved via the ionization energy with beam instruments, but by using measures to increase the density and the dispersion of the electron beam. The application of pairs of magnets to the ion source can be used, for example. 

Each mass spectrum is the quantitative analysis by the analyser system of the processes occurring during ionization. It is recorded as a line diagram. The involved fragmentation and rearrangement processes are extensively known 

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A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Figure Bl.7.1. Schematic diagram of an electron ionization ion source source block (1) filament (2) trap electrode (3) repeller electrode (4) acceleration region (5) focusing lens (6).

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

A further important use of El mass spectrometry lies in measuring isotope ratios, which can be used in estimating the ages of artifacts, rocks, or fossils. Electron ionization affects the isotopes of any one element equally, so that the true isotope ratio is not distorted by the ionization step. Further information on isotopes can be found in Chapter 46. 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

As described above, the mobile phase carrying mixture components along a gas chromatographic column is a gas, usually nitrogen or helium. This gas flows at or near atmospheric pressure at a rate generally about 0,5 to 3.0 ml/min and evenmally flows out of the end of the capillary column into the ion source of the mass spectrometer. The ion sources in GC/MS systems normally operate at about 10 mbar for electron ionization to about 10 mbar for chemical ionization. This large pressure... 

As each mixture component elutes and appears in the ion source, it is normally ionized either by an electron beam (see Chapter 3, Electron Ionization ) or by a reagent gas (see Chapter I, Chemical Ionization ), and the resulting ions are analyzed by the mass spectrometer to give a mass spectmm (Figure 36.4). 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

Note that many of the terms mentioned in this chapter are discussed in detail elsewhere in this book. For example, the theory and practical uses of electron ionization (El) are fully discussed in Chapter 3. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

The positive-ion electron-ionization spectra of BFB and DFTPP must exhibit molecular and specified fragment ions, the relative abundances of which must fall within a predefined range. Ion abundance criteria for BFB and DFTPP are shown in Table 41.1. 

This article should be read in conjunction with Chapter 3, Electron Ionization. ... 

In a high vacuum (low pressure 10 mbar), molecules and electrons interact to form ions (electron ionization, El). These ions are usually injected into the mass spectrometer analyzer section. 

Chemical ionization produces quasi-molecular or protonated molecular ions that do not fragment as readily as the molecular ions formed by electron ionization. Therefore, Cl spectra are normally simpler than El spectra in that they contain abundant quasi-molecular ions and few fragment ions. It is advantageous to run both Cl and El spectra on the same compound to obtain complementary information. 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

Molecules can interact with energetic electrons to give ions (electron ionization, El), which are electrically charged entities. The interaction used to be called electron impact (also El), although no actual collision occurs. 

The beam of substrate molecules then passes straight into the ion source (electron ionization, El, or chemical ionization. Cl) for ionization before entry into the mass analyzer. 

Metastable ions are most readily detected following electron ionization. 

In the few microseconds that molecular ions (M ) spend in an ion source following electron ionization, many have sufficient energy to decompose to give fragment ions (F, . .., F +). 

A normal, routine electron ionization mass spectrum represents the m/z values and abundances of molecular and fragment ions derived from one or more substances. 

Electron energy. The potential difference through which electrons are accelerated before they are used to bring about electron ionization. The term ionizing voltage is sometimes used in place of electron energy. 

Electron ionization. Ionization of any species by electrons. The process can be written for atoms or molecules as ... 

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