Capture electron

Electron capture is a rare type of nuclear decay in which an electron from the innermost energy level (the Is — see Chapter 3) is captured by the nucleus. This electron combines with a proton to form a neutron. The atomic number decreases by one, but the meiss number stays the same. The foUowdng equation shows the electron capture of Polonium-204 (Po-204)  

The electron combines with a proton in the polonium nucleus, creating an isotope of bismuth (Bi-204). 

The capture of the Is electron leaves a vacancy in the Is orbitals. Electrons drop down to fill the vacancy, releasing energy not in the visible part of the electromagnetic spectrum but in the X-ray portion. 

If you could watch a single atom of a radioactive isotope, U-238, for example, you wouldn t be able to predict when that particular atom might decay. It might take a millisecond, or it might take a century. There s simply no way to tell. 

If you want to find times or amounts that are not associated with a simple multiple of a half-life, you can use this equation  

Electron capture has the same effect as positron emission. In this process, an electron from an inner energy shell (especially the Is orbital) is captured by the nucleus. The captured electron is used to convert a proton into a neutron. This conversion can be shown thus  

Transmutation of7Be isotope into the 3Li isotope through an electron capturing. 

By considering the nuclear equation given above, find the number of neutrons in X. 

The sum of the atomic numbers on both sides must be equal in the nuclear equation. 

Number of Half-Lives Percent of the Radioactive isotope Remaining 

Unlike the forms of radioactive decay that we have discussed so far, electron capture involves a particle being absorbed by instead of emitted from an unstable nucleus. Electron capture occurs when a nucleus assimilates an electron from an inner orbital of its electron cloud. Like positron emission, the net effect of electron capture is the conversion of a proton into a neutron. 

We can represent electron capture with this nuclear equation  

When an atom undergoes electron capture, its atomic number decreases by 1 because it has one less proton. For example, when Ru-92 undergoes electron capture, its atomic number changes from 44 to 43  

EXAMPLE 19.2 Writing Nuclear Equations for Beta Decay, Positron Emission, and Electron Capture  

Write the nuclear equation for each type of decay. (a) beta decay in Bk-249 (b) positron emission in 0-15 (c) electron capture in I-111  

There is one further process of negative-ion formation that is of special interest, because it provides superior sensitivity towards many toxic and/or environmentally relevant substances [78-82] electron capture (EC) or electron attachment [72]. EC is a resonance process whereby an external electron is incorporated into an orbital of an atom or molecule [8]. Strictly speaking, EC is not a subtype of Cl because the electrons are not provided by a reacting ion, but are freely moving through the gas at thermal energy. Nonetheless, the ion source conditions to achieve EC are the same as for NICI [83]. 

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The chromatogram can finally be used as the series of bands or zones of components or the components can be eluted successively and then detected by various means (e.g. thermal conductivity, flame ionization, electron capture detectors, or the bands can be examined chemically). If the detection is non-destructive, preparative scale chromatography can separate measurable and useful quantities of components. The final detection stage can be coupled to a mass spectrometer (GCMS) and to a computer for final identification. 

Figure Bl.6.11 Electron transmission spectrum of 1,3-cyclohexadiene presented as the derivative of transmitted electron current as a fiinction of the incident electron energy [17]. The prominent resonances correspond to electron capture into the two unoccupied, antibonding a -orbitals. The negative ion state is sufficiently long lived that discrete vibronic components can be resolved.

K, electron capture IT, isomeric transition X, X-rays of indicated element (e.g., O-x, oxygen X-rays, and the type, K or L)... 

Schematic diagram of an electron capture detector for gas chromatography.

Environmental Analysis One of the most important environmental applications of gas chromatography is for the analysis of numerous organic pollutants in air, water, and wastewater. The analysis of volatile organics in drinking water, for example, is accomplished by a purge and trap, followed by their separation on a capillary column with a nonpolar stationary phase. A flame ionization, electron capture, or... 

An electron capture detector is relatively insensitive to nonhalogenated compounds, providing the additional selectivity. 

Selectivity Because it combines separation with analysis, gas chromatography provides excellent selectivity. By adjusting conditions it is usually possible to design a separation such that the analytes elute by themselves. Additional selectivity can be provided by using a detector, such as the electron capture detector, that does not respond to all compounds. 

Graham, R. C. Robertson, J. K. Analysis of Trihalomethanes in Soft Drinks, /. Chem. Educ. 1988, 65, 735-737. Trihalomethanes are extracted from soft drinks using a liquid-liquid extraction with pentane. Samples are analyzed using a packed column containing 20% OV-101 on 80/100 mesh Gaschrom Q equipped with an electron capture detector. 

Stemmier, E.A. and Hites, R.A., Electron Capture Negative Ion Mass Spectra of Environmental Contaminants and Related Compounds, VCH, Weinheim, Germany, 1988. 

The special problems for vaUdation presented by chiral separations can be even more burdensome for gc because most methods of detection (eg, flame ionization detection or electron capture detection) in gc destroy the sample. Even when nondestmctive detection (eg, thermal conductivity) is used, individual peak collection is generally more difficult than in Ic or tic. Thus, off-line chiroptical analysis is not usually an option. Eortunately, gc can be readily coupled to a mass spectrometer and is routinely used to vaUdate a chiral separation. 

Sulfur hexafluoride may be analyzed chromatographicaHy using a molecular sieve or a Porapak QS column. Using an electron-capture detector, a sensitivity of 10 to lO " ppb is possible (51—53). 

Pesticides. Chlorinated hydrocarbon pesticides (qv) are often found in feed or water consumed by cows (19,20) subsequently, they may appear in the milk, where they are not permitted. Tests for pesticides are seldom carried out in the dairy plant, but are most often done in regulatory or private specialized laboratories. Examining milk for insecticide residues involves extraction of fat, because the insecticide is contained in the fat, partitioning with acetonitrile, cleanup (FlorisH [26686-77-1] column) and concentration, saponification if necessary, and determination by means of paper, thin-layer, microcoulometric gas, or electron capture gas chromatography (see Trace and residue analysis). 

Solvent extraction followed by gas chromatographic analysis is used to determine paraffin wax antioxidants (qv), ie, butylated hydroxyanisole and butylated hydroxytoluene and other volatile materials. Trace amounts of chlorinated organic compounds, eg, polychlorinated biphenyls, can be deterrnined by using a gas chromatograph with an electron-capture detector (22). 

AH of the 15 plutonium isotopes Hsted in Table 3 are synthetic and radioactive (see Radioisotopes). The lighter isotopes decay mainly by K-electron capture, thereby forming neptunium isotopes. With the exception of mass numbers 237 [15411-93-5] 241 [14119-32-5] and 243, the nine intermediate isotopes, ie, 236—244, are transformed into uranium isotopes by a-decay. The heaviest plutonium isotopes tend to undergo P-decay, thereby forming americium. Detailed reviews of the nuclear properties have been pubUshed (18). 

Several gas-Hquid chromatographic procedures, using electron-capture detectors after suitable derivatization of the aminophenol isomers, have been cited for the deterrnination of impurities within products and their detection within environmental and wastewater samples (110,111). Modem high pressure Hquid chromatographic separation techniques employing fluorescence (112) and electrochemical (113) detectors in the 0.01 pg range have been described and should meet the needs of most analytical problems (114,115). 

In almost all cases X is unaffected by any changes in the physical and chemical conditions of the radionucHde. However, there are special conditions that can influence X. An example is the decay of Be that occurs by the capture of an atomic electron by the nucleus. Chemical compounds are formed by interactions between the outer electrons of the atoms in the compound, and different compounds have different electron wave functions for these outer electrons. Because Be has only four electrons, the wave functions of the electrons involved in the electron-capture process are influenced by the chemical bonding. The change in the Be decay constant for different compounds has been measured, and the maximum observed change is about 0.2%. 

For any nucHde that decays only by this electron capture process, if one were to produce an atom in which all of the electrons were removed, the effective X would become infinite. An interesting example of this involves the decay of Mn in interstellar space. For its normal electron cloud, Mn decays with a half-life of 312 d and this decay is by electron capture over 99.99% of the time. The remaining decays are less than 0.0000006% by j3 -decay and a possible branch of less than 0.0003% by /5 -decay. In interstellar space some Mn atoms have all of their electrons stripped off so they can only decay by these particle emissions, and therefore their effective half-life is greater than 3 x 10 yr. 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Electron Capture and /5" "-Decay. These processes are essentially the inverse of the j3 -decay in that the parent atom of Z andM transmutes into one of Z — 1 andM. This mode of decay can occur by the capture of an atomic electron by the nucleus, thereby converting a proton into a neutron. The loss of one lepton (the electron) requires the creation of another lepton (a neutrino) that carries off the excess energy, namely Q — — Z(e ), where the last term is the energy by which the electron was bound to the atom before it was captured. So the process is equivalent to... 

Table 6. Ratios for K and L Electron Capture, and for j3 -Emission and Electron Capture ...

Although protons and neutrons are not emitted from the ground states of these isotopes, there are many cases where particles are emitted from excited states. For example, Cs decays by electron capture and -emission to excited levels ia and ia 7% of these cases protons are emitted from... 

IT = isomeric transition, EC = electron capture, l3 = positron emission, and j3 = beta decay. 

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