Electron capture ionization is the ionization of a gas phase

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

by attachment of an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

to create an ion of the form A^-. The reaction is :A + e^- -> ^- where the M over the arrow denotes that to conserve

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

and

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

a third body is required (the

molecularity In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of stoichiometric coeffici ...

of the reaction is three). Electron capture can be used in conjunction with chemical ionization.

Electron-capture mass spectrometry

Electron-capture mass spectrometry (EC-MS) is a type of

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

that uses electron capture ionization to form negative

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s from

chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with positive electron affinities. The approach is particularly effective for

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries ...

s. In contrast to electron ionization, EC-MS uses low energy

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

in a

gas discharge Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied ...

. EC-MS will cause less fragmentation of molecules compared to electron ionization.

Negative ion formation

Resonance electron capture

Resonance electron capture is also known as nondissociative EC. The compound captures an electron to form a

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anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

. The energy of the electrons are about 0 eV. The electrons can be created in the Electron Ionization source with moderating gas such as H₂, CH₄, i-C₄H₁₀, NH₃, N₂, and Ar. After the ion captures the electron, the complex formed can stabilize during collisions and produce a stable anion that can be detected in a mass spectrometer. :AB + ''e''⁻ → AB^−•

Dissociative resonance capture

In dissociative resonance capture, the compound fragments resulting in

electron capture dissociation Electron-capture dissociation (ECD) is a method of fragmenting gas-phase ions for structure elucidation of peptides and proteins in tandem mass spectrometry. It is one of the most widely used techniques for activation and dissociation of mass sel ...

(ECD). ECD forms an anion fragment and a radical fragment. The energy of the electrons are from 0-15 eV, but the optimum energy can vary depending on the compound. :AB + e^- -> A^- + B^

Ion-pair formation

With electrons of energy greater than 10 ev, negative ions can also be formed through ion-pair formation. :AB + e⁻ → A⁻ + B⁺ + e⁻ Calibration of the mass spectrometer is important in electron capture ionization mode. A calibration compound is needed to ensure reproducibility in EC-MS. It is used to ensure that the mass scale used is correct and that the groups of ions are constant on a regular basis. Fragmentation in ECI has been studied by

tandem mass spectrometry Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more mass analyzers are coupled together using an additional reaction step to increase their abilities to analyse chemical samples. A com ...

. The technique can be used with

gas chromatography-mass spectrometry Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or com ...

Electron capture detector

An electron capture detector most often uses a

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

source to generate electrons used for ionization. Some examples of radioactive

isotopes Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

used are ³H, ⁶³Ni, ⁸⁵Kr, and ⁹⁰Sr. The gas in the detector chamber is ionized by the radiation particles. Nitrogen, argon and helium are common carrier gases used in the ECD. Argon and helium need to be combined with another gas, such as methane, in order to prevent immediate conversion into

metastable In chemistry and physics, metastability denotes an intermediate Energy level, energetic state within a dynamical system other than the system's ground state, state of least energy. A ball resting in a hollow on a slope is a simple example of me ...

ions. The combination will extend the lifetime of the metastable ions (10⁻⁶ seconds). The methane will cool the electrons during the collisions. The addition of methane will enhance the ability to form negative ions under high pressure because it will adjust the thermal energy to be similar to the energy distribution of the ions. Methane is the most common gas used because it can produce many positive ions when it collides with electrons. These positive ions will then form low energy electrons used for ionization: :2CH4+ + 2e^- -> CH4+ + CH3+ + H + \underbrace_ + \underbrace_ An ECD is used in some

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...

systems.

Applications

EC-MS (Electron-capture mass spectrometry) has been used for identifying trace levels of chlorinated

contaminants Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination ...

in the environment such as

polychlorinated biphenyls Polychlorinated biphenyls (PCBs) are highly carcinogenic chemical compounds, formerly used in industrial and consumer products, whose production was banned in the United States by the Toxic Substances Control Act of 1976, Toxic Substances Contro ...

(PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), and other polychlorinated compounds.

Pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...

derivatives, nitrogen containing

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, and phosphorus-containing

insecticides Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to b ...

have also been detected in EC-MS.

Bile acids Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Prima ...

can detected in various body fluids by using GC-EC-MS. Oxidative damage can also be monitored in trace amounts by analyzing oxidized phenylalanine using GC-EC-MS.

Advantages

EC-MS is a sensitive ionization method. Forming negative ions through electron capture ionization is more sensitive than forming positive ions through chemical ionization. It is a selective ionization technique that can prevent the formation of common

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found in environmental contaminants during ionization. Electron capture ionization will have less interference from these matrices compared to electron ionization. Electron capture mass spectra can distinguish between certain isomers that EI-MS cannot.

Limitations

Different energies in the ion source can cause variations in negative ion formation and make the mass spectra difficult to duplicate. Results shown in the mass spectrum can vary from instrument to instrument. The temperature of the ion source needs to be monitored. An increase in fragment ions occurs at higher temperatures. Lower temperatures will lower the energy of electrons. Set temperatures can vary, but it is important for electron energy to approach thermal levels for resonance electron capture to occur. Pressure of the added enhancement gas needs to be determined. Increasing the pressure will help stabilize the anions and extend the lifetimes of the negative ions. If the pressure is too high, not as many ions can exit the ion source. Analysis should be done using low sample loads for GC-EC-MS. The amount of sample will affect the ion abundance and cause variations in data.

References

{{Reflist Ion source