Organic compounds ionization

Organic compounds ionize when burned in a hydrogen air flame. If two electrodes at a potential difference of approximately 150 V are inserted into this flame, differences in conductivity of the flame can be measured as the solutes elute from the column and are burned. This isthe principle on which the flame ionization detector is based. In the usual flame detector, the column effluent ismixed with hydrogen. This mixture is fed into the flame jet of the detector. The jet is a thin-walled stainless steel tube that also acts as one electrode. The other electrode is a fine platinum wire held above the jet. The response of this detector is practically instantaneous. It is not affected as much as the thermal conductivity detector is by changes in temperature and carrier gas flow rate. It is very sensitive and can detect approximately 10 °-10 mol solute. 

Flame Ionization Detector Combustion of an organic compound in an Hz/air flame results in a flame rich in electrons and ions. If a potential of approximately 300 V is applied across the flame, a small current of roughly 10 -10 A develops. When amplified, this current provides a useful analytical signal. This is the basis of the popular flame ionization detector (FID), a schematic of which is shown in Figure 12.22. 

This experiment provides an alternative approach to measuring the partition coefficient (Henry s law constant) for volatile organic compounds in water. A OV-101 packed column and flame ionization detector are used. 

This thermal ionization process requires fiiament temperatures of about 1000-2000°C. At these temperatures, many substances, such as most organic compounds, are quickiy broken down, so the ions produced are not representative of the structure of the original sample substance placed on the filament. Ionization energies (1) for most organic substances are substantially greater than the filament work function (( )) therefore 1 - ( ) is positive (endothermic) and few positive ions are produced. 

An application of surface-assisted laser desorption-ionization (SALDI) method for practical, ultrahigh sensitivity detection of aromatic amines by GC-MS is reported. The prototype analytical device for trace detection of different organic compounds is created. 

Historically, measurements have classified ambient hydrocarbons in two classes methane (CH4) and all other nonmethane volatile organic compounds (NMVOCs). Analyzing hydrocarbons in the atmosphere involves a three-step process collection, separation, and quantification. Collection involves obtaining an aliquot of air, e.g., with an evacuated canister. The principal separation process is gas chromatography (GC), and the principal quantification technique is wdth a calibrated flame ionization detector (FID). Mass spectroscopy (MS) is used along with GC to identify individual hydrocarbon compounds. 

An acidity list covering more than 5000 organic compounds has been published E.I . Serjeant and B. Dempsey (eds.), "Ionization Constants ol Organic Acids in Aqueous Solution." IUPAC Chemical Data Series No. 23, Pergamon Press, Oxford, 1979. 

Hydrofluoric acid is a polar material, as water is, and it behaves as an ionizing solvent when it is scrupulously free of water. Salts that dissolve readily in liquid HF include LiF, NaF, KF, AgF, NaNOs, KNOa, AgNOj, Na2S04, K2S04, and Ag2S04. Liquid HF also dissolves organic compounds and is used as a solvent for a variety of reactions. 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

Faubert, D. Paul, G. J. C. Giroux, J. Bertrand, M. J. Selective fragmentation and ionization of organic compounds using an energy-tunable rare-gas metastable beam source. Int. I. Mass Spectrom. Ion Proc. 1993,124, 69-77. 

Water is the most effective solvent for promoting ionization, but most organic compounds do not dissolve appreciably in water. 

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