Ionisation in flames

If we accept the treatment of ionisation as an equihbrium process it is evident that the degree of ionisation will be affected by the presence of other ionisable species in the flame. As this interfering species B is also ionised to a certain degree in the flame, the degree of ionisation of the analyte atom A will be decreased, due to the law of mass-action, by the electrons formed from B. The charge balance now requires consideration of the ionisation of both species  

It can thus be concluded that in the presence of an easily ionisable element B in the flame the degree of ionisation of the analyte A is reduced and thereby its absorption is increased. Note, that at higher flame temperatures ionisation is increased which may counterbalance the increase in atomisation. Thus, a hotter flame does not necessarily result in an improved sensitivity of AAS measurements. 

The addition of an easily ionisable element at relatively high concentration as an ionisation buffer allows one to reduce the effect of shifts in ionisation equihbria. The ionisation buffer (often an alkali metal salt such as potassium chloride) creates a high concentration of electrons in the flame, resulting in suppression of the ionisation of the analyte. 

A generally useful and common approach to eliminate chemical interference in AAS is to use standard addition for quantitation. This will be described in more detail in the following section. 

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FID Flame ionisation detector Burning of compound in flame... 

Ionisation of elements in flames is very temperature-dependent, as shown in Table 8.2. 

Another type of interference that can arise in the atomiser is called ionisation interferences . Particularly when using hot atomisers, the loss of an electron from the neutral atom in metals with low ionisation energy may occur, thus reducing the free atom population (hence the sensitivity of the analyte determination, for which an atomic line is used, is reduced). These interferences can be suppressed in flames by adding a so-called ionisation suppressor to the sample solution. This consists in adding another element which provides a great excess of electrons in the flame (he. another easily ionisable element). In this way, the ionisation equilibrium is forced to the recombination of the ion with the electron to form the metal atom. Well-known examples of such buffering compounds are salts of Cs and La widely used in the determination of Na, K and Ca by FAAS or flame OES. 

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

Ramsteiner et al. [156] compared alkali flame ionisation, microcoulometric, flame photometric and electrolytic conductivity detectors for the determination of triazine herbicides in water. Methanol extracts were cleaned up on an alumina column and 12 herbicides were determined by gas chromatography with use of conventional columns containing 3% Carbowax 20m on 80-100 mesh Chromosorb G. 

The gases in flames are ionised as a rule, and it has been stated that the vivid combustion of phosphorus also gives rise to ionisation.6... 

This effect will obviously be greatest with elements having low ionisation potentials such as the alkali and alkaline earth metals, e.g. barium is approximately 80% ionised in the nitrous oxide flame. Since the ground state therefore becomes depopulated, the sensitivity will decrease. 

For the majority of elements commonly determined in water by AAS, an air—acetylene flame (2300°C) is sufficient for their atomisation. However, a number of elements are refractory and they require a hotter flame to promote their atomisation. Because of this, a nitrous oxide—acetylene flame (3000° C) is used for the determination of these elements. Refractory elements routinely determined in water are aluminium, barium, beryllium, chromium and molybdenum. Chromium shows different absorbances for chromium(III) and chromium(VI) in an air-acetylene flame [15] but use of a nitrous oxide-acetylene flame overcomes this. Barium, being an alkaline earth metal, ionises in a nitrous oxide—acetylene flame, giving reduced absorption of radiation by ground state atoms, however in this case an ionisation suppressor such as potassium should be added to samples, standards and blanks. 

This effect mostly occurs with alkali and alkaline earth metals. The low ionisation potentials of these elements cause them to be readily ionised in the flame with a resultant lowering of the population of ground state atoms and a suppression of sensitivity. The technique used to overcome this is to add an easily ionised salt such as potassium chloride to samples and standards. This ionises in preference to the analyte in the flame and enhances sensitivity. As an example, strontium, barium and aluminium are subject to ionisation in the flame. In water analyses, this is suppressed by adding potassium to the samples and standards so that the solution contains 2 000 mg l-1 potassium. 

Aluminium 237.3 nitrous oxide/ acetylene Aluminium is ionised in the nitrous oxide/acetylene flame add 2000 mg l1 potassium to the sample solution... 

The alkali and alkaline earth elements are easily ionised in the flame. It is therefore necessary to add an ionisation suppressant when analysing for these elements. The potassium salt of naphthasulphonic acid or a commercially available potassium standard solution must be added to give a final potassium concentration of approximately 1000/igml-1 in both samples and standards. 

This is the most useful technique for screening pesticides since it has wide applicability and sensitivity, and utilises equipment which is readily available in most laboratories. Over 95% of all pesticides may be chromatographed intact or as a simple derivative in some cases there is a clearly defined decomposition product although quantification may be difficult if the extent of decomposition is not reproducible. The sensitivity of the method is high using a flame ionisation detector when specific detectors are used, e.g. electron capture, alkali flame ionisation, or flame photometric detectors, even lower concentrations in body fluids may be detected. 

V. Hefter, K. Bergmann, Spectroscopic detection methods, in Atomic and Molecular Beam Methods, vol. 1, ed. by G. Scoles (Oxford Univ. Press, New York, 1988), p. 193 J.E.M. Goldsmith, Recent advances in flame diagnostics using fluorescence and ionisation techniques, in Laser Spectroscopy VIII, ed. by S. Svanbeig, W. Persson. Springer Ser. Opt. Sci., vol. 55 (Springer, Berlin, 1987), p. 337... 

Universal detection has not been an issue for GC, where the major limitation is the range of chemicals that can be rendered volatile and passed successfully through the column. The most common detector is the flame ionisation detector. The sample is ionised in a small flame of hydrogen, leading to an increase in conductivity. This is almost a universal detector since virtually everything can be ionised in this way. 

Calcium ionisation in the flame may be reduced or eliminated by adding an excess of sodium or potassium to the solutions. For the calcium suppression by phosphate, sulphate, silicate and alumina excess concentrations of a competing cation or anion may be added e.g, Sr, La, Mg, SO4, etc. Finally the protein interference of calcium in blood sera solutions has been overcome by the use of EDTA as a complexing reagent. 

Conversion to acetates, trifluoroacetates (178), butyl boronates (179) trimethylsilyl derivatives, or cycHc acetals offers a means both for identifying individual compounds and for separating mixtures of polyols, chiefly by gas—Hquid chromatography (glc). Thus, sorbitol in bakery products is converted to the hexaacetate, separated, and determined by glc using a flame ionisation detector (180) aqueous solutions of sorbitol and mannitol are similarly separated and determined (181). Sorbitol may be identified by formation of its monobensylidene derivative (182) and mannitol by conversion to its hexaacetate (183). 

Owing to poor volatihty, derivatization of nicotinic acid and nicotinamide are important techniques in the gc analysis of these substances. For example, a gc procedure has been reported for nicotinamide using a flame ionisation detector at detection limits of - 0.2 fig (58). The nonvolatile amide was converted to the nitrile by reaction with heptafluorobutryic anhydride (56). For a related molecule, quinolinic acid, fmol detection limits were claimed for a gc procedure using either packed or capillary columns after derivatization to its hexafluoroisopropyl ester (58). 

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

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