Nitrous oxide metal atoms

Flame methods are the conventional atomization sources used in MS for industrial hygiene (Table I). Air/acetylene at 2150-2400°C is used for the easily atomized elements like lead, cadmium, and zinc. Refractory metals such as tungsten or vanadium require hotter nitrous oxide/acetylene atomization at 2600-2800 C. The need for greater sensitivity and multielement analysis from a single filter has increased the use of electrothermal atomization for tin, vanadium, nickel, and other difficult elements. Formation of hydrides combined with flame atomization has been used in some cases to increase sensitivity. 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

As far as flame composition is concerned, it may be noted that an acetylene-air mixture is suitable for the determination of some 30 metals, but a propane-air flame is to be preferred for metals which are easily converted into an atomic vapour state. For metals such as aluminium and titanium which form refractory oxides, the higher temperature of the acetylene-nitrous oxide flame is essential, and the sensitivity is found to be enhanced if the flame is fuel-rich. 

The ratio, Nj/N0, can therefore be calculated. For the relatively easily excited alkali metal sodium, it is 9.9 x 10 6 at 2000 °K and 5.9 x 10 4 at 3000 °K this latter temperature is about the highest commonly obtained with flames used for atomic absorption or emission work. Hence, only about 1(T3 % of the sodium atoms are excited at 2000 ° and 6 x 1(F2 % at 3000°. For an element such as zinc,Nf/N0 is 5.4 x 10"10 at 3000 and so only 5 x 10"8% is excited. In spite of the small fraction excited, good sensitivities can be obtained for many elements by flame photometry if a high temperature flame is used, because the difference between zero and a small but finite number is measured. For example, seventy elements can be determined by flame photometry using the nitrous oxide-acetylene flame 1H. 

For the alkaline-earth metals, as noted earlier, a simple flame of almost any type can be used to excite the metals. However, to be able to determine a wide range of metals, it is common to use either an acetylene-air or acetylene-nitrous oxide flame as the source of energy to excite the atoms. The burner is long with a slot at the top and produces a long narrow flame that is situated end-on to the optics receiving the emitted light. 

Molybdenum may be identified at trace concentrations by flame atomic absorption spectrometry using nitrous oxide-acetylene flame. The metal is digested with nitric acid, diluted and analyzed. Aqueous solution of its compounds alternatively may be chelated with 8—hydroxyquinobne, extracted with methyl isobutyl ketone, and analyzed as above. The metal in solution may also be analyzed by ICP/AES at wavelengths 202.03 or 203.84 nm. Other instrumental techniques to measure molybdenum at trace concentrations include x-ray fluorescence, x-ray diffraction, neutron activation, and ICP-mass spectrometry, this last being most sensitive. 

The electron affinity of the metal surface is low in comparison with the tendency of the foreign molecule to receive electrons. This tendency is particularly high if the electron shell of the adsorbed molecule is incomplete (e.g. as in an oxygen atom) or if the bond of the atoms in the molecule is affected by an asymmetric electron shift (e.g, as in the molecules of nitrous oxide or carbon monoxide). In such cases the metal electrons become part of the electron shell of the foreign molecule. 

In the metal-catalyzed decomposition of nitrous oxide—corresponding to this conception—not the liberated 0 atom [C. Wagner (10)], but the N2O molecule receives electrons on adsorption (8,9,58). As the metal surface thereby loses electrons, it is to be expected that an increase of the work function will occur upon adsorption of N2O molecules, e.g., on platinum, even at such low temperatures that the thermal decomposition of N2O does not occur. 

According to this work the catalytic decomposition of nitrous oxide molecules proceeds in the following way a N2O molecule adsorbed by the catalyst binds metal electrons, and thus the bond between the 0 atom and N2 in the molecule is loosened, and N2 is thermally dissociated from O at sufficiently high temperature. The 0 atom is held to the surface through the influence of the metal electrons. It can combine with a neighboring... 

Nitrous Oxide (N20). In contrast to nitric oxide, N20 is not readily reduced by hydrogen atoms. However, it is a good O-atom transfer reagent and spontaneously reacts with metal surfaces at elevated temperatures. Under alkaline conditions with a freshly formed platinum surface (Pt ), N20 induces a vol-tammetric reduction peak (Figure 11.8) 10... 

A flame emission spectrometer therefore consists of an atom source, a monochromator and detector and is therefore simpler instrumentally than the corresponding atomic absorption system. Particular developments engendered by atomic absorption have restimulated interest in flame emission spectrometry after a dormant period. Chief of these is the use of the nitrous oxide—acetylene flame which is sufficiently hot to stimulate thermal atomic-emission from a wide range of metal elements. 

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. 

In the case of flame atomization, fuel-oxidant mixtures like acetylene-air are used for most easily atomized elements for the less easily atomized elements, for example, those that exist mainly as monoxides, the analytical sensitivity can be greatly enhanced by going to a higher-temperature flame, such as acetylene-nitrous oxide, thus shifting the equilibriiun M -F O MO toward the metal. 

Atomic absorption spectroscopy is commonly used to determine Fe, Al, Mn, Cr and other metals. Standard solutions should be prepared with the same acid concentration as that of the test solutions. Apart from Al which requires a nitrous oxide/acetylene flame, these cations may be measured using an air/acetylene flame. These metals may also be measured by inductive coupled plasma analysis (ICP). 

Atomic absorption measurements were made using standard conditions. Nearly stoichiometric flames were used for all metals but chromium, for which a reducing flame was used. The air-acetylene flame was used for all metals but vanadium, for which a nitrous oxide-acetylene flame was used. A single slot titanium burner was used for all of the metals investigated. Water saturated MIBK was used as the blank. Table I presents typical instrument parameters. 

The flame gases used for AAA can be pairs of air-acetylene, nitrous oxideacetylene or air-hydrogen. The nitrous oxide-acetylene flame has a maximum temperature of about 2900 °C and is used for the determination of elements which form refractory oxides. The air-hydrogen flame bums at a temperature of approximately 2000 °C and is used for the determination of alkali metals (Cs, Rb, K, Na) as its lower flame temperature reduces ionization interferences. Air-acetylene is the preferred flame, which has a temperature of approximately 2300 °C, for the determination of about 35 elements including chromium by atomic absorption (Perkin Elmer, 1982). 

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