Acetylene—nitrous oxide

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

Quantitative aluminum deterrninations in aluminum and aluminum base alloys is rarely done. The aluminum content is generally inferred as the balance after determining alloying additions and tramp elements. When aluminum is present as an alloying component in alternative alloy systems it is commonly deterrnined by some form of spectroscopy (qv) spark source emission, x-ray fluorescence, plasma emission (both inductively coupled and d-c plasmas), or atomic absorption using a nitrous oxide acetylene flame. 

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). 

Key L = fuel-lean R = fuel-rich AA= air/acetylene AP = air/ propane NA= nitrous oxide/acetylene AH = air/hydrogen Notes (1) If there are many interferences then NA is to be preferred. 

Spencer and Sachs [29] determined particulate aluminium in seawater by atomic absorption spectrometry. The suspended matter was collected from seawater (at least 2 litres) on a 0.45 tm membrane filter, the filter was ashed, and the residue was heated to fumes with 2 ml concentrated hydrofluoric acid and one drop of concentrated sulfuric acid. This residue was dissolved in 2 ml 2 M hydrochloric acid and the solution was diluted to give an aluminium concentration in the range 5-50 pg/1. Atomic absorption determination was carried out with a nitrous oxide acetylene flame. The effects of calcium, iron, sodium, and sulfate alone and in combination on the aluminium absorption were studied. 

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. 

Refractory compounds can be determined using a nitrous oxide-acetylene flame. The formation of refractory oxides with gases in flames might not be considered an interference, since it is constant under a given set of conditions but it does decrease the sensitivity markedly so that measurement of the element may not be possible. 

Ionization interference is particularly a problem in the high temperature nitrous oxide-acetylene flame, where even elements such as manganese can be appreciably... 

Boron in blood and tissue has recently been determined by Bader and Branden-berger 11 ) by dry ashing, and then aspirating the acidified solution into a nitrous oxide-acetylene flame. A limit of detection of 15 ppm in the solution was reported. 

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Atomic absorption also gave better results for aluminum using a nitrous oxide-acetylene flame. In general, optical emission was more rapid. 

Element Air-propane 2200 K Oxygen-hydrogen 2450 K Oxygen-acetylene 2800 K Nitrous oxide-acetylene 3230 K... 

Urine total calcium levels were analysed spectrometrically by atomic absorption (Perkin Elmer 2380) using a nitrous oxide-acetylene flame and standard conditions of the manufacturer (18). Urine samples and standards were appropriately diluted in 0.25 M KC1 to obviate ionization interferences. 

Pellenberg [114] analysed soils and river sediment for silicone content by nitrous oxide-acetylene flame atomic absorption spectrophotometry. He showed that total carbon and total carbohydrates both correlate well with silicone content and the correlation between sedimentary silicone and presumed sewage material is good enough to suggest silicone as a totally synthetic, specific tracer for sewage in the aquatic environment. 

FIGURE 9.9 A drawing of the top of a burner head (looking down). The slot is either 10 cm long (for air-acetylene flames) or 5 cm long (for nitrous oxide-acetylene flames). 

An atomic absorption method was published by AOAC Int. (2000) for determination of the anti-foaming agent polydimethylsiloxane in pineapple juice, that is based on extraction with 4-methyl-2-pentanone and aspiration into a nitrous oxide/acetylene flame. A silicone lamp was used for detection. 

ID Direct Nitrous Oxide-Acetylene Flame Method (Metals)... 

Recent advances in instrumentation have been primarily in improved atom reservoirs and automation. By far the most commonly used atom reservoir in AAS is the flame, either air acetylene (2400°K) or nitrous oxide acetylene (3200°K) for more... 

Add 20 ml 6 M HCI dropwise on to the ash and warm carefully to dissolve, and then make up with water to 50 ml in a volumetric flask. Using suitable dilutions where necessary, analyse for Ca, Fe and Al by AAS. It will be necessary to use a nitrous oxide-acetylene flame for aluminium, otherwise use the titration method as in Method 5.3. 

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. 

Rhenium can be analyzed by various instrumental techniques that include flame-AA, ICP-AES, ICP-MS, as well as x-ray and neutron activation methods. For flame-AA analysis the metal, its oxide, or other insoluble salts are dissolved in nitric acid or nitric-sulfuric acids, diluted, and aspirated directly into nitrous oxide-acetylene flame. Alternatively, rhenium is chelated with 8-hydroxy quinoline, extracted with methylisobutyl ketone and measured by flame-AA using nitrous oxide-acetylene flame. 

Ruthenium and its compounds are analyzed by flame AA method using nitrous oxide-acetylene flame. ICP-AES, ICP/MS, and neutron activation analysis are also applicable. The metal or its insoluble compounds may be solubilized by fusion with alkah and leached with water. 

Titanium can be meaured at trace concentations by flame-AA using a nitrous oxide-acetylene flame. The measurement can be done at 365.3 nm. ICP-AES and ICP/MS techniques also are apphcable. The metal or its compounds must be dissolved by digestion with HF and HCl and the solution diluted and analyzed instrumentaUy. 

Light source. A hollow cathode lamp coated with the element being analysed. Flame. The flame is usually air/acetylene providing a temperature ca 2500°C. Nitrous oxide/acetylene may be used to produce temperatures up to 3000°C, which are required to volatilise salts of elements such as aluminium or calcium. Monochromator. The monochromator is used to narrow down the width of the band of radiation being examined and is thus set to monitor the wavelength being emitted by the hollow cathode lamp. This cuts out interference by... 

In AAS, the flame is only required to produce ground-state atoms (cf AES, where a hot flame is preferred as atoms must also be excited). Frequently, an air-acetylene flame is sufficient to do this. For those elements which form more refractory compounds, or where interferences are encountered (see Section 2.4), a nitrous oxide-acetylene flame is preferred. In either case, a slot burner is used (100 mm for air-acetylene, 50 mm for nitrous oxide-acetylene) to increase the path length (this arises from Eqn. 2.3, Section 2.1) and to enable a specific portion of the flame to be viewed. Atoms are not uniformly distributed throughout the flame and, by... 

The nitrous oxide-acetylene flame is both hot and reducing. A characteristic red, interconal zone is obtained under slightly fuel-rich conditions. This red feather is due to emission by the cyanogen radical. This radical is a very efficient scavenger for oxygen, thus pulling equilibria such as... 

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