Nitrous oxide / acetylene flame

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

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. 

Increase in flame temperature often leads to the formation of free gaseous atoms, and for example aluminium oxide is more readily dissociated in an acetylene-nitrous oxide flame than it is in an acetylene-air flame. A calcium-aluminium interference arising from the formation of calcium aluminate can also be overcome by working at the higher temperature of an acetylene-nitrous oxide flame. 

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

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. 

Acetylene-nitrous oxide flame is suitable for elements such asAl, Be and rare earths ... 

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

The other major source of interelement interference is related to ionization. Since AAS depends on the absorbance of light by atoms, any change in the degree of ionization of an element will be reflected by a change in apparent concentration. Since the ionization of atoms in a flame represents an equilibrium with electrons within the flame, the ionization of one element affects the degree of ionization of another. The extent of ionization increases with flame temperature, so that these effects become exacerbated when the higher temperature acetylene-nitrous oxide flame is used to overcome the interference mentioned above. This ionization also leads to a lower sensitivity since a smaller proportion of the element is present in the atomic form. 

Research must be undertaken to demonstrate that LEI is adaptable to a wider variety of samples and analytically-useful flames. This will require further consideration of methods to discriminate against or remove low ionization potential interferents. Preliminary results have indicated that the use of an acetylene-nitrous oxide flame for the determination of metals which form refractory oxides exacerbates electrical interferences when samples contain IA elements 39). The much higher flame temperature produces higher concentrations of ions whose effects cannot be entirely mitigated by using an immersed electrode. 

Atomization of the sample is usually facilitated by the same flame aspiration technique that is used in flame emission spectrometry, and thus most flame atomic absorption spectrometers also have the capability to perform emission analysis. The previous discussion of flame chemistry with regard to emission spectroscopy applies to absorption spectroscopy as well. Flames present problems for the analysis of several elements due to the formation of refractory oxides within the flame, which lead to nonlinearity and low limits of detection. Such problems occur in the determination of calcium, aluminum, vanadium, molybdenum, and others. A high-temperature acetylene/nitrous oxide flame is useful in atomizing these elements. A few elements, such as phosphorous, boron, uranium, and zirconium, are quite refractory even at high temperatures and are best determined by nonflame techniques (Table 2). 

A Jarrell-Ash Model 82-532 MV spectrophotometer equipped with a Perkin Elmer nebulization system and a Leeds-Northrup Type W calibrated AZAR Recorder was used for the molybdenum analysis. The 3133 primary molybdenum absorption wavelength coupled with an acetylene-nitrous oxide flame were the basic parameters of the instrument. The recorder was set for a 5-fold signal expansion along with the amplifier at 3/4 damped position. 

A sample is weighed and base oil is added to 0.2S 0.01-g total mass. Fifty mililitres of a kerosine solution, containing potassium as an ionization suppressant, are added, and die sample and oil are dissolved. (Warning—See Note 1.) Standards are similarly prepared, always adding oil if necessary to yield a total mass of 0.2S g. These solutions are burned in the flame of an atomic absorption spectrophotometer. An acetylene/nitrous oxide flame is us. (Warning—See Note 2.)... 

For the barium determination, fit the barium hollow cathode lamp and set the monochromator at 553.6 nm. Make fine adjustments to the wavelength setting to give maximum output. Using the correct burner head for acety-lene/nitrous oxide, set up the acetylene/nitrous oxide flame. On instruments where applicable, adjust the gain control to... 

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