Flame techniques

The majority of flame analyses in water and effluents presents few problems. A pretreatment on the sample is performed only when necessary, as described earlier. Standards are prepared in the linear range of the analytical curve and blank solutions are also made up. It is preferable to acidify blanks, standards and samples to 1% with hydrochloric acid. Apart from acting as a preservative, it promotes atomisation of the analyte by forming volatile metal chlorides. The atomic absorption instrument is then set up and flame conditions and absorbances are optimised for the analyte. Following this, blanks, standards and samples are aspirated into the flame absorbances are recorded and results calculated. 

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. 

The relatively simple procedure of analysing waters for trace metals using AAS is complicated by the fact that a number of metal determinations suffer from interferences. These interferences are well documented in the literature and there are now standard methods for overcoming them. In the main, interferences encountered in the atomic absorption spectrophoto-metric analysis of water can be divided into four categories chemical interferences, matrix interferences, non-specific absorption and ionisation. 

This is the most common interference in flame AAS. The chemical interference prevents, enhances or suppresses the formation of ground state 

The technique for the determination of calcium and magnesium in waters and effluents is described below and is based on that reported in HMSO [16,17]. 

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For the sake of brevity, the so-called cool flame techniques based upon the use of an oxidant-lean flame such as hydrogen/nitrogen-air, have not been included. 

For the application of flame spectroscopic methods the sample must be prepared in the form of a suitable solution unless it is already presented in this form exceptionally, solid samples can be handled directly in some of the non-flame techniques (Section 21.6). 

Singhal J.S. T ien, Flammability Study of Polymer Fuels Using Opposed-Jet Diffusion Flame Technique, Rept No SQUID-TR-CWRU-3-PU, Contract N00014-67-A-0226-0005, Purdue Univ, Lafayette (1975)... 

Vagelopoulos, C.M., Egolfopoulos, E.N., and Law, C.K., Further considerations on the determination of laminar flame speeds with the counterflow twin flame technique, Proc. Combust. Inst., 25,1341,1994. 

Brenner et al. [ 169] applied inductively coupled plasma atomic emission spectrometry to the determination of calcium (and sulfate) in brines. The principal advantage of the technique was that it avoided tedious matrix matching of calibration standards when sulfate was determined indirectly by flame techniques. It also avoided time-consuming sample handling when the samples were processed by the gravimetric method. The detection limit was 70 ig/l and a linear dynamic range of 1 g/1 was obtained for sulfate. 

The chemiluminescent emission spectrum of GeCl2 was obtained by burning GeCl4 in potassium vapor using a diffusion flame technique 11 The spectrum consisted of a series of closely spaced diffuse bands in the region 4900—4100 A with an underlying continuum. The bands resemble those of SnCl2. 

Double-beam atomic absorption spectrophotometers are designed to control variations which may occur in the radiation source but they are not as effective as double-beam molecular absorption instruments in reducing variation because there is no blank sample in flame techniques. 

Conventional flame techniques present problems when dealing with either small or solid samples and in order to overcome these problems the electrothermal atomization technique was developed. Electrothermal, or flameless, atomizers are electrically heated devices which produce an atomic vapour (Figure 2.36). One type of cuvette consists of a graphite tube which has a small injection port drilled in the top surface. The tube is held between electrodes, which supply the current for heating and are also water-cooled to return the tube rapidly to an ambient temperature after atomization. 

Although electrothermal atomizers have certain advantages, they are slower than flame techniques particularly when large numbers of samples have to be analysed, and the transient readings which result from such methods may show poorer precision than do the steady readings obtained by sample aspiration. 

A microsampling system known as the Delve s cup is a hybrid of flameless and flame techniques. The sample is placed in a small crucible, which is held in the flame by means of a wire loop. The sample is ashed in a cooler part of the flame and then moved to the hotter part in order to cause the rapid vaporization of the element. The cup is held beneath an opening in a nickel or aluminium tube which is in the light path of the instrument. The atomic vapour... 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

Single element determination capability limited to fewer elements analysis time longer than flame the sensitivity and detection limits, however, are much greater than flame technique, and significantly better than ICP techniques. 

Flameless atomic absorption using an electrothermal atomiser is essentially a non-routine technique requiring specialist expertise. It is slower than flame analysis only 10—20 samples can be analysed in an hour furthermore, the precision is poorer (1—10%) than that for conventional flame atomic absorption (1%). The main advantage of the method, however, is its superior sensitivity for any metal the sensitivity is 100—1000 times greater when measured by the flameless as opposed to the flame technique. For this reason flameless atomic absorption is employed in the analysis of water samples where the flame techniques have insufficient sensitivity. An example of this is with the elements barium, beryllium, chromium, cobalt, copper, manganese, nickel and vanadium, all of which are required for public health reasons to be measured in raw and potable waters (section I.B). Because these elements are generally at the lOOjugl-1 level and less in water, their concentration is below the detection limit when determined by flame atomic absorption as a result, an electrothermal atomisation (ETA) technique is often employed for their determination. 

The detection limits obtained by flame and graphite furnace AAS and the concentration levels of the elements in seawater are summarized in Table 2. In general, graphite furnace AAS provides better sensitivities for many elements than the flame technique. Even so, AAS sensitivity is insufficient for the direct determination of most ultra-trace elements. Furthermore, concentrated salts and undissolved particulates cause severe interferences with the determination of trace elements by AAS. Therefore, it is necessary to concentrate the analytes before the determination, and, if possible, to separate the analytes from dissolved major constituents and particulates. Solvent extraction, coprecipitation and ion-exchange techniques are the most widely used techniques for the preconcentration of seawater. In the following sections, these techniques will be reviewed. It should be noted here that the efficiencies of the recovery of the analytes as well as the contamination from reagents and solvents must be carefully examined when the preconcentration techniques are applied. Chakrabarti et al. [10] have summarized the work on the application of preconcentration techniques to marine analysis by AAS. Hence, only some representative applications will be introduced hereafter. 

Usual flame techniques are often insufficiently detective to measure the low levels of As, Se and Hg present in foodstuffs. Mercury is commonly determined via the flameless cold-vapour technique, whereas there is much current activity in respect of the measurement of As and Se via their conversion to hydrides with subsequent decomposition in cool argon—hydrogen-entrained air flames or electrically-heated cells. Table 4 contains information on these techniques. 

During trace analytical analyses with the flame technique a separation and therefore concentration, e.g. via ion exchange, can be introduced if the concentrations during normal sample preparation are likely to be below the detection limit of the instrument [92],... 

Iron is best sampled by drillings (10 mg) which may be dissolved in aqua regia (1 ml). Standards should be prepared in a similar way and conventional flame techniques then usually suffice. 

The submerged-flame technique developed by BASF represents the latest de> cIopment in autothermal processes. Within a liquid hydrocarbon, a flame creates a suf ciently high temperature in its vicinity to cause the formation of light products, tnduding acetylene. The gases are quendied in the cold zones of the liquid, and the carbon black formed is sent with the hydrocarbon to the burner. The reactor can operate undo pressure with any hydrocarbon oomponnd, without the substandal producdon of carbon black. The weak point of the device is the control of the burner, which is difficult to achieve due to the bi gas flow velocity (Fig. 5.13). 

In addition to the normal flame technique, AAS may be carried out using a graphite furnace. This technique is capable of greater sensitivity than flame operation but usually shows poorer accuracy. The furnace technique requires the same steps for sample decomposition as do the flame techniques. 

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