Flame analyses

Flame Analyses. Smoothed temperature and mole fraction versus distance plots and the associated net reaction rate versus distance plots for a fuel-lean and fuel-rich flame are shown in Figures 1 and 2, respectively. The computer-generated symbols in the figures identify the species associated with each curve they do not denote data points. Approximately 16 data points from each flame were used to generate the curves. 

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. 

Sodium salt has a special connotation in describing these substances. The almost universal practice in analyses of heparin preparations is to bum a sample moistened with sulphuric acid in oxygen, weigh the ash, and calculate from the sulphated ash the equivalent sodium. This is evidently a questionable practice. The metal cations bound must be identified. As heparin and heparinoids complex with ions, there is interference with various colour and precipitation reactions for ions unless the heparin sample is first subjected to combustion. Flame analyses of Boots and Evans heparin for... 

In flame spectroscopy, the residence time of analyte in the optical path is < 1 s as it rises through the flame. A graphite furnace confines the atomized sample in the optical path for several seconds, thereby affording higher sensitivity. Whereas 1—2 mL is the minimum volume of solution necessary for flame analysis, as little as 1 pL is adequate for a furnace. Precision is rarely better than 5-10% with manual sample injection, but automated injection improves reproducibility to —1%. 

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. 

Two methods are given here for the determination of metals in crude and residual fuel oils the dilution—flame analysis method and an ashing procedure. Because of the nature of these oils simple dilution with solvent may leave undissolved solids in the solvent to be presented to the spectrometer. This could give rise to a decrease in the precision of the method. The ashing procedure overcomes this difficulty but is more time consuming. The method of choice will depend on whetiier the analytical emphasis is placed on speed of analysis or precision. Where very low levels of Ni or V are important it may be possible to modify the melhod given under fuel oils (Section IV.B.2.). 

The determination of metals in crude and residual fuel oil by dilution and flame analysis... 

The most commonly encountered additive elements in lubricating oils are Ca, Ba, Mg and Zn. They are normally present at relatively high concentrations such that a simple dilution—flame analysis procedure may be used to determine their concentrations. The control of the additive concentrations is important in the control of the physical and chemical properties of the lubricant. Problems associated with metallic particulate matter are not generally encountered with unused lubricating oils. 

In a number of cases, simple dissolution of a solid sample in an appropriate solvent is possible and some laboratory reagents may even be analysed without further treatment. Prior to flame analysis, the best solvent is dilute hydrochloric acid, provided of course that the major matrix elements are not silver, lead or another element which forms a sparingly soluble chloride. If additional oxidising ability is required, concentrated nitric acid may be added to the solvent. This acid is the preferred solvent when the analysis is to be completed by electrothermal atomisation. If the material contains large amounts of silica it may be necessary to add hydrofluoric acid after preliminary digestion with hydrochloric acid (see Chapter 4g). Care should of... 

Clearly the most popular separation and preconcentration technique for atomic absorption analysis is solvent extraction. In this case it is easy to identify extraction systems which will remove a broad range of impurities from matrices such as acids, bases and alkali metal salt solutions. In addition to the advantages to be gained from separation, especially valuable in furnace work, and the concentration factors available, solvent extraction confers an additional advantage (typically a factor of 3—5) in flame analysis arising from the favourable nebulisation characteristics of several organic solvents. 

Similar general remarks about inorganic industrial chemicals can be made as in section II.A. 1. Additionally, such chemicals and technological chemical solutions may be more complex and their major constituents less well defined. Thus interferences may constitute a greater problem. Fortunately, however, the levels of trace metals present may be higher than those of interest in laboratory chemicals and flame analysis is often appropriate. Again there is a need to exercise care over blanks and to match standards for major constituents and acid content. 

The analysis of brines perhaps deserves special mention as the high sodium chloride concentrations are extremely unfavourable for electrothermal atomisation and most troublesome in flame analysis. The preferred approach is probably solvent extraction with either oxine or APDC to remove the trace metals into a small volume of MIBK for flame atomisation or chloroform for electrothermal cells. Care must be taken to avoid interference from chloro-complexes in the extraction, and if this is suspected an ion-association extraction of these complexes might be preferable. 

When major components of plating solutions are determined, large dilutions may be required (e.g. a factor of 5000) to bring the sample into the normal working range for flame analysis. Such dilutions will, however, minimise any interference effects and viscosity effects from additives, and are thus to be preferred to the use of less sensitive lines or burner rotation. The above interference effects may be important in the determination of trace metal levels and attempts should be made to match the standards for major component levels, or to use the method of standard additions. Solvent extraction to remove the analyte from the matrix may be necessary. 

Paper and pulp, being based on plant materials represent particularly favourable matrices for atomic absorption analysis by flame or by furnace. Such applications have been reviewed in the specialist literature [165—168], Differences of opinion exist as to whether wet or dry ashing is to be preferred for flame analysis, but increasingly, paper samples are being added to graphite furnaces without any sample pre-treatment. 

A method has been reported for the determination of calcium, copper, iron, magnesium, potassium, sodium and zinc in cellulose [169]. The sample (10 g) was air-dried and then ashed at 575°C until all the carbon was removed. Hydrochloric acid (5 ml of 6M) was added to the residue and evaporated to dryness twice before taking up the sample in a third aliquot, diluting to 100 ml and aspiration into an air /acetylene flame. It is likely that volatile elements such as cadmium may be lost at such an elevated ashing temperature and temperatures below 500°C may be preferable. Alternatively wet ashing with nitric acid has been proposed for the determination of aluminium, cadmium, potassium and zinc in pressed boards [170] or sodium in gypsum glass board [171]. For the determination of lead in confection wrappers, the sample may be treated with concentrated nitric acid at 70—80°C and diluted for flame analysis [172]. In the full method, the wrapper was wiped clean with a damp tissue, cut up to 0.5 X 0.5 mm pieces and dried at 110°C (for paper, for plastic 80°C) for 1 h. The sample (0.5 g) was heated with concentrated nitric acid (1ml) at... 

Lead alloys (50 mg for flame analysis, 10 mg for furnace atomisation) can be dissolved in 1ml of distilled water with 0.5 ml nitric acid added. After warming, 5 ml of concentrated hydrochloric acid is added to dissolve the tin content, any precipitated lead chloride may be removed by centrifugation. Nickel, silver, tin and zinc may be determined in this solution using a conventional air/acetylene flame. Silver is usually the element of most interest in lead, but the proportion of tin in pewter is significant. The lead content can be determined in the original nitric acid solution but it is difficult to measure tin and lead in the same solution. 

For a given analysis, it is usual to distinguish between the error of precision of the method as a whole (including the preparation of the sample) and the error due to the atomic absorption measurement. For flame analysis, the precision level is approximately 0.5 to 3% of the concentration for the majority of elements and will obviously be poorer near the detection limit the presence of significant levels of salts leads to a reduction in the level of precision. The highest precision is obtained for absorption values of between 10 and 50% consequently, it is advisable to adjust the concentration of the solution to be analysed by using an appropriate dilution. 

In the early 1900s, scientists began to unravel the puzzle of chemical behavior. They had observed that certain elements emitted visible light when heated in a flame. Analysis of the emitted light revealed that an element s chemical behavior is related to the arrangement of the electrons in its atoms. In order for you to better understand this relationship and the nature of atomic structure, it will be helpful for you to first understand the nature of light. 

Although it appears that suppression may still be observed with the immersed electrode if the concentration of the concomitant is increased beyond 3000 pg/ml, other considerations due to the high salt concentration of the sample, such as burner clogging and arc-over, may be more important than signal suppression. When extremely high salt concentrations are present, flame analysis by other spectroscopic techniques becomes problematic as well. 

Results of the analysis (C) are presented in Table 1. Usually the display of the Perkin-Elmer 5000 instrument in flame atomic absorption analysis is autozeroed for the blank solution. For this reason the values read visually from the display and those stored by GIRAF differ by the value of the blank, a fact that has no influence on the results of the analysis. The uncertainties of the results (U) are shown in Table 1 also. The uncertainties of the calculated analyte concentrations in the test solutions UtT described in the Reference materials section are approximately one third of the corresponding uncertainties of the results of the analysis (for sodium it is not obvious because of rounding), and so the use of the term true values in the context of the validation is permissible. All these data were used for calculation of the acceptance criteria A and B formulated in the Validation plan (see Table 2). The criteria are satisfied for both graphite furnace and flame analysis. 

Figure 14.13 shows a TEM image of soot clusters sampled from an acetylene-in-air diffusion flame. Analysis shows these clusters are fractals with D — 1.8 and a = 23 nm (thus for X= 514.5 nm a= 0.28). This is typical of carbonaceous soot formed in a variety of flames. 

For satisfactory flame analysis it is essential that the analyst or his supervisor should be conversant with the theoretical basis of the method. Rule-of-thumb working will sooner or later result in serious errors being made. 

The elements rubidium and caesium are precipitated with sodium tetraphenylborate solution together with potassium, dissolved in a mixture of organic solvents (acetone and methyl isobutyl ketone) and subsequently determined by means of atomic absorption flame analysis. 

Apparatus for atomic absorption flame analysis Reagents... 

Final determination is by means of atomic absorption flame analysis. 

Atomic absorption method with flame analysis. 

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