Flames in atomic absorption spectroscopy

State the advantages and disadvantages of a furnace compared with a flame in atomic absorption spectroscopy. 

Amos M. D. and Willis J. B. (1966) Use of high-temperature pre-mixed flames in atomic absorption spectroscopy, Spectrochim Acta 22 1325— 1343. 

Use of glow-discharge and the related, but geometrically distinct, hoUow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

Although APDC complexes are soluble in many organic solvents, it is found that 4-methylpent-2-one (isobutyl methyl ketone) and heptan-2-one (n-pentyl methyl ketone) are, in general, the most satisfactory for direct nebulisation into the air/acetylene flame used in atomic absorption spectroscopy. 

A schematic diagram showing the disposition of these essential components for the different techniques is given in Fig. 21.3. The components included within the frame drawn in broken lines represent the apparatus required for flame emission spectroscopy. For atomic absorption spectroscopy and for atomic fluorescence spectroscopy there is the additional requirement of a resonance line source, In atomic absorption spectroscopy this source is placed in line with the detector, but in atomic fluorescence spectroscopy it is placed in a position at right angles to the detector as shown in the diagram. The essential components of the apparatus required for flame spectrophotometric techniques will be considered in detail in the following sections. 

Instead of employing the high temperature of a flame to bring about the production of atoms from the sample, it is possible in some cases to make use of either (a) non-flame methods involving the use of electrically heated graphite tubes or rods, or (b) vapour techniques. Procedures (a) and (b) both find applications in atomic absorption spectroscopy and in atomic fluorescence spectroscopy. 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

Eaithfull, N.T. (1971b) Flame interference in atomic absorption spectroscopy with a.c. modulated systems. Laboratory Practice 20(8), 641-643. 

How would emission intensity be affected by a 10 K rise in temperature In Figure 21-14, absorption arises from ground-state atoms, but emission arises from excited-state atoms. Emission intensity is proportional to the population of the excited state. Became the excited-state population changes by 4% when the temperature rises 10 K, emission intensity rises by 4%. It is critical in atomic emission spectroscopy that the flame be very stable or emission intensity will vary significantly. In atomic absorption spectroscopy, temperature variation is important but not as critical. 

Investigation of atomic spectra yields atomic energy levels. An important chemical application of atomic spectroscopy is in elemental analysis. Atomic absorption spectroscopy and emission spectroscopy are used for rapid, accurate quantitative analysis of most metals and some nonmetals, and have replaced the older, wet methods of analysis in many applications. One compares the intensity of a spectral line of the element being analyzed with a standard line of known intensity. In atomic absorption spectroscopy, a flame is used to vaporize the sample in emission spectroscopy, one passes a powerful electric discharge through the sample or uses a flame to produce the spectrum. Atomic spectroscopy is used clinically in the determination of Ca, Mg, K, Na, and Pb in blood samples. For details, see Robinson. 

In atomic absorption spectroscopy (AAS) both ionization and chemical interferences may occur. These interferences are caused by other ions in the sample and result in a reduction of the number of neutral atoms in the flame. Ionization interference is avoided by adding a relatively high amount of an easily ionized element to the samples and calibration solutions. For the determination of sodium and potassium, cesium is added. To eliminate chemical interferences from, for example, aluminum and phosphate, lanthanum can be added to the samples and calibration solutions. 

Until now we have used the database for a very simple purpose, namely to extract information from a single file. However, it is also possible to connect several files. Let us suppose that we want to use dBASE for the following problem. In atomic absorption spectroscopy (AAS), one has to choose between the flame and the (flameless) graphite tube methods. The flame methods does not have such a low detection limit as the graphite tube, but it is easier to handle, less prone to interferences and more robust. For that reason the user s strategy will often be to apply the flame method above a certain concentration limit and the flameless method below it. The flame method has its own experimental characteristics and we suppose that we have another database file in which the characteristics for flame methods are given per element. In that case, we would like the consultation to go like this ... 

In 1955, A. Walsh recognized this and showed how the absorption from the great preponderance of unexcited molecules could be exploited analytically.Thus, in atomic absorption spectroscopy (AAS) the light from a (usually modulated) somce emitting the spectrum of the desired analyte element is passed through a sample atomization cell (such as a flame or graphite tube furnace), a monochromator (to isolate the desired somce emission line) and finally into a detector to allow measmement of the change in somce line... 

This is a non-SI weight per volume (w/v) concentration term commonly used in quantitative analysis such as flame photometry, atomic absorption spectroscopy and gas chromatography, where low concentrations of solutes are to be analysed. The term ppm is equivalent to the expression of concentration as /igrnL" (10 gmL ) and a l.Oppm solution of a substance will have a concentration of 1.0/igmL (1.0 x 10 gmL ). A typical procedure for calculations in terms of ppm is shown in Box 6.2. 

It has been found, however, in practice that a perfectly straight analytical working curve (— log T plotted against concentration) is seldom obtained in atomic absorption spectroscopy. The reasons for this are usually a combination of instrumental problems broadening of the emission line of the light source due to self-reversal, Doppler and pressure broadening of the absorption lines of the atoms in the flame, failure to exclude flame emission entirely, use of a focused instead of a parallel... 

Bismuth added to urine was recovered by Willis (W14) with solvent extraction and determined by atomic absorption spectroscopy. An absorption interference rarely encountered in atomic absorption spectroscopy was seen from the absorption of the 3068-A line of bismuth by the OH radical in the air-coal gas flame. 

C14. Coker, D. T., and Ottaway, J. M., Formation of free atoms in air-acetylene flames used in atomic absorption spectroscopy. Nature (London) (Phys. Sci.) 239,156-157 (1971). 

N4. Nicolas, D., Spray chamber design for use in atomic absorption spectroscopy and flame photometry. J. Sd. Instrum. 4, 68-70 (1971). 

R4. Rann, C. S., Evaluation of a flame as the spectral source in atomic absorption spectroscopy. Speclrochim. Ada, Part B 23, 245-256 (1968). 

If the light comes from a source made from zinc it contains a very high proportion of wavelengths that are absorbed by zinc atoms. In atomic absorption spectroscopy the light source used is a hollow cathode lamp, specially made for each element to be determined. Measurement of absorbance of the light from a zinc hollow cathode lamp gives a very selective method for the measurement of the concentration of zinc in a solution introduced into the flame (Figure 6.1). 

The methods officially used in the wine trade transactions are summarized in Table 8.1. Generally, the OIV methods are officially adopted in the European Union without significant technical changes. The methods reported are mainly colorimetric, titrimetric, or use Atomic Emission Spectroscopy (AES, e.g. Flame Spectrophotometry), Atomic Absorption Spectroscopy (AAS), Hydride Generation-AAS (HG-AAS), Electrothermal-AAS (ET-AAS) and Vapour Atomic Flourescence Spectrophotometry (VAF). 

In atomic emission spectroscopy, the radiation source is the sample itself. The energy for excitation of analyte atoms is supplied by a plasma, a flame, an oven, or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. In atomic absorption spectroscopy, the radiation source is usually a line source such as a hollow cathode lamp, and the signal is the absorbance. The latter is calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomized sample. 

Heitland P. (2000) Verwendung von Spektrallinien in der ICP-OES im Wellenlangenbereich 125—190 nm, GIT Labor-Fachzeitschrijt 847—850. Christian G. D. and Feldmann F. J. (1971) A comparison study of detection limits using flame-emission spectroscopy with the nitrous oxide-acetylene flame and atomic-absorption spectroscopy, Appl Spectrosc 25 660-663. 

The next component part in the scheme of the atomic absorption spectrometer is the atomizer. Burners are often used as atomizers in atom absorption spectroscopy. Mixtures of air/acetylene, laughing gas/acetylene or hydrogen/argon are usually used as the fuel gas. The aim of the burner in atomic absorption spectroscopy is to evaporate the solution of the sample and to disintegrate the sample to the atomic state. Especially important for successful analysis is that the path of light through the flame of the burner be made as long as possible. Hence, the use of fissure burners. 

The essential difference between K-Ar and Ar-Ar dating techniques lies in the measurement of potassium. In K-Ar dating, potassium is measured generally using flame photometry, atomic absorption spectroscopy, or isotope dilution and Ar isotope measurements are made on a separate aliquot of the mineral or rock sample. In Ar-Ar dating, as the name suggests, potassium is measured by the transmutation of to Ar by neutron bombardment and the age calculated on the basis of the ratio of argon isotopes. 

Why is a high-temperature flame, for example, the nitrous oxide-acetylene flame, sometimes required in atomic-absorption spectroscopy ... 

The relative number of atoms in a particular energy state can be determined by use of the Boltzmann equation [refer to equation (2-23)]. Walsh has calculated these ratios for the lowest excited states of several typical elements and several flame temperatures. Table 9-2 indicates that the number of atoms in the ground state is much greater than the number in the lowest excited state at temperatures commonly used in atomic absorption spectroscopy. 

Early work in atomic absorption spectroscopy used the lower temperature flames thus the method was restricted to those elements that could be converted to atoms at lower temperatures. Since some metals, such as molybdenum, rhenium, and tin, are only partly converted to gaseous atoms in low temperature flames, higher temperature flames were developed with success. 

Amos and Willis suggested another combination of fuel and oxidant, acetylene and nitrous oxide, as another approach to the analysis of refractory elements. The combination of acetylene and nitrous oxide produces a high-temperature flame (2950°C) with little free oxygen to react with the metal elements. This flame is now very successfully used in atomic absorption spectroscopy and permits satisfactory atomic absorption analysis for many refractory elements. Use of nitrous oxide and acetylene requires a burner head that will withstand the temperatures produced by the flame. A common burner head for this combination of fuel and oxidant is 5 cm long and 0.05 cm wide. 

For analytical purposes it is essential that interference effects in atomic absorption spectroscopy be eliminated or minimized. If they cannot be eliminated, it is necessary to compensate adequately for their presence through use of proper standards or other compensating techniques. This section deals with this problem. Reference also should be made to Chapter 9, as many techniques used for flame emission also apply to atomic absorption. 

In atomic absorption spectroscopy, a sample is aspirated and atomized in a flame. A light beam from a hollow cathode lamp or an electrodeless discharge lamp is directed through the flame into a monochromator, and onto a detector that measures the amount of absorbed light. Absorption depends upon... 

---