Flame atomization

The flame is responsible for production of free atoms. Flame temperature and the fuel/oxidant ratio are very important in the production of free atoms from compounds. Many flame fuels and oxidants have been studied over the years, and temperature ranges for some flames are presented in Table 6.1. In modem commercial instruments, only air-acetylene and nitrous oxide-acetylene flames are used. 

In practice, the optimum position for taking absorbance measnranents is determined in exactly this way. A solution of the element to be determined is aspirated, the absorbance is monitored continually, and the burner head is moved up and down (and forward and back) with respect to the light beam until the position of maximum absorbance is located. Older instrnments have 

The maximum absorbance signal depends on the number of free atoms in the light path. These free atoms are in dynamic equilibrium with species in the flame they are produced continuously by the flame and lost continuously in the flame. The number produced depends on the original concentration of the sample and the atomization efficiency (Table 6.2). The number lost depends on the formation of oxides, ions, or other nonatomic species. The variation of atomization efficiency with 

Spectroscopy, Vol. 1, CRC Press, Boca Raton, FL, 1975. The efficiency of atomization in the flames has been measured as the fraction of total element in the gaseous state present as free neutral atoms or ionized atoms at the observation height in the atomizer. That is, efficiency of atomization = (neutral atoms + ions)/total element. Entries marked obtained using ionization suppression, discussed in Section 6.4.1. The data were obtained under a variety of conditions by multiple researchers and may not be directly comparable between elements. The reference should be consulted for details. 

To measure an atomic absorption signal, the analyte must be converted from dissolved ions in aqueous solution to reduced gas phase free atoms. The overall process is outlined in Fig. 6.16. As described earlier, the sample solution, containing the analyte as dissolved ions, is aspirated through the nebulizer. The solution is converted into a fine mist or aerosol, with the analyte still dissolved as ions. When the aerosol droplets enter the flame, the solvent (water, in this case) is evaporated. We say that the sample is desolvated . The sample is now in the form of tiny solid particles. The heat of the flame can melt (liquefy) the particles and then vaporize the particles. Finally the heat from the flame (and the combustion chemistry in the flame) must break the bonds between the analyte metal and its anion, and produce free M° atoms. This entire process must occur very rapidly, before the analyte is carried out of the observation zone of the flame. After free atoms are formed, several things can happen. The free atoms can absorb the incident radiation this is the process we want. The free atoms can 

The loss of free atoms in the atomizer is also a function of the chemistry of the sample. If the oxide of the analyte element is readily formed, the free atoms will form oxides in the flame and the population of free atoms will simultaneously decrease. This is the case with elements such as chromium, molybdenum, tungsten, and vanadium. On the other hand, some metal atoms are stable in the flame and the free atoms exist for a prolonged period. This is particularly so with the noble metals platinum, gold, and silver. Adjusting the fuel/oxidant ratio can change the flame chemistry and atom distribution in the flame as shown in Fig. 6.17(b). Atoms with small ionization energies will ionize readily at high temperatures (and even at moderate temperatures). In an air-acetylene flame, it can be shown that moderate concentrations of potassium are about 50% ionized, for example. Ions do not absorb atomic lines. 

The maximum absorbance signal depends on the number of free atoms in the light path. These free atoms are in dynamic equilibrium with species in the flame they are produced continuously by the flame and lost continuously in the flame. The number produced depends on the original concentration of the sample and the atomization efficiency (Table 6.2). The number lost depends on the formation of oxides, ions, or other nonatomic species. The variation of atomization efficiency with the chemical form of the sample is called chemical interference. It is the most serious interference encountered in AAS and must always be taken into account. Interferences are discussed in detail in Section 6.4. Table 6.2 can be used as a guide for choosing which flame to use for AAS determination of an element. For example, A1 will definitely give better sensitivity in a nitrous oxide-acetylene flame than in air-acetylene, where hardly any free atoms are formed the same is true of Ba. But for potassium, clearly an air-acetylene flame is a 

Source-. Modified from Robinson, Handbook of Spectroscopy. 

The emission spectrum of the analyte emitted from the radiation source is passed through an atomizer in which a portion of this radiation is absorbed by the free atoms of the analyte. The main function of the atomizer is to 

In the flame atomization technique the sample is sprayed into the flame in the form of an aerosol generated by means of a nebulizer. It is also possible to introduce solid samples directly or as suspensions into the flame. 

Flames employed in AAS may be divided into two groups the combustion flames and diffusion flames. In fuel-oxidant mixtures, the temperature of the flame varies generally from 2000 to 3000 K. Air and dinitrogen oxide (N2O) are the most widely used oxidants, and acetylene, propane, and hydrogen are the most common fuel gases. In diffusion flames, the fuel is also the carrier gas and it burns on coming into contact with the outer diffusion air. The temperatures of the diffusion flames are lower than those of the combustion flames. The characteristics of some commonly used flames are given in Table 5. 

1 Combustion Flames. The atomization is dependent on the temperature and the chemical environment i.e. chemical compounds and radicals existing in the flame) of the flame. The burning temperature of two different gas mixtures can be the same, but their analytical characteristics could be quite different. 

Pure oxygen is rarely used as an oxidant because its burning velocity is high and it is difficult to control. However, oxygen may be mixed with argon or helium to overcome these disadvantages. 

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With the exception of alkalis, the sensitivity is generally higher than that of atomic absorption (at least flame atomic absorption). Refer to Table 2.4. 

FLAME ATOMIC EMISSION, FLAME ATOMIC ABSORPTION,... 

The detection limits in the table correspond generally to the concentration of an element required to give a net signal equal to three times the standard deviation of the noise (background) in accordance with lUPAC recommendations. Detection limits can be confusing when steady-state techniques such as flame atomic emission or absorption, and plasma atomic emission or fluorescence, which... 

Element Wavelength, nm Flame emission Flame atomic absorption Electrothermal atomic absorption Argon ICP Plasma atomic fluorescence... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used flame atomization and electrothermal atomization. A few elements are atomized using other methods. 

Flame atomization assembly equipped with spray chamber and slot burner. The inset shows the nebulizer assembly. 

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. 

Absorbance profile for Ag and Cr in flame atomic absorption spectroscopy. 

Accuracy When spectral and chemical interferences are minimized, accuracies of 0.5-5% are routinely possible. With nonlinear calibration curves, higher accuracy is obtained by using a pair of standards whose absorbances closely bracket the sample s absorbance and assuming that the change in absorbance is linear over the limited concentration range. Determinate errors for electrothermal atomization are frequently greater than that obtained with flame atomization due to more serious matrix interferences. 

Precision For absorbances greater than 0.1-0.2, the relative standard deviation for atomic absorption is 0.3-1% for flame atomization, and 1-5% for electrothermal atomization. The principal limitation is the variation in the concentration of free-analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and atomization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization. 

Flame Sources Atomization and excitation in flame atomic emission is accomplished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and injected directly into the flame. 

Method for background correction in flame atomic emission. 

Description of Method. Salt substitutes, which are used in place of table salt for individuals on a low-sodium diet, contain KCI. Depending on the brand, fumaric acid, calcium hydrogen phosphate, or potassium tartrate also may be present. Typically, the concentration of sodium in a salt substitute is about 100 ppm. The concentration of sodium is easily determined by flame atomic emission. Because it is difficult to match the matrix of the standards to that of the sample, the analysis is accomplished by the method of standard additions. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

Hobbins reported the following calibration data for the flame atomic absorption analysis for phosphorus. ... 

This experiment describes a fractional factorial design used to examine the effects of flame height, flame stoichiometry, acetic acid, lamp current, wavelength, and slit width on the flame atomic absorbance obtained using a solution of 2.00-ppm Ag+. 

Small concentrations of iron can also be deterrnined by flame atomic absorption and inductively coupled plasma emission spectroscopies (see... 

The capacity factors of SN-SiO, for metal ions were determined under a range of different conditions of pH, metal ions concentrations and time of interaction. Preconcentration of Cd ", Pb ", Zn " and CvS were used for their preliminary determination by flame atomic absorption spectroscopy. The optimum pH values for quantitative soi ption ai e 5.8, 6.2, 6.5, 7.0 for Pb, Cu, Cd and Zn, respectively. The sorption ability of SN-SiO, to metal ions decrease in line Pb>Cu> >Zn>Cd. The soi ption capacity of the sorbent is 2.7,7.19,11.12,28.49 mg-g Hor Cd, Zn, Pb, andCu, respectively. The sorbent distribution coefficient calculated from soi ption isotherms was 10 ml-g for studied cations. All these metal ions can be desorbed with 5 ml of O.lmole-k HCl (sorbent recovery average out 96-100%). 

In this work, a simple, rapid and sensitive Flame Atomic Absolution Spectrometry (FAAS) method has been developed for the determination of trace amount of Co + in vaiious samples after adsoi ption of its complex on modified Analcime using a Schiff base Bis-[(2,2 -dihydroxy)-N,N -diethylen-triamino-l,r-naphtaldimine] by column method in the pH range (4-7) at flow rats 1 ml-minf... 

The possibility of preconcentration of selenium (IV) by coprecipitation with iron (III) hydroxide and lanthanum (III) hydroxide with subsequent determination by flame atomic absorption spectroscopy has been investigated also. The effect of nature and concentration of collector and interfering ions on precision accuracy and reproducibility of analytical signal A has been studied. Application of FefOH) as copreconcentrant leads to small relative error (less than 5%). S, is 0.1-0.2 for 5-100 p.g Se in the sample. Concentration factor is 6. The effect of concentration of hydrochloric acid on precision and accuracy of AAS determination of Se has been studied. The best results were obtained with HCl (1 1). 

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