Flame Excitation Source

A flame is the result of the exothermic chemical reaction between two gases, one of which serves as the fuel and the other as the oxidant. The reaction is an oxidation-reduction reaction, with the oxidant oxidizing the fuel. In the process, the reaction generates a great deal of heat. Common 

The ratio of the number of atoms in an upper excited state to the number of atoms in a lower energy state can be calculated from the Maxwell-Boltzmann equation (also called the Boltzmann distribution)  

AE is the energy difference between the upper and lower states (J) 

The Boltzmann distribution assumes the system is in thermal equilibrium. The emission intensity is related to the nnmber of atoms in the higher excited state, N, since we are looking at emission as the atom relaxes from a higher state to a lower state. 

Using the Boltzmann equation, we can calculate the ratio of the number of excited-state atoms at two different temperatnres. For potassinm atoms, the major atomic emission line occnrs at 766.5 nm. The energy of this transition in jonles is 

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The high stability of the flame source, when compared to arc or spark excitation, was recognized as the key to the construction of simple instruments for the determination of easily excited elements, such as the alkali metals. Thus the first flame photometer produced in the U.S. in 1945 by Barnes used filters rather than a prism or grating, and used a modified Meeker burner as the flame excitation source. The instrument was especially useful for sodium and potassium determinations and was also soon utilized for calcium and magnesium analyses despite the handicap of poor detection limits. 

The flame excitation source of a flame emission spectrometer must fulfill several requirements if it is to be satisfactory. These include the ability to (1) evaporate a liquid droplet sample, (2) vaporize the sample, (3) decompose the compound(s) in the evaporated sample, and (4) spectrally excite the ground state atoms. These processes must occur at a steady rate to achieve a steady emission signal. 

Atomization and Excitation Atomic emission requires a means for converting an analyte in solid, liquid, or solution form to a free gaseous atom. The same source of thermal energy usually serves as the excitation source. The most common methods are flames and plasmas, both of which are useful for liquid or solution samples. Solid samples may be analyzed by dissolving in solution and using a flame or plasma atomizer. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

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. 

New developments are, however, needed to make a major step forward in the field of speciation analysis. The first part, isolation and separation of species, may be the easiest one to tackle. For the second part, the measurement of the trace element, a major improvement in sensitivity is needed. As the concentration of the different species lies far below that of the total concentration (species often occur at a mere ng/1 level and below), it looks like existing methods will never be able to cope with the new demands. A new physical principle will have to be explored, away from absorption spectrometry, emission spectrometry, mass spectrometry, and/or more powerful excitation sources than flame, arc or plasma will have to be developed. The goal is to develop routine analytical set-ups with sensitivities that are three to six orders of magnitude lower than achieved hitherto. 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

In the past, flames used for atomic absorption spectrometry have also been used for atomic emission spectrometry, and these are described in some detail in Chapter 2. However, the advent of plasma excitation sources has resulted in the demise of flame atomic emission spectrometry, for the reasons discussed in Section 4.2.3. 

In the past, much atomic emission work has been performed on atomic absorption instruments which use a flame as the excitation source. However, these have been surpassed by instruments which utilise a high-temperature plasma as the excitation source, owing to their high sensitivity and increased linear dynamic range. 

Based on the configurations in Figure 1.5, many analytical techniques have been developed employing different atomisation/excitation sources. For example, two powerful AAS techniques are widespread one uses the flame as atomiser (FAAS) whereas the other is based on electrothermal atomisation (ETAAS) in a graphite furnace. Although the flame has limited application in OES, many other analytical emission techniques have evolved in recent decades based on dilTerent atomisation/excitation plasma sources. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

In FAES and FAAS, the analytical results will be totally degraded if there is a spectral overlap of an analyte transition. This can result from an interfering matrix element with a transition close to that of the analyte. This table presents a list of those overlaps that have been observed and those that are predicted to happen. In many cases the interferant element has been present in great excess when compared to the analyte species. Therefore, if the predicted interferant element is a major component of the matrix, a careful investigation for spectral overlap should be made. Excitation sources other than flames were not covered in this study. 

In AAS, the excitation source inert gas emission offers a potential background spectral interference. The most common inert gases used in hollow cathode lamps are Ne and Ar. The data taken for this table and the other tables in this book on lamp spectra are from HCLs however, electrodeless discharge lamps emit very similar spectra. The emission spectra for Ne and Ar HCLs and close lines that must be resolved for accurate analytical results are provided in the following four tables. This information was obtained for HCLs and flame atom cells and should not be considered with respect to plasma sources. In the Type column, I indicates that the transition originates from an atomic species and II indicates a singly ionized species. 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

When the image of elongated excitation sources, e.g., arcs, flames, plasmas, etc., is focused along the slit height of a polychromator, the spatial intensity information (vertical axis) is accurately relayed to the exit focal plane, concurrently with the horizontal spectral dispersion. Thus, by (electronically) dividing the target into a few tens of tracks, the entire spectral profile of these sources can be simultaneously observed and quantitatively studied. 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

Perhaps the most marked difference between flame AAS and flame AFS is the fact that, in the latter technique, the signal increases with the useful source intensity. Source intensity (for a constant line profile) has no effect in AAS, because absorbance is a ratio (see Chapter 1). For this reason, over almost three decades a great deal of research effort has gone into trying to produce more intense and stable sources for use in AFS. For some elements, very low detection limits have been obtained using lasers as excitation sources. A comparison of detection limits by AFS using diverse sources may be found in the useful critical and comprehensive review of AFS by Omenetto and Winefordner.7 Such sources have found very limited application in routine environmental analysis, primarily because of cost and lack of standard commercially available instrumentation they will not be considered further here. 

With the main resonance line for cadmium at 228.8 nm, it is hardly surprising that this element is not determined usefully by flame AES. However cadmium is a very easily atomized element, and the determination by flame AAS is sensitive, with detection limits sometimes as low as 1 ng ml-1 often being cited for the air-acetylene flame.1 Determination by flame AFS may result in detection limits two orders of magnitude lower than this, if a suitable excitation source is available.12 The determinations in acetylene flames are virtually free from chemical interference. Because of the ease of atomization, the element may be readily determined using atom-trapping techniques or boat or cup techniques, as discussed in Chapter 6. Recently a cold vapour sample introduction technique has also been suggested for cadmium determination.13,14... 

Since the hollow-cathode lamp spectra used in AAS are relatively simple, spectral bandwidths narrower than 0.1 nm are seldom if ever used. In atomic emission analysis, however, higher resolving power is often essential, particularly when the excitation source (e.g. the nitrous oxide—acetylene flame) is producing a complex spectrum. The instrument should, therefore, provide a wide range of slit settings and a convenient digital display of the wavelength in use for the operator. 

Since boron produces weak flame absorption signals, an echelle spectrometer equipped with a d-c plasma excitation source similar to that procurable from Spectrametrics, Inc., 204 Andover Street, Andover, Massachusetts-01810, can be used to determine this element by AES. 

Furthermore, it is desired that atomization and excitation occur in an inert chemical environment to minimize possible interferences. Different flame, spark, and arc somces have been used as the excitation sources since the beginning of the twentieth century however, none of these approximates the fiiU fist of conditions fisted above. It was not until mid-1960s when the analytically useful plasma sources were developed, subsfantially improving fhe capabilities of OES. The first commercially available inductively coupled plasma optical emission spectrometry (ICP-OES) was introduced in 1974 and since then the revival of OES can be noted. 

This technique uses a argon plasma induced by high frequency radiation as an excitation source. A plasma is a volume of luminous gas with a portion of its atoms or molecules in an ionised state. This definition can also be applied to a flame however, in analytical spectroscopy the term plasma is normally reserved for an electrical discharge. 

Traditional excitation sources included combustion flames, arcs, and sparks. Flames are limited by relatively low temperatures so that it is difficult to analyze refractory elements or elements with high excitation energies, particularly at low concentrations. In addition, combustion products and flame gases cause both chemical and spectral interferences. Arcs and sparks are capable of higher temperatures, but are strongly affected by the nature of the sample. Minor variations in sample composition can cause variation in the excitation conditions, requiring a close matching of samples and standards or the use of an internal standard. 

Emission spectrometry using chemical flames (flame atomic emission spectrometry, FAES) as excitation sources is the earlier counterpart to flame atomic absorption spectrometry. In this context emission techniques involving arc/spark and direct or inductively coupled plasma for excitation are omitted and treated separately. Other terms used for this technique include optical emission, flame emission, flame photometry, atomic emission, and this technique could encompass molecular emission, graphite furnace atomic emission and molecular emission cavity analysis (MEGA). 

Slavin (1971). The book by Slavin (Emission spectrochemical analysis) is an excellent book by the senior member of an atomic spectrometry family on emission spectrochemistry covering important advances since the 1950 s in theory and optical, electronic, measurement and computer technologies. The underlying technique is principally arc/spark emission spectrography/ spectrometry with mention of new excitation sources such as the flame, high-frequency discharge and plasma jet coming into play at that time. 

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