chemistry

1. Flame Atomic Emission Spectroscopy

2. Flame Atomic Emission Spectroscopy
Flame atomic emission spectroscopy, also called flame photometry
 Based on measurement of emission spectrum produced when a solution containing metals or
nonmetals such as halides, sulfur, or phosphorus is introduced into a flame.
In early experiments, the detector used was the analyst’s eye.
Elements that emitted visible light could be identified qualitatively
 These “flame tests” were used to confirm the presence of certain elements in sample,
particularly alkali &alkaline-earth metals.
A list of visible colors emitted by elements in a flame is given in Table 7.1.
Spectrometer & photon detector such as a photomultiplier tube (PMT) or charge-coupled
device (CCD) permits more accurate identification of elements present.

3. Continue…………
In addition, use of a photon detector permits quantitative analysis of sample.
The wavelength of radiation indicates what element is present, & radiation intensity indicates
how much of element is present.
Flame AES is particularly useful for determination of elements in first two groups of periodic
table, including sodium, potassium, lithium, calcium, magnesium, strontium, & barium.
Determination of these elements is often called for in medicine, agriculture, & animal science.

7. Instrumentation for Flame OES
Flame OES can be performed using most modern atomic absorption spectrometers.
No external lamp is needed as flame serves as both atomization source & excitation source.
A schematic diagram of a flame emission spectrometer based on single beam atomic
absorption spectrometer is shown in Figure 7.2.
For measurement of alkali metals in clinical samples such as serum or urine, only a low-
resolution filter photometer is needed because of simplicity of spectra.
Both instruments require a burner assembly, a flame, a wavelength selection device, & a
detector.

8. Burner Assembly
Central component of a flame emission spectrometer is burner assembly which has a device to
nebulize sample & then introduce sample aerosol into flame.
In the flame, free atoms are formed & then excited, which causes them to emit radiant energy.
It is essential that emission intensity be steady over reasonable periods of time (1–2 min).
Lundegardh (Air-Acetylene) or premix (efficiency of 20,000 to 70,000 Btu/hr and produces a
short flame) burner is most commonly used and is depicted in Figure 6.8a.
In premix burner, sample, in solution form, is first aspirated into a nebulizer where it forms an
aerosol or spray.
An impact bead or flow spoiler is used to break droplets from nebulizer into even smaller
droplets.

9. Larger droplets coalesce on the sides of the spray chamber and drain away.
Smaller droplets and vapor are swept into the base of the flame in the form of a cloud.
An important feature of this burner is that only a small portion (about 5%) of aspirated sample
reaches flame.
The droplets that reach the flame are, however, very small & easily decomposed.
High atomization efficiency leads to increased emission intensity & increased analytical
sensitivity compared with other burner designs.
The process that occurs in burner assembly & flame is outlined in Table 7.2.
Common nebulizers used in FOES are pneumatic nebulizer & cross-flow nebulizer
Continue…………

11. Wavelength Selection Devices
Two wavelength selectors are used in flame OES, monochromators & filters.
Monochromators: have slits & dispersion elements.
The common dispersion element in modern flame atomic absorption & emission
spectrometers is a diffraction grating.
Filters: Alkali metals in a low-temperature flame emit only a few lines & therefore have a
simple emission spectrum.
In this case, wider wavelength ranges may be allowed to fall on detector without causing
errors, so an optical filter may replace more expensive diffraction grating.
Filters are built with materials that are transparent over a narrow spectral range.
Transparent spectral range is designed to be one in which emission from a given element
occurs.

12. When a filter is placed between the flame & detector, radiation of desired wavelength from the
sample is allowed to reach the detector & measured.
Other radiation is absorbed by filter & is not measured.
Therefore, separate filter is required for each element to be measured.
Instruments that use filters as wavelength selectors are convenient for simple repetitive
analysis but limited with regard to number of elements for which they can be used.
Most flame photometers are single channel instruments.
Multichannel instruments have been designed where emission from flame falls on two or more
filters.
Each filter transmits radiation for which it has been designed, and transmitted radiation falls on
a PMT behind the filter.
Continue…………

13. Intensity of radiation is measured for calibration standards, blanks, and samples.
A calibration curve of concentration versus emission intensity is made for standards & the
concentration of sample is determined by comparison to calibration curve.
This multichannel approach permits use of an internal standard (IS) to improve precision.
Filter photometers designed for hospital or veterinary laboratories often have autosamplers &
autodilutors attached, permitting the unattended analysis of many samples per hour.
Continue…………

14. Detectors
The detectors in common use for these systems are the PMT or solid-state detectors such as
CCDs and charge injection devices (CIDs).

15. Flame Excitation Source
Flame is result of the exothermic chemical reaction between two gases, one of which serves as
fuel & other as the oxidant.
Common oxidants for modern AAS/OES systems are air & nitrous oxide.
Only fuel used in modern AAS/OES systems is acetylene, although commercial filter
photometer systems can use propane, natural gas, or butane as fuel.
When a liquid sample is introduced into a flame, a complex process to produce excited-state
atoms occurs.
Spectral emission lines are generated by excited atoms formed during the process of
combustion in a flame.
Emission lines are characterized by wavelength & intensity.

16. The wavelengths emitted depend on atoms present.
Each element has a different set of quantized energy levels (such as those shown schematically
in Figure 7.1) & will emit different, characteristic wavelengths of light.
The intensity of emission depends on several factors including
concentrations of elements in sample
rate at which excited atoms are formed in flame
the rate at which sample is introduced into the flame
the composition of the flame
the temperature of the flame.
Continue…………

17. The intensity–concentration relationship is the basis for quantitative analysis by flame OES.
Flame temperature is probably the most important single variable in flame photometry.
In general, increase in flame temperature causes an increase in emission intensity.
This does not happen with elements that ionize easily, such as sodium, potassium, & lithium.
If these elements are heated at too high, they become ionized.
If the atoms ionize, the valence electrons are lost & therefore cannot return to ground state &
emit atomic radiation in process.
Results in loss of atomic emission intensity.
These elements must be determined in low-temperature flames to minimize ionization.
Continue…………

18. Ratio of number of atoms in an upper excited state to number of atoms in a lower energy state
can be calculated from Maxwell–Boltzmann equation;
𝑁1
𝑁𝑂
=
𝑔1
𝑔𝑜
𝑒−∆𝐸/𝑘𝑇Eq.7.1
Where; N1 is the number of atoms in the upper state
N0 is the number of atoms in the lower state
g1 &g0 are the number of states having equal energy at each level 0, 1, etc.
ΔE is the energy difference between the upper and lower states (J)
k is the Boltzmann constant = 1.381 × 10−23 J/K,
T is the absolute temperature (K)
Continue…………

19. The Boltzmann distribution assumes the system is in thermal equilibrium.
The emission intensity is related to the number of atoms in the higher excited state, N1
By this equation, we can calculate ratio of number of excited-state atoms at two different
temperatures.
For potassium atoms, major atomic emission line occurs at 766.5 nm.
The energy of this transition in joules is ∆E = hc/λ
∆E = (6.626 ∗ 10−34Js) (2.998 ∗ 108m/s)/(766.5 nm)(1 ∗ 109m/nm)
∆E = 2.59 ∗ 10−19J
Continue…………

20. The temperature in a typical air–acetylene flame is about 2200°C or 2473 K.
The ratio of excited state potassium atoms at 2498 versus 2473 K is calculated by dividing the
Boltzmann equation at 2498 K by that at 2473 K.
N0 and the degeneracy terms cancel and we are left with

𝑁2498
𝑁2473
= exp[−2.59 ∗ 10−19
(J)/1.38*10−23
(J/K)(2498K)]/exp[-−2.59 ∗ 10−19
(J)/1.38*10−23
(J/K)(2473K)]

𝑁2498
𝑁2473
= 1.08
This tells us that a 25 K increase in flame temperature results in an 8% increase in the excited-
state population of potassium atoms that give rise to this emission line.
The intensity of emission line is directly proportional to the excited-state population, even for
systems not in thermal equilibrium.
Continue…………

21. Relationship between emission intensity, S, & excited-state population can be expressed as
S = kN (7.2)
Where; S is the intensity, k is a proportionality constant & N is the excited-state atom population
S is related directly to the number of atoms in excited state.
As absolute temperature increases, number of atoms in excited state increases.
AES is very sensitive to changes in temperature.
Temperature in atomizer must be carefully controlled for quantitative measurement of
emission intensity.
Table 7.3 lists representative maximum temperatures for some common flames.
Continue…………

23. As energy required to cause excitation increases, it is more difficult to excite atoms, & the
number of atoms in excited state decreases.
As a consequence of relationship ΔE = hν = hc/λ, a decrease in wavelength of emission line
indicates that more energy is required to excite the atom.
The process becomes difficult, fewer atoms are excited, & intensity of radiation decreases.
Consequently, elements with emission lines in short-wavelength part of spectrum give weak
emission signals in low-temperature flames.
For these elements, high-temperature nitrous oxide–acetylene flame is favored, or the high-
energy electrical or plasma excitation sources should be used.
Continue…………

24. Another factor that influences emission intensity is the ratio of fuel to oxidant in flame.
Highest flame temperature is obtained when a stoichiometric mixture of the two is used.
In a stoichiometric flame, number of moles of fuel & oxidant present react completely.
Any excess of oxidant or fuel results in decrease in temperature of flame.
However, some atoms are unstable in certain kinds of flames.
Al atoms oxidize very quickly in a stoichiometric flame or a flame with an excess of oxidant.
Al2O3 emits molecular radiation not at same wavelength as line emission associated with Al.
This results in a direct loss of atomic emission intensity.
To prevent formation of Al2O3, flame is usually run in a “reducing state,” with an excess of fuel.
Continue…………

25. The excess fuel “mops up” free oxidant & minimizes oxidation of Al in flame.
Some elements emit more strongly in oxidizing flames.
Moreover, excess O2 decompose other materials present in sample & reduces molecular
background.
Manufacturers of flame emission instruments provide a list of recommended flame
compositions for elements measured by OES.
Table 7.A.1 in Appendix 7.A provides flame OES detection limits (DLs) for many elements and
identifies the flame used.
Continue…………

26. Interferences
Radiation intensity measured may not represent the concentration of analyte in sample
accurately because of the presence of interferences.
Interferences fall into two categories: spectral & nonspectral.
Three principal sources of interference are encountered in flame OES.

27. Chemical Interference
If analyte is present in sample with anions combines strongly, it will not decompose to free
atoms.
If anions present in solution combine only weakly with analyte, decomposition & formation of
free atoms will be easier. This effect is called chemical interference.
Chemical interference is a nonspectral interference.
It can be reduced or eliminated in a number of ways.
Analyte ion may be extracted away from sample matrix by using a chelating agent or the
interfering anion may be removed by ion exchange.
Standards can be made from same salt present in sample.
A releasing agent may be added to sample solution.

28. Excitation & Ionization Interferences
Excitation & ionization interferences are nonspectral interferences.
When sample is aspirated into a flame, elements in sample may form neutral atoms, excited
atoms, & ions.
These species exist in a state of dynamic equilibrium that gives rise to a steady emission signal.
If samples contain different amounts of elements, the position of equilibrium may be shifted.
This may affect the intensity of atomic emission.
This is excitation interference & is generally restricted to the alkali metals.
Can be overcome by matrix matching the samples & standards, where sample composition is
known.
In M.M is not practical, the methods of standard additions (MSA) can be used.

29. A related problem is ionization interference.
If analyte atoms are ionized in the flame, they cannot emit atomic emission wavelengths, & a
reduction in atomic emission intensity will occur.
It can be prevented by the phenomena of ionization suppressant.
Use of ionization suppressants is recommended for best sensitivity & accuracy when
determining these elements.
Continue…………

30. Spectral Interferences
There are two types of spectral interferences in flame OES
background radiation
overlapping line emission from different elements.
An example of background emission from an air–acetylene flame is illustrated in Figure 7.3.
In order to accurately determine intensity emitted by atom of interest, the intensity due to
background emission must be measured & subtracted from the total intensity.
A blank solution is aspirated & intensity at the analyte wavelength is measured.
This intensity is due to the background emission, since no analyte is present.
Then the sample & standard solutions are aspirated & their intensity measured.

32. Sample or standard intensity is due to sum of background emission plus analyte emission.
Subtraction of blank intensity from sample or standard intensity results in intensity due to
analyte.
Second method for measuring background intensity is to measure background intensity of
actual sample at a wavelength very close to analyte emission line.
This intensity is then subtracted from intensity of sample measured at analyte emission
wavelength.
This approach can be less accurate than first method if the background spectrum is not “flat” in
region of interest.
Continue…………

33. Other type of spectral interference is emission by another element of same wavelength as
analyte .
Analyst may choose another analyte wavelength, extract the interfering element, or apply a
correction factor if concentration of interfering element is known.
By “same wavelength” mean that instrument cannot resolve the interfering line from analyte
line, even if actual wavelengths are slightly different.
Spectral bandpass is such that both wavelengths pass through the exit slit of system to
detector.
For this reason, high-resolution spectrometers are needed for nonflame OES.
Continue…………

34. Analytical Applications of Flame OES
Qualitative Analysis: atomic emission spectroscopy is an excellent qualitative method for
determining multiple elements in samples.
Presence of elements in a sample is determined qualitatively by observing emission at
wavelength characteristic of element.
Flame OES is a fast, simple method for qualitative identification of the group 1 and 2 elements
Can be used for any element that emits radiation in a flame provided care is taken to
discriminate emission lines from any spectral interference.

35. Quantitative Analysis
Flame OES can be used to determine the concentrations of elements in samples.
The sample usually must be in solution form.
Generally, one element is determined at a time if using an AAS system in emission mode.
Multichannel instruments are available for the simultaneous determination of two or more
elements.

36. Recommended book
Undergraduate Instrumental Analysis
James W. Robinson (7th Edition)
Chapter 7: Atomic Emission Spectroscopy
(7.1 Flame Atomic Emission Spectroscopy)