Pabitra Kumar Mani; Assoc. Prof.; BCKV;

Class - 17

Atomic absorption Spectroscopy

ACSS-501ACSS-501

Absorption, emission, fluorescence

Schematic representation of absorption, emission, and fluorescence.

Atomic Absorption Spectrometry (AAS)In this method, the atomic vapours containing free atoms of an element in the ground state are subjected to a UV-VIS radiation source emitting a characteristic frequency of the element present in atomic vapours. The radiation is absorbed and the intensity of the radiation is attenuated. The absorbed radiation causes excitation of electrons from the ground state to an excited level. The extent of absorption is a quantitative measure of the concn. of the atomic vapours of the element in the ground state. It is an electronic excitation and the energy of transitions lies in the to UV-VIS region of the electromagnetic spectrum.

Atomic Emission Spectrometry (AES)

In this method, a sample is normally excited by the thermal energy of a flame, argon plasma or an electrical discharge. The atoms in the sample absorb thermal energy, causing the excitation of the outer orbital electrons. As the excited state is short lived, the excited atoms return back to the ground state after a very short lifetime (typically10-6 to 10-9 s). This is accompanied by the emission of EMR, normally in the form of light in the UV-VIS region. The wavelength of the emitted radiation and its intensity provide the qualitative and quantitative information about the analyte. The atomic emission spectroscopy employing flame as a means of excitation is called flame photometry or flame emission spectroscopy (FES).

A blackbody emits a continuous spectrum If you look directly at a blackbody, you will see this continuous spectrum

Clouds of gas absorb certain wavelengths (colors) of light A continuous spectrum that hits a cloud of cool gas will be partially absorbed The transmitted spectrum is called an absorption line spectrum (because certain lines are absorbed), and is continuous except for the colors that were absorbed by the gas.

•Anything that absorbs also emits. •A cloud of cool gas that absorbs certain colors from a blackbody will emit exactly those colors as the gas atoms de-excite •If we look at the cloud without the blackbody in our line of sight, we will see an

emission line spectrum. •The lines of emission have the same color as the absorption lines in the absorption line spectrum •If you added an emission line spectrum and an absorption line spectrum, you

would get a continuous spectrum.

• Atomic spectra: single external electron

Slightly different in energy

Basic components of an atomic spectrophotometer

Molecular spectra are generally broad. These consist of a number of closely spaced lines constituting what is called a band spectrum. The band nature of the spectrum is due to a number of factors like, the quantisation of the rotational and vibrational motions of the molecules along with the quantisation of electronic energy levels. In addition, the width of the spectrum is also dependent on some of the instrumental parameters.

In contrast to the above, the atomic spectra consist of a number of very sharp lines, characteristic of the atomic species. It implies that these give rise to line spectra. The atomic spectra are generally much sharper because in atomic systems, the rotational and the vibrational motion are not quantised and the transitions are observed amongst the electronic energy levels of the absorbing species. A typical atomic spectrum is shown in Fig. 7.2.

Similar to the molecular spectrum, the signals in an atomic spectrum are also characterised in terms of three parameters. These are given below.

• position• intensity• width

Position of the SignalThe position of the spectral signals is determined by the difference in the energies of different energy levels. If E2 and E1 represent the energy of the higher and lower energy levels concerned, the energy change during the transition from E2 to E1 levelmay be defined as follows.

where, h is the Planck’s constant, c is the velocity of light, v the frequency and λ the wavelength of the emitted radiation. The wavelength of the radiation is characteristic of the atoms of the particular element from which it is emitted.

When flame photometry is employed as an analytical tool, the wavelength of the radiation emerging from a flame tells us about the elements that are present in that flame

Intensity of the SignalThe intensity of a signal depends on the population of the energy level from which the transition is originating and the probability of such a transition. According to the Boltzmann statistical distribution, the population of the ground state i.e., the number of species in the ground state is highest and it keeps on decreasing as we go to higher energy levels. In case of the atoms the population of any excited state relative to that of the ground state is given by the following formula.

N* and N0 = Number of atoms in excited state and ground state, respectively,

E = Energy difference between ground and excited state (in J),

g* and g0 = Statistical factors that depend on the degeneracies of the levels involved,

k = Boltzmann constant (= 1.28 × 10-23 J k-1), T = Absolute temperature.

According to Eq. after the ground state, the lowest energy excited state will be most populated and the population of the higher excited states would decrease progressively. Eq. can be used to determine the population of an excited state with respect to the ground state.

Let us illustrate this with the help of an example.

Q: The characteristic yellow emission of sodium vapours consists of a pair of lines at 589 nm and 589.6 nm. These arise from the emission of radiation by the gaseous sodium atoms in the 3p excited state to 3s ground state. Compute the ratio of the sodium atoms in the excited state to the ground state.

Therefore, to evaluate the ratio of the atoms in the excited state to that in the ground state we need to know the statistical factors g* and g0 and ΔE.

As the 3s and 3p levels have two and six quantum states i.e., the statistical factors are 2 and 6 respectively, the ratio of g* and g0 comes out to be 6/2 = 3

The change in energy, ΔE, can be calculated by using the formula,

The wavelength is taken as an average of the two,

Substituting the value of ΔE in the above eqn, we get the following.

We have learnt above that the intensity of a signal depends on the population of the level from which the transition originates and the probability of such a transition.These are the intrinsic parameters of the analyte being determined. In addition to these, the intensity of a signal does depend on an imp. external parameter, viz., the concn. of the analyte.

Intensity Concentration Relationship

The intensity of the emitted radiation (P) is proportional to the no. of excited atoms N*.

we see that the number of atoms in the excited state, N*, at a given temperature are proportional to N0. Therefore, we can write the following.

P = K. C (as, N0 is directly proportional to metal concn)

Thus, the intensity of the emitted light will be directly proportional to the concentration of the element introduced into the flame.

e = charge of the electron, m = mass of the electron c = the speed of light, N = the no. of free absorbing atoms in the light path f = is the oscillator strength of the absorption line.It can be seen that there are a number of constants in this equation. The only variables are N, the total number of atoms in light path, and f the oscillator strength.

The relationship between them and the total amount of light absorbed is a basis for quantitative analysis. The degree of absorption for each element and each absorption line depends on the oscillator strength which is a direct measure of how strongly each atom will absorb at that wavelength.

To a first approximation, absorption by free atoms is similar to absorptionby molecules and there is a linear relationship between absorbance and the“concn” of the sample. This relationship is given by the Beer-Lambert Law

it is found that B-L relationship cannotbe sustained because flame atomizers are generally used as the “sample cell”and the population of free atoms in a flame is far from homogeneous.Homogeneity of the sample is a basic requirement for the application ofBeer’s Law.

The oscillator strength in emission spectroscopy is a measure of how closely an atom resembles a classical oscillator in its ability to emit radiation.The greater the oscillator strength the greater the emission intensity for a given set of conditions.

A , = the transition probability between energy levels i and j , f, = the oscillator strength of the associated emission line; h = wavelength of the emission line.The relationship between the absorption oscillator strength and the emission oscillator strength is given by the equation

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Atomic Line Widths• Widths of atomic lines are quite

important in atomic spectroscopy.• Narrow lines in atomic and emission

spectra reduce the possibility of interference due to overlapping lines.

• Atomic absorption and emission lines consists of a symmetric distribution of wavelengths that centers on a mean wavelength (0) which is the wavelength of maximum absorption or maximum intensity for emitted radiation.

• The energy associated with 0 is equal to the exact energy difference between two quantum states responsible for absorption or emission.

• A transition between two discrete, single-valued energy states should be a line with line-width equal to zero.

• However, several phenomena cause line broadening in such a way that all atomic lines have finite widths.

• Line width or effective line width (1/2) of an atomic absorption or emission line is defined as its width in wavelength units when measured at one half the maximum signal.

Sources of broadening:(1) Uncertainty effect(2) Doppler effect(3) Pressure effects due to collisions(4) Electric and magnetic field effects

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Uncertainty Effect• It results from the uncertainty principle postulated in 1927

by Werner Heisenberg.• One of several ways of formulating the Heisenberg

uncertainty principle is shown in the following equation:Δt x ΔE = h/2π

• The meaning in words of this equation is as follows: if the energy E of a particle or system of particles – photons, electrons, neutrons or protons – is measured for an exactly known period of time Δt, then this energy is uncertain by at least h/ 2πΔt.

• Therefore, the energy of a particle can be known with zero uncertainty only if it is observed for an infinite period of time.

• For finite periods, the energy measurement can never be more precise then h/ 2πΔt.

• The lifetime of a ground state is typically long, but the lifetimes of excited states are generally short, typically 10-7 to 10-8 seconds.

• Line widths due to uncertainty broadening are called natural line widths and are generally 10-5nm or 10-4Å.

Note: = 1/2

The "natural line width" indicates the lower limit of the absorption lines width. It can be calculated from the uncertainty principle which states that

where Δτ is the lifetime of the excited state and ΔE the range of energy over which the line emits, i.e. the line width in terms of energy.

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Doppler Effect• In a collection of atoms in a hot environment, such as an atomizer, atomic motions occur in every direction.

• The magnitude of the Doppler shift increases with the velocity at which the emitting or absorbing species approaches or recedes the detector.

• For relatively low velocities, the relationship between the Doppler shift (Δλ) and the velocity (v) of an approaching or receding atom is given by:

Δ λ / λ 0 = v / cWhere λ 0 is the wavelength of an un-shifted line of a sample of an element at rest relative to the transducer, and c is the speed of light.

• Emitting atom moving: (a) towards a photon detector, the detector sees wave crests more often and detect radiation of higher frequency; (b) away from the detector, the detector sees wave crests less frequently and detects radiation at lower frequency.

• The result is an statistical distribution of frequencies and thus a broadening of spectral lines.

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Pressure Effects Due to Collisions

• Pressure or collisional broadening is caused by collisions of the emitting or absorbing species with other atoms or ions in the heated medium.

• These collisions produce small changes in energy levels and hence a range of absorbed or emitted wavelengths.

• These collisions produce broadening that is two to three orders of magnitude grater than the natural line widths.

• Eg. :Hollow-cathode lamps (HCL):• Pressure in these lamps is kept

really low to minimize collisional broadening.

• Glass tube is filled with neon or argon at a pressure of 1 to 5 torr.

’ ’

Ene

rgy

(eV

)

Atom 1 Atom 2

E1

E2

E1

E2

Light Source: Hollow Cathode Lamp

anode

cathode

+

-

window

light

Power Supply

• The electric potential ionizes rare gas atoms and accelerates them into the cathode where they sputter metal atoms into the gas phase

• Collisions with gas atoms or electrons excite the metal atoms

• On decay the metal atoms emit light

Cup made of metal of interest

C. Hollow cathodesAn illustration of the hollow cathode is given in Figure 2. In this system the metal of interest is used as the material from which the cathode is made.

The light source is filled with an inert gas, such as neon, which is ionized by the anode. The positively charged neon ions are then attracted by the negative charged cathode and accelerated towards it. On arrival at the cathode the neon strikes the surface of the cathode. If it has sufficient energy it causes atoms of the cathode to be ejected. This process is called “sputtering”.

The sputtered atoms are invariably excited and emit radiation characteristicof the cathode metal(s). The emitted lines are generally very narrow in band width .The sample atoms absorb only at their own characteristic wavelengths. It is therefore essential that the light source emit at Exactly the same wavelength. This can be accomplished by using a hollow cathode made of the same element as the element being determined.

The exact mechanism of the excitation process in the hollow cathode lamp (hcl) is of interest. Figure 11 is a close-up view of a typical lamp and of the mechanism. The lamp itself is a sealed glass envelope filled with argon or neon) gas. When the lamp is on, neon atoms are ionized, as shown, with the electrons drawn to the anode (+ charged electrode), while the neon ions, Ne+, "bombard" the surface of the cathode (- charged electrode). The metal atoms, M, in the cathode are elevated to the excited state and are ejected from the surface as a result of this bombardment. When the atoms return to the ground state, the characteristic line spectrum of that atom is emitted. It is this light, which is directed at the flame, where unexcited atoms of the same element absorb the radiation and are themselves raised to the excited state. As indicated previously, the absorbance is measured and related to concn.

Fig11. The hcl and the process of metal atom excitation and light emission

Illustration of how the light emitted by the hollow cathode lamp is the exact wavelength needed to excite the atoms in the flame.

Hollow Cathode Lamp As stated before, the light from this lamp is exactly the light required for the analysis, even though no monochromator is used. The reason for this is that atoms of the metal to be tested are present within the lamp, and when the lamp is on, these atoms are supplied with energy, which causes them to elevate to the excited states. Upon returning to the ground state, exactly the same wavelengths that are useful in the analysis are emitted, since it is the analyzed metal with exactly the same energy levels that undergoes excitation.

The light emitted by such a lamp consists of the line spectra of all the kinds of atoms present. No interference will usually occur as long as the sufficiently intense line for a given metal can be found which can be cleanly separated from all other lines with the monochromator.

PRINCIPLE OF FLAME PHOTOMETRYThis method is based upon the measurement of intensity of radiation emitted, in the visible region, when a metal atom is introduced into a flame. The wavelength of the radiation (or the colour), emitted tells us what the element is, and the intensity of the radiation tells us how much of the element is present. IUPAC Committee on Spectroscopic Nomenclature has recommended abbreviation FAES (flame atomic emission spectrometry) for this technique.

In a typical flame photometric experiment, a solution containing the relevant substance to be analysed is aspirated into the burner and dispersed into the flame as a fine spray. This process is called nebulisation.

The five processes occurring in the flame can be summarised as follows.i) Desolvationii) Vapourisationiii) Atomisationiv) Excitation v) Emission of radiation: Electrons in the excited state are very unstable and move back down to the ground state or a lower energy state quite quickly. As they do so, they emit the energy in the form of radiation of characteristic wavelength, which is measured by a detector.

After Manning T.J. and Grow W.P., 1997

Inductively Coupled Plasma Source

A plasma is a hot, partially ionized

gas. It contains relatively high

concentrations of ions and electrons.

Argon ions, once formed in a plasma, are

capable of absorbing sufficient power

from an external source to maintain the

temperature at a level at which further

ionization sustains the plasma

indefinitely. The plasma temperature is

about 10 000 K.

A plasma is an ionized gas that is macroscopically neutral (i.e. with the same number of positive particles (ions) and negative particles (electrons)). If a monoatomic gas, X, is used, a plasma can be described by the following eqlm.:

where Xn+ is an ion with n charges and e is the electron.In contrast to a flame, it is necessary to supply an external energy in the form of an electrical field in order to ionize the gas and to sustain the plasma, which, in turn, will transmit part of this energy to the sample to atomize, ionize and excite it.

● direct current plasma (DCP) is obtained when a direct current field is established across electrodes,

● ICP is obtained when a high-frequency (hf) field is applied through a coil,

● microwave-induced plasma (MIP) is obtained when a microwave field is applied to a cavity

The gas that is used to generate the plasma (plasma gas) is argon. Like any noble gas, argon is a monoatomic element with a high ionization energy (15.76 eV), and is chemically inert.

Consequently:(i) a simple spectrum is emitted by argon in contrast to a flame

where primarily molecular spectra are observed;(ii) argon has the capability to excite and ionize most of the

elements of the Periodic Table;(iii) no stable compounds are formed between argon and the analytes.

In atomic emission spectrometry (AES), a source will have actually two roles:

1st step consists of the atomization of the sample to be analyzed so as to obtain free analyte atoms, usually in the ground state,

2nd step consists of a partial ionization of the analyte atoms, and of the excitation of the atoms and the ions to higher-energy states.

The plasma acts as reservoir of energy provided by the rf field, and transfers this energy to an analyte, M. It should be noted that the atomization of a sample is a relatively long process (of the order of a few ms), while ionization and excitation are very fast processes.

The major species are not only the argon ions, Ar+, and the electrons, e, but also the excited argon atoms, Ar*,with the special case of the metastable levels, Arm .

The main ionization processes are:

charge-transfer ionization

electron-impact ionization

Penning ionization

while the main excitation processes for the analyte atom are:

electron impact excitation

ion–electron radiative recombination

Quantitative analysis will be possible if the intensity of the line can be related to the concentration of the emitting species. The intensity of a line is proportional to:(i) the difference in energy between the upper level, Em, and the lower level, Ek, of the transition,(ii) the population of electrons, nm, in the upper level, Em,(iii) the number of possible transitions between Em and Ek per unit time.

This value is expressed by the transition probability A, and has been defined by Einstein. Therefore, the intensity I is proportional to:

As seen above, it is possible to relate the population nm to the total population N through the Boltzmann equation. The intensity of a line can be, therefore, written as:

where Φ is a coefficient to account for the emission being isotropic over a solid angle of 4 steradian.

Inductively Coupled Plasma Source

ICP-AES: Plasma

Q n AThanks