Instrumentation, P K MANI

Spectrophotometry property is mainly concerned with the following regions of the spectrum:

ultraviolet, 185-400 nm; visible 400-760 nm; and infrared, 0.76- 15 µm.

Colorimetry is concerned with the visible region of the spectrum.

Plane-polarized electromagnetic radiation showing the electric field, and the direction of propagation.

Electric field component of plane-polarized electromagnetic radiation.

Attenuation of a beam of radiation by an absorbing solution.

Reflection and scattering losses with a soln contained in a typical glass cell.

• Transmission• Reflection • Refraction• Scattering• Luminescence• Chiro-optical phenomena

When light impinges on a cuvette containing our molecule of interest (solute) in a soln (solvent), other optical processes do or can occur:

THEORY OF SPECTROPHOTOMETRY AND COLORIMETRY

When light (monochromatic or heterogeneous) falls upon a homogeneous medium, a portion of the incident light is reflected, a portion is absorbed within the medium, and the remainder is transmitted. If the intensity of the incident light is expressed by Io, that of the absorbed light by Ia, that of the transmitted light by It and that of the reflected light by Ir , then:

For air-glass interfaces arising from the use of glass cells, it may be stated that about 4 per cent of the incident light is reflected. Ir is usually eliminated by the use of a control, such as a comparison cell, hence:

Lambert's Law. This law States that “when monochromatic light passes througha transparent medium, the rate of decrease in intensity with the thickness of the medium is proportional to the intensity of the light”. (= the intensity of the emitted light decreases exponentially as the thickness of the absorbing medium increases arithmetically). We may express the law by the differential eqn:

I is the intensity of the incident light of wavelength λ, l (ell) is the thickness of the medium, and k is a proportionality factor. Integrating equation (1) and putting I = Io when l = 0, we obtain:

or, stated in other terms,

Eqn........ 1

Eqn..... 2

ll

l

l0

0

kI

dI

ti

i

kll 0I

In

t

where Io is the intensity of the incident light falling upon an absorbing medium of thickness(l ), It is the intensity of the transmitted light, and k is a constant for the wavelength and the absorbing medium used. By changing from natural to common logarithms we obtain:

where K = k / 2.3026 and is usually termed the absorption coefficient. The absorption coefficient is generally defined as the reciprocal of the thickness (l cm) required to reduce the light to 1/10 of its intensity. This follows from equation (3), since:

Eqn....... 3

The ratio It/Io is the fraction of the incident light transmitted by a thickness (l)

of the medium and is termed the transmittance T.

Its reciprocal Io/It is the opacity, and the absorbance A of the medium (formerly

called the optical density ) is given by:*

Beer's Law.The intensity of a beam of monochromatic light decreases exponentially as the concn. of the absorbing substance increases arithmetically. This may be written in the form:

where c is the concentration, and k/ and K/ are constants. Combining equations (3) and (4), we have:

This is the fundamental equation of colorimetry and spectrophotometry, and is often spoken of as the Beer-Lambert Law

Eqn......4

The value of a will clearly depend upon the method of expression of the concn. If c

is expressed in mole L-1 and l in cm then a is given the symbol E and is called the molar absorption coefficient or molar absorptivity (formerly the molar extinction coefficient).

It will be apparent that there is a relationship between the absorbance A, the

transmittance T, and the molar absorption coefficient, since:

• Linearity is observed in the low concentration ranges(<0.01), but may not be at higher concentrations.

• This deviation at higher concentrations is due to intermolecular interactions.

• As the concentration increases, the strength of interaction increases and causes deviations from linearity.

• The absorptivity not really constant and independent of concentration but e is related to the refractive index (h ) of the solution :

• At low concentrations the refractive index is essentially constant-so e constant and linearity is observed.

Real Limitations

Absorption, emission, fluorescence

Schematic representation of absorption, emission, and fluorescence.

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.

Consider the simplified energy-level diagram shown in Fig. where E0 represents the ground state in which the electrons of a given atom are at their lowest energy level and E1, E2, E3, etc., represent higher or excited energy levels.Transitions between two quantised energy levels, Say from E0 to Et, correspond to the absorption of radiant energy, and the amount of energy absorbed (ΔE) is determined by Bohr's equation

The relationship between the ground-state and excited-state populations is given by the Boltzmann equation

Nt = no. of atoms in the excited state,No = number of ground state atoms,g /go = ratio of statistical weights for ground and excited states,

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 flame photometry or flame emission spectroscopy photometry or flame emission spectroscopy (FES).(FES).

Electrons in a molecule can be classified in 3 different types.

1. Electrons in a single covalent bond (σ-bond): these are tightly bound and radiation of high energy (short wavelength) is required to excite them.

2. Electrons attached to atoms such as chlorine, oxygen or nitrogen as 'lonepairs': these non-bonding electrons can be excited at a lower energy (longer wavelength) than tightly bound bonding electrons.

3. Electrons in double or triple bonds (π-orbitals) which can be excited relatively easily. In molecules containing a series of alternating double bonds (conjugated systems), the π-electrons are delocalised and require less energy for excitation so that the absorption rises to higher wavelengths.

A diagram showing the various kinds of electronic excitation that may occur in organic molecules is shown above. Of the six transitions outlined, only the two lowest energy ones (left-most, colored blue) are achieved by the energies available in the 200 to 800 nm spectrum. As a rule, energetically favored electron promotion will be from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the resulting species is called an excited state

Basic components of an atomic spectrophotometer

After G.Ma and G.W. Gonzales, http://www.cee.vt.edu

Operation principle of AAS

Light source – hollow cathode lamp. Each element has its own unique lamp.

Atomic cell – flame (gas mixture) or graphite furnance (accepts solutions,

slurries, or even solids).

Detector – photomultiplier.

At the temperature of an air/acetylene flame (~2300oC) atoms of many elements exist largely in the ground state. When a beam ofradiant energy that consists of the emission spectrum for the element that is to be determined is passed through the flame, some of the ground state atoms absorb energy of characteristic wavelengths (resonance lines) and are raised to a higher energy state. The radiation not removed by absorption is isolated by a monochrometer and detected by a photomultiplier. For example, at 283.3 nm,

Pb ---> Pb* by absorption of a photon

The amount of radiant energy absorbed as a function of concentration of an element in the flame is the basis of atomic absorption spectroscopy. The amount of light absorbed is proportional to the elemental concentration, assuming Beer‘s Law holds.

PRINCIPLE OF AAS

The exact mechanism of the excitation process in the hollow cathode lamp (hcl) is of interest. Figure 1. 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.

Fig1. The hcl and the process of metal atom excitation and light emission

Hollow Cathode Lamp:

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.

Illustration of how the light emitted by the hollow cathode

lamp is the exact wavelength needed to excite the atoms in the flame.

Kν =absorption coefficient at the frequency νe = 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.

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.

For a few elements, such as the alkali metals Na and K, an air/ acetylene flame is hot enough not only to produce ground state atoms, but to raise some of the atoms to an excited electronic state. The radiant energy emitted when the atoms return to the ground state is proportional to the concentration and is the basis of flame emission spectroscopy. i.e., at 589 nm, Na -------> Na* (energy from flame)

Na* -------> Na + hn (at 589 nm)

PRINCIPLE OF FLAME PHOTOMETRY

PRINCIPLE OF FLAME PHOTOMETRY

This 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.

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) Desolvation ii) Vapourisationiii) Atomisation iv) 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.

Within the flame, there are many more atoms in the ground state than in the excited state. For Zn, for instance, in a 2000K flame, there are 7.3 X 1015 atoms in the ground state for every excited atom.The alkali metals are elements with unocuppied atomic orbitals of low enough energy to be sufficiently populated by a flame.

The intensity of the light emitted could be described by the Scheibe-Lomakin Equation: I = k × c n

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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 thatwhere 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.

36

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

Intensity of the Signal

The 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.

Intensity Concentration RelationshipIntensity 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.

Quantum or Wave Mechanics• Light has both wave & particle

properties• de Broglie (1924) proposed that

all moving objects have wave properties.

• For light: E = hν = hc / λ• For particles: E = mc2 (Einstein)

L. de BroglieL. de Broglie(1892-1987)(1892-1987)

for particles is called the de Broglie wavelength

Therefore, mc = h / and for particles (mass)x(velocity) = h /

Origins of Atomic SpectroscopySpectroscopy of atoms or ions do not involve vibrations or rotation transitions. Transition involves promoting an electron from a ground state to a higher empty atomic state orbital, this state is referred to as the excited state. Shown to the right is the three sodium absorption and emission process and the emission lines. Atomic p-orbitals are in fact split into two energy levels for the multiple spins of the electron. The energy level is so small however that a single line observed. A high resolution would show the line as a doublet.

For the element sodium, two inner shells are completely filled and there is one electron in the outer third shell.This electron is said to be in an s orbital. However, the remaining orbitals of the third shell and all the orbitals of the fourth, fifth and sixth shells, etc., are empty. When the outer electron of sodium is in the s orbital the atom is said to be in the “ground state” or the “unexcited state”. If the atom absorbs radiation the electron undergoes a transition to one of the empty orbitals at the higher energy levels.

From Grotrian diagram it can be seen that the s electron can undergo transition to various p orbitals. These in turn exhibit fine structure as a result of the electron in a p orbital spinning in either oftwo possible directions within the orbital. There is a slight difference in the energy of such an electron depending on its direction of spin, i.e. the spin quantum number.

Optical Atomic Spectra• Figure 8-1a shows the energy level diagram for sodium.• A value of zero electron volts (eV) is arbitrarily assigned to

orbital 3s.• The scale extends up to 5.14eV, the energy required to

remove the single 3s electron to produce a sodium ion. 5.14eV is the ionization energy.

• A horizontal line represents the energy of and atomic orbital.

• “p” orbitals are split into two levels which differ slightly in energy:

3s → 3p: l = 5896Å or 5890Å

3s → 4p: l = 3303Å or 3302Å

3s → 5p: l = 2853.0Å or 2852.8Å• There are similar differences in the d and f orbitals, but their

magnitudes are usually so small that are undetectable, thus only a single level is shown for orbitals d.

Spin-orbit coupling

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.