Dr. P. K. ManiBCKV

E – energy difference between two levels;

h – Plank’s constant, 6.626068 × 10-34 m2kg/s;

c – speed of light, 299 792 458 m/s;

λ – wavelenght, nm

Ion Emission

Atom Emission

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).

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

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.

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.

Inductively coupled plasmas

By definition, a plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electrons. (The concentrations of the two are such that the net charge approaches zero.) In the argon plasma employed for emission analyses, argon ions and electrons are the principal conducting species, although cations from the sample will also be present in lesser amounts.

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; temperatures as great as 10,000 K are encountered.

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 equilibrium:

Ar : 1s22s22p63s23px23py

23pz2

[Ne]3s23px23py

23pz2

The gas that is used to generate the plasma (plasma gas) is Argon. Like any noble gas,Argon is a mono atomic 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.

The plasma acts as reservoir of energy provided by the radio frequency field, and transfers this energy to an analyte, M.

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.

Various ionization and excitation processes have been suggested resulting from the presence of species that are obtained during the plasma generation.

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 hf field is produced by a rf generator and accelerates the electrons. These electrons will ionize the plasma gas:

Then, through the process of radiative recombination, the argon ions are recombined with electrons to lead to excited argon atoms and to a significant background:

This background is mainly produced by the process of radiative recombination in the ultraviolet (UV) region, while bremsstrahlung must also be taken into consideration in the visible part of the spectrum . Except for the resonance lines that are located at 106.7 nm and 104.8 nm, there are no argon atomic lines below 300 nm.

the kinetic energy of the projectile electron must be greater than theionization potential of the bound electron to be ionized

In addition to ionization and excitation, beta particles may also transfer energy by a process called Bremsstrahlung. As negative beta particles approach the positively charged nucleus of an atom, some are slowed down and pulled into a new direction (like the gravitational pull of the earth on a satellite). As the electrons are slowed down some energy is released in the form of X-rays called Bremsstrahlung. These X-rays are more penetrating than the beta particles and may require more shielding. Plastic is commonly used to shield beta particles because low density materials result in less production of Bremsstrahlung

Electron ionization (EI, formerly known as electron impact) is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions.

where M is the analyte molecule being ionized, e− is the electron and M+• is the resulting ion.

Radiative recombination (RR) is a non-resonant, one step recombination process in which a free electron recombines with an ion, emitting excess energy in the form of a photon:

The Penning ionization process, in which a metastable atom or molecule A* collides with another atom or molecule B, to leave A unexcited and B+ as an ion,

is one member of a class of irreversible energy conversion processes in which irreversibility is due to the loss of an electron into its continuum.

Penning ionization refers to the interaction between a gas-phase excited-state atom or molecule G* and a target molecule M resulting in the formation of a radical molecular cation M+●, an electron e−, and a neutral gas molecule G

Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom

Two laws are used to describe equilibrium in a plasma:

the Boltzmann law to describe the equilibrium between the population of the various levels within the same ionization state, including both excited and ground states, and

the Saha law to describe the equilibrium between the population of two successive ionization states.

If we consider the population nm and nk of the excited level Em and Ek, respectively, their ratio can be given by the Boltzmann law:

The Saha EquationIn 1920, Meghnad Saha derived an equation for the relative number of atoms in each ionization state. Here, just present the result:

Note:

This depends on the number density of electrons, ne. This is because as the number of free electrons increases, it is more likely that they can recombine with an atom and lower the ionization state.

The Boltzmann factor exp(-Ε i/kT) means it is more difficult to ionize atoms with high ionization potentials

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For a gas at a high enough temperature, the thermal collisions of the atoms will ionize some of the atoms. One or more of the electrons that are normally bound to the atom in orbits around the atomic nucleus will be ejected from the atom and will form an electron gas that co-exists with the gas of atomic ions and neutral atoms. This state of matter is called a plasma.

The Saha equation describes the degree of ionization of this plasma as a function of the temperature, density, and ionization energies of the atoms.

The Saha equation only holds for weakly ionized plasmas for which the Debye length is large. This means that the “screening” of the coulomb charge of ions and electrons by other ions and electrons is negligible. The subsequent lowering of the ionization potentials and the “cutoff” of the partition function is therefore also negligible.

In general, the ICP is not in true thermodynamic equilibrium because the various processes that are occurring in the plasma are individually not in equilibrium. Therefore, the ICP cannot be characterized by a single equilibrium temperature. The Saha equation can be used to make a reasonable estimate of the degree of ionization:

where ni, na, and ne are the number densities of ions, atoms, and free

electrons, respectively; m is the electron mass; Za and Zi are partition

functions; Ei is the ionization potential; and T is the temperature.

Using this equation, Houk (1986) calculated the degree of ionization of elements for a plasma temperature of 7500 K and an electron density of 10l5 cm-3.

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PLASMA FORMATION :Inductively coupled plasmas are formed by coupling energy produced by a RF generator to the plasma support gas with an electromagnetic field. The field is produced by applying an RF power (typically 700-1500 W) to an antenna (load coil) constructed from 3-mm-diameter copper tubing wrapped in a two- or three-turn 3-cm-diameter coil, positioned around the quartz torch assembly designed to configure and confine the plasma. An alternating current field is created that oscillates at the frequency of the tuned RF generator.The plasma is initiated by the addition of a few "seed” electrons, generated from the spark of a Tesla coil or a piezoelectric starter, to the flowing support gas in the vicinity of the load coil. After the plasma is initiated it is sustained by a process known as inductive coupling.

As these seed electrons are accelerated by the electromagnetic RF field, collisions with neutral gas atoms create the ionized medium of the plasma. The mean free path of accelerated electrons in atmospheric pressure argon gas is about1 μm before a collision occurs with an argon atom. These collisions produce additional electrons. This cascading effect creates and sustains the plasma.Once the gas is ionized, it is self-sustaining as long as RF power is applied to the load coil. The ICP has the appearance of an intensely bright fireball-shaped discharge.

Inductively Coupled Plasma Spectroscopy

A photograph of the ICP torch

Schematic view of an ICP.Schematic view of an ICP.Manning T.J. and Grow W.P., 1997

Induction coil

Magnetic field

Plasma Generation The ICP is generated by coupling the energy from a radio frequency

generator into a suitable gas via a magnetic field which is induced through a two- or three-turn, water -cooled copper coil.

The radiofrequency energy is normally supplied at a frequency of 27.12 MHz, delivering forward power at between 500 and 2000 W.

Two gas flows, usually argon, flow in a tangential manner through the outer tubes of a concentric, three-tube quartz torch which is placed axially in the copper coil.

Because the outer and intermediate gases flow tangentially (i.e. they swirl around as they pass through the torch), the plasma is continually revolving and has a 'weak spot' at the centre of its base, through which the inner gas flow, containing the sample, can be introduced.

When the gas is seeded with electrons, usually by means of a spark, the electrons accelerate in the magnetic field and reach energies sufficient to ionize gaseous atoms in the field.

Subsequent collisions with other gaseous atoms causes further ionization and so on, so that the plasma becomes self-sustaining. This occurs almost instantaneously.

The magnetic field causes the ions and electrons to flow in the horizontal plane of the coil, thereby heating the neutral argon by collisional energy exchange, and a hot fireball is produced.

The hottest part of the ICP has a temperature between 8000 and 10000 K, which is the temperature of the surface of the Sun, though the analytically useful region is in the tail-flame with a temperature between 5000 and 6000 K.

Inductively Coupled Plasma Source

ICP-AES: Plasma

Advantages of an ICP source

1. The analytes are confined to a narrow region.

2. The plasma provides simultaneous excitation of many elements.

3. The analyst is not limited to analytical lines involving ground state transitions but can select from first or even second ionization state lines. For the elements Ba, Be, Mg, Sr, Ti, and V, the ion lines provide the best detection limits.

4. The high temperature of the plasma ensures the complete breakdown of chemical compounds (even refractory compounds) and impedes the formation of other interfering compounds, thus virtually eliminating matrix effects.

5. The ICP torch provides a chemically inert atmosphere and an optically thin emission source.6. Excitation and emission zones are spatially separated : this results in a low background. 7. Low background, combined with a high S/N ratio of analyte emission, results in low detection limits, typically in the parts-per-billion range.

list of some of the most beneficial characteristics of the ICP source.

Additional information

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.

ICP-AES: Plasma

The main ionization processes are:

the charge-transfer ionization,

the electron-impact ionization

the Penning ionization,

while the main excitation processes for the analyte atom are:

the electron impact excitation,

And the ion–electron radiative recombination,

Ionization statesConsider an atom in an ionization state r, for example a carbon ion with only 3 electrons (C IV) when normally it would have six in its neutral state. In its ground state, 2 electrons would occupy its n = 1 state and 1 would occupy its n = 2 state. This atom can be raised to the next higher ionization state (C V) with the ejection of the latter electron if a collision or photon provides the appropriate energy, the ionization potential,

Such a transition is indicated as Transition 1 in Fig. 2. It takes the atom from the ground state of the r ionization state to the ground state of the r+1 ionization state. The ejected electron will take on any excess energy p the exciting photon may have had beyond r. The photon energy would be

Ar 1s22s22p63s23px23py

23pz2 [Ne]3s23px

23py23pz

2

Energy states of an atom in two ionizations states. The r+1 state has one less bound electron less than the r state. In the r+1 state, that electron is free.

if the final momentum of the ejected electron is p2/2m.

In Transition 2 the energy of the exciting photon would be

In Transition 1 the photon energy would be

More generally, the transition can initiate in any energy state of the original ion and terminate in any energy state of the final ion as illustrated by Transition 2 of Fig. 2. Theenergy of the exciting photon would be

if the final momentum of the ejected electron is p2/2m.

Mn 1s22s22p63s23p63d54s2

Fe 1s22s22p63s23p63d64s2

Co 1s22s22p63s23p63d74s2

Ni 1s22s22p63s23p63d84s2

Cu 1s22s22p63s23p63d104s1

Zn 1s22s22p63s23p63d104s2

d5 means

d8 means

A transition element is defined as one which has partially filled d orbitals either in the element or any of its compounds. Zinc (at the right-hand end of the d-block) always has a completely full 3d level (3d10) and so doesn't count as a transition element.

3 power sources have been employed in argon plasma spectroscopy.

(i)dc electrical source capable of maintaining a current of several amperes between electrodes immersed in a stream of argon. The second and third utilize powerful

(ii)radio-frequency and

(iii)microwave-frequency fields through which the argon flows.

Of the three, the radio-frequency, or inductively coupled plasma (ICP), source appears to offer the greatest advantage in terms of sensitivity and freedom from interference.

On the other hand, the dc plasma source (DCP) has the virtue of simplicity and lower cost.

The microwave induced plasma source (MIP) is not widely used for analysis

• If we confine ourselves to a discussion of an argon ICP then we can say that it consists mainly of the following species: