Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on...

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International UniversityUpdated on 8/28/2008 Chapter 2 Plasma Atomic Emission Spectrometry

1.References: John R. Dean, Atomic Absorption and Plasma Spectroscopy, 1997. Wiley. Steve J. Hill, Inductively Coupled Plasma Spectrometry and Its

Applications, 1999. CRC Press. Ed. Metcalfe: Atomic Absorption and Emission Spectroscopy, Analytical

Chemistry by Open leaning, 1987. A. Montaser and D.W. Golightly, Inductively Coupled Plasma in

Analytical Atomic Spetcrometry, 1987, VCH. J.W. Robinson: Atomic Spectroscopy, Second Edition, Revised and

Expended, 1996 Peter C. Uden, Element-Specifc Chromatographic Detection by Atomic

Emission Spectrometry, 1992. ACS Symposium 479. C. Vandecasteele, and C.B. Block: Modern methods for Trace Element

Determination, 1997. C. M. Barshick, et al. Inorganic Mass Spectrometry, 2000. And many others

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International University Updated on 9/6/2008 Chapter 2 Plasma Atomic Emission Spectrometry

Plasma Atomic Emission Spectrometry (AES)1. Introduction

1.1 Limitations of Flame AES

Chemical interferences (matrix interferences): the degree of the atomization of the sample in the flame is affected by the chemical nature of the sample.

Ionization interferences: the degree of ionization of some easily ionizable elements, and hence the degree of atomization, is affected by the presence of other easily ionizable elements.

Spectral interferences: the presence of spectral lines of other elements present close to the spectral line of the analyte can give misleading results.


1.1.2 Advantages of high temperature Greater concentration of emitting atoms, particularly

for those atoms with emission lines in the ultra-violet. And effects of high temperature on the various

interferences: Chemical interferences will be reduced as the temperature is raised.

E.g. the well-known suppression of calcium signals by phosphate Ionization interferences are not always severe in plasma, due to the

very high electron densities present in the plasma. The high concentration of electrons result from ionization of the argon, which is much larger than the ionization of sample component.

Spectral interferences will be increased. Since more atoms will be in very highly excited states, there will be an increase in the number and intensity of non-resonance emission transitions (i.e. transitions to lower-lying excited state rather than the ground state.


1.2. Direct current plasma (DCP)

The DCP is less widely used because (1) its poor detection limits compared to ICP; (2) ionization interferences; and (3) graphite electrodes must be

replaced every few hours.


1.3. Microwave-induced plasma (MIP)

Consists of a quartz tube surrounded by a microwave waveguide or cavity.

Microwaves produced from a microwave generator fill the cavity and cause the electrons in the plasma support gas (e.g. He) to oscillate.

The oscillating electrons collide with other atoms in the flowing gas to create and maintain a high-temperature plasma.

A spark (Tesla coil) is needed to create some initial electrons to create the plasma.

A 200 W power supply at a frequency of 2.45 x 109 Hz is used to generate the plasma.


1. Thermodynamic equilibrium is usually not attained with microwave plasma. (gas temperature is about 1000 – 3000 K)

Low atomization efficiency, greater concentrations of excited atoms (corresponding to an excitation temperature of 7000-9000 K).

The excited atoms of interest are formed in collision with metastable helium atoms. (A metastable atom is a relatively long-live excited state).

The following reactions are a simplified version of what is thought to occur in the plasma.

He* + X → He + X+ + e-

X+ + He + e- → He + X*

X* → X + hυ

An asterisk indicates an excited state. The excited atom is formed from electron-ion recombination, and so will be in a very highly excited electronic state.


1.Why Helium?

A metastable argon atom has less energy than a

metastable helium (high excitation temperature)

For non-metal atoms analysis, which need much more energy of produce excited states than metal atoms. Another way of thinking of this is that since the atoms are ionized first, then the recombination process must form highly excited atoms states.


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Few examples of ionization potentials:

Element 1st ionization potential (eV) 2nd ionization potential (eV)

He 24.58 54.40

Ar 15.76 27.62

F 17.42 34.98

Cl 13.01 23.80

Se 9.75 21.5

Ni 7.63 18.15

Co 7.85 17.05


1.Advantages of MIP over ICP and DCP: It is cheap (relatively) High excitation temperature (allows non-metals to be detected)

Limitations: Relatively poor detection limits (for non-metals). Plasma is easily quenched by water (so that the introduction of

aqueous samples is a problem). Chemical interferences can be a problem.

These problems are largely due to the low power of the plasma. There is usually insufficient plasma enthalpy to desolvate and vaporize aerosols (liquid samples) efficiently, leading to plasma instability and extinction.


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1.4. Inductively coupled plasma (ICP)

4.1 ICP generation

What is plasma?

A plasma is an electrically neutral, highly ionized gas composed of ions, electrons and neutral particles.

ICP is also known as the radio-frequency (RF) plasma. Operated at high power levels, 0.5 to 3.0 kW and a frequency of 15-50

MHz. Temperatures over 10,000 K are reached in the plasma.


1.Process Theory: The radio-frequency electrical current is

passed through a metal induction coil. The current has an associated magnetic field, with lines of force passing along the axis of a quartz tube placed inside the coil.

Ionization of the flowing argon is initiated by a spark from a Tesla coil.

Electrons are accelerated by the magnetic field to travel in circular orbits inside the quartz tube.

Energy is transferred from the electrons to the gas by collisions so that the gas heats up.

The temperatures reached produce high concentration of both excited atoms and ions.

Plasma is a type of discharge not flame


1.Why Ar?

Monoatomic element with a high ionization energy (15.76 eV), chemically inert, Consequently:

A simple spectrum in contrast to a flame where primarily molecular spectra are observed.

Has the capacity to excite and ionize most of the elements of the periodic table (but not for some non-metals).

No stable compounds are formed between argon and the analytes.

Why usually not us He?


1.Three gases (All Ar in most cases):Three gas flows along three concentric tubes.

Inner gas (or nebulizer gas, or aerosol carrier gas). Carrier gas

Ar support gas (or intermediate gas, or auxiliary gas).

Outer gas (or coolant gas or plasma gas) Prevent the quartz tube from melting. This flow is introduced tangentially. Large amounts of gas (> 10 L/min) are used in most current design.


1.Torch design:• An annular plasma is formed, allowing

effective and reproducible introduction of aerosol. A stream of gas with a relatively small cross-section can thus bore a hole without disturbing the stability of the plasma.

• The aerosol travels through a narrow axial channel, surrounded by the high temperature core. The temperature in this channel (6000-8000 K) (dark channel) is still sufficiently high to give rise to efficient desolvation, volatilization, excitation and possibly ionization of the sample component.

• The analytical measurements are made in the cooler tail plume of the plasma. The spectral background in the plume, particularly in the useful 190-300 nm region, is relatively simple, consisting mainly of argon emission lines.

NORMAL ANALYTICAL ZONE (blue)

INITIAL RAD. ZONE (red)

INDUCTION REGION

OUTER GAS FLOW

AEROSOL GAS FLOW INTO AXIAL CHANNEL

LOADCOIL

TORCH

From Houk ICP Course


YO, Y(I), Y(II) EMISSION ZONES COURTESY VARIAN


1.By the time the sample atoms reached the observation point: Temperature: 4000-8000 k Flow rate: 15-20 m/s Residence time: about 2 ms

These times and temperatures are roughly two or three times greater than those found in the hottest flames (acetylene/nitrous oxide) employed in flame methods

As a consequence: More complete atomization Fewer chemical interference problems Ionization interference effects are small (high electron concentration from

Ar ionization). Small Self-absorption and self-reversal effects (atomization occurs in a

chemically inert environment, enhancing the lifetime of the analyte by preventing oxide formation and the temperature cross section of the plasma is relatively uniform; as a consequence)

Large linear range (over several orders of magnitude)


1.4.2 Excitation and ionization mechanisms

Ionization the plasma gas (Ar) by the accelerated electrons :

e + Ar → Ar+ + e + e

Radiative recombination leads to excited Ar atoms and a significant background (e.g. two resonance lines 106.7 and 104.8 nm ):

Ar+ + e → Ar* + hv

In the plasma are argon ions (Ar+), electrons (e), excited argon atoms (Ar*), and with special case the metastable atoms (Arm).

Roles of source in AES: Atomization of the sample to obtain free atoms, usually in the

ground state (slow process). Partial ionization of the analyte atoms, and the excitation of the

atoms and ions to higher-energy states (fast process).


1.4.2.1 Penning processArm + X → Ar + X+ + e-

Arm + X → Ar + X+* + e-

In this case, ionization energy plus excitation energy of the ion < the excitation energy of Arm.

Otherwise, a two-step reaction, including electron collision, should be considered:

Arm + X → Ar + X+ + e-

X+ + e- → X+* + e-

High level of excited atoms of analyte are from 1) direct excitation by Arm or 2) a three-body recombination:

Arm + X → Ar + X*

X+ + 2e- → X* + e-


1.4.2.2 Charge transfer ionization

Charge transfer has attracted attention as a possible reaction for analyte excitation and ionization:

Ar+ + X → Ar + X+

Ar+ + X → Ar + X+*

Ionization (or ionization plus excitation) potential of X must be lower than that of Ar (15.76 eV).


1.4.2.3 Electron impact ionization

e (fast) + M → M+ + e (slow) + e (slow)

4.2.4 Electron impact excitation

e + M → M* + e

4.3.5 Ion-electron radiative recombination M+ + e → M* + hv


1.4.3 How to measure temperature?

If a monoatomic gas, X, is used, a plasma can be described by the following equilibrium:

q q

X = ∑Xn+ + ∑n.e

n=1 n=1

Where Xn+ is an ion with n charges and e is the electron. When argon is used as plasma gas, the only Ar ions that are observed are singly ionized ions (n=1).


1. How do we measure temperature?

Measure ICP Plasma temperature is not very easy because

Temperature is too high so that no thermometer can be used

Plasma is NOT in thermodynamic equilibrium conditions.


1.Measuring Plasma Temperature (Cont’d):

Plasmas are composed of species with large differences in mass: electrons and heavy Ar particles.

Two different kinetic energies Te and Tkin, the kinetic temperature of the electrons and the heavy particles, respectively.

Poor efficiency of the energy transfer between the electrons and the heavy particles.

The processes of excitation, ionization and radiation are undergoing so fast that no enough time for those sub-systems to reach equilibrium.


1. Four temperatures used to characterize the plasma:

Excitation temperature (Texc): A measure of the population density of the energy levels (described by Boltzmann equilibrium).

Ionization temperature (Tion): A measure of the population density of the different ionization states (described by Saha equilibrium)

Electron temperature (Te) is a measure of the kinetic energy of the electrons (related to the velocity of the free electrons).

Gas temperature (Tg or Tkinetic) is a measure of the kinetic energy of the atoms.

Generally: Tg < Texc < Tion < Te


1. In each case, temperatures range from 7000 – 10,000 K. Alternative approaches must be used to measure temperature.

Spectroscopic methods. E.g. the excitation temperature can be measured by means of the Boltzmann equation.

N1/N0 = g1/g0 exp(-∆E/kT)

Where

N1 = the number of the atoms in the excited state

N0 = the number of the atoms in the ground or lower state

g1 = is the number of energy levels having the same energies for the upper (excited) energy levels.

g2 = is the number of energy levels having the same energies for the lower (ground) energy levels.

∆E = is the difference in energy between the lower and upper energy levels

k = is the Boltzmann constant (8.314 J K-1mol-1


1.4.4. Instrumentation

4.4.1 Sample Introduction

Normally samples are introduced as solutions, but direct introduction of solid and gases is also possible.

Plasma AES or MS can be used as chromatography detector


4.4.1.1 Nebulization

The basic function of a nebulizer is to transform a stream of liquid into a cloud of droplets.

The smaller the droplets formed, the more sample reaches the plasma (typically less than 2 μm)


Pneumatic nebulizer

• The most common type of nebulization. Several types of Pneumatic nebulizer are used: the concentric nebulizer (Meinhard nebulizer); The cross-flow nebulizer; the Babington nebulizer; and the fritted nebulizer.

• The sample solution is supplied to the nebulizer either by forced feed (e.g. with a peristaltic pump) or by aspiration resulting from the Venturi effect. Only a small portion of the introduced liquid is nebulized, the larger fraction flowing to waste.


• A Venturi meter is shown in a diagram, the pressure in "1" conditions is higher than "2", and the relationship between the fluid speed in "2" and "1" respectively, is the same as for pressure.

A Venturi meter is shown in a diagram, the pressure in "1" conditions is higher than "2", and the relationship between the fluid speed in "2" and "1" respectively, is the same as for pressure. http://en.wikipedia.org/wiki/Venturi

http://en.wikipedia.org/wiki/Fluid

http://en.wikipedia.org/wiki/Speed

http://en.wikipedia.org/wiki/Fluid

http://en.wikipedia.org/wiki/Speed


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Concentric glass nebulizer Meinhard type


Spray chamber

Functions:

Droplet-size filter. Remove any droplets larger than 10 μm to waste.

Dampen noise originating from the peristaltic pump.


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Ultrasonic nebulizer

Transducer vibrating at a frequency of up at 10 MHz.

Fine aerosol produced

Heated

Refrigerated (condense out the solvent)

Fine & dry particles

Less cooling effect on plasma!


Thermospray nebulization

Limitation:High temperature &High Pressure process(e.g. Selenite and selenate)

Advantages:High transport efficiency (20-50%)


1. Direct injection nebulizer (DIN)

Directly connected to microbore-HPLC


4.4.1.2 Electrothermal evaporation


4.4.1.3 Hydride generation


1.4.4.1.4 Direct solid sampling• Generally, Arc, spark erosion, or laser ablation, are used to sample

these solid samples.

• Laser ablation is also used as a sampling method in ICP-MS.


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