1. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2008Chapter 2Plasma 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

2. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/6/2008Chapter 2Plasma Atomic Emission Spectrometry Plasma Atomic Emission Spectrometry (AES) 1.Introduction 1.1Limitations 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.

3. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2008Chapter 2Plasma Atomic Emission Spectrometry 1. 1.2Advantages 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.

4. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/6/2006Chapter 2Plasma Atomic Emission Spectrometry 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.

5. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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 10 9 Hz is used to generate the plasma.

6. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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.

7. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/6/2006Chapter 2Plasma Atomic Emission Spectrometry 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.

8. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/06/2006Chapter 2Plasma Atomic Emission Spectrometry 1. Few examples of ionization potentials: Element1st ionization potential (eV)2nd ionization potential (eV) He24.5854.40 Ar15.7627.62 F17.4234.98 Cl13.0123.80 Se9.7521.5 Ni7.6318.15 Co7.8517.05

9. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/06/2006Chapter 2Plasma Atomic Emission Spectrometry 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.

10. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/06/2006Chapter 2Plasma Atomic Emission Spectrometry 1.

11. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.Inductively coupled plasma (ICP) 4.1ICP 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.

12. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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

13. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/3/2008Chapter 2Plasma Atomic Emission Spectrometry 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?

14. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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.

15. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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 LOAD COIL TORCH

16. From Houk ICP Course Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2004Chapter 2Plasma Atomic Emission Spectrometry YO, Y(I), Y(II) EMISSION ZONES COURTESY VARIAN

17. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 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)

18. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.2Excitation 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 (Ar m ). 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).

19. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/11/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.2.1Penning process Ar m + X → Ar + X + + e- Ar m + X → Ar + X + * + e- In this case, ionization energy plus excitation energy of the ion < the excitation energy of Ar m. Otherwise, a two-step reaction, including electron collision, should be considered: Ar m + X → Ar + X + + e- X + + e- → X + * + e- High level of excited atoms of analyte are from 1) direct excitation by Ar m or 2) a three-body recombination: Ar m + X → Ar + X* X + + 2e- → X* + e-

20. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.2.2Charge 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).

21. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.2.3Electron impact ionization e (fast) + M → M + + e (slow) + e (slow) 4.2.4Electron impact excitation e + M → M* + e 4.3.5Ion-electron radiative recombination M + + e → M* + hv

22. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.3How to measure temperature? If a monoatomic gas, X, is used, a plasma can be described by the following equilibrium: q q X = ∑X n+ + ∑n.e n=1 Where X n+ 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).

23. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2004Chapter 2Plasma Atomic Emission Spectrometry 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.

24. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 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 T kin, 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.

25. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2004Chapter 2Plasma Atomic Emission Spectrometry 1. Four temperatures used to characterize the plasma:  Excitation temperature (T exc ): A measure of the population density of the energy levels (described by Boltzmann equilibrium).  Ionization temperature (T ion ): A measure of the population density of the different ionization states (described by Saha equilibrium)  Electron temperature (T e ) is a measure of the kinetic energy of the electrons (related to the velocity of the free electrons).  Gas temperature (T g or T kinetic ) is a measure of the kinetic energy of the atoms. Generally:T g < T exc < T ion < T e

26. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2004Chapter 2Plasma Atomic Emission Spectrometry 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. N 1 /N 0 = g 1 /g 0 exp(-∆E/kT) Where N 1 = the number of the atoms in the excited state N 0 = the number of the atoms in the ground or lower state g 1 = is the number of energy levels having the same energies for the upper (excited) energy levels. g 2 = 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 -1 mol -1

27. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.4.Instrumentation 4.4.1Sample 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

28. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 4.4.1.1Nebulization 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)

29. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry  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.

30. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 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.fluid speed 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.fluidspeed http://en.wikipedia.org/wiki/Venturi

31. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. Concentric glass nebulizer Meinhard type

32. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry Spray chamber Functions: Droplet-size filter. Remove any droplets larger than 10 μm to waste. Dampen noise originating from the peristaltic pump.

33. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1.

34. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry  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!

35. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry  Thermospray nebulization Limitation: High temperature & High Pressure process (e.g. Selenite and selenate) Advantages: High transport efficiency (20-50%)

36. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1.  Direct injection nebulizer (DIN) Directly connected to microbore-HPLC

37. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 4.4.1.2Electrothermal evaporation

38. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 4.4.1.3Hydride generation

39. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1. 4.4.1.4Direct 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.

40. Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 8/28/2006Chapter 2Plasma Atomic Emission Spectrometry 1.