Atomic Emission Spectroscopy

1. Atomic Emission Spectroscopy Arcs, Sparks And Plasma

2. Atomic Emission Spectroscopy )AES) - (AES), in contrast to AAS, uses the very high temperatures of atomization sources to excite atoms. • Thus excluding the need for lamp sources. Emission sources, which are routinely used in AES: • plasma, 2) arcs and 3) sparks, 4) flames. We will study the different types of emission sources, their operational principles, features, and operational characteristics. Finally, instrumental designs and applications of emission methods will be discussed.

3. Plasma Sources The term “plasma” is defined as a homogeneous mixture of (gaseous atoms, ions and electrons) at very high temperatures. Two types of plasma atomic emission sources are frequently used: • Inductively coupled plasma (ICP). • Direct current plasma (DCP).

5. Inductively Coupled Plasma (ICP) A typical ICP consists: • Three concentric quartz tubes (متداخلات بنفس المحور) through which streams of argon gas flow at a rate in the range from 5-20 L/min. • The outer tube is about 2.5 cm (1 inch) in diameter and the top of this tube is surrounded by a radiofrequency powered induction coil (RF) producing a power of about 2 kW at a frequency in the range from 27-41 MHz. • This coil produces a strong magnetic field as well.

6. - Ionization of flowing argon is achieved by a spark. - The ionized argon interacts with the strong magnetic field and is thus forced to move within the vicinity of the induction coil at a very high speed. • A very high temperature is obtained as a result of the very high resistance experienced by circulating argon (collisions of e- and cations with ambient gas) (ohmic heating). • Why Ar used??: 1- inert 2- few emission lines

7. - The top of the quartz tube will experience very high temperatures and should, therefore, be isolated and cooled. Cooling: This can be accomplished by passing argon tangentially around the walls of the tube. A schematic of an ICP (usually called a torch plasma) is shown below:

8. Approach spark or arc to ionize Ar and so Ar+ ions circulate fatly and suffers from resistance so temperature increases Current passes in the coil and form MF  to the current in the coil

9. Inductively Coupled Plasma Torch Radio frequency induction coil 27-41 MHz 0.5-2KW Plasma torch F = 2.5 cm Tangential flow isolates the torch from plasma Plasma argon Sample argon Argon flow rate = 5-20 L/min Tangential argon plasma support flow

10. The torch is formed as a result of the argon emission at the very high temperature of the plasma. The temperature gradients in the ICP torch can be pictured in the following graphics:

12. the Radio Frequency generator • the Load coil • the Torch Energy Source

13. Problems: • Needs very large amount of Ar. • Needs large amount of Energy • Main Advantage: • Pure Atomization Cross flow nebulizer

14. Viewing region for working

15. The viewing region: Used in elemental analysis is usually about 6000 oC, which is about 1.5-2.5 cm above the top of the tube. • High cost of the ICP torch: Because Argon consumption is relatively high which makes the running. • Argon is a unique inert gas for plasma torches: Since it has few emission lines. This decreases possibility of interferences with other analyte lines.

16. Sample Introduction Several methods for sample introduction: The most widely used is: The nebulization of an analyte solution into the plasma. However, other methods, as described earlier, are fine where vapors of analyte molecules or atom from electrothermal or ablation devices can be driven into the torch for complete atomization and excitation. For your convenience, sample introduction methods are summarized here again:

17. Samples in Solution Pneumatic Nebulizers • Samples in solution are usually easily introduced into the atomizer by a simple nebulization, aspiration, process. • Nebulization converts the solution into an aerosol of very fine droplets using a jet of compressed gas. • The flow of gas carries the aerosol droplets to the atomization chamber or region.

19. Ultrasonic Nebulizers • In this case samples are pumped onto the surface of a piezoelectric crystal that vibrates in the kHz to MHz range. • Such vibrations convert samples into homogeneous aerosols that can be driven into atomizers. • Ultrasonic nebulization is preferred over pneumatic nebulization since finer droplets and more homogeneous aerosols are usually achieved. • However, most instruments use pneumatic nebulization for convenience.

20. Electrothermal Vaporization (Only for sample introd. Not for atomization) • An accurately measured quantity of sample (few mL) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. • Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. • The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation.

21. Electrothermal Vaporization To ICP Sample Graphite rod Heater power Water coolant Argon inlet

22. Hydride Generation Techniques • Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead can be vaporized by converting them to volatile hydrides by addition of sodium borohydride. • Volatile hydrides are then swept into the plasma by a stream of an inert gas.

24. Introduction of Solid Samples A variety of techniques were used to introduce solid samples into atomizers. These include: 1. Conductive Samples • If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. • Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. • Upon drying, this solid composite can be used as an electrode. • The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are swept into the plasma by argon flow.

25. Laser Ablation • Sufficient energy from a focused intense laser will interact with the surface of samples (in a similar manner like arcs and sparks) resulting in ablation. • The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation. • Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples.

26. The Glow Discharge Technique The technique is used for sample introduction and atomization as well. • The electrodes are kept at a 250 to 1000 V DC. • This high potential is sufficient to cause ionization of argon, which will be accelerated to the cathode where the sample is introduced. • Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called sputtering. • Samples should thus be conductive to use the technique of glow discharge. • The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation by flowing argon. • However, nonconductive samples were reported to be atomized by this technique where they were mixed with a conductor material like graphite or powdered copper.

27. Plasma Appearance and Spectra • A plasma torch looks very much like a flame but with a very intense nontransparent brilliant لامع white color at the core (less than 1 cm above the top of the tube). • In the region from 1-3 cm above the top of the tube, the plasma becomes transparent. • The temperatures used are at least two to three orders of magnitude higher than that achieved by flames which may suggest efficient atomization and fewer chemical interferences.

28. Ionization in plasma is not a problem: It is may be thought to be a problem due to the very high temperatures But fortunately: the large electron flux from the ionization of argon will suppress ionization of all species.

29. 2) The Direct Current Plasma (DCP) • The DCP is composed of three electrodes arranged in an inverted Y configuration. • A tungsten cathode resides at the top arm of the inverted Y. • The lower two arms are occupied by two graphite anodes. • Argon flows from the two anode blocks and plasma is obtained by momentarily bringing the cathode in contact with the anodes. • Argon ionizes and a high current passes through the cathode and anodes.

30. It is this current which ionizes more argon and sustains the current indefinitelyلأجل غير مسمى . • Samples are aspiratedيسحب into the vicinity of the electrodes (at the center of the inverted Y) where the temperature is about 5000 oC. • DCP sources usually have fewer lines than ICP sources, require less argon/hour, and have lower sensitivities than ICP sources. • In addition, the graphite electrodes tend to decay with continuous use and should thus be frequently exchanged. A schematic of a DCP source is shown below:

33. DCP advantages: • Less argon consumption about 1/3 the ICP and less E of power supply. • Simpler instrumental requirements. • less spectral line interference (lower atomization temp. about 5000 0C). However: ICP sources are more convenient to work with: - ICP is free from frequent consumables (like the anodes in DCP’s which need to be frequently changed) - More sensitive than DCP sources.

34. Advantages of Plasma Sources • No oxide formation as a result of two factors including: • Very high temperature • Inert environment inside the plasma (no oxygen) 2. Minimum chemical interferences (no or few ionization) e’s from the ionized Ar suppress ioniztion. 3. Minimum spectral interferences except for higher possibility of spectral line interference due to exceedingly large number of emission lines (because of high temperature).

35. 4. Uniform temperature which results in precise determinations 5. No self-absorption is observed which extends the linear dynamic range to higher concentrations 6. No need for a separate lamp for each element 7. Easily adaptable to multichannel analysis (simultaneous measurements of many elements).

36. Plasma Emission Instruments Three classes of plasma emission instruments can be presented including: 1. Sequential instruments قياس تتابعي In this class of instruments a single channel detector is used: • The signal for each element is read using the specific wavelength for each element sequentially. • each element is measured after the another. Two types of sequential instruments are available:

37. Linear sequential scan instruments: بنفس السرعة The wavelength is linearly changed with time. Therefore, the grating is driven by a single speed during an analysis of interest. b. Slew scan instruments: The monochromator is preset to provide specific wavelengths: • moving very fast in between wavelengths. • while moving slowly at the specific wavelengths. Therefore, a two-speed motor driving the grating is thus used.

38. Radial مقابل vs. and axial عرضي Viewing Radial – traditional side view, better for concentrated samples. Axial – direct view into plasma, lower sensitivity, shifts detection range lower.

39. Much more light available: • Sometimes called a “Horizontal Plasma” • This gives you the opportunity to achieve Lower Detection Limits than Radial Plasma. • Useful analysis in many different sample matrices • Excellent Detection Limits • Disadvantages: • Has certain limitations Matrix Interferences • Na and K can have problems The solution complicates sample preparation Axial view

40. Sometimes called “Vertical” or “Side-On” Plasma • Accommodates all ICP matrices • Detection Limits similar to Flame AA Better for Refractory Si, Ti, B, W, Mo • Can analyze S, P and Halogens • Advantages: • Robust, fewer interferences. • Wide dynamic range, best sensitivity Radial view

41. Detector PMT Filter wheel, to remove orders of radiation

42. Sequential vs. multichannel • Sequential instrument • PMT moved behind aperture plate,(slits found for elements at their  at the focal plane, so PMT moves to slits according to line of each element at each slit). • or grating + prism moved to focus new l on exit slit • Pre-configured exit slits to detect up to 20 lines, slew scan • Characteristics • Cheaper • Slower • Multichannel instrument • Polychromators (not monochromator) (multiple PMT's) • Array-based system(1D or 2 D) - FT Instruments • Charge-injection device/charge coupled device • characteristics • Expensive ( > $80,000-150,000) • Faster

43. Slew scan spectrometer • Two slew-scan gratings • Two PMTs for VIS and UV • Most use holographic (3-D) grating

44. Slew Scan Spectrometer Exit slit Composite grating Mirror Filter wheel Photomultiplier tubes Motorized observation height Plasma torch Mirrors Hg lamp Entrance slit

45. 2. Multichannel Instruments This class of instruments is also referred to as simultaneous instruments in which all signals are reported at the same time using two types of configurations:

46. a. Polychromators (do not confused with monochromator) Multiple detectors: each measure 1  • Usually photomultiplier tubes are used. • Beams of radiation emerging from the grating are guided to exit slits (each representing the wavelength of a specific element) are focused at several PMTs for detection. • Detection, thus, takes place simultaneously

47. Detectors Grating Many slits: at each a detector present simultaneous measuring

48. Grating Many PMT for the elements to be analyzed Radio frequency generator Measuring electronics Dedicated computer Instrument control electronics Schematic of an ICP polychromator

49. b. Array-based systems • This multichannel type instrument uses a multichannel detector like a charge injection device or a charge-coupled device. • Diffracted beams from a grating pass through a prism where further resolution of diffracted beams takes place by a prism. • The prism will disperse the orders of each diffracted beam. • The multichannel detector can also be a linear photodiode array as in the figure below:

50. Diode Array Detector Grating PDA Problems: Spectral line interferences should be eliminated; the detector need to be very small to prevent more than one line to hit any detector