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Atomic emission spectroscopy
- 1. Atomic Emission Spectroscopy MAHENDRA G S M.Pharm
- 2. Introduction • Technique is also known as OPTICAL EMISSION SPECTROSCOPY (OES) • The study of radiation emitted by excited atoms and monatomic ions • Relaxation of atoms in the excited state results in emission of light • Produces line spectra in the UV-VIS and the vacuum UV regions
- 3. Introduction • Used for qualitative identification of elements present in the sample • Also for quantitative analysis from ppm levels to percent • Multielement technique • Can be used to determine metals, metalloids, and some nonmetals simultaneously • Emission wavelength and energy are related by ΔE = hc/λ
- 4. Introduction • Does not require light source • Excited atoms in the flame emit light that reaches the detector (luminescence) Techniques Based on Excitation Source • Flame Photometry (flame OES) • Furnace (Electrical Excitation) • Inductively Coupled Plasma (ICP)
- 5. Principle Atomic emission spectroscopy is also an analytical technique that is used to measure the concentrations of elements in samples. It uses quantitative measurement of the emission from excited atoms to determine analyte concentration.
- 6. The analyte atoms are promoted to a higher energy level by the sufficient energy that is provided by the high temperature of the atomization sources . The excited atoms decay back to lower levels by emitting light. Emissions are passed through monochromators or filters prior to detection by photomultiplier tubes.
- 7. The instrumentation of atomic emission spectroscopy is the same as that of atomic absorption, but without the presence of a radiation source . In atomic Emission the sample is atomized and the analyte atoms are excited to higher energy levels.
- 8. Schematic Diagram of an Atomic Emission spectrometer
- 9. Sources • Flame – still used for metal atoms • Alternating current arc (AC arc) • Direct current (DC arc) • Alternating current spark (AC spark) • Direct current Plasma • Microwave Induced Plasma • Inductively Coupled Plasma – the most important technique
- 10. Flame • It is used for those molecules which do not require very high temperatures for excitation into atoms during quantitative analysis. • Sample in solution form and sprayed into the flame of a burner. Fuel Oxidant Flame temp (O C) H2 O2 2800 H2 Air 2100 Acetylene O2 3000 Acetylene Air 2200
- 11. Alternating current spark (AC spark) • High voltage transfer 10 – 50 KV across two electrodes gives a spark. • The use of condenser increases the current. • Brilliant spark is obtained at 0.005 microfarad capacity to the capacitor. Advantages: • Reproducible • Less material is consumed • High concentration solution can be used • Heating effect is less which is useful for analysis of low melting materials. • No interference
- 12. AC Arc • It employs potential difference of 1000 volts or more. • The arc is drawn at a distance of 0.5 – 3 mm. Advantages: • Best source for qualitative analysis • Stable • Reproducible
- 13. DC Arc • The DC arc is generated with a potential gradient of 50 – 300 volts. • The current used is about 1 to 300 amps and emission is due to neutral atoms. • The temperature in the arc stream ranges from 2273 – 5273 K. • Used for identification and determination of elements present in very small concentrations.
- 14. Three types of high-temperature plasma Plasma • an electrically conducting gaseous mixture containing significant concentrations of cations and electrons. • The inductively coupled plasma (ICP) • The direct current plasma (DCP) • The microwave induced plasma (MIP) The most important of these plasmas is the inductively coupled plasma (ICP).
- 15. Sources Advantages of plasma • Simultaneous multi-element Analysis – saves sample amount • Some non-metal determination (Cl, Br, I, and S) • Concentration range of several decades (105 – 106 ) Disadvantages of plasma • very complex Spectra - hundreds to thousands of lines • High resolution and expensive optical components • Expensive instruments, highly trained personnel required
- 16. The Direct Current Plasma Technique • The direct current plasma is created by the electronic release of the two electrodes. • The samples are placed on an electrode. • In the technique solid samples are placed near the discharge to encourage the emission of the sample by the converted gas atoms.
- 17. Inductively Coupled Plasma (ICP) • Plasma generated in a device called a Torch • Torch up to 1" diameter • Argon cools outer tube, defines plasma shape • Rapid tangential flow of argon cools outer quartz and centers plasma • Rate of Argon Consumption 5 - 20 L/Min • Radio frequency (RF) generator 27 or 41 MHz up to 2 kW • Telsa coil produces initiation spark • Ions and electrons interact with magnetic field and begin to flow in a circular motion. • Resistance to movement (collisions of electrons and cations with ambient gas) leads to ohmic heating. • Sample introduction is analogous to atomic absorption.
- 18. A typical inductively coupled plasma source called a torch
- 19. Inductively-coupled plasma atomic emission spectrometer
- 20. Arc and Spark AES •Arc and Spark Excitation Sources • Limited to semi-quantitative/qualitative analysis (arc flicker) • Usually performed on solids • Largely displaced by plasma-AES •Electric current flowing between two electrodes
- 21. Sample holder •The function of sample holder is to introduce the sample into the electrical discharge. •Two types, for • Solid samples • Liquid samples
- 24. Detectors • For Quantitative analysis photographic plate is used on which all the emission lines from the samples are recorded. • This photograph of the emission spectrum helps in the measurement of wavelength of radiation lines. • From these lines emitting elements can be identified. • The instrument with photographic recording is called spectrograph and the one using photoelectric device as spectrometer.
- 25. Advantages of Photographic plates over Photomultiplier tubes • Large number of spectral lines can be recorded. • Photographic plates provides a permanent record of the spectrum. • Emission spectrum can be integrated by a photographic emulsion for long time. • Photographic emulsions are preferred because of their high sensitivity in UV and visible region.
- 26. Disadvantages of Photographic plates over Photomultiplier tubes • Photographic plates do not show quick response to spectral lines and their interpretation is not easy. • In Photographic detection, controlled photographic development is required. This requires much time and increases the risk of error.
- 27. WAVELENGTH SELECTION AND DETECTION FOR AES •Arc and spark instruments normally contain non scanning monochromators. •Either a series of slits is cut in the focal plane of the monochromator and a photomultiplier tube is placed behind each slit that corresponds to the wavelength of a line that is to be measured, or one or more photographic plates or pieces of film are placed on the focal of the monochromator.
- 28. QUALITATIVE ANALYSIS WITH ARC AND SPARK AES •Qualitative analysis is performed by comparing the wavelengths of the intense lines from the sample with those for known elements. •It is generally agreed that at least three intense lines of a sample must be matched within a known element in order to conclude that the sample contains the element
- 29. QUANTITATIVE ANALYSIS WITH ARC AND SPARK AES • Regardless of the type of detection used for the assay, the precision of the results can be improved by matrix-matching the standards with the sample. • Use of the internal-standard method also improves precision. • Usually a working curve is prepared by plotting the ratio or logarithm of the ratio of intensity of the standard's line to the internal standard's line as a function of the logarithm of the concentration of the standard. • The corresponding ratio for the analyte is obtained and the concentration determined from the working curve.
- 30. AES WITH ELECTRICAL DISCHARGES • An electrical discharge between two electrodes can be used to atomize or ionize a sample and to excite the resulting atoms or ions. • The sample can be contained in or coated on one or both of the electrodes or the electrode(s) can be made from the analyte. • The second electrode which does not contain the analyte is the counter electrode. • Electrical discharges can be used to assay nearly all metals and metalloids. • Approximately 72 elements can be determined using electrical discharges.
- 31. AES WITH ELECTRICAL DISCHARGES • For analyses of solutions and gases the use of plasma is generally preferred although electrical discharge can be used. • Solid samples are usually assayed with the aid of electrical discharges. • Typically it is possible to assay about 30 elements in a single sample in less than half an hour using electrical discharges. • To record the spectrum of a sample normally requires less than a minute.
- 32. ELECTRODES FOR AES • The electrodes that are used for the various forms of AES are usually constructed from graphite. • Graphite is a good choice for an electrode material because it is conductive and does not spectrally interfere with the assay of most metals and metalloids. • In special cases metallic electrodes (often copper) or electrodes that are fabricated from the analyte are used. • Regardless of the type of electrodes that are used, a portion of each of the electrodes is consumed during the electrical discharge. • The electrode material should be chosen so as not to spectrally interference during the analysis.
- 33. Sample introduction • Nebulizer • Electrothermal vaporizer
- 34. Nebulizer •convert solution to fine spray or aerosol •Ultrasonic nebulizer • uses ultrasound waves to "boil" solution flowing across disc •Pneumatic nebulizer • uses high pressure gas to entrain solution
- 35. Electro-thermal vaporizer ETV • electric current rapidly heats crucible containing sample • sample carried to atomizer by gas (Ar, He) • only for introduction, not atomization
- 36. Plasma structure •Brilliant white core • Ar continuum and lines •Flame-like tail • up to 2 cm •Transparent region • where measurements are made (no continuum)
- 37. Plasma characteristics • Hotter than flame (10,000 K) - more complete atomization/ excitation • Atomized in "inert" atmosphere • Ionization interference small due to high density of electrons • Sample atoms reside in plasma for ~2 msec and • Plasma chemically inert, little oxide formation • Temperature profile quite stable and uniform.
- 38. Direct Current plasma (DC) • First reported in 1920s • DC current (10-15 A) flows between anodes and cathode • Plasma core at 10,000 K, viewing region at ~5,000 K • Simpler, less Ar than ICP - less expensive • Less sensitive than ICP • Should replace the carbon anodes in several hours
- 39. Atomic Emission Spectrometer • May be >1,000 visible lines (<1 Å) on continuum • Need • higher resolution (<0.1 Å) • higher throughput • low stray light • wide dynamic range (>1,000,000) • precise and accurate wavelength calibration/intensities • stability • computer controlled
- 40. AES instrument types Three instrument types: • Sequential (scanning and slew-scanning) • Multichannel - Measure intensities of a large number of elements (50-60) simultaneously • Fourier transform FT-AES
- 41. Desirable properties of an AE spectrometer
- 42. Sequential vs. multichannel Sequential instrument • PMT moved behind aperture plate, • 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 • charge-injection device/charge coupled device • characteristics • Expensive ( > $80,000) • Faster
- 43. Sequential monochromator Slew-scan spectrometers • even with many lines, much spectrum contains no information • rapidly scanned (slewed) across blank regions (between atomic emission lines) • From 165 nm to 800 nm in 20 msec • slowly scanned across lines • 0.01 to 0.001 nm increment • computer control/pre-selected lines to scan
- 44. Slew scan spectrometer •Two slew-scan gratings •Two PMTs for Visible & UV •Holographic grating
- 45. Scanning echelle spectrometer • PMT is moved to monitor signal from slotted aperture. • About 300 photo-etched slits • 1 second for moving one slit • Can be used as multi channel spectrometer • Mostly with DC plasma source
- 46. Multichannel photoelectric spectrometer •multichannel PMT instruments • for rapid determinations (<20 lines) but not versatile • For routine analysis of solids •metals, alloys, ores, rocks, soils • portable instruments •Multichannel charge transfer devices • Recently on the market • Orignally developed for plasma sources
- 47. Multichannel polychromator AES • Rowland circle • Quantitative determination of more than 20 elements within 5 minutes
- 48. Applications of AES • Qualitative analysis is done using AES in the same manner in which it is done using FES. The spectrum of the analyte is obtained and compared with the atomic and ionic spectra of possible elements in the analyte. Generally an element is considered to be in the analyte if at least three intense lines can be matched with those from the spectrum of a known element. • Quantitative analysis with a plasma can be done using either an atomic or an ionic line. Ionic lines are chosen for most analyses because they are usually more intense at the temperatures of plasmas than are the atomic lines.
- 49. Applications of AES • In practice ~60 elements detectable • 10 ppb range most metals • In determining the impurities of Ni, Mn, Cu, Al etc., in iron and steel in metallurgical processes. The percentage determined is 0.001% in iron to 30 in steel. • Lubricating oils can be analysed for Ni, Fe, Mn etc., • Solid samples and animal tissues have been analysed for several elements including K, Na, Ca, Zn, Ni, etc., • To detect 40 elements in plants and soils, thus metal deficiency in plants and soils can be diagnosed.
- 50. Detection power of ICP-AES
- 51. ICP/OES INTERFERENCES • Spectral interferences: • caused by background emission from continuous or recombination phenomena, • stray light from the line emission of high concentration elements, • overlap of a spectral line from another element, • or unresolved overlap of molecular band spectra. • Corrections • Background emission and stray light compensated for by subtracting background emission determined by measurements adjacent to the analyte wavelength peak. • Correction factors can be applied if interference is well characterized • Inter-element corrections will vary for the same emission line among instruments because of differences in resolution, as determined by the grating, the entrance and exit slit widths, and by the order of dispersion.
- 52. Physical interferences of ICP • cause • effects associated with the sample nebulization and transport processes. • Changes in viscosity and surface tension can cause significant inaccuracies, • especially in samples containing high dissolved solids • or high acid concentrations. • Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization. • Reduction • by diluting the sample • or by using a peristaltic pump, • by using an internal standard • or by using a high solids nebulizer.
- 53. Interferences of ICP •Chemical interferences: • include molecular compound formation, ionization effects, and solute vaporization effects. • Normally, these effects are not significant with the ICP technique. • Chemical interferences are highly dependent on matrix type and the specific analyte element.
- 54. Memory interferences • When analytes in a previous sample contribute to the signals measured in a new sample. • Memory effects can result • from sample deposition on the uptake tubing to the nebulizer • from the build up of sample material in the plasma torch and spray chamber. • The site where these effects occur is dependent on the element and can be minimized • by flushing the system with a rinse blank between samples. • High salt concentrations can cause analyte signal suppressions and confuse interference tests.
- 55. Typical Calibration ICP curves
- 56. Calibration curves of ICP-AES
- 57. Advantages of AES • Highly specific • Sensitive (low concentration 0.0001%) • Metalloids (arsenic, silicon, selenium) have been identified by this technique • Samples in solid or liquid state and rarely gas samples can be used • Techniques requires minimum sample preparation • No preliminary treatment of sample is required • Spectra can be taken simultaneously for more than 2 elements and no separation is required. (1 – 10 mg) • Rapid results, if automated, times required is 30 – 60 seconds
- 58. Disadvantages of AES •Equipment is costly and experience is required for handling and interpretation of spectra. •Recording is done on a photographic plate which takes sometime to develop, print and interpret the results. •The spectrograph is essentially a comparator. For quantitative analysis, standards of similar composition are required. •Radiation intensities are not always reproducible •Method limited to the analysis of elements
- 59. Comparison Between Atomic Absorption and Emission Spectroscopy Absorption - Measure trace metal concentrations in complex matrices . - Atomic absorption depends upon the number of ground state atoms . Emission - Measure trace metal concentrations in complex matrices . - Atomic emission depends upon the number of excited atoms .
- 60. - It measures the radiation absorbed by the ground state atoms. - Presence of a light source ( HCL ) . - The temperature in the atomizer is adjusted to atomize the analyte atoms in the ground state only. - It measures the radiation emitted by the excited atoms . - Absence of the light source . - The temperature in the atomizer is big enough to atomize the analyte atoms and excite them to a higher energy level.