Introduction Basic Schematic Atomizer Wavelength Selector Detector Scanning instruments can detect multiple elements. Many lines detected so sometimes it is a quantitatively difficult method. Source can be flame, but more commonly plasma because it is much hotter.
Each element emit its own characteristics line spectrum Quantitative analysis can be performed here by observing what are emitted & comparing these with various standards. Detector permits qualitative as well as quantitative analysis Wavelength of emitted radiation indicates what element is present and the radiation intensity indicates how much of the element is present
Intensity of the emitted light increase with concentration Relationship between intensity and concentration is usually linearI I = kc Unknown concentration can be detected by comparison with one or a series of standards in the same manner for the molecular techniques c
Types of Atomizer Flame Plasma Arc and spark
Process Sample is sprayed by the nebulizer into the burner. Carried into the flame Atomized & excited The emission from the excited atoms passes into the monochromator where the selected wavelength is passed through for measurement. Intensity of the emitted wavelength is measured by the detection system & indicated on the readout/computer.
Relationship Between AA and FES Flame Emission it measures the radiation emitted by the excited atoms that is related to concentration. Atomic Absorption it measures the radiation absorbed by the unexcited atoms that are determined. Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame. The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.
Flame Emission Spectroscopy Flame Emission Spectroscopy is based upon those particles that are electronically excited in the medium. The Function of Flame To convert the constituents of liquid sample into the vapor state. To decompose the constituents into atoms or simple molecules: M+ + e- (from flame) M + hv To electronically excite a fraction of the resulting atomic or molecular species M M*
Interference INTERFERENCES Spectral Chemical interference interference NOTE: same interference which occur in AAS
Comparison btw AAS & AES (Based on Flame) Flame Atomic Flame Atomic Absorption EmissionProcess measured Absorption (light Emission (light absorbed by emitted by excited unexcited atoms in atoms in a flame) flame)Use of flame Atomization Atomization & excitationInstrumentation Light source No light sourceBeer’s Law Applicable Not applicableData obtained A vs c I vs c
2. Plasma Plasma – highly ionized, electrically neutral gaseous mixture of cations and electrons that approaches temperature 10, 000 K. There are three types of plasma sources: a) Inductively coupled plasma (ICP) b) Direct current plasma (DCP) c) Microwave induced plasma (MIP) ICP is the most common plasma source.
Inductively Coupled Plasma (ICP) Constructed of three concentric quartz tube. RF current passes through the water-cooled Cu coil, which induces a magnetic field. A spark generates argon ions which are held in the magnetic field and collide with other argon atoms to produce more ions. Argon in outer tube swirls to keep plasma above the tube. The heat is produced due to the formation of argon ions.
Inductively Coupled Plasma (ICP) Plasma Appearance a. Excitation Region The bright, white, donut shaped region at the top of the torch. Radiation from this region is a continuum with the argon line spectrum superimposed. Temperature: 8000 – 10 000 K b. Observation Region The flame shaped region above the torch with temperatures 1000 – 8000 K. The spectrum consists of emission lines from the analyte along with many lines from ions in the torch.
Inductively Coupled Plasma (ICP) 1. Sample Introduction a. Liquid Sample - Nebulizer similar to FAAS - Sample nebulized in a stream of argon with a flow rate of 0.3 – 1.5 L/min. - Sample aerosol enters the plasma at the base through the central tube. b. Solid Samples - Sample atomized by electrothermal atomization a and carried into the plasma by a flow of argon gas.
Advantage of ICP-AES overFlame AES 1. Temperature is two to three times higher than in a flame or furnace, which results in higher atomization and excitation efficiencies. 2. There is little chemical interference. 3. Atomization in the inert (argon) atmosphere minimizes oxidation of the analyte. 4. Short optical path length minimizes the probability of self-absorption by argon atoms in the plasma. 5. Linear calibration curves can cover up to five orders of magnitude.
ICP-AES over Flame AES Much lower detection limit because: Higher temperature with the plasma will increase the population of excited state atoms. The plasma environment is relatively chemically inert due to the higher population of electrons which will minimize the interference of ionization.
AAS and AES Both methods use atomization of a sample and therefore determine the concentrations of elements. For AAS, absorption of radiation of a defined wavelength is passed through a sample and the absorption of the radiation is determined. The absorption is defined by the electronic transition for a given element and is specific for a given element. The concentration is proportional to the absorbed radiation. In AES, the element is excited. A rapid relaxation is accompanied by emission of UV or visible radiation is used to identify the element. The intensity of the emitted photon is proportional to element concentration.