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Based on the breakdown of a substance into atoms in a flame, furnace or plasma. (^) (Plasma is a gas hot enough to contain ions and free electrons) (^) Samples are vaporized at temperatures between 2000-6,000K, liquid evaporates and the remaining solid is atomized (^) atomization: process in which the sample is broken (decomposed)into gaseous atoms (^) The conc of atoms in the vapour are measured by absorption and emission of a characteristic wavelength of radiation (Max Planck).(The wavelength for each transition is specific to each element (^) Analysis provides information on elemental composition of sample or compound (^) Used for analysis of metals and some non-metals (quantitative and qualitative)
Atomic Spectroscopy
Atomic absorption
- (^) Gaseous state atoms absorb UV/VIS light and make transitions to higher electronic energy levels.
- (^) The wavelength for each transition is specific to each element
- (^) Conc are measured at mg/L or μg/L range (ppm or ppb)
- (^) Low limits of detection (LOD) can be measured- highly sensitive. -No overlap of line spectra
- (^) To analyze major constituents of an unknown the sample must be diluted to lower the concentrations to ppb- LOD depends on the instrument being used and the analyte.
- (^) Trace constituents can be analyzed without the need for pre- concentration.
- (^) Beer’s lamberts law can be used to find concentrations of unknown using a calibration curve prepared from standards (quantitative).
- (^) Absorbance is directly proportional to path length and concentration
Partial Groatrian diagram of Na
Summarizes the energy of states and transitions between them
Horizontal line labelled 3s correspond to the lowest ground state energy of the atom Eo 3p 4p 4d are three higher energy electronic state The single 3S (groundstate) in Na can be excited into either of these higher levels of Na by absorption of energy whose energy wavelength =hc/∆E (correspond to the energy difference) A few nanoseconds after excitation to 3p, the e returns from the excited state to the ground state by emitting a photon whose energy wavelength =hc/∆E transitions from 4p, would be more energetic than from 3p Only transitions between adjacent columns are allowed.
Exercise
(i) Using the partial Groatrian diagram of Na, explain the origin of line spectra/atomic spectra
Na-
- (^) Before external source is applied the Na atoms are in their lowest energy or ground state. The applied energy causes outer e to be promoted to higher energy levels. With Na in the ground state, with the single valence e in the 3S orbital. Adsorption of External energy (Photons) (ΔE=h = hc/λ) can promote this e to the 3p, 4p, 4s, 5s, etc depending on the E of photon absorbed. The excitation last for a short time (nanoseconds) according to Frank Cordon Principle. excited e relax to ground state giving up energy as photons of UV and Visible radiation. This transition to and from groundstate gives a characteristic wavelength as shown in the diagram. Transition can be from different energy state all of which yields an atomic spectra on emission of radiation. Some transition have shorter λ others have longer λ. A transition to and from ground state is called resonance transition is the most intense transition. 3p to 3S 590 nm 4p to 3s 330 nm 5p to 3S 285 nm
components atomic Emission Spectrometer
1. Light source Spectroscopy
Line source and not contionous sources used in molecular spectroscopy- Hollow cathode lamp (HCL) is the most commonly
used, alternatively electrodeless discharge lamp (EDC) is employed for
compounds that form poor HCL.
- (^) Atomic spectroscopy lines are appr 10-4^ nm
- (^) Monochromators cannot isolate lines of narrower than 10- (^3) and 10-2nm
- (^) We use a HCL or EDL containing the same vapor as the element to be analyte.
- (^) Line sources are preferred because they represent precise wavelengths needed for the absorption by the analytes.
Hollow cathode lamp (HCL)
- (^) the emitted radiation is focused into a beam which passes through a quartz window to the vaporized sample.
- (^) Analyzing for a particular element (e.g Pb. Ca, Mg ) requires using light from that element.
Hollow Cathode Lamp
Electrodeless discharge lamp
- (^) an intense field of radiofrequency or microwave radiation is applied
around the quartz lamp.
- (^) Ar gas gains kinetic energy from the RF, causing the gas to be ionised.
- (^) The cations (Ar+) accelerates and collides with the metal atoms which
becomes excited and as they decay to the ground state they emit radiation
characteristic to the element.
- (^) Microwave electrodeless discharge lamps lies around 100 MHz (5-
times more intense than HCL) and more reproducible.
EDL are available for other metals Including Sb, As,Bi, Cs, Ge, Pb, Hg, P, K, Rb, SE, Sn etc
Atomizers
- (^) sample to be analyzed needs to be in atomic state
- (^) The role of atomizer is to dissolvate a liquid sample and then
the solid particles are vaporized and decomposed into free
gaseous atoms in their ground state form.
- (^) The gaseous atoms absorb radiation emitted from the HCL or
EDL (light source) and thus generate a measurable signal.
- (^) types of atomization for absorption spectroscopy:
Flame- Flame atomic spectroscopy (FAAS)
Electrothermal atomization (Graphite furnace atomization)-
ET/GF-AAS
Temperature Effects on Atomic Spectra
- (^) Temperature has a significant effect on the population of excited vs
ground atomic species thus increases the sensitivity. The
distribution is described by the Boltzman equation ;
- (^) Temp affect the degree to which a sample breaks into atoms and
the extent to which a given atom is found in its ground, excited or
ionized state.
- (^) Each of these influences the signal we obtain.
Temperature Effects on Atomic Spectra:
- (^) Emission spectra are very dependant on the temperature of the
atomizer.
- (^) However, even in very hot flames only a small fraction of atoms are
in excited states.
Excitation/Emission sources
• Flame
• Plasma sources- Inductively Coupled
Plasma(ICP), Direct Current Plasma (DCP),
Microwave induced plasma (MIP)
• Electrical Excitation-Arc or spark
