Prabhakar singh sem-ii 4.4 - atomic absorption and plasma

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Transcript of Prabhakar singh sem-ii 4.4 - atomic absorption and plasma

Slide 1

Atomic Spectroscopy:Basic Principles And Instruments

Atomic SpectroscopyAbsorption Spectroscopy: AAS Emission Spectroscopy: FES, ICP-AES(OES)

Atomic Spectroscopy

E energy difference between two levels;h Planks constant, 6.626068 10-34m2kg/s;c speed of light, 299792458 m/s; wavelenght, nm

Ion EmissionAtom Emission

Atomic spectroscopyAtomic spectroscopyis the study of the electromagnetic radiation absorbed and emitted by atoms.

Since unique elements have characteristic (signature) spectra, atomic spectroscopy, specifically theelectromagnetic spectrumormass spectrum, is applied for determination of elemental compositions.

It can be divided byatomizationsource or by the type of spectroscopy used.

Optical spectroscopyElectrons exist in energy levels (i.e.atomic orbitals) within an atom. Atomic orbitals are quantized, meaning they exist as defined values instead of being continuous (see:atomic orbitals).

Electrons may move between orbitals, but in doing so they must absorb or emit energy equal to the energy difference between their atom's specific quantized orbital energy levels.

In optical spectroscopy, energy absorbed to move an electron to a higher energy level (higher orbital) and/or the energy emitted as the electron moves to a lower energy level is absorbed or emitted in the form ofphotons(light particles).

Because each element has a unique number of electrons, an atom will absorb/release energy in a pattern unique to its elemental identity (e.g. Ca, Na, etc.) and thus will absorb/emit photons in a correspondingly unique pattern.

The type of atoms present in a sample, or the amount of atoms present in a sample can be deduced from measuring these changes in light wavelength and light intensity.

Optical spectroscopy is further divided intoatomic absorption spectroscopyandatomic emission spectroscopy. In atomic absorption spectroscopy, light of a predetermined wavelength is passed through a collection of atoms. If the wavelength of the source light has energy corresponding to the energy difference between two energy levels in the atoms, a portion of the light will be absorbed. The difference between the intensity of the light emitted from the source (e.g., lamp) and the light collected by the detector yields an absorbance value. This absorbance value can then be used to determine the concentration of a given element (or atoms) within the sample. The relationship between the concentration of atoms, the distance the light travels through the collection of atoms, and the portion of the light absorbed is given by theBeerLambert law. In atomic emission spectroscopy]], the intensity of the emitted light is directly proportional to the concentration of atoms.

Atomic Spectroscopy

Nebulizer converts the solution into a sprayFlame (or Plasma) causes the solvent to evaporate, leaving dry aerosol particles, then volatilizes the particles, producing atomic, molecular and ionic species

Atomic absorption spectroscopy (AAS) Gases mixture flame (1800 4500 C): air-propane, air-acetylene etc. ; Atomic absorption spectrometry quantifies the absorption of ground state atoms in the gaseous state ; The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels . The analyte concentration is determined from the amount of absorption.

ICP-AES

Inductively Coupled Plasma - Atomic Emission Spectrometry

Atomic emission spectroscopy

ICP-AES

BasicsAtomic emission spectroscopy measures the intensity of light emitted by atoms or ions of the elements of interest at specific wavelengths;Inductively Coupled Plasma spectrometers use emission spectroscopy to detect and quantify elements in a sample; ICP-AES uses the argon plasma (6000-10000 C) for atomization and excitation of the sample atoms; ICP-AES determines approximately all of the elements except gases and some non-metals (C, N, F, O, H).

Schematic diagram of the processes in the ICPAfter Spectro Gmbh, Germany

ICP spectrometerMain SystemsSample Introduction System: to deliver the sample solution to the plasma. Consists of pump, nebulizer and spray chamber. Plasma: to generate the signal. Plasma is forming in the torch from gas argon. Optics: to measure the signal. Computer with appropriate software: for controlling the instrument and measuring process.

After Manning T.J. and Grow W.P., 1997Inductively Coupled Plasma SourceA plasma is a hot, partially ionizedgas. It contains relatively highconcentrations of ions and electrons. Argon ions, once formed in a plasma, are capable of absorbing sufficient power from an external source to maintain the temperature at a level at which further ionization sustains the plasma indefinitely. The plasma temperature is about 10 000 K.

ICP-AES: Plasma

ICP-AES: Radial (SOP) and axial (EOP)

SOP: Side-on-PlasmaEOP: End-on-Plasma more suitable for hard matrices (concentrated samples); alkali metals (Na, K, Li) calibration is more linear; less spectral interferences;lower sensitivity (Limit-of-Detection is higher);

more suitable for light matrices; alkali metals (Na, K, Li) calibration is less linear; more spectral interferences;higher sensitivity (Limit-of-Detection is lower);

ICP-AES, FEs and AAS: application Clinical Analysis: metals in biological fluids (blood, urine); Environmental Analysis: trace metals and other elements in waters, soils, plants, composts and sludges; Pharmaceuticals: traces of catalysts used; traces of poison metals (Cd, Pb etc); Industry: trace metal analysis in raw materials; noble metals determination. Forensic science: gunshot powder residue analysis, toxicological examination ( e.g., thallium (Tl) determination)