From molecular to elemental analysis there are three major techniques used for elemental analysis: Optical spectrometry Mass spectrometry X-ray spectrometryChapter 9: Atomic Absorption and fluorescence Spectroscopy- Both AA and AF require a light source - Like Molecular Absorption & Fluorescence, in AA high intensity is not required, in AF high intensity results in greater sensitivity. • Sample Atomization • Atomic Absorption (AA) • Atomization Fluorescence (AF)Absorption Molecular Absorption Emol = Eelectronic + Evibration + Erotation Atomic Absorption Emol = Eelectronic E0 E1 Ei Ej9/17/11 9:01 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/Docs/Document-Detail-Google.aspx?doc_id=423316159/17/11 9:01 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/Docs/Document-Detail-Google.aspx?doc_id=423316159/17/11 9:18 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/docs/42331615/Atomic-Absorption-Spectrometry-pyWhat happens inside the flame?9/17/11 9:36 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/docs/42331615/Atomic-Absorption-Spectrometry-pySample processes occurring during atomization1.#Atomiza+on#in#Flame#9/17/11 9:01 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/Docs/Document-Detail-Google.aspx?doc_id=42331615MgCl2 (solution) MgCl2 (S), MgO (S) Evaporation MgCl2 (S) or MgO (S) MgCl2 (g) or MgO (g) Volatilization MgCl2 (g) or MgO (g) Mg0 +Cl0 + O0 AtomizationFLAMESRegions in a flame Rich in free atomsTemperature profile oxidation Variation due to the degree of oxidation for a given element A different portion of the flame should be used for the determination of each elementAtomiza+on#cell#Transport sample aerosol to flame Desolvation AtomizationCheap Simple Flame stability Low temperature 1.#Flame#Atomizers#Disadvantages: Low sampling efficiency n Only about 5% reach the flame and large portion of the sample flows down the drain. n The residence time of individual atoms in the optical path in the flame is brief ( ~10-4 s). Used for atomic absorption, fluorescence and emission spectroscopy Flame#Atomizers#• Short atomization period. • Long average residence time of the atoms (~ s). Why#are#electroThermal#atomizers?#• The sample is contained in a heated, graphite furnace. • The furnace is heated by passing an electrical current through it (thus, it is electro thermal). • To prevent oxidation of the furnace, it is sheathed in gas (Ar usually) • There is no nebulziation, etc. The sample is introduced as a drop (usually 5-20 uL), slurry or solid particle (rare) 2.#ElectroThermal#Atomizers#Advantages: n High sensitivity for small volumes of sample Used for atomic absorption, fluorescence Electrothermal#Atomizers#Disadvantages: n 5% - 10% relative precision n Furnace methods are slow n Narrow analytical range• The furnace goes through several steps… – Drying (usually just above 110 deg. C.) – Ashing (up to 1000 deg. C) – Atomization (Up to 2000-3000 C) – Cleanout (quick ramp up to 3500 C or so). Waste is blown out with a blast of Ar. • The light from the source (HCL) passes through the furnace and absorption during the atomization step is recorded over several seconds. This makes ETAAS more sensitive than FAAS for most elements. ElectroThermal#Atomizers#9/17/11 9:01 PMAtomic Absorption Spectrometry pyPage 1 of 1http://www.docstoc.com/Docs/Document-Detail-Google.aspx?doc_id=42331615Radiation Sources for AASRadiation Sources for AAS Requirement of radiation source in AAS Give light at the specific wavelength Emission bandwidth is narrower than atomic absorption line (0.01 nm) Sufficient intensityHollow Cathode Lamp Conventional HCLNe or Ar at 1-5 TorrHollow-Cathode LampsHollow Cathode Lamp (Cont’d) ü High potential, and thus high currents lead to greater intensities ü Doppler broadening of the emission lines from the lamp ü Self-absorption: the greater currents produce an increased number of unexcited atoms in the cloud. The unexcited atoms, in turn, are capable of absorbing the radiation emitted by the excited ones. This self-absorption leads to lowered intensities, particular at the center of the emission bandElectrodeless Discharged LampsElectrodeless discharge lamps (EDL) ü Constructed from a sealed quartz tube containing a few torr of an inert gas such as argon and a small quantity of the metal of interest (or its salt). ü The lamp does not contain an electrode but instead is energized by an intense field of radio-frequency or microwave radiation. ü Radiant intensities usually one or two orders of magnitude greater than the normal HCLs. ü The main drawbacks: their performance does not appear to be as reliable as that of the HCL lamps (signal instability with time) and they are only commercially available for some elements (~ 15).Atomic#Spectrophotometers#Elimina+on#of#Emission#from#Flame#• Problem#####?#Con+nuous#emission#from#flame#####?#Atomic#line#emission#from#atom#in#flame#• How#to#solve#the#problem#####?#Place#monochromator#between#flame#and#detector######?#modula+on#of#radia+on#source##########Chopper##########AC#operated#source##Single-Beam DesignNote: the Ref bean does not pass through the flame thus does not correct for the interferences from the flame! synchronized Double-Beam DesignInterferences in AAS and AFS 1. Spectral Interferences 2. Chemical InterferencesSpectral Interference • Spectral Interferences – Overlapping – Broadening absorption for air/fuel mixture – Scattering or absorption by sample matrixSpectral#Interference#Overlapping:#Atomic#absorp+on#line#of#other#elements#308.211 nm 308.215 nm 309.27 nm Vanadium Aluminum AluminumSpectral#Interference#Overlapping:#Analyte#absorp+on#line#with#matrices#absorp+on#(non?specific,#molecular#absorp+on)#Background Correction • Two-line Correction (like Internal Standard, not very common) • Use of D2 or H2 lamp (Continuum-Source Correction, very common) • Source Self-Reversal (Smith –Hieftje, relatively new) • Zeeman Effect (common for graphite furnace instruments)1. Two-line Correction Reference line should have the following properties: 1. Very close to analyte line 2. Not adsorbed by analyte Simple