Atomic Spectroscopy Atomic Spectroscopic Methods Covered in Ch 313: Optical Atomic Spectrometry (Ch 8-10) Atomic X-ray Spectrometry (Ch 12) Atomic Mass Spectrometry (Ch 11) is combined later on with Molecular Mass Spectrometry (Ch 20)
the elements in a sample are converted to gas-phase atoms or ions detected based on UV-Vis absorption, emission, or fluorescence detection limits in the part-per- billion (ppb) range
Energy Level Diagrams - valence electron transitions Na 1s 2 2s 2 2p 6 3s 1 Mg + 1s 2 2s 2 2p 6 3s 1
Atomic Emission Spectra - valence electrons excited by a flame, plasma, or electric spark to higher energy levels; emission to ground state produces the emission spectra. Na excitation Na emission
Atomic Absorption Spectra - the gas phase atoms and ions can also absorb radiation directly from an outside source monitor missing wavelength Light Source PoPo PoPo absorption
Atomic Fluorescence Spectra - atoms in a flame made to fluoresce by irradiation with an intense light source, e.g. laser
1. Doppler Broadening - frequency shift of light due to source motion 2. Pressure Broadening - increased pressure increases the number of atomic collisions Na Atomic collisions can activate or deactivate an excited state collisions shift ground state energy because of electron cloud interactions
Boltzmann Distribution N j = excited state population N o = ground state population P j and P o = degeneracy terms E j = excited state energy K = Boltzmann’s constant = 1.38 x 10 -23 J/K T = Kelvin temperature For the 3p level of Na – 2500 K 2510 K 0.5 % change 4 % change
Flame Backgrounds O 2 – H 2 O 2 – C 2 H 2 N 2 O – C 2 H 2 Increasing T
Processes leading to flame atomization - 1.aspiration 2.Nebulization 3.Desolvation 4.Volatilization 5.Free atoms
Further reactions with O 2 and N 2 in the atmosphere = BACKGROUND Non-equilibrium unstable combustion products NOT USED Homogenous T and composition equilibrium free atoms OBSERVE HERE
Temperatures in the primary combustion zone are the hottest
oxides Must choose the height within the flame to do the analysis without creating oxides
Laminar Flow Burner Oxidant (air or O2) nebulizes sample Aerosol mixed with fuel and past baffles that remove larger drops Mixture ignited in slotted burner head Longer path length Danger - “flashback” explosion
Graphite Furnace Atomizer - all parts made from graphite 1.Drying or desolvation step – 110 o C, evaporates solvent 2.Ash step – 350-1200 o C, organics converted to CO 2 + H 2 O 3.Atomization step – 200 Amps, 2000-3000 o C, vaporization L’vov Platform 1 x 5 cm Liquid sample injected using a syringe
Graphite Furnace Atomizer – cont’d Constant inert gas flow (Ar) protects the graphite from oxidation and removes analyte from chamber walls.
1.Radiation Sources – require a very narrow linewidth because of negative deviations in calibration curve due to polychromatic radiation effect. To solve this problem, the lamp is constructed out of the same metal element being analyzed. As long as the temperature of the lamp is less than the flame temperature, Doppler and collisional broadening will be greater in the flame, and the source wavelength will be narrower in the sample.
Hollow Cathode Lamps 500 Volts 1.Cathode consists of metal to be analyzed 2.500 Volts across electrodes ionizes inert gas 3.Cations migrate towards the negative hollow cathode 4.Collisions with cathode “sputters” metal from surface 5.Metal atoms in excited states emit characteristic wavelengths 6.Metal redeposited on cathode or glass
2. Instrumentation – single beam; dual beam also available
3. Interferences A. Spectral – unresolveable peaks The 308.211 nm line of V interferes with the 308.215 nm line of Al. To resolve them – If = 1.0 µm then D -1 = 2.0 nm/mm but the slit widthis so narrow that diffraction will cause loss of signal.
3. Interferences B. Chemical (i) Releasing Agents – a cation that preferentially reacts with the interferent and prevents interaction with the sample. e.g. Ca (analyte) in the presence of PO 4 3- 3 Ca 2+ + 2 PO 4 3- Ca 3 (PO 4 ) 2 insoluble product that passes through flame without atomizing the Ca. Add La 3+ or Sr 2+ (both of which form even more insoulbe compounds with PO 4 3- ) (ii) Protecting Agents – prevent interference by forming a stable but volatile species with the analyte e.g. EDTA combines with Ca while leaving interferents behind like Al, PO 4 3-, and SO 4 2-
3. Interferences B. Chemical (iii) Ionization in Flames M(g) = M + (g) + e
1.less interelemental interference (many emission lines to choose from) 2.simultaneous detection of dozens of elements 3.good for compounds with high H vap (difficult to vaporize) 4.can detect elements that form "refractory compounds" (difficult to thermally decompose) such as the oxides of B, P, W, U, Zr, and Nb 5.can detect nonmentals such as Cl, Br, I, and S 6.wide dynamic range 7.no extensive sample pretreatment Plasma, arc and spark emission spectrometry have advantages over flame and electrothermal atomization techniques: