General Chemistry II

General Chemistry IIAtomic SpectroscopyModule 7Atomic Energy level DiagramEnergy level diagram for the outer electrons of an element (in this case sodium)Horizontal lines indicate the energies of several atomic orbitalsp orbitals are split into two levels of slightly different energy due to coupling of the orbital motion with the spin of the electron (for d and f orbitals these differences are very small and can be neglected)Multiplet states are formed by systems with n valence electronsEnergy levels are characteristic of the nuclear charge and therefore the element

NebulizationTo ensure the instantaneous evaporation of the sample it must be introduced into the flame in a finely dispersed stateThe mist from a pneumatic atomizer may be either admitted directly into the flame (total consumption burner) orit may beforehand pass through a cloud chamber, where the bigger droplets are separated by coagulation and sedimentation (laminar flow burner)

Flame FunctionsEvaporate the sample (liquid droplets)Vaporize the sample (solid to gas transformation)Decompose vaporized compoundsExcite ground state atoms (emission mode)These processes must occur at a steady rate to achieve a steady signalFlame PropertiesFlame temperatures ar not uniform, and thus, atomic excitation will not be uniform throughout the flameFlame is not homogeneous chemically. In the lower regions the reaction between fuel and oxidant is incomplete and in the higher regions there will be higher concentrations of combustion productsWhat happens inside the flame?

Small DropletsDry ParticlesVaporizationMoleculesDissociation(Atoms)SolutionIonizationExcitationEmissionExcitation Processes in Flames10Flame Background EmissionSuperimposed over the line spectra of the excited atoms are other spectral components due to the flame background including line and band spectra of molecules and radicals contained in the flameOH band280-295, 306-320, and 340-348 nmCH radicals387-439 nmC2 radicals 563, 517, 460, and 478 nmCN band 360, 390, 420, and 610 nmSpectral InterferenceOverlapSpectral linesFe 324.7 and Cu 324.8 nm (too close to be resolved by the monochromator)DispersionSpectral lines and spectral band systems produced by molecules or radicals in the flameSpectral line and a continuous background (incandescent particles)IonizationIf M0 or M* is provided sufficient energy it is converted into M+ which emits/absorbs light at a different frequency to M0/M* M0 M* M+ + e-Equilibrium is temperature dependent and varies with position (temperature in the flame)Increasing the [e-] in the flame by adding excess of an easily ionized substance can be used to shift the equilibrium to the left (suppress M+ formation)

Self-AbsorptionIf an element at relatively high concentration is excited and emits its energy near the center or hotter regions in a flame, that energy may be reduced in intensity as it passes through the cooler, outer regions of the flameNeutral atoms of the element, in cooler flame regions, can absorb the energy emitted by the excited atoms in the hotter regions of the flameOrigin of Fraunhofer lines in the spectrum of the sunLargely responsible for nonlinear calibration curves observed at higher concentration levelsCation-Anion InterferencesIf an element forms a stable molecule with an anion in the flame, the element will not be available for the atomic excitation processCalcium in the presence of phosphate or borate forms a stable complex so that the [Ca] signal is depressedHigh-temperature flames and plasmas minimize this effect (e.g., N2O-C2H2)Releasing agents and protective chelating agents can also minimize interferences due to formation of thermally stable compoundsIn the case of calcium phosphate the addition of lanthanum forms a more stable lanthanum phosphate complex and the calcium is free to form excited atomsAlternatively addition of EDTA to the sample forms a more stable Ca(EDTA) complex which inhibits formation of calcium phosphate in the flame and the calcium is free to form excited atomsOxide-Hydroxide FormationAs compounds decompose in a flame a variety of species are produced and a variety of reactions can occurFormation of metal oxides and hydroxides in the flame reduces the population of M*The dissociation energy of these complexes is often high and the flame may not have insufficient energy for this processUse high-temperature flames, oxygen-deficient (shielded) flames or plasmas to minimize effectCarbon Furnace AtomizersConsists of a graphite cylinder about 5 cm long and 0.8 cm in diameter into which the liquid sample is deposited through a hole in the centerThe graphite cylinder is surrounded by an insulated from an outer water-cooled cylinderAn inert gas passes over and through the graphite cyclinder to protect against atmospheric oxidationThe furnace is heated by passing a high current through the graphite cylinder using a three step programDrying (evaporation of solvent at a low temperature)Ashing (destruction of the sample matrix at a modest temperature)Atomization (vaporization and decomposition of compounds at a high temperature)

A deuterium lamp is often used for background correction to eliminate the effect of non-specific absorption and scatteringCHAPTER 20: Figure 20.6

Carbon Furnace

(a) Three stage atomization process employedwith graphite furnace atomization

This example shows the use of a matrix modifierto minimize interference in the determinationof Mn in sea water due to the presence of excess sodium chloride

(b) Sodium chloride blank monitored at Mn Absorption wavelength

(c) Background after addition of nitric acidT othe sample

Three Stage Heating ProgramPlasma Sources for Emission SpectroscopyElectrically conducting gaseous mixture consisting of an inert gas and its ions and electronsIons, once formed in a plasma, can absorb sufficient power from an external source to maintain the temperature at a level where further ionization sustains the plasma indefinitelyTemperatures up to 10,000 KInductively coupled plasma (ICP)Direct current plasma (DCP)

Typical ICP SourceConsists of 3 concentric quartz tubes through which streams of argon gas flow (Total consumption 5 to 20 L/min)Surrounding the top of the largest tube is a water-cooled induction coil powered by a radio-frequency generatorIonization of the flowing argon is initiated by a spark from a Tesla coil. The resulting ions and their associated electrons, then interact with the fluctuating magnetic field produced by the induction coil. This interaction causes the ions and electrons within the coil to flow in closed annular paths. The resistance of the ions and electrons to this flow of charge causes ohmic heating of the plasma

CHAPTER 20: Figure 20.12

CHAPTER 20: Figure 20.11

Argon Plasma

CHAPTER 20: Table 20.4

Single-beam designSingle-Beam AA SpectrometerDOUBLE BEAM FAA SPECTROMETER

Note: the Ref bean does not passthrough the flame thus does not correct for the interferences from the flame!synchronizedCHAPTER 20: Figure 20.16

Hollow Cathode Lamp

a tungsten anode and a cylindrical cathodeneon or argon at a pressure of 1 to 5 torrThe cathode is constructed of the metal whose spectrum is desired or served to support a layer of that metal Ionize the inert gas at a potential of ~ 300 VGenerate a current of ~ 5 to 15 mA as ions and electrons migrate to the electrodes. The gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud.A portion of sputtered metal atoms is in excited states and thus emits their characteristic radiation as they return to the ground sateEventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and are re-deposited Continuum-Source Correction

Zeeman Effect Background Correction:

Sequential ICP Optical Emission SpectrometerProgrammed to move from the line of one element to that of a second, pausing long enough to measure line intensities with a satisfactory S/N ratioScanning involves rotating the grating with a digitally controlled stepper motor so that different wavelengths are sequentially and precisely focused on the exit slit