6 Line Broadening Uncertainty Effects Natural line width Heisenberg uncertainty principle:The nature of the matter places limits on the precision with which certain pairs of physical measurements can be made.One of the important forms Heisenberg uncertainty principle:t ≥ p156To determine with negligibly small uncertainty, a huge measurement time is required.Natural line width
7 Superposition of tw sinusoidal wave of different frequencies but identical amplitudes. n Should be Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007
8 Line Broadening Doppler broadening Doppler shift:The wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer, and increases if the motion is receding from the transducer.In flame, Doppler broadening is much larger than natural line width
16 Sample Introduction Pneumatic Nebulizers Break the sample solution into small droplets.Solvent evaporates from many of the droplets.Most (>99%) are collected as wasteThe small fraction that reach the plasma have been de-solvated to a great extent.
29 Boltzmann Equation: Relates Excited State Population/Ground State Population Ratios to Energy, Temperature and Degeneracy
30 Flame AAS/AES Spray Chamber/Burner Configurations Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillaryOnly about 5% reach the flameLarger droplets are collectedSome of the solvent evaporatesFlow spoilersCheaper, somewhat more ruggedImpact beadsGenerally greater sensitivity
32 ElectroThermal AAS (ETAAS or GFAAS) 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)
33 ElectroThermal AAS (ETAAS or GFAAS) The furnace goes through several steps…Drying (usually just above 110 deg. C.)Ashing (up to 1000 deg. C)Atomization (Up to 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.
41 Hollow Cathode Lamp (Cont’d) 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 metalIonize 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
42 Hollow Cathode Lamp (Cont’d) High potential, and thus high currents lead to greater intensitiesDoppler broadening of the emission lines from the lampSelf-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 bandDoppler broadening ?
43 Improvement…….Most direct method of obtaining improved lamps for the emission of more intense atomic resonance lines is to separate the two functions involving the production and excitation of atomic vaporBoosted discharge hollow-cathode lamp (BDHCL) is introduced as an AFS excitation source by Sullivan and Walsh.It has received a great deal of attention and a number of modifications to this type of source have been conducted.
45 Operation principle of BDHCL A secondary discharge (boost) is struck between an efficient electron emitter and the anode, passing through the primary atom cloud.The second discharge does not produce too much extra atom vapor by sputtering the walls of the hollow cathode, but does increase significantly the efficiency in the excitation of sputtered atom vapor.This greatly reduces the self-absorption resulting from simply increasing the operating potential (increase Doppler broadening and self-absorption) to the primary anode and cylindrical cathode.
47 Electrodeless discharge lamps (EDL) 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).The lamp does not contain an electrode but instead is energized by an intense field of radio-frequency or microwave radiation.Radiant intensities usually one or two orders of magnitude greater than the normal HCLs.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.
55 (The draw is not to scale) B0.04 nmThe light from the HCL is absorbed by both the sample and the background, but the light from the D2 lamp is absorbed almost entirely by the backgroundA: HCL lamp, the shaded portion shows the light absorbed from the HCL. The emission has a much narrower line width than the absorption line.B: D2 lamp, the shaded portion shows the light absorbed by D2 lamp. The lamp emission is much broader than the sample absorption, and an averaged absorbance taken over the whole band pass of the monochromator.
57 Source Self-Reversal (Smith –Hieftje) Self-absorptionLine broadeningA relative new technique
58 Source Self-Reversal (Smith –Hieftje) Absorbed by sample reduced, not complete eliminate!But the background absorbs the same portion of light.Absorbed by sample and backgroundVandecasteele and Block, 1997, p126
59 Interferences in AAS and AFS Chemical InterferencesFormation of compounds of low volatilityCalcium analysis in the presence of Sulfate or phosphateSolutionsHigher temperatureReleasing agents: cations that react preferntially with the interference ions.Protection agents: form stable but volatile species with the analytes (i.e. EDTA,APDC….)
60 Chemical Interferences Atom ionizationM ↔ M+ + e
61 Commercial AFS instruments are on the market! Atomic Fluorescence SpectrometryCommercial AFS instruments are on the market!Learn more in CHM 6157