Optical Atomic Spectroscopy Optical Spectrometry Absorption Emission Fluorescence Mass Spectrometry X-Ray Spectrometry

Optical Atomic Spectroscopy Atomic spectra: single external electron Slightly different in energy

Atomic spectrum Mg Spins are paired No split Spins are unpaired Energy splitting Singlet ground state Singlet excited state Triplet excited state

Atomic spectroscopy Emission Absorption Fluorescence

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 ≥ 1 p156 To determine  with negligibly small uncertainty, a huge measurement time is required. Natural line width

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

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

Line Broadening Doppler broadening

Line Broadening Pressure broadening Caused by collisions of the emitting or absorbing species with other ions or atoms High pressure Hg and xenon lamps, continuum spectra

Temperature Effects Bolzmann equation Effects on AAS, AFS, and AES

Atomic spectroscopy Interaction of an atom in the gas phase with EMR Samples are solids, liquids and gases but usually not ATOMS!

Atomic Spectroscopy Sample Introduction Flame Furnace ICP Sources for Atomic Absorption/Fluorescence Hollow Cathode Lams Sources for Atomic Emission Flames Plasmas Wavelength Separators + Slits +Detectors

How to get things to atomize?

How to get samples into the instruments?

Sample Introduction Pneumatic Nebulizers Break the sample solution into small droplets. Solvent evaporates from many of the droplets. Most (>99%) are collected as waste The small fraction that reach the plasma have been de-solvated to a great extent.

What is a nebulizer? SAMPLE AEROSOL

Concentric Tube

Cross-flow

Fritted-disk

Babington

What happens inside the flame?

FLAMES Rich in free atoms

FLAMES T  E

GOOD AND BAD THINGS oxidation

Boltzmann Equation: Relates Excited State Population/Ground State Population Ratios to Energy, Temperature and Degeneracy

Flame AAS/AES Spray Chamber/Burner Configurations Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillary Only about 5% reach the flame Larger droplets are collected Some of the solvent evaporates Flow spoilers Cheaper, somewhat more rugged Impact beads Generally greater sensitivity

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)

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 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.

Radiation Sources for AAS Hollow Cathode Lamp Conventional HCL

Ne or Ar at 1-5 Torr

Hollow Cathode Lamp (Cont’d) a tungsten anode and a cylindrical cathode neon or argon at a pressure of 1 to 5 torr The 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 V Generate 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 sate Eventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and are re-deposited

Hollow Cathode Lamp (Cont’d) High potential, and thus high currents lead to greater intensities Doppler broadening of the emission lines from the lamp 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 band Doppler broadening ?

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 vapor Boosted 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.

Boosted discharge hollow-cathode lamp (BDHCL)

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.

Electrodeless Discharge Lamps (EDL)

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.

Single-beam design

DOUBLE BEAM FAA SPECTROMETER Note: the Ref bean does not pass through the flame thus does not correct for the interferences from the flame! synchronized

Interferences in AAS and AFS Spectral Interferences Overlapping Broadening absorption for air/fuel mixture Scattering or absorption by sample matrix

Background Correction Two-line Correction (like Internal Standard) Continuum-Source Correction Zeeman Effect Source Self-Reversal (Smith –Hieftje)

Continuum-Source Correction

Continuum-Source Correction

(The draw is not to scale) B 0.04 nm The 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 background A: 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.

Zeeman Effect Background Correction:

Source Self-Reversal (Smith –Hieftje) Self-absorption Line broadening A relative new technique

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 background Vandecasteele and Block, 1997, p126

Interferences in AAS and AFS Chemical Interferences Formation of compounds of low volatility Calcium analysis in the presence of Sulfate or phosphate Solutions Higher temperature Releasing agents: cations that react preferntially with the interference ions. Protection agents: form stable but volatile species with the analytes (i.e. EDTA,APDC….)

Chemical Interferences Atom ionization M ↔ M+ + e

Commercial AFS instruments are on the market! Atomic Fluorescence Spectrometry Commercial AFS instruments are on the market! Learn more in CHM 6157