Atomic Spectroscopy

Types of Atomic Spectrometry

A class of spectroscopic methods in which the species examined in the spectrometer are in the form of ATOMS (not molecules or ions as in solution spectrophotometry & spectrofluorimetry)

Three important methods based on spectroscopy of atomic species are:

  1. Flame Emission Photometry (FEP)
  2. Atomic Absorption Spectrophotometry (AAS)
  3. Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES)

Only 1 and 2 will be considered in this subject
The atoms measured are most commonly those of mineral elements such as Na, K, Mg, Cu, Fe etc.

Flame Emission Photometry (FEP)

THEORY

  1. Sample solution sprayed or aspirated as fine mist into flame.
    Conversion of sample solution into an aerosol by atomiser (scent spray) principle.
    No chemical change in the sample in this stage. [NB atomiser does not convert anything into atoms]. 
  2. Heat of the flame vaporizes sample constituents. Still no chemical change.
     
  3. By heat of the flame + action of the reducing gas (fuel), molecules & ions of the sample species are decomposed and reduced to give ATOMS.
    eg Na+ + e- --> Na 
  4. Heat of the flame causes excitation of some atoms into higher electronic states. 
  5. Excited atoms revert to ground state by emission of light energy, hn, of characteristic wavelength; measured by detector.

Flame Photometer

Atomic Line Spectra

Atoms in the vapour state give LINE SPECTRA (Not band spectra, because no covalent bonds hence no vibrational sub-levels to cause broadening).

Coloured glass filter usually able to isolate the line of analyte element if well separated from other emission lines.
eg To measure sodium and potassium separately in samples containing both

Emission
of		  Na                  ||
 K                            ||

___________________________________

400    500    600   700   800

l (nm)

Quantitative Flame Photometry

Plot of emission intensity vs concentration of ionic species in the solution being measured is linear over wide range but with deviation at both LOW and HIGH concentrations.
  1. Very low concentration - emission falls below expected. Due to IONIZATION. (Some atoms converted back to ions.)
    eg K --> K+ + e-
    Insignificant ionization at higher c. 
  2. Linear region 
  3. Negative deviation at high concentration due to SELF ABSORPTION.
    Photons emitted by excited atoms partly absorbed by ground state atoms in flame.

Experimental Aspects of Flame Photometry

  1. Propane-air or natural gas-air give good flame - strong heat, minimal background light emission. But always need to run a solvent blank for setting zero emission. 
  2. Solutions diluted to fall within linear part of emission curve. Can calibrate with standards accordingly (eg from 0.05 -0.25 mM Na+). 
  3. Use of very low conc Na+ and K+ solutions ---> problems of avoiding contamination.
    Especially Na+, leaches slowly from glass, contact with skin.
     
  4. Anion and cation interference effects can cause errors (enhancement or suppression) . "Radiation buffer" for dilution of standards and samples to swamp out inconsistencies.
     
  5. Internal standard (lithium) useful to counter random flame instability and random dilution errors.

Atomic Absorption Spectrophotometry (AAS)

Principle
FEP limited (for most purposes) to Na & K.
With non-alkali metals most atoms remain in the ground state at normal flame temperature ---> no emission.
AAS measurement is based on the ground state atoms; has much wider applicability than FEP.
Flame can be used as in FEP to reduce & decompose ions/molecules in solution to atoms in flame.
Then measure conc. of GROUND STATE atoms by spectrophotometric principle - absorption of light from a beam passing through flame.
Use elongated burner - flame light path ~10cm - to enhance absorption.

Atomic Absorption Spectrophotometer

Hollow Cathode Lamp

Absorption in the flame is by vapour phase atoms, giving line spectra (see 7.2), in this case absorption lines.
A continuous spectrum light source, even with high quality monochromator cannot achieve sufficiently narrow band pass width for absorption line spectra.
Use special lamps, each emitting line spectrum matched to the line spectrum of the analyte atoms in the flame. The type of lamp is a hollow cathode lamp.

Different lamp for each analyte element, but some multi-element lamps available.

  • At high voltage, ions of He or Ar gas form at anode and bombard cathode.
    SPUTTERING occurs - atoms dislodged from the surface and produce an atomic cloud. Some sputtered atoms are in excited state and emit their characteristic line spectrum as they revert to the ground state.
     
  • Cylindrical shape of the cathode gives direction to emerging beam, and helps re-deposit sputtered atoms back on cathode. 
  • Monochromator isolates particular spectral line & eliminates stray radiation eg emissions from inert gas in lamp. 
  • Modulation of light beam upstream of flame (by rotating chopper) allows detector to reject emission generated within flame.

Experimental Aspects of AAS

  • Wide application and high sensitivity for metallic elements, eg Ca & Mg in clinical labs, heavy metal pollutants ( Pb, Hg, Cu etc) in environmental labs. 
  • Some metal ions in samples are present as strong complexes - not easily decomposed to atoms in the flame ---> low result
    eg Ca interference by phosphate, overcome by adding lanthanum chloride (LaCl3) to samples (& standards & blank). La3+ ions are a RELEASING AGENT for Ca.
    Phosphate ions trapped as more stable lanthanum phosphate complexes; calcium released as free Ca2+ ions - more easily reduced to atoms in the flame.
  • Flame is most common but not the only way of forming atomic vapour of an element to make use of its absorption.

    Flameless AAS methods have advantages for many applications (better sensitivity for elements not easily vaporised in flame). Methods include electric arcs, hydride generators but most important is the high temperature graphite furnace. 

  • Quantitative analysis by AAS
    Beer's law usually holds for absorbances up to about 1.0, due to highly monochromatic light.
    Sample bracketing method relies on standards of most similar concentration to sample (about 10% above and below in absorbance) - useful if standard curve non linear or has non-zero intercept.
    cS = cL + (cU - cL) .

    (AS - AL)

    (AU - AL)

    where c = concentration, A = absorbance, and subscripts S, U, L denote sample, upper standard, lower standard respectively.