Metal Analysis by Flame and Plasma Atomic Spectroscopy

FlameA. Atomization

1. Types of Atomization Processesa.) Nebulizersb. Electrothermal atomization

2. Line Width3. Effect of Temperature

B. InterferencesC. Sample Preparation

Plasma Emission Spectroscopy

Continuous Atomizers

• Used in AA and DCP (direct current plasma)

Discrete Atomizers

• Sample is atomized all at once, allowing for better detection limits

Line Width

Which is wider, atoms or molecules spectra?

Factors contributing to line width:

1.) Uncertainty Principle

Lifetimes of excited states are only a finite amount of time. There are uncertainties in transition time.

Called natural line width. Overall 10-4A

Doppler Broadening

Detector

Detector

longer

shorter

Pressure Broadening

• Arises from collisions between analyte and other atoms or ions in heated media, result in small changes in g.s. energy and hence a spread in wavelength

• In high pressure Hg and Xe lamps pressure broadening is so extensive that continuous radiation is produced.

Temperature

• Extremely important to have it consistent in order to get

consistent amount of atomization

Spray

Solution of Analyte

Solid/ gas aerosol

Gaseous molecules

Atoms

Atomic ions

Excited molecules

Excited atoms

Excited ions

Nebulization

Desolvation

Volatilization

Dissociation

Ionization

h molecular

h atomic

h atomic

Why does a reproducible temperature matter so much?

1. Temperature effects population size of ground and excited states

2. 10K change in temperature for Na results in a 4% change in excited state population

3. Emission spectroscopy more sensitive to small temperature changes in flame than are absorption and fluorescence because they are based on excited state populations

Flames and Atomization

• Flame temperature and position are critical for achieving reproducible atomization

1o combustion region: Not at thermal equilibrium, blue due to C2CH, & other radicals, not used for analytical spectroscopy

Outer zone: Atoms from inner core are converted to stable molecular oxides, cooler

Interconal region: Pretty narrow in stoichiometric flames, rich in free atoms, widely used for analytical spectroscopy

Gases used

Common Fuels Common oxidants

Natural gas Air

H2 O2

Acetylene Nitrous Oxide

• Why is the burner the shape that it is?

• Sensitivity of metal with burner height varies by metal, see transparency

Electrothermal atomizers a.k.a. graphite furnaces

• Sample is introduced into graphite tube, solvent evaporated, & then heated rapidly to 2000-3000K w/high current

• Sample residence time up to 1 second

• Detection limit 10-10 – 10-13g/Sample

Disadvantages of Graphite Furnaces

• 5-10% precision vs. 1% for flame or plasma

• Slow/sample• Linear range <2

orders of magnitude

Interferences

Spectral Interferences: Arises when absorption or emission of other species in solution lies very close to the same wavelength

1. Can result from combustion products of flame fuel or oxidant

2. Molecular oxides from sample itself

Ways to correct for matrix interferences

1. Two line correction method: Pick line very close to analyte spectral line but that analyte does not absorb at, subtract 2 lines

2. Background correction: Subtract continuous source (such as D2) from sample

3. Zeeman correction: Magnetic field applied produces plane polarized light, light goes through polarizer only when sample is introduced

Chemical Interferences

Result from chemical processes occurring during atomization that alter the absorption characteristics of the analyte: Easier to correct for then spectral interferences!

1. Anion Interference

• Most common: Anions will react w/analyte to produce a species of low volatility ex. PO43- or SO42-. This will significantly reduce Ca (or other metal’s) absorption by making Ca3(PO4)2 and CaSO4

• Cation interference also possible• Can be minimized w/ releasing

agents or protective agents which react preferentially with interfering species, for ex. Sr will react preferentially w/PO43- making it possible to determine Ca

Protective Agents

• Form stable but volatile complexes w/ analyte

2. Ions in Flames

• Can be minimized w/ ionization supressor which produces an excess of ions so L’Chatlier’s principle is employed

• M === M+ + e-

3. Formation of Stable Compounds

• Some analytes are atomized with difficulty, i.e. Hg or Pb For these you must use a hotter flame or a fuel rich flame

Cool flame Hot

flame

Sample Preparation for Metal Analysis

• Dissolved metals: What passes through a 0.45um membrane filter of an unacidified sample

• Suspended metals: What is retained on a 0.45um membrane filter of an unacidified sample

• Total metals: Sum of dissolved and suspended metals

• Acid extractable metals: [ ] of metals in solution after treatment of unfiltered sample with hot acid

Sample Handling for Metal Analysis

1. Filter immediately

2. Preserve with acid to pH = 2-3

3. Can store up to 6 months

4. Containers: teflon > polypropylene > linear polyethylene > glass

{ Avoid glass for trace levels}

• Detergent wash, tap water rinse, soak in acid, rinse with metal free water

• Avoid paints, rubber, paper, and metal objects

Sample Extraction

• Sample is digested in concentrated HCl, HNO3, H2SO4, etc. by boiling to lowest volume before precipitation, cover w/ watch glass to avoid spattering

• Purpose of acid digestion is to oxidize the organic materials in sample and dissolve all the metals

• Continuous atomizers require samples to be in solution but discrete atomizers do not

• Organic solutions will affect outcome, increase sensitivity b/c less surface tension resulting in finer drop size