Atomic Absorption Spectroscopy Flashcards
Atomic Spectroscopy
A substance is decomposed into atoms in a flame, furnace, or plasma
Free gaseous atoms absorb or emit energy
Absorbance/emission used to measure concentration of atoms
Atomic absorption spectroscopy (types)
FAAS, GFAAS
Optical emission spectroscopy (types)
FOES, ICP-OES
Plasma
Gas that is hot enough to contain ions and free electrons
What is atomic spectroscopy used for?
Elemental analysis (atoms/elements in free gas)
Trace metal analysis (metals analyzed as atoms)
Atomic spectroscopy (description)
Liquid samples need to be converted into free gas phase atoms in order to be detected
Free gas phase atom has no vibrational or rotational energy
Each element has different wavelength of absorption or emission
Select unique λmax for each element
(no need to separate the analytes before detection)
Advantage: can analyze a sample containing multiple elements
Disadvantage: no speciation of metal (i.e. total copper), can’t distinguish forms of metal (Cu2+, Cu3+ etc., would need HPLC-AAS)
Describe wavelength selection for atomic spectroscopy
Most elements have discrete wavelengths of absorption or emission, and a few lines are typically available in AAS
(most instruments have bandwidth of 0.1 nm)
An elements emission spectrum is ______ to its absorption spectrum?
Inverse
Energy emitted by photons = energy absorbed to excite back to previous levels
Atomizer
Burner head usually 10 cm long (long path length = max absorption)
Small droplet of sample travel to burner head (large ones eliminated)
Responsible for producing free gaseous atoms of analyte
Atomizer steps
M+ + A-(solution) to M+ + A-(aerosol) (nebulization)
M+ + A- to MA(s) (desolvation)
MA(s) to MA(l) (liquefaction)
MA(l) to MA(g) (vaporization)
MA(g) to Mo + Ao (atomization)
Mo + Ao to M* (excitation)
M* to M+ + e- (ionization)
Nebulization
Convert liquid samples to droplets
(cloud of droplets hit obstruction, small ones carry on to baffle, large ones removed to waste)
Desolvation
Once in burner head flame, solvent is evaporated leaving dry salt particles
MA(s) = solid salt of metal cation and associated anion
Liquefaction
In flame, salt is liquefied by flame temperature
Vaporization
In flame, liquid salt is vaporized to gas by flame temperature
Atomization
Gas salt particles dissociate to form neutral gaseous atoms (radicals)
Mo, Ao = neutral, ground state free atoms
Excitation
If sufficient energy present, atoms can be excited to higher electron state - only wanted in OES not AAS
Ionization
Once excited, atom of element can be ionized if enough thermal energy, collisions of excited atom with partially burnt flame gases can cause ions to form
Not desired in OES or AAS
Other steps in FAAS/FOES
Metal oxides and hydroxides can form (reduces response)
Regions of flame
Outer core (secondary combustion zone): more likely to see metal oxides/hydroxides
Interconal region (hottest region of flame): salt particles are atomized in gas phase
Blue cone (primary combustion zone): desolvation of small droplets
What are low temperature flames more subject to?
Low temp. flame subject to more interferences from incomplete atomization
Acetylene/air flame (common), 2400-2700 K
Acetylene/NO2, 2900-3100 K
Acetylene/O2, 3300-3400 K
Hydrogen/Air, 2300-4200 K
Effect of temperature on atomic spectroscopy
Increasingtemperature increases the number of excited metal atoms (M*) in OES
Elements common with OES?
Elements common with AAS?
OES = groups 1 and 2 (less energy needed to excite)
AAS = transition metals
What has better detection limits, GFAAS or FAAS?
GFAAS
What are Cr, V, Al, Si prone to?
What are Cs and Rb prone to?
Cr, V, Al, Si prone to formation of metal oxides/hydroxides in cooler flame (use hotter)
Cs, Rb prone to ionization at higher temp (use cooler)
Excess fuel (rich) vs excess oxidant (lean) flame
Excess fuel = reduced flame = hotter
Excess oxidant = oxidizing flame = cooler
Types of lamps
Hollow cathode lamp
Electrodeless discharge lamp
Types of interferences (AAS)
Spectral interferences
(atomic spectral interferences from other elements)
(spectral/chemical interferences from oxides)
(background absorption)
Chemical Interferences
Matrix Interferences
Atomic spectral interferences (from other elements) (AAS)
How to reduce it?
Mainly a problem when trying to measure absorption of one element in presence of high concentration of another element with similar absorption line
Overlap occurs when lines separated by <0.01 nm
Reduce by selecting wavelength with no overlap (maybe second or third most intense instead of first)
Extract interference (but ensure no loss of analyte)
Spectral/chemical interferences (from oxides) (AAS)
How to reduce it?
Molecular band from metal oxide/hydroxide
(eg CaOH interferes with Na and Ba lines)
Reduce by using hotter flame (or increase fuel in fuel/oxidant ratio)
Background absorption (AAS)
How to reduce it?
Absorption of radiation from hollow cathode lamp by molecules or polyatomic species in atomizer
More common problem for low wavelengths
(eg incomplete combustion of organic solvent in flame, more common in GFAAS)
Reduce by continuum-source correction method or Zeeman background correction method (generally better)
Continuum-Source correction method
For corrections < 350 nm a deuterium lamp is used
Absorbance(hallow cathode) - Absorbance(continuum) is the corrected absorption
Zeeman background correction method
Atomic vapour in strong magnetic field, splitting of electronic energy levels of the atoms takes place - several absorption lines for each electronic transition
Absorbance(magnetic field off) - Absorbance(magnetic field on) is the corrected absorption
Better for lower detection at higher wavelength
Chemical interferences (AAS)
How to reduce it?
Most commonly observed in presence of anions that are found in sample matrix (PO4 3-, SO4 2-)
Anions affect stability of metal
More thermally stable complexes Ca3(PO4)2 decrease atomization (not enough energy in flame to break these bonds)
If known matrix, add ions to standards
Add another metal ion that forms a more stable complex with anions (La 3+)
Add another complexing agent that does not interfere with atomization (EDTA)
Use hotter flame
Matrix interferences (AAS)
How to reduce it?
Important to GFAAS
Organic solvents enhance nebulization
High acid % in aqueous solutions can increase viscosity and decrease nebulization
Reduce by matching solutions to sample matrix or adding releasing agents
Use standard addition approach
Matrix modification
Types of interferences (OES)
Chemical interferences
Excitation interferences
Ionization interferences
Spectral interferences
(background radiation)
(overlapping emission lines different elements)
Chemical interference (OES)
How to reduce it?
(same as AAS)
Reduce by adding supressing agent such as EDTA to form a complex with Ca2+, sample preparation, add a releasing agent (La3+)
Excitation interferences (OES)
How to reduce it?
Presence of other element in solution increases the normal population of excited state of analyte of interest
(eg Na and K can go to excited state, can excite eachother)
Reduce by matrix matching, use standard addition approach
Ionization interferences (OES)
How to reduce it?
Excited atoms ionized, they cannot emit light of desired wavelength, observed for easily ionized alkali metals and alkali earth metals)
Reduce by adding large amount of ionization suppressant (like Cs which is easily ionized)
Background radiation (OES)
How to reduce it?
Excited molecules and radicals from combustion products in the flame can emit light over broad region
Reduce by correcting absorbances against a blank
(external standard calibration, standard addition calibration)
Overlapping emission lines of different elements (OES)
How to reduce it?
(self explanatory)
High-res OES see less
Reduce by selecting different wavelength with no overlap, or extracting interference prior to analysis
