Atomic Absorption Spectroscopy

Question 1

Atomic Spectroscopy

Answer 1

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

Question 2

Atomic absorption spectroscopy (types)

Answer 2

FAAS, GFAAS

Question 3

Optical emission spectroscopy (types)

Answer 3

FOES, ICP-OES

Question 4

Plasma

Answer 4

Gas that is hot enough to contain ions and free electrons

Question 5

What is atomic spectroscopy used for?

Answer 5

Elemental analysis (atoms/elements in free gas)
Trace metal analysis (metals analyzed as atoms)

Question 6

Atomic spectroscopy (description)

Answer 6

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)

Question 7

Describe wavelength selection for atomic spectroscopy

Answer 7

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)

Question 8

An elements emission spectrum is ______ to its absorption spectrum?

Answer 8

Inverse
Energy emitted by photons = energy absorbed to excite back to previous levels

Question 9

Atomizer

Answer 9

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

Question 10

Atomizer steps

Answer 10

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)

Question 11

Nebulization

Answer 11

Convert liquid samples to droplets
(cloud of droplets hit obstruction, small ones carry on to baffle, large ones removed to waste)

Question 12

Desolvation

Answer 12

Once in burner head flame, solvent is evaporated leaving dry salt particles
MA(s) = solid salt of metal cation and associated anion

Question 13

Liquefaction

Answer 13

In flame, salt is liquefied by flame temperature

Question 14

Vaporization

Answer 14

In flame, liquid salt is vaporized to gas by flame temperature

Question 15

Atomization

Answer 15

Gas salt particles dissociate to form neutral gaseous atoms (radicals)
Mo, Ao = neutral, ground state free atoms

Question 16

Excitation

Answer 16

If sufficient energy present, atoms can be excited to higher electron state - only wanted in OES not AAS

Question 17

Ionization

Answer 17

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

Question 18

Other steps in FAAS/FOES

Answer 18

Metal oxides and hydroxides can form (reduces response)

Question 19

Regions of flame

Answer 19

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

Question 20

What are low temperature flames more subject to?

Answer 20

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

Question 21

Effect of temperature on atomic spectroscopy

Answer 21

Increasingtemperature increases the number of excited metal atoms (M*) in OES

Question 22

Elements common with OES?
Elements common with AAS?

Answer 22

OES = groups 1 and 2 (less energy needed to excite)
AAS = transition metals

Question 23

What has better detection limits, GFAAS or FAAS?

Answer 23

GFAAS

Question 24

What are Cr, V, Al, Si prone to?
What are Cs and Rb prone to?

Answer 24

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)

Question 25

Excess fuel (rich) vs excess oxidant (lean) flame

Answer 25

Excess fuel = reduced flame = hotter Excess oxidant = oxidizing flame = cooler

Question 26

Types of lamps

Answer 26

Hollow cathode lamp Electrodeless discharge lamp

Question 27

Types of interferences (AAS)

Answer 27

Spectral interferences (atomic spectral interferences from other elements) (spectral/chemical interferences from oxides) (background absorption) Chemical Interferences Matrix Interferences

Question 28

Atomic spectral interferences (from other elements) (AAS) How to reduce it?

Answer 28

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)

Question 29

Spectral/chemical interferences (from oxides) (AAS) How to reduce it?

Answer 29

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)

Question 30

Background absorption (AAS) How to reduce it?

Answer 30

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)

Question 31

Continuum-Source correction method

Answer 31

For corrections < 350 nm a deuterium lamp is used Absorbance(hallow cathode) - Absorbance(continuum) is the corrected absorption

Question 32

Zeeman background correction method

Answer 32

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

Question 33

Chemical interferences (AAS) How to reduce it?

Answer 33

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

Question 34

Matrix interferences (AAS) How to reduce it?

Answer 34

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

Question 35

Types of interferences (OES)

Answer 35

Chemical interferences Excitation interferences Ionization interferences Spectral interferences (background radiation) (overlapping emission lines different elements)

Question 36

Chemical interference (OES) How to reduce it?

Answer 36

(same as AAS) Reduce by adding supressing agent such as EDTA to form a complex with Ca2+, sample preparation, add a releasing agent (La3+)

Question 37

Excitation interferences (OES) How to reduce it?

Answer 37

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

Question 38

Ionization interferences (OES) How to reduce it?

Answer 38

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)

Question 39

Background radiation (OES) How to reduce it?

Answer 39

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)

Question 40

Overlapping emission lines of different elements (OES) How to reduce it?

Answer 40

(self explanatory) High-res OES see less Reduce by selecting different wavelength with no overlap, or extracting interference prior to analysis