metal analysis by flame and plasma atomic spectroscopy

8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy

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Metal Analysis by Flame and

Plasma Atomic SpectroscopyFlame

A. Atomization

1. Types of Atomization Processes

a.) Nebulizersb. Electrothermal atomization

2. Line Width

3. Effect of Temperature

B. InterferencesC. Sample Preparation

Plasma Emission Spectroscopy



Continuous Atomizers

• Used in AA

and DCP

(direct

currentplasma)



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 PrincipleLifetimes of excited states are only a finite

amount of time. There are uncertainties in

transition time.Called natural line width. Overall 10-4 A



Doppler Broadening

Detector

Detector

l longer

l 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

Gaseousmolecules

Atoms

Atomic

ions

Excited

molecules

Excited

atoms

Excited

ions

Nebulization

Desolvation

Volatilization

Dissociation

Ionization

hn molecular

hn atomic

hn 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

reproducibleatomization

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 analyticalspectroscopy



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 to2000-3000K 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 linevery close to analyte spectral line butthat analyte does not absorb at, subtract

2 lines2. Background correction: Subtract

continuous source (such as D2) fromsample

3. Zeeman correction: Magnetic fieldapplied produces plane polarized light,light goes through polarizer only when

sample is introduced



Chemical Interferences

Result from chemical

processes occurring

during atomization

that alter theabsorption

characteristics of the

analyte: Easier to

correct for thenspectral interferences!



1. Anion Interference

• Most common: Anions will reactw/analyte to produce a species oflow volatility ex. PO43- or SO42-.This will significantly reduce Ca (or

other metal’s) absorption bymaking Ca3(PO4)2 and CaSO4

• Cation interference also possible

• Can be minimized w/ releasingagents or protective agents which

react preferentially with interferingspecies, for ex. Sr will reactpreferentially w/PO43- making itpossible 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 mustuse 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 ofdissolved and suspended

metals

• Acid extractable metals: [

] of metals in solution

after treatment ofunfiltered 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 6months

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 tolowest 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 willaffect outcome, increase

sensitivity b/c less

surface tension resulting

in finer drop size

metal analysis by flame and plasma atomic spectroscopy

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