Atomic Spectroscopy

Atomic spectroscopy measures the spectra of elementsin their atomic/ionized states (gas phase).

Atomic spectrometry, exploits quantized electronic transitions characteristic of each individual element in their atomic or ionicstate. Technique for elemental analysis.

Transitions occur in the UV, VIS and NIR regions of the electromagnetic spectrum.

The valence electronic transitions of atoms/ions are being exploited.

This requires each atom to be isolated from all others, so the transitions are not perturbed by neighboring atoms or by bonding effects.

Otherwise the resulting spectra are representative more of molecules or molecular fragments than of atoms themselves.

AS spectral ‘bands’ are very narrow; ‘lines’.

No vibrational or rotational quantization in atomsor their ions. Therefore spectral output lines from individual atomic species are very narrow; rarely overlap with one another.

Emission spectrum of H

The background is significant, it depends on the flame temperature, composition etc., and requires correction.

http://physics.nist.gov/cgi-bin/AtData/lines_form?XXRwq0qFe In AS absorption and emission spectral profiles are very narrow.

typical overlapped

Origins of AS traced to ‘flame test’.

AES

AAS

AFS

‘Analytecell’

Atoms/ions of elements absorb and excited atoms/ions emit a range radiation of characteristic frequencies unique to each element.

ICP-OAES

6000-8000oCplasmaArgon

Argon

Conventionalsample cell -flame.

2300-2600oCO2-C2H2 flame

Sample must be decomposed to the greatest extent possible into its constituent atoms/ions. Ideally, this atomization step should be quantitative; there should be no residual bonding in the gas-phase ‘atomic’ cloud.

Atomization of analytes start with the sample nebulization(spraying) process.

Sample: salts - element exists as an ion or in covalent compound form where the element of interest is bonded.

Sample atomizationin the heat source

aspirationThe suction caused by high flow rates of oxidant gas - Venturi effect.

for thoroughmixing

Larger drops

aerosol

Solution impinging on the bead aerosolizes the sample.

Once formed, wet droplets (aerosol) in the nebulized spray are sent into the high-temperature environment such as a (chemical) flame or flowing rare-gas plasma - ICP.

Nebulization increases the surface area of the solution sample, so solvent evaporation (desolvation in ‘flame’) can proceed rapidly and the resulting dried solute particles that can be volatilized better. (flame atomic absorption, flame emission,and plasma emission spectrometry).

Nebulized spray

In the flame; de-solvation and solute-particle vaporization takesplace, the resulting vapor converted into free atoms and ions.

2

M (g) e(g) M(g)

M (g) e(g) M (g)

..

+

+ +

+ =+ =

The environment in the ‘flames’ often hot enough that many of the atoms that are formed exist as positive ions.

Also, the environment in these atomization sources is energetic to yield sufficient population in the exited state, strong emission from either the free atoms or their ionic counterparts.

For each atomic/ionic species the population distributioncan be calculated using the Boltzmann’s Law, nu, nl excited andground state populations, respectively :

Eu u RT

l l

n ge

n g

−∆

=

Statistical weightings

−∆

=E

l

Ru Tu

l

n ge

n g

Large flame T makes exponent on e → 0, despite high ∆E; makes nu (excited state) significant making AES possible, from the radiative decay of the excited states.

Flame Atomic Absorption block diagram

Light source

AAS AES

Flame AAimplementation

AESI=kcanalyte

SourceAASLambert-Beer Law

A=log(P0/P)=k’canalyte

In reality P0 is theintensity of light reachingwith the ‘blank’.

HCLAtomSource

AAS

LaserSource

AtomSource

AFS

AtomSource

AES

Atomic Spectral lines are sharp (narrow). AAS mode:

The source is unique. The narrow band width of the spectralabsorption/(emission) lines precludes the employment ofbroad band source-monochromator combination to irradiatethe sample.

The irradiating beam profile must be comparable or narrower band-width to that of the absorption profile.

The strategy is to use the emission lines of high intensity fromthe element of interest. This is accomplished in HCLs.

Atomic absorption lines are very narrow.

For the Beer-Lambert law to be applicable, the bandwidth of the source should be narrow in comparison with the width of the absorption peak.

Otherwise, the signal-to-noise ratio and the slope of the calibration curve would be low; the resulting in poor sensitivity.

Therefore, the use of ‘line sources’ with bandwidths even narrower than the width of absorption peaks is preferred.

Continuous radiation sources use monochromators to acquire a narrow frequency band; but even monochromators have effective bandwidths that are significantly greater than the width of atomic absorption lines.

Hollow Cathode Lamp (HCL)

Vi

discharge Ne or Ar at1-5 torr

made of theelement of

of analyte.

At low pressure and applied high voltage a discharge is created.

Discharge ionizes the atoms; ionized particles accelerated toward electrodes. High energy particles sputter atoms and ions into gas phase.

Ions and atoms are exited by high energy particle impact.

As they relax, they produce radiation of the same frequencyof analyte of interest.

~nm

~10-2nm

Spectral Line widths

The AA line profile is broadened due to three factors:1. Natural Decay leads to natural broadening2. Doppler effect leads to Doppler broadening

Where T is temperature and M is atomic mass.

7 T(Hz) (7.0 10 )

M−∆ ≈ ×ν ν

3. Pressure broadening due to collisions. Collisions lowersthe excited state life time thus broadens the spectral line.

The sample is at a higher temperature than the HCL.Therefore they will have two different profiles due to different broadening.

Background in Atomic Spectroscopy is significant.

SourceAASLambert-Beer Law

A=log(P0/P)=k’c

In reality P0 is the intensity of light reaching with the ‘blank’.

The significant background makes the absolute absorbance value for calculations, erroneous.

The “background” errors are due to;1. scattering of the HCL light beam, mainly from the sample

matrix; tends to reduce the power of the beam reaching the detector (false absorbance, P).

Sample/blank in

D

time

2. emissions from sample molecular species and from the fuel component emissions, generally broad spectral emissions; tends to increase the power reaching the detector (false P0) .

Flame only

D

time

Correction for the background emission/absorption is an essential feature in atomic spectroscopy.

Flame only

Blank in

Sample in

D

time

D

time

D

time

Background correction methods:

Beam chopping: Simple method to produce an alternating current.

Deuterium lamp: Use D2 lamp to simulate the background intensity.

Smith-Hieftje: Run HCL at high and low currents to produce different emission profiles of the lamp. First run lamp at low current measure absorbance of analyte and background. Then lamp is pulsed with a high current. During the pulse, the analyte absorbance is reduced but not to zero. Most of absorbance is due to background.

Zeeman Background Correction: A magnetic field is used tosplit the degenerate energy levels. Under the field the species absorb polarized light only. This difference is exploited.

Beam chopping

Background+ analyte present

Background present

P0

P

‘No background’

Pow

er r

each

ing

the

dete

ctor

0PA log kc

P = =

The background = blank (everything but analyte)

Abackground

~nm

~10-2nm

D2 Lamp correction

Deuterium lamps generate a broad profile.Thus the absorbance from the D2 lamp by analyte line is negligible. D2 beam power loss is mainly due to background.

aspirate solvent first; set zeroaspirate sample; measure AD2aspirate sample; measure AHCL

AD2=AbackgroundAHCL=Abackground+ AanalyteAHCL-AD2=Aanalyte

D2Lamp

Measure the A for HCL and D2lamps, alternatively.

10nm

.01nm

AD2 for all practical purposes is due to background.

AD2=AbackgroundAHCL=Abackground+ Aanalyte

AHCL-AD2=Aanalyte

Smith-Hieftje Background correction: Conceptually similar toD2 correction (which uses a broad spectral band). Broad band is generated from the HCL itself.

HCL at high currents produce a wide emission spectral line.

Thus alternating the current density through the HCL (modulation), to account for the background (absorbance at highcurrent density) and measure line absorbance and background( absorbance at low current density) makes the correction possible

Ahigh=Abackground

Alow=Abackground+ AanalyteAlow-Ahigh=Aanalyte

7 T(Hz) (7.0 10 )

M−∆ ≈ ×ν ν

HCLlow

HCLhigh

HCL output variation with current

HCLlow

HCLhigh

Ahigh=Abackground

Alow=Abackground+ AanalyteAlow-Ahigh=Aanalyte

B

Zeeman Background Correction

Triplet

B

ν ν-δν ν ν+δν

One peak three peaks

un-polarized light

σ- π σ+

Interaction with polarized light:

π absorbs lightif polarized parallelto the B.

σ absorbs lightif polarized normalto the B.

Zeeman Background CorrectionPolarization w.r.t. B

B

A||B =Abackground+ analyte

A⊥B =AbackgroundA||B -A⊥B = Aanalyte

B

Un-polarizedHCL

Note: polarized radiation used.

No absorption by π component Zeeman Background Correction

B

Un-polarizedHCL

Absorption by π component + bkg

B

When atoms are subjected to a B, the light component polarized parallel to B is absorbed by the atoms, but the light component polarized normal to B is hardly absorbed.

The background contribution to absorption of light remains unchanged, however, from the any (polarized/unpolarized) light.

Measuring the power of light parallel to B and normal to B measured at the same wave length the background absorption can be excluded by subtraction of signals of these modes.

A||B =Abackground+ analyte

A⊥B =AbackgroundA||B -A⊥B = Aanalyte

Abackground

Abackground+Aanalyte

Alternatively:

Polarization⊥ to B.

L’vov platform

Graphite Furnace AA

Introducing a sample also introduces a matrix that will forma partially burnt ashy material. Such material blocks thelight path and interfere with absorption measurements.

To minimise such detrimental effects the temperature ofThe graphite tube temperature is increased starting at room temperature a in a stream of inert gas.

In graphite furnace AAS, element determinations are carried out with matrix modifiers. Chemical modification should be considered if an analyte is highly volatile, or if the analyte and matrix volatilize at similar temperatures.

Such modification would allow ashing at higher (i.e. atomization at lower) furnace temperatures, resulting in elimination of the matrix effects with no loss of the analyte.

Depending on the element to be determined, various substances are used; e.g. palladium nitrate. Modifiers should not contain the of the element to be analyzed.

4500CyanogenOxygen

2677HydrogenOxygen

2740Natural gasOxygen

1577HydrogenEntrained air/Ar

2045HydrogenAir

1725PropaneAir

1825Coal gasAir

2955AcetyleneNitrous oxide

2250AcetyleneAir

Maximum Temp CFuelOxidant

Oxidant-Fuel Combinations

Interferences:

Spectral interference: overlap of analyte signals from other elements. Select a different line to monitor.

Fuel Background: Interference from partially oxidized fuel molecules Emission from N2O-C2H2 flame

Interference from the formation of molecular (refractory) species Chemical interference:

Reduction of atomization because of the formation of non-volatile salts; in the presence of sulfates, phosphates.

Use a complexing agent (protecting agent, EDTA, 8HQ)to protect the ion.

Add La+3 (releasing agent) because LaPO4 more stable, frees other atoms such as Ca2+ nonvolatile salts.

Use of fuel rich flame minimizes ionization, increase atom population.

High temperatures make ion population significant however, increase ion population.

Ionization Interference:

Often encountered with easily ionizable elements (alkali metals). Ionization very high, making atom population low.

Reverse equilibrium by adding an ionization suppressor,CsCl (1000ppm).

M(g) M (g) e(g)

[M ][e]K

[M]

+

+

= +

=

Effect of Ionization buffer on Atom population

M0 (g) MY (g)

M+ (g)

M* (g)

excitation

M + hν

emission

ionizatio

n

+ ion

izatio

n bu

ffer

association

+ Releasing Agent

Improving excited state species

M+* (g) M+* (g)+ hν

Buffer against chemical interference with P, Al, Si for Calcium (10g/l high pure La2+ added to blank, standard and samples ) or use of a N2O/C2H2flame.

Ionization interference is possible - ionization buffer for Potassium (1g/l high pure CsCl– added to blank, standard and samples)

Electrode-less Discharge Lamps

For volatile elements high energy throughput electrode-less discharge lamps are used. The output power of the EDL lamp is higher than the HCL.

A quartz bulb filled with an inert gas containing the element or a salt of the element for which the lamp is to be used. The bulb is placed inside a ceramic cylinder on which antenna for a RF generator is coiled, which ionizes the inert gas and the coupled energy excites the vaporized atoms inside the bulb and causes emission of characteristic light.