Atomic Emission Spectroscopy (Chapter10.ppt

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1. ATOMIC EMISSION SPECTROSCOPY

2. ATOMIC EMISSION SPECTROSCOPY
The sample is converted into excited gaseous atoms and ions
which emit light at characteristic wavelengths (wavenumbers).
The analyte is identified by emission at a known wavelength
(qual. analysis) and its concentration is determined from the
intensity of the emission at that wavelength (quant. analysis).
Atoms or ions in the gas phase can be excited by high
temperatures or by light absorption.
A fraction of the atoms or ions lose their excitation energy by
giving off a photon.
The most intense emission line for a neutral atom is the
resonance line (same line for absorption and emission e.g.
589nm for Na atom).

3. Atomic Emission Spectrometers.
Emission sources: The source must
a. vaporize the sample,
b. break down all compounds to atoms and ions,
c. and excite the atoms and ions.
•Very high temperatures are required, much higher than in atomic absorption
Methods to atomize and excite the analytes:
a. electrical arcs (low voltage, high current)
b. electrical sparks (high voltage, low current
c. flames have been used
.

4. Monochromators/ Polychromators
A polychromator has multiple exit slits with multiple detectors
PMT Because there are many emission lines from the plasma,
often separated by only a few Angstroms, high resolution is
required in monochromators and polychromators.
Designs are based on an echelle grating operating at high
orders combined with an order-sorting prism

5. Detectors
•Usually a PMT (multiple PMT’s in a polychromator)
•Both the polychromator + multiple PMTs and the echelle can
monitor concentrations of 10 or more elements from one sample
within a few minutes.
•These devices are used heavily by the metallurgy industry to
monitor composition of alloys.

6. Internal Standard vs. external standard method
•BecauseAEScaneasilyperformmulti-element
analysis,aninternalstandard(anelementnot
naturallyfoundinthesamples)isoftenincorporated
ataconstantconcentrationintoeachsample.
•Tocompensateforfluctuationofplasmaemission
intensity(causedbyfluctuationsinplasma
temperature),theemissionintensityofeachelement
lineisdividedbytheemissionintensityofaninternal
standard.

7. Internal standard Method
AnInternalStandardisasubstancethatisaddedina
constantamounttoallsamples,blanksandcalibration
standardsinananalysis.
Calibrationinvolvesplottingtheratiooftheanalytesignal
totheinternalstandardsignalasafunctionofanalyte
concentrationofthestandards.
Thisratioforthesamplesisthenusedtoobtaintheir
analyteconcentrationsfromacalibrationcurve.
Internalstandardcancompensateforseveraltypesof
bothrandomandsystematicerrors.

8. Internal Standard Method
Most convenient when variations in analytical sample size,
position, or matrix limit the precision of a technique.
Or when the sensitivity of instrument changes with time

9. Internal Standard Procedure
Prepare a set of standard solutions for analyte (A) as with the
calibration curve method, butadd a constant amount of a
second species (B called internal standard) to each solution.
Add the same amount of IS to the blank and sample solution.
Prepare a plot of S
A/S
Bversus [A].

10. Example: Pb by ICP Emission
Each Pb solution contains 100 ppm Cu as IS[Pb]
(ppm) Pb Cu Pb/Cu
20 112 1347 0.083
40 243 1527 0.159
60 326 1383 0.236
80 355 1135 0.313
100 558 1440 0.388
Signal

12. No Internal Standard Correction
y = 5.02x + 17.6
R² = 0.9413
0
100
200
300
400
500
600
020406080100120

13. Internal Standard Correction
y = 0.0038x + 0.0066
R² = 1
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
020406080100120

14. unknown sample of Pb (containing100 ppm Cu
as IS) gives a ratio of 0.241.
Conc. of Pb in the unknown = 78.3 ppm

15. Back to sources: flame

16. Flame photometer:
flame emission spectrometry is used for the
quantitative determination of small amounts of metal
salts in solution.
Flam emission is commonly used in clinical labs for
determination the concentration of Na or K ions in
blood.
In this method:
A. sample solution sprayed or aspirated as fine mist
into flame;
B. conversion of sample solution into an aerosol by
atomizer (scent spray)

17. principle.
Then Heat of the flame vaporizes sample constituents.
Still no chemical change. By heat of the flame + action of
the reducing gas (fuel), molecules & ions of the sample
species are decomposed and reduced to give ATOMS.
Na+ + e---> Na
Heat of the flame causes excitation of some atoms into
higher electronic states.Excited atoms revert to ground
state by emission of light energy, h, of characteristic
wavelength; measured by detector.
The reading that the instrument gives is used to calculate
the concentration of the element to be determined.

18. The Sherwood Model 41O

19. Experimental Aspects of Flame Photometry
•Propane-air or natural gas-air gives good flame -strong heat,
minimal background light emission.
•But always need to run a solvent blank for setting zero emission.
•Solutions diluted to fall within linear part of emission curve.
•Anion and cation interference effects can cause errors
(enhancement or suppression). "Radiation buffer" for dilution of
standards and samples to swamp out inconsistencies.
•Internal standard (lithium) useful to counter random flame
instability and random dilution errors.

23. Plasma not flame
•ICPtorchisthemostimportant.
•LiketheflameAAexperiment,thesampleis
introducedintotheinstrumentasasolutionfedintoa
nebulizer,creatingafinesprayofdropletsinaflowing
gasstream

26. Inductively coupled plasma (ICP):
•A plasma (partially ionized gas) of argon is formed and
sustained by intense electric and magnetic fields inside a coiled
radio antenna (27 MHz, 1-2 kW power).
A triple-tube quartz torch is used to:
•(a) generate annular plasma,
•(b) introduce the sample as droplets in an Argas stream.
•Temperatures are about 10,000 K at hottest part of plasma.
Observation is made 1-3 cm above hottest part; local
temperatures are still high (ca. 6000 K).

27. Advantages:
(a) a very wide dynamic range (3 to 6 orders-of-magnitude) on
calibration curves;
(b) stable operation over long periods of time,
(c) almost no chemical interferences due to the very high
temperatures.
Disadvantages:
(a) expensive to buy and operate;
(b) rapid consumption of high purity Ar gas (several L/min).