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Analytical Chemistry –Atomic
absorption Spectroscopy
KR
LSU
IIntroduction to Atomic Spectroscopy
AAtoms exhibit interactions with EM radiation. We will discuss some of the spectroscopic methods used in their determinations.
An absorption of a photon causes an excited state, where an atom 's electron goes from a lower energy state to one of higher energy. As
in molecular spectroscopy, we can monitor the difference in the absorption or monitor the subsequent photon emission.
Let's look at the excitation of an atom by energy absorption. As review from Freshman Chemistry:
Remember the energy levels allowed in an atom: 1s,2s,2p,3s,3p,4s,3d,4p, etc.
Shorthand Notation
1H
1s1
6C
1s2 2s2 2p2
28Ni
1s2 2s2 2p6 3s2 3p6 4s2 3d8
Filled Shell Configuration
1H
1s1
6C
[He] 2s2 2p2
28Ni
[Ar] 4s2 3d8
•Every element has its own signature of energy levels where certain quantized energies are required to cause excitations, i.e. the energy spacings
are different.
•Excited atoms may lose energy and emit a photon of light and return to a lower energy state.
•An atom's emission of the quantized wavelengths of light provide a series of narrow bands termed line spectra.
Atomic instrumental methods typically monitor either the atomic specie's absorption or emission of light energy.
the emission spectra for Na and H in the visible region are shown.
ABSORPTION
Or laser
EMISSION/FLUORESCENCE
DETECTORS: PMT, photodiode
Figures of Merit
· Analyte concentrations: part per million (ppm) to part per trillion (ppt) level
· Precision: 1-2%
· Because spectrum is so sharp(narrow), little overlap of different elements (can do multiple analysis).
· Able to Measure over 60 elements in a sample.
Atomic spectra produce optical spectra of gaseous atoms with bandwidth ~0.001 nm; whereas molecular species have optical spectra
with bandwidth ~100 nm, SEE TEXTBOOK !!!
Two methods used in atomic spectroscopy.
i) Atomic absorption spectroscopy (AAS)
AA Instrumentation:
1) hollow-cathode lamp
2) monochromator
3) flame or graphite furnace(sample cell)
4) Beam chopper {not shown (modulation)}
5) detector
6) read out display
The flame converts the sample to atomic vapor and the atoms absorb light produced from the Hollow cathode lamp. The sample cell can be either
the burner flame or a graphite furnace! The monochromator allows the selection of the wavelength of light that reaches the detector. There is no
reference cell, the resonance line light is modulated using a mechanical chopper which alteranetly blocks the light beam reaching the detector.
Thus the burner flame light reaching the detector can be compensated.
The detector typically is a PMT.
A working calibration curve involving standards is used to determine analyte concentrations.
Can use a dual beam instrument, where the lamp source light is split. The sample light beam is directed through and reference beam around the
sample cell. In this setup, the baseline should be more stable due to decrease in source intensity fluctuations of the instrument.
The AAS light source:
Hollow cathode Lamp: Since atomic absorption is a Quantized process, we need light at specific narrow line wavelength. Therefore, the HC
lamp must contain the element we are trying to determine. The lamp consists of inert gas, an anode and a cathode sealed in quartz glass. The gas
is ionized, which collide with the metal and produce a sputtering effect. These excited metal atoms give the light energy, which the sample
analyte can absorb.
[-]  e-  [+] --->
cathode anode
Ar°--->Ar+--->M°--->M*--->M° + 
1) Flame burner head cell path--
AA sample cells:
·
·
·
·
A liquid sample is introduced into an atomizer whose temperature is 2000–3000K.
Sample, oxidant, and fuel are combined and nebulized–broken into small droplets
Droplets entering the flame lose their water through evaporation; then the remaining sample vaporizes and decomposes into atoms
The most common fuel-oxidizer combination is acetylene and air (flame temp: 2400–2700K)
· Hotter flames are needed for refractory elements (those with high vaporization temperatures) or to decompose species such as metal
oxides or hydroxides.
· When a hotter flame is required, the acetylene–nitrous oxide combination is usually used (flame temp: 2900–3100K)
· Sample amounts: minimum needed is 1–2 mL.
Graphite furnace cell path--
Furnaces
•Graphite furnace offers greater sensitivity than a flame and requires a smaller volume of sample.
•A graphite furnace (at ~2000K) confines the atomized sample in the optical path for a residence time of several seconds resulting in high
sensitivity.
•Sample amounts: minimum needed is 1–2 µL.
· The GF Instrumental configuration is difficult to operate.
· Monochromator: wavelength selection ~ diffraction grating
Detector: photomultiplier tube
Data:
Calibration Curve: Usually make a "best fit" plot using different concentrations of standards and their absorbances. For example:
Copper in sample: using standards and linear regression curve.
Data from "CU6 96 DATA"
0.6
y = 3.3071e-3 + 5.3150e-2x R^2 = 0.998
0.5
ABS
0.4
0.3
AB S
0.2
0.1
0.0
0
2
4
6
8
10
conc(ppm)
Standards
Std1  1.0 ppm
Std2  3.0 ppm
Std3  5.0 ppm
Std4  7.5 ppm
Std5  10 ppm
Abs 0.046
Abs 0.168
Abs 0.281
Abs 0.401
Abs 0.529
unknown reading: Abs= 0.156  Conc = ?
What if we had taken an absorbance reading of 0.769?
Is this a problem? What would you suggest we do?
12
ii. Emission, Inductively Coupled Plasma
DETECTORS:
Any for fluorescence
Also an option:
ICP +MS
SEE TEXTBOOK !!!
Inductively Coupled Plasma AE (or MS, see later)
AE-atomic emission, excited atoms emit
-higher temperatures of plasma ( 6000 K +,
see Boltzman distribution textbook)
-ultrasonic nebulizer, better detection limit (ICP/AE 0.3 ng/g,
ICP/MS 0.001ng/g, ultra trace)
-better stability for fluorescence
-inert Ar+ environment
-<50MHz induction coil
Ar producing plasma
Ar coolant
-
(ii) Atomic emission spectroscopy (AES): overview
Similar to AAS but no light source (HC) is needed.
Some of the atomized atoms are promoted to excited electronic states by collisions with other atoms. The excited atoms emit their
characteristic radiation as they return to their ground state.
· an emission technique
· Allows simultaneous analysis of different species.
· Several methods of excitation are possible: flame, a spark, or Inductively coupled Plasma (ICP).
The other methods for excitation are not as efficient in exciting the analyte (atoms) as ICP. The analyte signal increases as the number of
same excited atoms increases. As According to the Boltzman Distribution;
N*/No = (g*/go) e-E/RT
COMPARISON OF AAS vs AES
Atomic absorption (AAS)
•an absorption technique
•one analyte at a time
•cheaper analysis than AES
•requires analyte specific HC lamp source
•can use a flame or graphite furnace to heat sample
•works with metals and a few metalloids
•some matrix effects
•less signal variation due to temperature effects
•for a graphite furnace, it confines the sample to light path for longer times giving enhanced signal, greater sensitivity, less sample requirements
Atomic Emission (AES)
•AES is more expensive
•ICP instrumentation somewhat more complicated to operate
•AES require less sample preparation and gives simultaneous analysis
•doesn't require different lamps( light sources).
•Uses PDA or a series of detectors to monitor different wavelengths (different element ID's)
Interferences in Atomic Absorption Spectroscopy
The detection limit is the concentration of an element that gives a signal equal to twice the peak-to-peak noise level of the baseline.
IInterference is any effect that changes the signal when analyte concentration remains unchanged.
Types of Interferences
1. Spectral – unwanted signals overlapping analyte signal
2. Chemical – chemical reactions decreasing the concentration of analyte atoms
3. Ionization – ionization of analyte atoms decreases the concentration of neutral atoms
What about interferences? How do we over come interferences?
Chemical interference -- most common in AA, due to a thermally stable compound present, which influences the analyte's energy absorption.
Two ways of controlling: use hotter flame or add a releasing agent (something that ties up the competing species).
Ionization interference-- flame too hot causes ionization of analyte. Add alkali metals, which have lower ionization potentials to suppress analyte
ionization.
Matrix interferences -- due to nature of sample, the junk in it! Dilute the sample if possible to reduce effects or use "method of standard
addition", the sample is spiked with a standard, readings plotted and the concentration is extrapolated from plot.
SEE TEXTBOOK
ICP MS
Mass spectroscopy
CHAPTER 21 - textbook