Atomic Emission - AES
Thermal excitation
M → M*
Radiative decay to lower energy level
M* → M + hn
Emission signal directly proportional to concentration
Holy Grail of Atomic Spectroscopy
For one sample:
The ability to measure all
elements at all ranges of
concentration at one time.
Excitation Source
The atoms are excited by energy provided by
the source.
The energy created by a flame can excite only
a few atoms, e.g. alkali metals
Other atoms (especially non-metals) need
much higher energy - plasma
If you only have a flame instrument, you can
use AES for alkali metals (and a few others),
otherwise you should use AAS to achieve
good detection limits.
Types of high energy analytical
plasmas
DC Arc
4000-5000 K
HV Spark
40,000
Direct Current Plasma
6000-10,000
Inductively Coupled Plasma (ICP)
6000-8000
Microwave Induced Plasma (MIP)
electrodeless
5000-7000
Capacitively Coupled Microwave Plasma
(CMP) electrode
5000-7000
Plasmas
Ionized gas that is electrically neutral
Very high temperature and energy
Contains ions, electrons, neutral atoms &
molecules
Inductively Coupled Plasmas
Ionized Ar flow,
sustained in a
torch by the RF
field generated by
induction coils.
Up to 20 mL/min
Ar flow
Annual cost of
several thousand
dollars
Characteristics of Plasma AES
Sufficient energy to excite all elements
Capable of doing solids, liquids, or gases
-sample introduction via nebulizer, ETV, laser ablation,
others
Tolerant to variety of solvents and solutions
Simultaneous multielement analysis
Large Linear Dynamic Range (LDR)
Low LOD
ICP-AES spectrum
ICP AES Calibration Curve
If you can interpret
your spectrum,
you can get great
quantitative
results.
Calibration curve is
plotted as log/log,
because the LDR
spans several
orders of
magnitude.
Internal Standard
An internal standard is used to compensate for
various random (and even systematic)
errors.
A big random error in plasma emission
spectroscopy is power/intensity fluctuations
of the plasma.
Reasoning: fluctuations effect on analyte will
be the same as the effect on the internal
standard.
Quantitative Analysis - Calibration
with Internal Standard
Internal standard must be
something not present
in your standards or
sample
(in this example, Y)
The signal plotted is the
ratio:
Intensity ratio = Analyte signal
Yttrium signal
Homework problem
3
500
y = 4.2781x + 7.1758
450
y = 0.7619x + 1.0441
2.5
log intensity ratio
intensity ratio
400
350
300
250
200
150
2
1.5
1
100
0.5
50
0
0
0
20
40
60
80
100
Cu co ncentration (p pm)
linear-linear plot
120
-0.5
0
0.5
1
1.5
2
2.5
log co ncentration (ppm)
log-log plot
When your LDR spans more than 2 orders of magnitude,
it can be helpful to do a log-log plot so you can see your
data points better.
ICP
Advantages
Analysis of solutions or dissolved solids
Disadvantages
ionization leads to
complex spectra
need high resolution
monochromator
LDR spans several orders of magnitude
Expensive
Detection limits in the parts per billion
range
Multielement analysis: Determine up to 70
elements in two minutes per sample
Plasma source leads to
messy background fluctuations