Chapter 20 Atomic Spectroscopy
Problems: 1, 2, 6, 8, 9, 10
20-1 What is Atomic Spectroscopy
Atomic Spectroscopy is principal tool for measuring metallic elements at ppm
(parts per million level - :g/ml).
A block diagram is shown in figure 20-1.
-Liquid sample is aspirated (sucked) through a plastic tube into a flame
-Flame evaporates all liquid, breaks all molecules into atoms, and excites many
atoms into high energy states
-Concentration of elements measured by absorption or emission of light from
atoms in flame (figure 20-2)
Atomic Absorption - measure concentration by absorption of light
Requires a light source of proper wavelength
Atomic Emission - measures concentration by emission of light
No light source required
Both require monochrometer to tune to proper wavelength
-Compare figures 18-5 or 18-6 with figure 20-3
spectra of gaseous atoms incredibly sharp when compared to molecules
in solution (line width 100 nm in solution, .01-.001 nm in atomic vapors)
Makes these methods very selective, can often analyze for many different
elements in one sample with no interference
-Measures concentrations down to ppm so very sensitive (In fact you may have
to dilute a sample down to get it acceptable for analysis)
-precision is only about 2% so not terribly precise
19-2 Atomization: Flames, Furnaces, and Plasmas
Atomization - Process of breaking analyte into gaseous atoms
Older or inexpensive machines (like ours) use a combustion flame
Newer, more expensive instruments use either a electrically heated graphite
furnace or inductively coupled argon plasma to atomize samples
Flame (our machine)
Typical premix burner shown in figure 20-4
Sample, oxidant, fuel all mixed together before hits the flame
Flow of oxidant gas draws liquid through small opening and makes into a
mist (this part called a nebulizer) similar to air brush for painting or old
fashioned perfume bottle
Mist directed at a glass bead to further break droplets apart
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Then mist directed down a series of baffles
Mixes gases and mist together
Catches larger droplets and pulls them out of the gas
Only about 5% of original sample actually makes it into the flame!
In flame, solvent first evaporates from around each mist droplet
Leaves individual atoms in flame
Many metal atoms (M) oxidized to oxides (MO) or hydroxides (MOH)
In MO or MOH form, no longer atomic, so no longer absorbs or
emits at same wavelength, so signal decreases
‘Rich’ flames (yellow)- rich in fuel - presence of excess C in fuel
tend to reduce MO and MOH to M so can get better signal.
However, rich flames are cooler, so not a many M atoms are
excited state
‘Lean’ Flames (Blue) - low in fuel, high in oxidant, more tendency to
oxidize metals, but hotter flame so more M in excited state
Choosing right flame condition is often a trade-off, will use different
flames for different elements or for absorbtion vs. emission
Common flame gases and temperatures shown in table 20-1
Our machine set up for Acetylene/air (2,400-2,700K) or
acetylene/nitrous (2,900-3,100K)
Other variables include flow of gases to flame, height above the
flame that you do your measurement, ratio fuel to oxidant. So lots
of variable to tweak if you want to get optimum conditions for a
particular experiment
Furnace
Typical electrically heated graphite furnace shown in
More sensitivity than flame
Less sample used than in flame
1-100 ul of sample injected in hole in middle
Windows at ends keep sample from escaping
Furnace heated to 2,250K for <7 sec
(Furnace heats up at rate of 2000K/sec!)
Furnace made of graphite - so kept in inert atmosphere so doesn’t burn
up!
Why more sensitive?
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Sample constrained to light path for several seconds
(In flame passes through light in a fraction of a second)
Less sample - here on 1-100 ul of sample in furnace - In flame may
aspirate 10 ml through flame to get a good reading
Look a bit more on construction. Sample actually sits on a platform called
a L’vov platform. This allows to heat more uniformly and reproducibly
Look a bit more at heating. Not simply heating the bejasus out of it.
Typically in 3 phases (similar to broccoli lab)
Heat for 125 fo 20 s to evaporate water
Heat to 1400 for 60 s to char (burn all organic matter)
Heat to 2,100 for 10 sec to atomize - during this time you
turn on the detector and integrate the reading over the entire
10 seconds
Heat to 2,500 for 3 sec to burn off any residue and start over
So you can see there are some variables in optimizing heating
Also variables in what the sample is dissolved in
This is called the matrix
Can add different compounds to matric called matric
modifiers to change evaporation characteristics of sample
Can also have things in the sample itself that affect sample
evaporation. Bottom line - not a simple and just stick it in
and heat it up
Inductively Coupled Plasma (ICP)
Photo at start of chapter on page 410 and figure 19-6
Here we are going to have a ’flame’ at about 10,000K 3-4X normal flame
To get it we are going to use an inert gas, usually Ar Will explain in a
minute
Of all techniques has best signal because of high temperature and
stability. However is last technique you will see at a place like this
because very expensive to buy and to run.
How does it work
Start with inert gas, Ar
Put some coils of wire around it
Run radio frequency AC current through the wire
Current running in wire makes AC electric field outside the
coil (this is the inductively coupled part)
AC field rips electrons off of Ar atoms to make Ar+ ions and
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e-. (Now have a gas of Ar+ ions so this is the plasma )
Electrons ramming back and forth in AC field heat up to
10,000 K
So hot coils will melts so need to have the coils
cooled with water
By convection the hot plasma will move up, so need
to replace Ar in induction area with fresh Ar
Also blow Ar past to help cool!
That is why looks like a flame
Can use a nebulizer system similar to that used in a flame to
introduce sample into ICP flame. But can do even more
high tech with a ultrasonic nebulizer. Spray sample onto a
crystal vibrating at 1 MHz. Gases then mixed, and heated to
evaporate solvent. Get sensitivity up by a factor of 10
Picture of apparatus figure 20-7
20-3 How temperature affects Atomic Spectroscopy
Have mentioned temperature several time as important. Why?
Has to do with number of atoms in excited state
Need to look at Boltzmann distribution
N* is number of atoms in upper (excited) state
N0 is number of atoms in lower (ground) state
g* is number of upper states with the same energy (>1 if degenerate and
more than one state have the same energy)
g0 is the number of lower states with the same energy (again is >1 if the
state is degenerate)
)E is the energy difference between the state
k is the Boltzmann constant (1.381x10-23 J/K)
T is the temperature in K.
Let’s calculate the Boltzmann distribution for the lowest state of a sodium atom
at 298K, 2500K (a typical air/acetylene flame) and 10,000K (a typical ICP ‘flame’)
The )E for this transition is 3.371x10-19 J/atom
(Homework problem from previous chapters. What is the wavelength of
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this transition?)
Assume the excited state has a degeneracy of 2, and the ground state is nondegenerate (degeneracies should always be given in a problem)
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298K
So you would need about 1036 atoms (that is 1013 moles) before you would have
a single atom in the excited state at room temperature
2,500K
So at 2,500K only .01% of our atoms are in the excited state. Still not much, but
it is a lot better than at RT
10,000K
What does this mean for absorption or emission signals?
In absorption you start with the ground state, so the more atoms in the
ground state, the more signal. Absorption will work well at either RT or
even 2,500 because most atoms are in the ground state
In emission you start in the excited state. The more atoms in the excited
state the more signal, and yo can see that here the 10,000K of the ICP
gives you literally .174/.000115 about 1,500 time more atoms in the
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excited state or 1,500 times more signal.
Also note, if you follow the presentation in the book, it shows that at 2,600 K just
a change of 10K in the temperature will change the number of atoms in the
excited state by 4%, so if you use flame for emission, small changes in the flame
temperature can change your answer be several %, so you need a very stable
flame.
Bottom line the flame is OK for atomic absorbtion, the ICP is best for emission
(But we will still do emission with a flame because it does work, it just isn’t as
sensitive or as accurate as it could be with the fancy expensive instrument)
20-4 Instrumentation
Since we are dealing with spectroscopy, our instrument has many of the same
elements as any other spectrometer (light source, sample holder, monochrometer,
detector etc.)
The main places we will see difference are
1. Light source
2. Sample holder
3. How we subtract the background
Linewidth Problem
One of the things you should have picked up on in the previous chapters
is that to accurately measure an absorbance you need to be at an
absorbance maximum in the spectrum. Do you remember why?
(This is the only region where Beer’s Law applies...If you are on a
shoulder then the absorbance varies as a function of wavelength over the
region and Beer’s Law doesn’t work)
In our typical UV/Vis spectrometer the width of the overall transition was
20-100 nm, and the bandwidth (region that the machine could be tuned
to) was on the order of 1 nm or so.
In Atomic spectroscopy we have already seen that the transition lines are
much sharper, on the order of .01-.001 nm, so the bandwidth of our
spectrometer need to be at least 10x smaller!
In theory the bandwidth of atomic absorbtions is actually about .0001 nm,
but there are two effect that broaden the transitions into the .01-.001nm
range
Doppler effects (figure 20-11)
Just as a train whistle has a different pitch when the train is coming
toward you or away from you, and atom ‘sees’ a slightly different
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wavelength depending on whether it is traveling toward a source or
away from it, to this spread the frequencies out slightly.
Pressure Broadening
As the pressure inside the flame increases, the number of
collisions of atoms in the flame increases. The energy of these
collisions can be added or subtracted to the energy of the
transition itself, and this also can broaden out the overall range of
transitions observed.
Hollow-Cathode Lamps (figure 20-12)
This is the light source used in Atomic absorbtion
Tube filled with inert gas (Ne or Ar)
Hollow cathode (negative) made with metal we want to detect
Run a high voltage between anode and cathode
This makes Ne or Ar ionize
Ne+ or Ar+ attracted to hollow metal cathode
As these ions hit the metal, atoms of metal are ejected into the gas
As metal atom interact with energetic electrons atoms are excited,
so generate light at that metals wavelength
Since excitation is not by flame the linewidth is extra sharp
Thus we don’t need a monochrometer!
It only emits the wavelength we need
It emits is in such a narrow line (compared to the absorbtion
spectrum) that we are right at the ‘sweet-spot’ of absorbance max
Only disadvantage is that we need a different lamp for each element
Background Correction
Along with the signal from the metal you are interested in, there is always
a signal from the background, that is a signal from absorption, emission
and/or optical scattering from the sample matrix, the flame plasma, or the
white hot furnace itelf. This is especially true in the graphite ovens,
where the oven itself holds in smoke that can affect your readings. A
typical signal from a furnace/oven is not shown in text. Note that the
overall background it about .3, while the signals are from .4 to 1, so the
baseline correction, or a subtraction of .3 from all values is very
significant.
With an older machine like ours, the background correction is fairly
simple, run a blank with no metal in it as a control on the baseline for
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each experiment
With the fancier newer machines there are fancier correction techniques
Zeeman Background correction (Zay-man)
Used in absorption
Apply strong magnetic field parallel to light path
Magnetic field changes energy of atoms
Some with higher E, some lower, some no change
One with lower E emit at longer wavelength
Ones with higher E emit at shorter wavelength
Ones with same E don’t interact with light because is is in
the wrong polarization
Thus when turn magnetic field on, signal from sample
disappears (higher and lower E atoms cannot absorb the
light at original frequency, atoms at same E cannot interact
with light due to polarization)
So turn on magnetic field to get background, then turn off to
get signal. Kind of like the chopper we had on a double
beam spectrophotometer.
This text is a bit weak here. Look as Skoog, Holler & Neiman
Multielement Analysis with ICP
In our low end Flame AA/emission machine, you have to manually change
the monochrometer to the correct wavelength for each element you want
to analyze. With the more expensive ICP machines you can have it scan
the entire wavelength range and see many elements in a single sample,
but it takes time to complete a scan, so sometimes the machines are
designed with the diffraction grating fixed, and detectors set at the correct
angles for the elements you think you want to analyze for. This is makes
an individual analysis much faster, but the machine then can’t analyze
any different element.
The best solution is to match an ICP instrument with a photodiode array
detector so you can analyze many element at once without scanning.
However, the resolution in a single, linear, photodiode array is not good
enough to separate all the lines of all the element. Instead, as shown in
color plate 20, the light dispersed in 1 dimension using a grating and in a
second dimension using a prism, so the spectrum get spread out in two
dimensions, and read of using a 2 dimensional charge injection device
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(CID)
One of the more sensitive alternative is to take the output of atoms from
an ICP flame and send it directly to a mass spectrometer, that detects
atomic masses and separates elements by their atomic mass. This give
you even more sensitivity, and retains the ability to identify all elements.
This hybrid machine the ICP/MS is about the ultimate machine for
elemental analysis at this time
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Detection Limits Figure 20-16
Detection limit is defined as the amount of material needed to give a
signal that is 2x the peak to peak noise of the baseline (see figure 20-15)
The table in figure 20-16 summarizes the detection limits for various
elements using methods described in this chapter
In general Flame AA is least sensitive with detection limits in the range of
1 - 100 ng/ml (1-100 ppb)
Furnace detection limits are about 100X better (do you remember why?)
ICP emission is in between, but can approach the furnace levels
ICP/MS is even 100-1000x better, with detection limits in the ppt-parts per
trillion range!
20-5 Interference
Interference is defined as any effect that changes the signal of an analyte while
its concentration remains the same (note can increase or decrease the signal)
Type of interference
Spectral Interference - analyte signal overlaps signals from wither
element in sample, flame or furnace. Typically increases signal, so need
to subtract a background correction. Best solution is to choose a different
wavelength for analysis where interference doesn’t occur
Chemical Interference - Chemical reactions between analyte and matrix
that prevent the atomization of the analyte. One common one is that
Sulfate or phosphates anions will form nonvolitile salts with Ca+2, so its
signal decreases. Can be treated in two ways
1. Releasing agents - can add to metal to prevent formation of
interference complexs. This this example EDTA or 8hydroxyquinoline. Can also add to anion - in this case La+3
2. Use a fuel rich flame to reduce oxides or use a hotter flame
Ionization interference - mostly for alkali metals
Ionization E is lowest for these metals (remember from Gen
Chem?) . Thus the E of the flame provides enough E that a sizable
portion of atoms are in ionic instead of atomic state.
Usual fix is to add an ion suppressor, an element that ionizes more
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easily than the analyte This gives you a high concentration of the
ion of the suppressor in the flame, and this then pushes the
equilibrium of the analyte to favor the un-ionized state
Method of Standard Addition
There are all kinds of interferences. Often can take weeks to identify and
fix each different interference that might be occurring in a sample. The
method of standard addition is a way of doing an experiment that corrects
for many interferences, so you don’t have to worry about them
See section 5-3
Essentially a different kind of calibration curve
Usually make calibration curve by dissolving a standard in water
In method of standard addition make standard curve by adding measured
amounts of analyte to sample (see figure 20-17), and then extrapolate
back to the zero point, where you haven’t added anything
Why does this work? When we make a standard curve you want the
standard to resemble the sample in every way. Complex, real world
samples contain all sorts of junk, and it is hard to make a standard that
exactly matches it. Thus your artificial standard may easily leave out
some trace element that is causing an interference and changing the
signal in your analysis. In the method of standard addition, since each
sample you analyze was made from the original sample + a little more of
the standard, you know that is has all the original components, so all the
interferences are there. When you make your standard cure with the
interferences intact, the interferences cancel out from the analysis so you
don’t have top worry about them!
Virtues of ICP
Once again ICP is superior. At the high T of the ICP flame, most common
interferences disappear. Add to that the better signal, atomization, lack of
oxide formation, and low background, and you have a superior method