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Chapter 8 Intro to Optical Spectrometry
Problems: 1, 2, 4, 6, 7, 9
Three major spectrometric techniques for identification of elements in a sample
1.
Optical Spectrometry
a.
convert sample to gaseous atoms or elementary ions Atomization
b.
Observe UV/vis absorption, emission or fluorescence
2
Mass Spectrometry
a.
Again atomize sample
b.
Convert to singly charged positive ion
c.
Separate on mass/charge ratio to identify
3.
X-ray Spectrometry
a.
Atomization not needed
b.
Directly measure absorption, fluorescence or emission of X-rays
This chapter
Theoretical discussion of properties of optical spectra
Look at how to atomize samples
Next chapter (9) atomic absorption methods
Chapter 10 Atomic emission techniques
8A Optical Atomic Spectra
8A-1 Energy Level Diagrams
Figure 8-1
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Energy level diagrams of valence electrons is useful place to start
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E is linear
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Ground state is set to 0
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Upper end is set by ionization E
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Units of eV, electron volts
1eV=1.602x10-19 J
1 mole of 1 eV = 1.602x10-19 x 6.02x1023 = 96,000J or 96kJ
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Note there should be 3 p orbitals, but they are degenerate and so should
have the same E
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Instead you have 2 E’s for P based on the spin of the electron
P1/2 and P3/2 refer to spin of electron itself is same or different than
spin of orbital
If spins opposite, make opposite magnetic field and are attracted
so overall E is lower
If spins the same, make magnetic fields in same direction, and are
opposed to each other so E is slightly higher
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Same thing happens for d and f, but here E differences are so small in Na
that can be ignored, and just have a single d line
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Figure 8-2 Energy levels of Mgo (not Mg+1)
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different than Mg+1
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Na and Mg+1 had 1 electron in valence
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Mg0 has 2 electrons in valence
So no have singlet and triplet transitions as well
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Singlet state, 2 electrons opposite spin (higher E) (1S or 1P)
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Triplet 2 electrons have same spin (lower E) (3S or 3P)
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Note that Na and Mg+1 were 2S or 2P
Doublet state, 2 energy levels, that was why we had 2 p’s
Single unpaired electron
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triplet state means that see 3 energy levels
See three different P levels
D and F levels so close don’t’ worry about here?
Different levels due to spin spin interactions of electrons
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More electrons, get more complex
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As gets more complex, harder to model and theorize
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Lines in diagrams show only ‘allowed’ transitions
Other transitions are ‘forbidden’
Quantum mechanical selection rules can tell you which are which,
beyond the scope of this class
Atomic Emission Spectra
At RT all atoms are in ground state
Excite by heat of flame, electric arc or spark
Atoms or ions quickly emit radiation and return to ground
Line on previous figures indicated common emission transitions and their
wavelengths
For example two Na lines 5890 and 5896Å responsible for yellow color of
Na in a flame
Emission (fig 8-4) of Na has lots of peaks, some very intense, others
weak, some properly resolved, others unresolved
Peaks that correspond to transitions from the ground state are
called resonance lines
Atomic Absorption Spectra
In hot gaseous medium can also absorb light
Primarily observe only resonance lines from the ground state
Atomic Fluorescence Spectra
Put atoms or ions in the flame, illuminate the flame with light
corresponding to absorbance bands, will now fluoresce and get
even more band to observe
8A-2 Atomic Line widths
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Important for 2 reasons
1. if narrow then low overlap so can do several compounds at once
with no overlap
2. If narrow then need to design instrument so it can properly
monitor these narrow bands
Atomic absorption and emission lines usually symmetric, gaussian shape
around maximum value
Maximum value corresponds exactly to E needed to make quantum
transition
Based on pure quantum theory would say that width should be 0
Actually slightly broader
1.
Uncertainty effects
2.
Doppler shifts
3.
Pressure effects from atomic collisions
4.
Electric and magnetic field effects
(specialized application, not used here)
Uncertainty effects
life time of before and after transition state is finite
Gives uncertain transition time
One of our pairs of variables that are covered by uncertainty are
frequency and time
)< @)t>1
Overall can be shown (example 8-1) that uncertainty broadening makes
line widths about .0001Å.
This is call natural line widths
Doppler Broadening
Have probably already seen examples of Doppler shifts
(Train whistles, car horns)
Same thing happens with light
If the source of a light beam is moving toward you, light waves pile up,
appear to have a shorter wavelength, is the source of the light is
moving away the waves are more spread out and the wavelength
appears longer
In a hot flame atoms and ions are moving all directions in a wide
distribution of velocities
Doppler shift give line widths about .01Å (100x greater than natural line
width)
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Pressure Broadening
occurs as emitting or absorbing molecules collide with other atoms or ions
in flame
Usually between analyte ions and other combustion product in flame
Broadening in .1 to .01Å range
This is why high pressure Xenon lamps or Mercury lamps give off a
continuum instead of lines, the high pressure makes the molecules
collide and the collisions broaden distinct lines into the continuum.
8A-3 Effects of T on Atomic Spectra
Can calculate relative number of atoms in excited state over ground state using
Boltzmann equation:
Nj = number of atoms in exited state
N0 = number of atoms in ground state
Ej is the )E (in Joules) between excited and ground state
k = Boltzamnn constant (1.28x10-23J/K)
T=temp(in K)
Pj statistical factor for the number of different excited state
P0 statistical factor for number of different ground states
Let’s try this calculation following example 8-2
What is ratio of excited (Atoms in 3p state)/ground atoms(atoms in 3s state) at
RT (298 K) and 2500 K
From Fig 8-1 have two transitions 5890 and 5896Å
will use average, 5893Å
E=h< =hc/8
= (6.626x10-34 x 3x108)/5893x10-10
=3.37x10-19 J
There are 2 different quantum state for the 3s so Po = 2
(I’m not certain how they got this but I suspect, +1/2 and -1/2 electron
spins?)
There are 6 different quantum state for 3p so Pj=6
(Again I’m not absolutely certain but there are 3 P orbitals (Ms = -1,0,+1)
and +/- ½ spin on the electron for a total of 6 state)
at 298K
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So, roughly for every 1036 molecules (1013 moles - I don’t know maybe for as
many atoms as there are on the earth?) there is about 1 molecule in the excited
state at RT
At 2500K
So for every 10,000 molecules in the ground state
there are 10,000x1.72x10-4 =1.7, or about 2
molecules in the excited state. 1 in 10,000 a lot
more! And if you do the example in the book you will
se that raising the temp by 10K raises the number in
the excited state by about 5 %!
Bottom line T can be critical
Also other indirect effects
as T increases, atomization works better, so get more molecules in the
vapor phase
as T increases, peaks gt broader due to Doppler and pressure effects
As T changes, amount of ions in gas can change
All things put together Absorption and Emission techniques are largely
complementary; elements that can’t be easily analyzed using one technique, can
be analyzed using the other
Fluorescence (in theory) is most sensitive
8A-4 Band and Continuum Spectra associated with Atomic Spectra
In general at atomic absorption spectrum will contains both band and continuum
spectra in addition to the line spectra (Fig 6-13 page 132)
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Bands usually due to molecules
Continuum due to thermal radiation of hot flame gas
Sometimes the band spectra can be used to determine a concentration
More often band and continuum radiation ends up interfering with what you want,
so you must minimize artifacts by changing wavelengths, doing background
corrections, or changing atomization conditions to remove the interference
8B Atomization Methods
Atomization - process of converting sample into atomic vapor
Critical step in these methods
Several methods given on table 8-1
details of method given in various chapters
8C Sample Introduction Methods
Sample introduction - getting sample to the atomizer
For analytical technique needs to be:
a representative sample
must be reproducible
Actually is weak part in many of these methods
For solid, refractory samples - major problems
For solutions and gaseous samples - usually trivial!
First 5 atomization methods table 8-1, samples commonly introduced as
solutions, or, maybe a slurry
Methods to introduce solids of fine powders not as reproducible and has more
errors
8C-1 Introduction of Solutions Table 8-2
First 3 are most common, first four are for solutions, second four are for solids
Nebulization - converting a sample into a mist of finely divided droplets through
use of a jet of compressed gas
Mist is called aerosol
Flow of gas carries aerosol of sample to place where atomization takes place
Pneumatic Nebulizers Figure 8-9
Uses flow fo gases and liquids to get aerosol
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Have high pressure gas flow, drip fluid into flow to make into a mist
a and b are simplest
c, with fritted glass disk give the finest aerosol
d is least prone to clogging, so used with solutions of high salt content
Ultrasonic Nebulizers
Sample pumped onto surface of crystal that vibrates at >20kHz
(ultrasonic)
More complicated to build but give denser, more homogeneous aerosol
Electro thermal Vaporizers (ETV)
Put sample in closed container filled with argon gas
Quickly heat sample with electric current to turns into gas
Pump a burst of gas into analyzer and analyze quickly
Hybrid Generation Techniques
Make solution to be analyzed acidic
react acidic solution with BH4This creates a metal hydride that is usually volatile
Use gas to carry volatile hydride into heated chamber to convert hydride
to atoms
Run analysis on atoms
This technique also make discrete burst of atoms for analysis
Used for Arsenic, Antimony, Tin, selenium, bismuth, lead
This technique enhances detection limits by a factor of 10-100
Most of these elements are poisonous in trace amounts, hence detecting
at low levels is important
Also, after you have made the aerosol with the poisonous metal, you have
to dispose of aerosol safely
8C-2 Introduction of Solid Samples
Introduction of solid in form of powder, metal, or particles into atomizer is more
difficult, but can avoid tedious sample preparation, sample decomposition, or
sample precipitating out of solution
Often less precise and less accurate, Need special calibration as well
Direct Insertion
Put solid sample directly in atomizer
Arc or spark atomizer, put powder directly on electrode that spark
comes from
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Electro thermal Vaporizers
Similar to those used for solutions, just place solid sample in a chamber
heat sample in a graphite or tantalum boat with AC current
sweep atoms into instrument with burst of inert gas
Arc or Spark Ablation
make a spark to the surface of the solid
spark makes a plume of particulate metals and metal atoms
transport plume to instrument with burst of inert gas
whole process called ablation
Sample must conduct electricity so can make spark
Laser Ablation
Zap surface with a focused laser burst
again make a plume of particulate matter and atoms
sweep plume into instrument with inert gas
Can be used for conducting and nonconducting samples
Glow Discharge Figure 8-10
Glow discharge, - a glow that appears in low pressure (1-10 torr.
.001-.01 atm) Ar gas between a pair of electrodes at 250 to 1000V
DC potential
Glow arises from V breaking Ar into Ar+ and eIf make - electrode the sample, as Ar+ is generated, it is attracted to
electrode, where it slams into electrode with enough force to eject
neutral metal atoms
Ejection of metal atoms called ‘sputtering’
Glow of gas can then be analyzed for presence of atoms