INTRODUCTION INTRODUCTION TO OPTICAL TO OPTICAL
METHODSMETHODS
Many analytical methods are based on the interaction of radiant energy with matter.
Recall: hE
hcE
c
THE NATURE OF RADIANT ENERGY
Dual nature of electromagnetic energy – behaves as:- waves or- discrete packets of energy (photons)
h = Planck’s constant = 6.62610-34 J s = Frequency = Wavelengthc = velocity of radiation = 2.998108 m s-1 through a vacuum
All electromagnetic radiation travels at the same speed, c
Energy
The interactions of radiations with chemical systems follow different mechanism and provide different kinds on information.
Valence electrons
Molecular vibrations
Molecular rotations
Atomic/ molecular transitions:
Partial energy level diagram for valence electrons in sodium atoms.
Outer valence electrons can absorb photons and move to higher energy level
Ground state
INTERACTION OF RADIATION WITH MATTERElectron configuration of Na: 1s2 2s2 2p6 3s1
wavelengths
Irradiated with light containing wavelengths 589.00 and 589.59 nm outer valence electrons absorb photons and transfer to 3p levels
excited state -absorb photons
h
Excited electrons have a strong tendency to return to ground state emit photons of definite amount of energy
Na line (589 nm): 3p 3s transition
Analytical application of resonance absorption and radiation = atomic absorption spectrometry
Ground state
h
Other alkali metals also emit characteristic colours when placed in a high temperature flame.
Li line: 2p 2s transition
K line: 4p 4s transition
With a highly energetic source, many electrons (not only outer electrons) can be excited to varying degrees
Resulting radiation contains many discrete and reproducible wavelengths
Mostly in UV-Vis regions
Analytical application = emission spectrometry
EMISSION
FLUORESCENCE
The energy gained by a molecule on the absorption of a photon does not remain in that molecule, but is lost by several mechanisms.
Part of the energy is converted to heat, lowering the net energy of the molecule to the lowest vibrational and rotational level within the same electronic level
The remainder of the energy is the radiated, returning the molecule to the ground state
For example:
FLUORESCENCE
h
heatLowering of energy to the lowest vibrational and rotational level within the same electronic level
The remainder of the energy is the radiated, returning to the ground state
Radiation source
The intensity of the response for each analyte must be calibrated with standard solutions of known concentration of each analyte. A calibration curve of signal vs concentration of analyte is then drawn for each analyte.
Use concentration range where:- calibration is linear i.e. concentrations must not be too high to prevent curvature- concentrations are high enough to give good signal-to-noise ratios
Intensity will depend on instrument parameters, therefore need to calibrate each time instrument is turned on or the setting are changed.If a large batch of samples are being analysed at once, check signal of standard periodically to ensure the is no drift in the signal.Analyse reference materials to check accuracy.
Match standards to samples as far as possible.
QUANTITATIVE ANALYSIS
Resolution:
Sensitivity:
Related to
- signal-to-noise ratio
- detection limit
Related to - peak overlap- selectivity
NOMENCLATURE
ATOMIC ABSORPTION ATOMIC ABSORPTION SPECTROMETRYSPECTROMETRY
At sufficiently high temperatures most compounds decompose into atoms in the gas phase.
Samples vapourised at 2000-6000 K
Signal measured - atomic absorption or emission at characteristic wavelengths
In atomic spectroscopy:
High sensitivity – ppm levels
High resolution – ability to distinguish one element from another in complex samples
Ability for simultaneous multi-element analysis
1-2% precision – not as good as some wet chemical methods
1 ppm = 1mg/kg 1 mg/L for aq
solutions
Electronic transitions can then occur when energy is absorbed or emitted.
Flame temperature = 2000-3000 K
Solution is aspirated into a flame
causes solvent to evaporate
remaining solid is atomised in flame
FLAME AAS
Some of these atoms can absorb radiant energy of a characteristic wavelength and become excited to a higher electronic state.
In atomic absorption, energy from a light source is absorbed the radiant power decreases as it is transmitted through the flame
The higher the concentration of a solution the more atoms there are the more radiation is absorbed.
Atomic absorption
FLAME
SOURCE
bkPPo
ln
or ebkPPA o loglog
k = absorption coefficient
b = path length
Po = intensity of source
P = intensity of radiation measured
A = absorbance
Therefore:A k concentration
Recall:The higher the concentration of a solution the more atoms there are the more radiation is absorbed.
~10 cm
INSTRUMENTATION
HOLLOW CATHODE LAMP
To create frequencies of radiation that are absorbed by the analyte, the cathode must be of the same element as the analyte.
Energetic Ne+ or Ar+ ions accelerated towards and bombards cathode where atoms vapourise and emit radiation
Apply potential such that currents of 1-50 mA flow
Inert gas ionises at anode.
Contains inert gas (Ne or Ar)
measures the amount of light that passes through the flame
(the rest is absorbed)
tuned to a specific wavelength and slit width separates the selected absorption line from other lines emitted
from the source
MONOCHROMATOR
DETECTOR
NEBULISER AND BURNERSample must be in the form of small droplets when it passes into the flame – done by the nebuliser.
(support gas)
Droplets are mixed with combustion gasSample drawn up in
capillary by decreased pressure of expanding gas – Venturi effect
Large droplets condense
(Larger drops)
Maximum flame temperatures:
!!!
Air-acetylene flame – most common, BUT…- Some elements need hotter flame to atomise fully- Some elements form refractory oxides in the flame which are not
atomised at the lower temperatures
Acetylene-nitrous oxide flame:- reducing flame prevents oxide formation- high temperatures remove many chemical interferences
BUT increased ionisation of many elements occurs at higher temperatures: e.g. Na Na+ + e-
Results in loss of sensitivity (fewer neutral atoms)
NB: Careful when lighting and turning off the burners – the order is important!
For example:First light and air-acetylene flame, then convert to nitrous oxide-acetylene flame. Reverse order for turning off.
Use the correct burner for the type of flame used hotter flame, narrower and shorter slot.
Choice of wavelength
Ratio of gases in mix
Aspiration rate of solution
Height of burner position of measurement in flame
OPTIMISATION OF SIGNAL
Monitor absorption while aspirating solution of test element and adjusting conditions.
FURNACE ATOMISERS
Instead of using a flame to atomise the sample, a furnace can be used.
Produces significantly lower detection limits than flame AAS.
Much smaller sample size is required.
Heating occurs in an inert atmosphere to prevent oxide formation
BUT:
Interferences are great
Precision is poorer
Graphite furnace
QUANTITATIVE ANALYSIS
See section under Optical Methods!
Interferences:
There are a range of interferences which can affect the absorption signal which could lead to erroneous results. A few of these are mentioned here.
Analyte element combines with other elements and production of neutral atom in flame is decreased.e.g. Ca2+ combines with PO4
3- to produce calcium pyrophospate in the flame Add releasing agent
e.g. EDTA complexes with Ca2+
Matrix match standards
Acids frequently cause depression in signal Matrix match standards
Chemical interferences:
Some elements ionise easily in the flame e.g. alkali metals cause decrease in no. of atoms in flame decrease in sensitivity Add ionisation suppressant
high concentration (~200-1000 ppm) of other easily ionisable elements e.g. Na, K to (suppresses ionisation of analyte element)
Matrix match standards
Altering physical properties of sample solution
e.g. viscosity affects aspiration, nebulisation etc.
Ionization interferences:
Physical interferences:
INDUCTIVELY COUPLED INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION PLASMA OPTICAL EMISSION SPECTROMETRY (ICP-OES)SPECTROMETRY (ICP-OES)
Excitation sources powered by electrical energy (we will consider the ICP source)
• Excitation source transforms the sample to a plasma of atoms, ions etc. that can be electronically excited.
• Deactivation of these excited states produces radiation which are sorted by wavelength.
Recall: Every element has characteristic spectra.
Simultaneous multi-element determinations!!
EMISSION SPECTROMETRY
Atomic emission
ICP DISCHARGE
ICP discharge is caused by the effect of a radio frequency field on a flowing gas.
Coil is energised by radio frequency generator (5-75 MHz).
Ar(g) flows upward and transports sample through a quartz tube inside a copper coil or solenoid.
The radio frequency signal causes a changing magnetic field inside the coil in the flowing Ar(g).
The changing magnetic field induces a circulating (eddy) current in the Ar(g) which in turn heats the Ar(g).
Coolant gas to protects quartz tube from hot plasma
Forms a stable plasma that is extremely hot.
Radio frequency load coil
Quartz tube
The solvent is evaporated from the solution droplets.
Only dried particles flow with the argon to the plasma.
Solution droplets formed in the spray chamber.
NOTE:
There are other sources of radiation other than ICP that are used in emission instruments, e.g.:
- AC or DC arc
- Spark
- Microwave plasma dicharge
- Laser microprobe
QUANTITATIVE ANALYSIS
Internal standards used to minimise effect of variation in instrument response - useful for multi-element techniques
See section under Optical Methods!
Spectral overlap as light is emitted by many different elements inthe sample (at the same wavelength)
Interferences:
Some chemical interferences are reduced due to high temperatures of the plasma
Spectral interferences:
DETECTION LIMITS OF SOME SPECTROMETRY DETECTION LIMITS OF SOME SPECTROMETRY TECHNIQUES TECHNIQUES
NOTE:
GFAAS is more sensitive than FAAS
ICP-MS has extremely low detection limits