INTRODUCTION TO OPTICAL METHODS

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