| Analytical Instrumentation
II CTEC 2431 |
 |
| Analytical Instrumentation
II CTEC 2431 |
|
| ATOMIC ABSORPTION SPECTROPHOTOMETRY and INDUCTIVELY COUPLED PLASMA | Powerpoint |
| Atomic Absorption Components |  |
| Atomic Absorption Sample and Standards Preparation | |
| Inductively Coupled Plasma (ICP) |
ATOMIC ABSORPTION 
Atomic absorption measures metallic elements.
NEBULIZATION OF LIQUID SAMPLES
Nebulization is where the sample is converted to a fine mist of finely divided droplets but using a jet of compressed gas. The flow carries the sample into the atomization region.
PNEUMATIC NEBULIZERS (most common)
Four types of pneumatic nebulizers:
- Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary (Bennoulli effect). This is also referred to aspiration. The high velocity breaks the sample into a mist and carries it to the atomization region.
- Cross-flow - the jet stream flows at right angles to the capillary tip. The sample is sometimes pumped through the capillary.
- Fritted disk - the sample is pumped onto a fritled disk through which the gas jet is flowing. Gives a finer aerosol than the others.
- Babington - jet is pumped through a small orifice in a sphere on which a thin film of sample flows. This type is less prone to clogging and used for high salt content samples.
ULTRASONIC NEBULIZER
The sample is pumped onto the surface of a vibrating piezoelectric crystal. The resulting mist is denser and more homogeneous than pneumatic nebulizers.
ELECTRO-THERMAL VAPORIZERS (ETV)
An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer.
INTRODUCTION OF SOLID SAMPLES
If one uses solid powders, one may avoid the problem of dissolving the sample. But the procedures by which a solid sample is introduced into the atomization device are very difficult to calibrate and there are problems with accuracy and precision.
Various methods have been used but none of these techniques match the results of using a solution to introduce the sample into the atomization region.
Some the the proposed techniques are as follows:
- direct manual insertion of the sample into the atomization device. The sample must be ground into a fine powder on placed on a probe that is inserted directly into the atomization device.
- electrothermal vaporization (via a carbon rod or boat) of the sample which is carried by an inert gas into the atomization region.
- arc and spark ablation where the interaction of a spark with the surface of a solid sample (the sample must be conductive or mixed with a conductive) produces a "plume" of vaporized sample which, in turn, is carried off by an inert gas.
- laser ablation in which a focused laser beam is directed onto the surface of a solid sample producing a "plume".
- glow discharge (GD) technique where both sample introduction and sample atomization occur simultaneously. Argon gas is placed between two electrodes with a DC potential of 250 - 1000 V. The potential causes the argon gas to break down into charged argon ions and electrons. The charged ions are accelerated and attracted to the cathode which contains the sample. Neutral sample atoms are ejected by a process known as "sputtering".
FLAME ATOMIZATION
The sample is aspirated through a nebulizer in which the jet of gas is a mixture of oxidant and fuel. The flame acts as the atomization region.
Within the flame is a complex set of processes.
The nebulizated mist is desolvated (the solvent is evaporated) leaving a finely divided solid molecular aerosol. The solid is then volatized which produces an atomic gas. These gaseous molecules can be excited themselves or dissociated into atoms or atomic ions which in turn can be excited.
With the multitude of processing occurring in the flame, the atomization step is very critical to the success of the analysis. The type of flame is thus critical to process.
Types of Flames
Types of Flames (Fuel/Oxidant) Temperature oC Velocity (cm/sec) methane/air 1700 - 1900 39 - 43 methane/oxygen 2700 - 2800 370 - 390 hydrogen/air 2000 - 2100 300 - 440 acetylene/air 2100 - 2400 158 - 266 acetylene/oxygen 3050 - 3150 1100 - 2480 acetylene/nitrous oxide 2600 - 2800 285 At temperatures of 1700o - 2400oC, only easily decomposed samples can be atomized. More refractory samples require higher temperatures.
Burning velocities are also important since flames are stable within only certain gas flame rates. The gas flow rate must exceed the burning velocity otherwise a flashback occurs. As the gas flow rate increases, the flame rises above the burner to a point where the gas velocity is equal to the burn velocity. This is the region where the flame is the most stable. At higher flow rates, the flame moves to high and is blown off the burner. The flow rate of the fuel/oxidant is very important.
Flame Structure
Three important regions of the flame are:
Primary combustion zone - thermal equilibrium it is not reached in this zone so is not used. Interzonal region - rich in free atoms and is widely used. Secondary combustion zone - stable oxides are formed. Oxides do not absorb at the designed wavelength. Therefore, adjustment of the region of the flame to the entrance slit is critical.
Flame Absorbance Profiles
Adjustment of the position of the flame relative to the entrance slit is very important.
A different portion of the flame may be needed to be used for different elements.
It is best to adjust the flame in relation to the entrance slit to obtain the maximum absorbance.
Flame Atomizers
A typical atomizer is a concentric tube nebulizer with a laminar flow burner.
The aerosol, which uses the oxidizer flow, is mixed with the fuel and passes a series of baffles which remove the larger mist and only passes the finer solution droplets. The removal of the coarse mist means that most of the sample is actually drained from the mixing chamber.
The aerosol / oxidant / fuel mixture are then combusted in a slotted burner that provides a flame from 5 to 10 cm in length.
These laminar flow burners provide a relatively quiet reproducible flame with a long sample path length.
Gas Regulators
The flow of oxidant and fuel is very important and needs close control. It is desirable to vary the ratio of fuel to oxidant for various metals. For example, a metal which readily forms oxides may work better with an excess of fuel in the mixture.
The rate of flow is normally controlled by rotameters.
PRINCIPLE OF OPERATION
Atomic absorption spectrophometry is based on the simple operating principles as visible spectrophotometry.
The radiant energy is measured using a solvent blank.
The sample dissolved in the solvent is atomized into a flame. The metal of interest absorbs energy at a particular wavelength.
The absorbance is then measured by the detector. Absorbance is equal to the radiant energy as measured through the blank minus the radiant energy remaining after absorbance by the sample.
The system follows the Beer-Lambert Law
A = abc
The amount of light absorbed is a function of the number of absorbing atoms in the light path. The number of atoms is controlled by the cell path length and the number of atoms in the sample.
Concentration of the sample is directly proportional to the absorbance but not % T.
SOURCES OF RADIANT ENERGY
Atomic absorption lines are very narrow (0.002 - 0.005 nm) and the excitation energies are unique for each element. For Beer’s Law to work, the bandwidth of the radiant source must be narrow relative to the absorption peak. The problem is that monochrometers produce bandwidths significantly greater than the narrow bandwidth needed for atomic absorption. This, in turn, would give rise to deviations in Beer’s Law.
What needs to be produced are bandwidths from the radiant source narrower than the absorption peaks. To produce these narrow bandwidths the metal to be analyzed is used as a source of the radiant energy thus, a separate source is needed for each element.
HOLLOW CATHODE LAMPS
The radiant energy source for an atomic absorption spectrophotometer is a hollow cathode lamp which gives off radiation at specific wavelengths.
A hollow cathode lamp uses a tungsten anode and a cathode coated with the metal of interest. The tube is filled with neon or argon at a pressure of 1 - 5 torr.
The inert gas is ionized when an electrical potential of about 300 V is applied across the electrodes. This generates a current of about 5 - 15 ma as the ions migrate to the electrodes. If the potential is great enough, the ionized cations have enough kinetic energy to dislodge some of the metal atoms on the cathode surface. This produces an atomic cloud (sputtering). Some of the sputtered atoms are in an excited state which emit the metals characteristic spectra when they return to their ground state. The metal atoms eventually diffuse back to the cathode or the glass walls.
To reduce the area they can diffuse back to, the cathode is normally shielded by an open glass tube.
The hollow cathode lamp consists of two electrodes (cathode and anode) with a potential voltage across them. The glass container is evacuated and filled with an inert gas (normally Ne or Ar). There is a small transparent window at the business end of the lamp.
The electrical potential between the anode and cathode causes the inert gas to ionize and bombard the cathode with ions. Atoms on the cathode are ejected and raised to an excited energy state. When they return to the ground state they emit radiation at a specific wavelength based on the cathode coating.
Hollow cathode lamps are selected on the basis of the metallic elements of interest.
SOURCE MODULATION
The flame also emits radiation. Most of those wavelengths are eliminated by the instruments monochrometer, but radiation emitted corresponding to the desired wavelengths would not be eliminated.
To eliminate the effects of flame radiation, the source output is modulated so that its intensity fluctuates at a constant frequency. The detector then receives two signals, an alternate one from the source and a continuous one from the flame. The unmodulated DC signal is removed by a highpass RC filter and passes the AC signal. A chopper is used to chop the beam to the desired frequency.
