Atomic Absorption Spectroscopy

• Atomic Absorption Spectroscopy is a common and reliable technique used to identify metals and metalloids.
• It can analyze over 62 elements within a sample and measure their concentrations.
• The first atomic absorption spectrometer was created by CSIRO scientist Alan Walsh in 1954.
• The periodic table highlights elements detectable by atomic absorption in pink.

Principles

• Free gas atoms generated in an atomizer absorb radiation at specific frequencies to quantify ground state atoms in the gaseous state
• Atoms transition to higher electronic energy levels by absorbing ultraviolet or visible light.
• Analyte concentration is determined by the amount of absorption.
• Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

Advantages

• High sensitivity levels are around [10-10g]
• Good accuracy with a relative error of 0.1 ~ 0.5%
• High level of selectivity

Disadvantages

• A resonance line source is needed for each element to be determined.

Components

• Common radiation source is Hollow Cathode Lamps in AAS
• Hollow Cathode Lamps contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined.
• These are sealed in a glass tube filled with an inert gas (neon or argon) and each element requires its own lamp.
• Nebulizers suck up liquid samples at a controlled rate in order to create a fine aerosol spray for introduction into flame
• This mixes the aerosol, fuel, and oxidant.

Atomizers

• Elements must be in atomic state to be properly analyzed
• Atomization is essentially the process of separating tiny particles into individual molecules and breaking molecules into atoms
• This is done via high temperatures within a flame or graphite furnace,
• Flame atomizers mix an oxidant gas and a fuel gas to create a flame.
• An air-acetylene or nitrous oxide-acetylene flame is often used
• These atomizers are typically used with liquid or dissolved samples.
• Graphite tube atomizers use a graphite-coated furnace to vaporize samples.
• GFAAS samples are deposited in a small graphite coated tube.
• They are heated to vaporize and atomize the analyte with a high current power supply

Monochromator

• It is very important in an atomic absorption spectrometer
• Monochromators select the specific wavelength of light the sample absorbs
• It also excludes other wavelengths, which allows for determination of the selected element in the presence of others.

Detector

• The detector is typically a photomultiplier tube.
• The photmultiplier function is to convert the light signal into an electrical signal proportional to the light intensity.
• Signal amplifiers process the electrical signal which can be displayed for readout, or fed into a data station for printout.

Calibration Curve

• Calibration curves determine the unknown concentration of an element in a solution.
• The instrument is calibrated using several known solutions.
• The absorbance of each known solution is measured and generates a concentration versus absorbance curve
• After the sample solution is fed into the instrument, the absorbance of the element in this solution is measured.
• An elements unknown concentration is then calculated from the calibration curve.

Applications

• It Determines small amounts of metals like lead, mercury, calcium, and magnesium
• It is used in environmental studies to check the components of drinking water, ocean water, and soil.
• It is used in the food industry and pharmaceutical industry

Lead Determination in Contaminated Soil

• Collect approximately 50g samples from specific sampling points on the site.
• The sampling point should include surface soil and two further samples taken at 0.5 and 1.0 m depth.
• Document the exact location of these points to enable further sampling.
• Then weigh around 1g of sieved soil before transferring to a 100 ml beaker.
• Afterwards, add 20 ml of 1:1 nitric acid.
• Then boil gently on a hot plate until volume of nitric acid is reduced to 5 ml.
• Further steps include adding 20 ml of deionized water and boiling until the volume is 10 ml before cooling the suspension and filtering through whatman filter paper.
• Then, wash both the beaker and filter paper using deionized water until a volume of about 25 ml is obtained before transferring the filtrate to a 50 ml flask.
• Then make up to the mark with deionized water.
• Finally, setup acetylene-air flame with resonance line 217 nm where Standard lead solutions containing 1-10 mg ml-¹ are suitable for measurement.