Spectrophotometry - Atomic Absorption Spectrophotometry

Atomic absorption spectroscopy (AAS) is similar to flame photometry with the difference that it measures the absorption of a beam of monochromatic light by the atoms in the flame. This technique was first introduced by Alan Walsh in Australia in 1954. We will discuss the principle, instrumentation and applications one by one.

Principle:

The basic principle behind the AAS is that the free atoms normally remain in the ground state which are capable of absorbing the energy of their own specific resonance wavelength. If light of the resonance wavelength is passed through the flame containing the atoms (in sample), then part of the light will be absorbed. The atoms absorb UV or visible light and make the transitions to higher energy levels. The absorption will be directly proportional to the number of atoms in the ground state in the flame.

Instrumentation:
The major difference in the instrumentation of AAS and flame spectrophotometry is the presence of a radiation source (a particular resonance wavelength cannot be isolated from the continuous source using a prism or diffraction gratings). So, for this purpose, a hollow cathode lamp is used.

Light Source: (Hollow Cathode Discharge Lamp): It contains a tungsten anode and cathode (as can be seen in the diagram on the right) is a hollow cylindrical tube which is lined by the element to be determined. These are sealed in the glass tube filled with an inert gas like neon or argon at a low pressure. At the end of the cylinder is a window, made up of quartz or pyrex, transparent to the emitted radiation. Each element in question will thus emit monochromatic radiation characteristic of the emission spectrum of that particular element involved. So, each element has its own unique lamp which must be used for the analysis.

Nebulizer: It creates a fine spray of the sample for the introduction in the flame. The aerosol and the fuel and oxidant are mixed thoroughly for the introduction into the flame.

Atomizer: The elements which needs to be analysed needs to be in the atomic state. Here comes the role of atomizer. It breaks down the molecules into the atoms by exposing the analyte to high temperatures in a flame of graphite furnace (as explained in previous post, here).

Monochromator: A monochromator is used to select the specific wavelength of light which is absorbed by the sample and to exclude other wavelengths. The selection of the specific wavelength allows the determination of the element.

Detector: The light selected by the monochromator is directed onto the detector that typically is a photomultiplier tube that converts the light signal to electrical signal proportional to the light intensity.

Applications of Atomic Absorption Spectrometry

• It is highly sensitive technique and can measure upto parts per billion of a gram (ugdm-3)
• It is used to detect the presence of metals as impurity or in alloys.
• The minute levels of the metals could be detected in biological samples like copper in the brain tissues.
• The quantity of elements can be determined be agricultural and food products.
• It can also be used to determine the impurity in the environmental water sources like in the ocean water, river and stream water, waste water, sludge and suspensions.

Spectrophotometry - Flame Photometry

Flame spectrophotometry is a technique in which the intensity of the radiations emitted by a chemical into the flame is determined.  This basic concept of working of flame spectrometer is that, a flame, through its heat, can raise the atoms from a lower energy state to a higher energy state and when it comes back to its ground state, there is emission which is in the form of radiations. And determination of these radiations is by flame spectrophotometer.

Flame photometry can be applied in two ways as emission flame photometry or simple flame photometry and atomic absorption spectrophotometry. We will discuss the principle, instrumentation and applications of the two one by one.

Lets start with emission flame photometry or simply, flame photometry.

Emission Flame Photometry:

Principle:

Here, the solution containing the metallic salt (to be analyzed) is placed into the flame, whereby the solvent is evaporated, leaving behind only the solid. The solid is then dissociated by vaporization. The volatilization of the molecules in the solid produces free atoms which then, due to heat, excites to a higher energy level.  The emission spectrum is produced when the atoms return back to the ground state (as a result of radiation). This is the basic principle of the emission flame photometry.

Instrumentation:

Below is the basic representation of the components which are involved in flame photometry.

Nebulizers: Before the samples get into the flame, they must be converted to a fine spray, i.e., they must be nebulized. This is necessary as the large drops will not be able to stay in the hottest area of the flame for a long time and hence, will be difficult to volatize and excite.

Atomizers or Flames: It converts the sample or the analyte to free atoms. The atomizers can be flame atomizers or graphite rod atomizers.

Flame atomizers: To create flame, we need to mix an oxidant gas and a fuel gas. Generally, air-acetylene flame or nitrous oxide-acetylene flame is used (in the above diagram, this type is depicted). 

Graphite rod atomizers: These uses graphite rod instead of the flame.  The graphite rod is a small cavity in which the sample can be pipetted. These tubes are heated using a high current power supply such that the temperature can raise as high as 2500 degree Celsius. As a result, the sample is vaporized or atomized.

Monochromators: A monochromator is used to select a specific wavelength of light which can be absorbed by the sample while excluding other wavelengths. Generally, a simple filter is used. However, in sophisticated instruments, the prisms or diffraction gratings are used.

Detectors:  The light selected by a monochromator is directed into a detector, which is generally a photomultiplier. It converts the light signal into the electrical signal which is proportional to the intensity of the light.

Applications of flame photometry:

• It is used to determine even the small quantities of metals like lead, calcium, mercury etc.
• So, it is used in the determination of sodium, potassium, calcium, lithium etc. in the biological samples (like serum, interstitial fluids etc.).
• It is used in the determination of lead in the petrol.
• It is used in determination of calcium and magnesium in the cement.

In the next post, we will discuss about atomic absorption spectrometry.