Thursday, May 12, 2016

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:
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.

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.