Atomic Absorption Spectrometry

Atomic Absorption Spectrometry

    Atomic spectroscopy is used for the qualitative and quantitative determination of 70 to 80 elements. The methods can be based on absorption, emission or fluorescence. Here , we briefly consider the most important of these techniques .

1. This technique is applied for direct analysis of metals, but it can be applied in indirect determination of non-metal.
2. It based on using of line source as excitation source.

3. The sample should be analysed in the gaseous state, so atomizers are required.

High-performance liquid chromatography

High-performance liquid chromatography

HPLC is the most versatile and widely used type of elution chromatography. The technique is used by chemists for separating and determining species in a variety of organic , inorganic and biological materials.

Liquid chromatography (LC)used to separate analytes in solution including metal ions and organic compounds. The mobile phase is a solvent and the stationary phase is a liquid on a solid support, a solid, or an ion-exchange resin.

Ion-Exchange chromatography

Ion-Exchange chromatography

Based upon exchange equilibria between ions in solution and ions of like sign on the surface of an insoluble , high-molecular-weight solid .

Clay and zeolites ,

synthetic ion-exchange resins  .

Formed during emulsion copolymerization of styrene and divinylbenzene.

The mobile phase are aqueous solutions that may contain moderate amount of methanol or other water-miscible organic solvents .

Application of Thin-Layer chromatography

Application of Thin-Layer chromatography

( a ) Qualitative Analysis :

     Retardation factor Rf

The Rf value for each substance is the distance it has moved divided by the distance the solvent front has moved .

                  Travel distance of analyte

 Rf  =         Travel distance of solvent

(b) Quantitative Analysis

1 – Comparing the area of a spot with that of standard .

2 – Scraping the spot from the plate , and measuring the analyte by suitable physical or chemical method .

3 – Scanning densitometer

 

Applying Molecular Spectroscopic Methods

Applying Molecular Spectroscopic Methods

    Molecular spectroscopic methods are among the most widely used of all instrumental analytical methods. Molecular spectroscopy based on ultraviolet and visible radiation is used for the identification and determination of a huge number of inorganic, organic and biochemical species.

o                                                                                                                                                                                                                                                                       The electronic transitions are due to absorption of radiation by specific types of groups or bonds.

o                                                                                                                                                                                                                                                                       The wavelength of absorption and the intensity of the absorption are dependent on the type.

Deviation from Beer’s Law

Deviation from Beer’s Law

Instrumental deviations

•       Fluctuation in the electric power.

•       Stray radiation that can strike the detector without passing through the sample.(-ve)

•       The use of polychromatic light.

•       Reflection loss error resulting from an inaccurate response of detector.

How to Minimize the instrumental deviation

•       Use constant power supply.

•       Using narrow bands of radiation.

•       Restricting absorbance measurements varies from about 0.2 to 1.

•       Painting the cell compartment with black paint.

Chemical Deviations.

Deviations from beer's law appear when the absorbing species undergoes:

Association-dissociation reactions

Acid-base reactions

Polymerization reactions

Complexation reactions

Reactions with solvent

To minimize chemical deviations

•       Adjust the pH by using buffer solutions.

•       Adding a large excess of complexing agent.

•       Ionic strength adjustment.

•       Preparation of a calibration graph over the measurement range.

•       By choosing wavelength for the analysis at which all of the absorbing species have the same absorbitivity .

chemical analysis

Qualitative Analysis :

Retention times should be useful for the identification of compounds in mixture

Quantitative Analysis:

Quantitative column chromatography is based upon a comparison of either the height or the area of the analyte peak with that of one or more standards.