Atomic Absorption Spectroscopy, Chemistry tutorial

Introduction:

Atomic Absorption Spectroscopy (or simply AAS) is a method in which the absorption of light by free gaseous atoms in flame or furnace is employed to evaluate the concentration of atoms. As the atoms are single, they don't vibrate or rotate, only electronic transitions take place.

Assume a solution having a metallic salt, example: sodium chloride, if it is aspirated to a flame for illustration, acetylene burning in air, a vapor that includes atoms of the metal might be formed. A few of these gaseous metal atoms might be increased to an energy level that is adequately high to permit the emission of radiation features of the metal example: the characteristics yellow colour imparted to flames via compounds of sodium. Though, a much larger number of the gaseous metal atoms will generally remain in an unexpected state or, in another word in the ground state. Such ground state atoms are able of absorbing radiant energy of their own particular resonance wavelength, that is, the wavelength of the radiation which the atoms would emit if excited from the ground state. Therefore, if light of the resonance wavelength is passed via a flame having the atoms in question, then portion of the light will be absorbed, and the extent of absorption will be proportional to the number of ground state atoms present in the flame.

Principle of Atomic Absorption Spectroscopy:

The sample solution is aspirated to a flame and the sample element is transformed to atomic vapor. Most of the atoms in a flame remain in the ground state and it is such ground state atoms which are measured in the atomic absorption. Such ground state atoms can absorb radiation of a specific wavelength which is generated through a special source made from that element (that is, the element been analyzed). The wavelength of radiation given off via the source is similar that these sample elements would give off if it were to emit the radiation, as it is the similar element by the source. The absorption follows Beer's law, which is, the absorbance is directly proportional to the path length in the flame and to the concentration of atomic vapor in the flame.

Experimental Preliminaries:

The given procedures are followed whenever carrying out determinations of the concentration of elements by AAS; preparation of calibration curve, preparation of sample solutions and preparation of standards.

Preparation of calibration curve:

The calibration curve for use in atomic absorption measurements is plotted via aspirating to the flame samples of solutions having known concentrations of the element to be found out, measuring the absorption of each and every solution, and then making a graph in which the measured absorption is plotted against the concentration of the solutions. Whenever we are dealing by a test solution that includes a single component, then the standard solutions are made by dissolving a weighed quantity of a salt of the element to be found out in a known volume of distilled (or deionized) water in the graduated flask. However if other substances are present in the test solution, they must as well be incorporated in the standard solutions and at same concentration to whatever exists in the test solution.

At least four standard solutions must be employed covering the optimum absorbance range 0.1 to 0.4; and if the calibration curve confirms to be non- linear (this frequently occurs at high absorbance values), then measurements by additional standard solutions must be carried out. In general having all absorbance measurements, the readings should be taken after the instrument zero has been adjusted against a blank, which might be distilled water or a solution of same composition to the test solution however minus the component to be found out. This is general to observe the standard solutions in order of increasing concentration, and after making the measurements by one solution; distilled water is aspirated to the flame to eliminate all traces of solution before proceeding to the subsequent solution. At least two, and preferably three, separate absorption readings must be made by each and every solution, and an average value taken. If essential, the test solution should be correctly diluted by using a pipette and a graduated flask, therefore it too gives absorbance readings in the range 0.1 to 0.4.

By using the calibration curve it is a simple matter to interpolate from the evaluated absorbance of the test solution the concentration of the relevant element in the solution. All modern instruments comprise a microcomputer that stores the calibration curve and lets a direct read-out of concentration.

Preparation of sample solutions:

For application of the flame spectroscopic procedures the sample should be prepared in the form of an appropriate solution unless it is already represented in this form.

Aqueous solutions might at times be analyzed directly devoid of any pretreatment; however it is a matter of chance which the given solution must have the right amount of material to provide a satisfactory absorbance reading. Whenever the existing concentration of the element to be found out is too high, then the solution should be diluted quantitatively before commencing the absorption measurements. On the contrary, whenever the concentration of the metal in the test solution is to low, then a concentration process should be taken out which entails by employing separation methods. The separation processes most generally employed by flame spectrophotometric processes are solvent extraction and ion exchange.

The solid samples will require some form of dissolution method prior to measurement. Most of the dissolution methods are available; here are some of them.

1) Wet ashing:  The general method is to treat the solid sample via acid digestion, generating a clear solution without loss of the element to be found out. Hydrochloric acid, nitric acid or aqua regia (3:1 hydrochloric acid: nitric acid) will dissolve most of the inorganic substances. Hydrofluoric acid should be employed to decompose silicates, and perchloric acid is frequently employed to break up the organic complexes. The instruction manual generally supplied by the instrument will provide guidance on acceptable acid concentrations. Biological samples generally only need simple dilution prior to measurement, or they can be evaluated directly by using furnace atomic absorption.

2) Fusions: A weighed sample is mixed via a flux in a metal or graphite crucible. The sample and flux mixture is heated over a flame, or in a furnace, and the resultant fused material is leached by either water or a suitable acid or alkali. The most broadly employed flux is sodium peroxide. Fusions by this substance are generally taken out in a zirconium crucible and cooled melt is then leached by dilute mineral acid. Lithium metaborate is a good flux for the silicate rocks. 

3) Dry ashing: The sample is weighed to a crucible, heated in the muffle furnace and then the residue is dissolved in an appropriate acid. This method is frequently employed to eliminate organic substances from the analyte material. Care should be taken to make sure that the volatile elements like mercury, arsenic and even lead are not eliminated in the ashing procedure.

4) Microwave dissolution: Microwave ovens have been employed for the sample dissolution. The sample is sealed in a particularly designed microwave digestion vessel by a mixture of the suitable acids. The high frequency microwave temperature, generally 100 to 250o C, and the increased pressure help in the considerable reduction in the time taken for sample dissolution. The process has been employed for the dissolution of samples of coal, fly ash, biological and geological materials.

Preparation of standard solutions:

In the flame spectrophotometric measurements we are mainly concerned by the solutions having extremely small concentrations of the element to be found out. This follows that the standard solutions that will be needed for the analyses should as well have very small concentrations of the relevant elements, and it is rarely practicable to form the standard solutions by directly weighing out the needed reference substance. The general practice, thus, is to make stock solutions that contain around 1000ugml-1 of the needed element, and then the working standard solutions are made by appropriate dilution of the stock solutions. Solutions that have less than 10ugml-1 are often found to deteriorate on standing, owing to adsorption of the solute on to the walls of glass vessels. As a result, standard solutions in which the solute concentration is of this order must not be stored for more than 1 to 2 days. The stock solutions are preferably made from the pure metal or from the pure metal oxide via dissolution in an appropriate acid solution; the solids use should be of the highest purity.

Uses of Atomic Absorption Spectroscopy:

Atomic Absorption Spectroscopy is employed specifically for finding out the concentrations of metal ions in solutions.

Experiment: Determination of concentration of magnesium in tap water

Purpose: To find out the concentration of magnesium in tap water

Discussion: The determination of magnesium in potable water is extremely straightforward; very little interference is encountered whenever employing an acetylene-air flame.

Equipment/Materials:

 Magnesium metal, Deionized water, Hydrochloric acid, Tap water, 1 L graduated flask, Atomic Absorption spectrophotometer, Distilled water, Magnesium hollow cathode lamp, Pipette and Analytical weighing balance.

Preparation of the standard solutions:

A magnesium stock solution (1000mgL-1) is made by dissolving 1.000g magnesium metal in 50ml of 5M hydrochloric acid. After dissolution of the metal, the solution is transferred to a 1 L graduated flask and prepared to the mark with distilled water. An intermediate stock solution having 50mgL-1 is made by pipetting 50mL of the stock solution to a 1 L graduated flask and diluting to the mark. Dilute precisely four parts of this solution to provide four standard solutions of magnesium with known magnesium concentrations lying in the optimum working range of the instrument to be employed (generally 0.1 to 0.4ugmL-1 Mg2+).

Experimental Procedure:

However the precise mode of operation might differ according to the particular instrument employed, the given procedure might be regarded as typical. Place a magnesium hollow cathode lamp in the operating position, adjust the current to the suggested value (generally 2-3Ma), and choose the magnesium line at 285.2 nm by employing the suitable Monochromator slit width. Connect the suitable gas supplies to the burner following the instructions thorough for the instrument, and adjust the operating conditions to provide a fuel-lean acetylene-air flame.

Beginning by the least concentrated solution, aspirate in turn the standard magnesium solutions into the flame and for each take three readings of the absorbance; between each and every solutions and remember to aspirate deionized water into the burner. Lastly, read the absorbance of the sample of tap water; this will generally need considerable dilution in order to provide an absorbance reading lying in the range of values recorded for the standard solutions. Plot the calibration curve and utilize this to find out the magnesium concentration of the tap water. Whenever the magnesium content of the water is more than 5ug mL-1 it may be considered preferable to work by the less sensitive magnesium line at wavelength of around 202.5 nm.

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