DETERMINATION OF MANGANESE BY ATOMIC ABSORPTION SPECTROMETRY WITH ELECTROTHERMAL
ATOMIZATION (ET-AAS)
Introduction
In this experiment, manganese is determined by graphite furnace atomic absorption spectrometry. The atomic absorption spectrometry with electrothermal atomization is much more sensitive than flame methods for low concentrations of manganese and does not require an extraction step for very low concentrations.
In this work manganese in a multielement vitamin tablet will be determines, by the method of atomic absorption spectrometry with electrothermal atomization (GFAAS).
Reagents and Apparatus
-Nitric acid 65 % Suprapur (Aldrich), distilled water (conductivity 4 µS/cm).
-Nitric acid 0.2 %. It is prepared by diluting 3.076 mL HNO3Suprapur with 1000 mL distilled water.
All the measurements will be done using an atomic absorption spectrophotometer “Perkin Elmer”, model “AA 600”, equipped with :
-radiation source – consisting in a multielement light source emitting the line spectra characteristic for the element being analyzed (Mn): a lamp with a hollow cathode (placed on an automated transport system).
-correction source for the background: Zeeman unit.
-monochromator- used for the isolation of an absorption line from the characteristic line spectra for the analyzed element.
-solid detector associated with an electronic amplifying system and measuring equipment.
-atomization source represented by the graphite furnace with traverse heating.
The graphite tube is maintained in position in the furnace by two graphite contacts that ensure the electrical contact for the heating of the tube. The graphite tube is manufactured from pyrolytic graphite and presents an integrated platform L’vov that determines a thermal equilibrium between the sample and the atmosphere inside the tube during the atomization step.
To prevent the oxidation of the graphite tube at high temperatures and to purge the vapors and fumes from the tube, an inert gas (argon) is used both inside the tube and around its outer shell. The cooling of the graphite tube is done by coupling it to a cold water source, an optimal temperature and flow rate of the water could achieve the cooling of the furnace in 20 seconds.
- theautosampler – contains all the mechanical and electrical components required for the intake of a correct volume of solution from a selected container, its injection in the graphite tube and the washing of the injection needle.
The use of the autosampler for the injection of the solutions in the graphite tube leads to a considerable improvement of the analytical results compared to the manual pipette measuring.
The measured volume will be 20 µL.
- computer – with “AA WinLab”, software controls the spectrophotometer.
Working Procedure
After turning the Ar cylinder is opened and the Av starts to flow, the instrument is turned on, and the software is then turned on as well “AA Win Lab”. An automated equipment verification test starts, and takes few minutes to allow you into the system.From the method library, we select the method to be used, or use any method and start editing all parameters, then from file select save as "method file". Then create a sample information file as per your samples and save it as your method name. The hollow cathode lamp for manganese is then turned on – and allowed to warm up for 15 min.
In the method editor are established: the quantity of sample/standard injected (20 µL), number of replicates, type of calibration curve (linear/nonlinear), the concentrations of the standards used for the calibration and the position of the standardsand the program of the graphite furnace.
TheTemperature Program of the Graphite Furnace
20 µL of blank/standard/sample are introduced in the atomization unit and the temperature is increased gradually to remove the solvent and interferents before the atomization. In the table below is presented the default temperature program of the graphite furnace optimised for the determination of manganese. (Fill in the gaps from the default program)
Step # / Temperature(ºC) / Ramp
(time-s) / Hold
(time-s) / Internal gas flow (mL/min)
1 / 250
2 / 250
3 / 250
4 / 0
5 / 250
The five steps in the temperature program of the graphite furnace have the following purpose:
1.2. -drying – takes place after the injection of the sample into the furnace. The sample must be dried at a corresponding low temperature to avoid its sputtering. For the aqueous solutions, the drying is applied at temperatures of 100 –140 ºC. The use of a temperature ramp ensures the increase of the temperature in a well determined interval. After the temperature ramp, the furnace is maintained at the selected drying temperature, until full drying. Because only a few microliters of sample are used, the time for maintaining at constant temperature is under 1 minute. During the drying process, the internal gas flow is maintained at its maximum value (250-300 mL/min) for the purging the vaporized solvent from the tube.
3. pyrolysis – is used for the decompositionand ashingof the organic and inorganic compounds of the matrix, leaving the analyzed element in a less complex matrix for analysis. During this step, the temperature is increased as much as possible to ash the matrix components, but up to the analytevolatilization temperature. The internal gas flow is maintained at 250-300 mL/min for the removal of the volatilized matrix components.
4. atomization – is used for the production of the atom population corresponding to the element to be analyzed, that will allow the measuring of the atomic absorption. In this step, the temperature is increased up to the point of dissociation of the volatilized molecular species. The atomization temperature is a characteristic of the analyzed element. The fastest possible temperature increase For the atomization is desired. That is why the time corresponding to the temperature ramp will be set to the minimum values that will ensure the highest temperature increase rate. At the beginning of this step, the “reading” operation of the spectrometer is activated, thus measuring the absorbance of the light radiation.
5. cleaning – after the atomization, the graphite furnace can be heated to higher temperatures, so that any sample residue remaining in the furnace will be burned, and purged out by the flowing argon.
Generating of the Calibration Curve
“Zero” point of the instrument is read by measuring the absorbance corresponding to the calibration blank. The value of the absorbance corresponding to the calibration blank will be automatically subtracted from the absorbances corresponding to the standards and the samples.
Preparation of the Sample
- Obtain a multielement vitamin tablet from your instructor, weigh it accurately to the nearest 0.1 mg and record the weight. Reweigh your tablet and record the weight once again, repeat this process till you have 4 weight values.
- Crush the tablet using a clean mortar and pestle, till you obtain a very fine powder. Weigh about half that tablet powder and dissolve it in about 40 mL of 0.2% nitric acid and transfer to a 100 mL volumetric flask. Continue shaking and complete to volume.
- Use a 2 micron disk filter to filter about 20 mL of the solution in 2, and accurately transfer 10 mL of the filtrate into a 1L volumetric flask, and complete to mark using 0.2% HNO3. This is your sample solution.
Preparation of Standard Solutions
From the 10 ppm stock standard manganese solution, prepare a 100 ppb solution by diluting 10 mL of the 10 ppm solution to 1L, using 0.2% nitric acid.
Part I:
- Make the following solutions:
Soln # / Sample volume (mL) / Standard volume
(mL) / 0.2% Nitric Acid (mL) / Total (mL)
1 / 10 / 0 / 40 / 50
2 / 10 / 5 / 35 / 50
3 / 10 / 10 / 30 / 50
4 / 10 / 15 / 25 / 50
5 / 10 / 20 / 20 / 50
6 / 10 / 25 / 15 / 50
Measurements:
- Place the blank (0.2% nitric acid)in vial number 1, and your solutions in the appropriate locations in the autosampler, as per your method.
- Define all these solutions as samples, and click "analyze samples", and record the blank corrected signal for each solution.
- Draw a calibration curve with the mL standard on the x-axis and the corrected absorbance on the y-axis.
- Calculate the slope and the intercept of the linear plot, and calculate the concentration of the unknown manganese in the sample, using the equation:
Cx = bCs/mVx
Where, Cx is the concentration of manganese in solution, Cs is the standard concentration, Vx is the sample volume, b, m are the intercept of the straight calibration line and its slope, respectively.
- Find the concentration of manganese in the initial stock solution, and finally how many mg of manganese per tablet.
Part II:
- Make the following solutions:
- Standard solutions
Soln # / Standard volume
(mL) / 0.2% Nitric Acid (mL) / Total (mL)
1 / 0 / 40 / 50
2 / 5 / 35 / 50
3 / 10 / 30 / 50
4 / 15 / 25 / 50
5 / 20 / 20 / 50
6 / 25 / 15 / 50
- Dilute 10 mL of the final sample solution into 50 mL of 0.2% nitric acid, or simply use the first solution in the previous part.
Measurements:
- Place the blank (0.2% nitric acid) in vial number 1, and your solutions in the appropriate locations in the autosampler, as per your method.
- Define all standard solutions, and click "Calibrate", and construct a calibration curve after selecting nonlinear through zero in method editor.
- Flush the autosampler tip and analyze your sample in triplicate.
- Calculate the slope and the intercept of the linear plot, and calculate the concentration of the unknown manganese in the sample.
- Find the concentration of manganese in the initial stock solution, and finally how many mg of manganese per tablet.
Group I will do Part I while Group II will do Part II