Docstoc

Atomic Absorption - DOC

Document Sample
Atomic Absorption - DOC Powered By Docstoc
					                         Atomic Absorption Analysis of Metals

OBJECTIVES
1.   Select a metal to be analyzed and determine the best procedure and instrument
     parameters.
2.   Learn to operate an AAS instrument.
3.   Do an analysis of an unknown natural sample, including a sample preparation
     procedure.
4.   Do a standard addition calibration method.

REFERENCES
Read Harris, chapter 22.

INTRODUCTION
Atomic Absorption is a selective and sensitive techniques for analysis of metal ions.
The detection limit is less than 1 ppm for most metals. The general method is to
dissolve the metal in the sample and measure its atomic absorbance in comparison
with a set of standards. The basic instrument has an element specific source , a
monochromator, an atomizer and a detector, with some display system. A hollow
cathode source for the element being analyzed produces a bright line spectrum. This
signal is modulated by a mechanical chopper. The sample or standard is aspirated into
a flame where it is reduced to the neutral atoms and absorption of the signal by the
atoms occurs. The signal is then dispersed by a grating monochromator, detected by a
photomultiplier and displayed.

Standard addition method
In this method known, but different amounts, of standard are added to several samples
of the analyte solution. A plot is made of the signal detected (absorbance) vs. amount
of standard added, as shown below. The concentration of the analyte is obtained by
extrapolating back to zero signal.




This method has the advantage that if an interfering substance is in the analyte
solution, it will cause proportional interference for the standard and analyte, and will be
corrected for automatically. The interference only causes a loss in sensitivity of the
method. A disadvantage is that it is more work because a set of standard additions


CHM 367, Atomic Absorption                   24                                  F-03
must be prepared for each sample. This method is especially suited to AA analysis
where the analyte contains the acid used in digestion of the sample matrix and
unknown materials derived from the sample matrix, both of which may cause
interference.

Detection limit. In AAS the detection limit is often defined as the concentration which
gives a 1 % absorbance, i.e. A = 0.004. However, a better definition is the
concentration that gives an absorbance 2-3 times the background noise level, for which
we can use the standard deviation of the absorbance for the calibration blank.

Digestion methods.
The proper digestion method will depend on the nature of the sample. Digestion
methods are discussed in the text and other sources, such as G. D. Christian and F. J.
Feldman, Atomic Absorption Spectroscopy. Applications in Agriculture, Biology and
Medicine, New York, Wiley-Interscience Division, 1970. Consult with the instructor to
determine the best digestion method for your sample.

Interference
Chemical interference can be diminished in several ways. Addition of La(N0 3)3 or
Sr(NO3)2 to give a 1 % solution will often correct for anion interference. Addition of
EDTA to make a 0.05 M solution will also correct for anion interference and enhance
the sensitivity. One can also use a higher energy atomization method such as an
N2O/C2H2 flame or a graphite furnace.

Ionization interference results for easily ionized metals, when the gas phase atoms are
converted to gas phase cations. Ionization interference can be eliminated by adding
KNO3 to make the standard and analyte solutions 2000 ppm in K +.

The analytical procedure for this experiment will be to prepare a sample for analysis
using a digestion procedure, prepare a normal standard curve, and analyze a sample.
You will then prepare a standard curve and analyze the same sample by the standard
addition method.

Experiment objective.
Chlorophyll molecules contain Mg2+ within the heme structure. When leaves turn color
in the fall, does the tree retain the Mg2+, or is the Mg2+ lost as the brown leave fall to the
ground? Good ecological reasons can be advanced for each strategy. The class will
design and conduct an experiment, and share data, in order to answer this question.

EXPERIMENTAL
This procedure is in outline form only. You will have to fill in the details by consultation
with the instructor or with other references.




CHM 367, Atomic Absorption                    25                                   F-03
Sample selection and treatment.
1.     Select an appropriate material and metal ion for analysis.

2.     Consult with the instructor to determine the proper digestion method. A commonly used method
       is at the end of this section.

3.     Weigh out a sample of about 0.1 to 1.0 g. Digest the sample by the chosen method. Transfer to
       a 100 mL volumetric flask, add any needed reagents to control interference and dilute to the mark.
       Also prepare a digestion blank in the same manner.

Preparation of standard solutions for direct linear calibration.
4.     Prepare or obtain a standard stock solution 10 times more concentrated than the highest
       concentration standard needed for the calibration curve.

5.     Prepare 5 standards in the range recommended in the methods manual. Be sure to add any
       interference control reagents used in step 3. Prepare an appropriate calibration blank which
       contains the same level of reagents to control interference.

6.     Set up the instrument with a file for direct analysis and another for standard addition. Run the
       analyses using the Automatic Run mode.

7.     Run the standards and prepare a standard curve, using the calibration blank for the zero. Re-zero
       with the calibration blank and then run the unknown and the digestion blank. If the unknown is off
       scale, dilute the unknown solution and the digestion blank, so that the absorbance of the unknown
       is in the middle portion of the calibration curve. Be sure to add reagents for interference control
       as was done before.

Preparation of standard solutions for the standard addition calibration.
8.     Prepare the standards for the standard addition in a series of 6 test tubes using the standards
       prepared for the direct calibration and the final dilution of the sample used above. Be sure to mix
       these solutions fully.

       Solution         Standard         Volume           Sample
       Addition 1           #1           5.00 ml          5.00 mL
       Addition 2           #2           5.00 mL          5.00 mL
       Addition 3           #3           5.00 mL          5.00 mL
       Addition 4           #4           5.00 mL          5.00 mL
       Addition 5           #5           5.00 mL          5.00 mL
       Addition 0       calb. blank      5.00 mL          5.00 mL

9.     Prepare a solution to be used to zero the instrument by combining 5.00 mL of the final dilution of
       the digestion blank with 5.00 mL of the calibration blank.

10.    Run the standard addition method on this series of solutions.

11.    Print out the calibration data, the results and the plot for each analysis method.

DIGESTION OF ORGANIC MATTER SAMPLES FOR AAS ANALYSIS
1.     Collect the following items -- a wash bottle with Q-water, a bottle of con. H2SO4, 3 disposable
       pipets, two digestion flasks, and a beaker for the H2SO5.

2.     Prepare 15 mL of H2SO5 for each sample to be digested by adding 5 mL of con. H2SO4 to 10 mL
       of H2O2.

3.     Weigh out 0.1 g to 1.0 g of sample, depending on the % of metal expected in the sample.

4.     Transfer the sample to a digestion flask.



CHM 367, Atomic Absorption                          26                                        F-03
5.     Bring everything to the hood with the digestion apparatus. Clamp the digestion flasks in nearly
       vertical positions on the heating coils.

6.     Add 1-2 mL of Q-water to the sample and the blank flasks and add 1 mL of con. H2SO4 to each
       flask, using a disposable pipet.

7.     Turn the heater control to 3. As the digestion begins, the sample will form a dark carbon char. It
       may be necessary to reduce the heat to keep the frothing under control. When a significant
       amount of char has formed, add 1 mL of H2SO5. This should cause most of the char to be
       oxidized to CO2. Be sure to make the same addition to the blank flask. You may have to adjust
       the heat level to keep this process under control.

8.     Heat the digestion solution until more char develops or until dense, white fumes of SO3 are
       observed. If any char develops, add another 1 mL of H2SO5, and repeat the heating, treating the
       blank in the same manner. It may be necessary to increase the control setting as the digestion
       proceeds.

9.     When no char or yellow color develops before the SO3 fumes are observed, remove the flask
       from the heater and allow it to cool.

10.    When the solutions have cooled, quickly add 30-40 mL of Q-water to each. Then allow the
       solution to cool again. They are now ready for transfer to volumetric flasks, addition of
       interference control reagents and dilution to volume.

SAFETY AND DISPOSAL NOTES
Most metal ions are toxic. Avoid consumption. Wash your hands after working with the
solutions.

Air/Acetylene mixtures are very explosive. Be sure to turn the acetylene on last and off
first.

The oxidizing acid mixtures used for digestions are all very powerful oxidizing agents.
Handle these with great care. Clean up carefully after using them. Instantly wash off
any spills on your skin.

Determine if the metal ion used is a hazardous waste, or if can be disposed down the
drain. If it is hazardous, obtain a proper waste collection bottle and label it with the
metal ion, the predominant anion present, and collect all of your solutions containing
metal ions, including the drip from the AAS burner, but not the stock standard solution.
Leave the bottle in the waste collection area. If it is not hazardous, add Na2CO3 to
neutralize the excess acid and dump the solution down the drain.

REPORT
Consult the report guide for this experiment as you prepare your report.

QUESTIONS FOR THOUGHT
1.  Compare the advantages and disadvantages of the direct calibration method and
    the standard addition method.
2.  What would be the effect on the detection limit if you changed the number of
    replicates and/or the signal averaging time?



CHM 367, Atomic Absorption                         27                                        F-03