LAB NO. 6
ATOMIC ABSORPTION and FLAME EMISSION SPECTROSCOPY
NOTE: Answers to Preliminary Questions must be initialled by your instructor
before you begin the experiment.
INTRODUCTION
Flame Emission Spectroscopy (F.E.S.) and Atomic Absorption
Spectroscopy (A.A.S.) are techniques for trace metal analysis. Both of these
methods use a flame to convert a solution to free, un-ionized atoms. In F.E.S.,
however, the flame also serves to excite valence electrons to an energy state
above the ground state, and then the emission of this energy is measured. In
A.A.S., the ability of free atoms to absorb rather than emit light is utilized. The
energy source which causes the excitation of atoms in A.A.S. is the hollow
cathode lamp.
In order to obtain the best results and the highest sensitivity with these
techniques, one of the requisites is that as many solution atoms as possible
reach the flame. This process occurs by the solution being drawn into a burnernebulizer by a stream of air and then being mixed with air and fuel. Most of the
sample solution passes into the nebulizer chamber in the form of rather large
droplets which immediately settle out and pass into a drain tube. Only a small
fraction of the original sample remains in suspension in the gas mixture and
passes to the flame. Here the solvent evaporates and the solute is dissociated
into atoms by the heat of the flame. Some of these atoms will be excited above
the ground state. The intensity of the light emitted by these atoms as they relax
back to the ground state serves as the basis for their detection in F.E.S. In
A.A.S., ground state atoms in the flame absorb radiation from a hollow cathode
lamp. The amount of light absorbed provides the basis for the analysis.
Most metals may be analyzed by either A.A.S. or F.E.S. A.A.S. is the
more frequently used technique, since many analysts believe, perhaps
erroneously, that more skill is required to carry out a successful F.E.S. analysis.
The instrumentation required for F.E.S is same as that required for A.A.S. with
the exception that in the former technique the hollow cathode lamp is not utilized.
Thus, most commercial instruments intended for A.A.S. will also facilitate F.E.S.
measurements.
The choice of which technique to utilize depends on the circumstances
pertaining to the analysis. A.A.S. affords better detection limits for some
elements, while F.E.S. is superior for others. With respect to chemical
interferences, - refractory compound formation and ionization interference being
the most common - both techniques suffer equally. The steps that must be
taken to rectify chemical interferences in A.A.S. are the same as those required
6-2
in F.E.S. Spectral interference from nearby emission or absorption lines is a
frequent problem in F.E.S. but is rarely a consideration in A.A.S. On the other
hand, interference associated with a shift of the background underlying the
absorbance or emission lines of interest is much more readily corrected for in
F.E.S. than in A.A.S. A final consideration is the availability of a hollow cathode
lamp for the particular element you wish to determine. These devices are
expensive and a great number have been purchased by laboratories to carry out
A.A.S. analyses which could be done equally well or better by F.E.S.
In this experiment, A.A.S. will be investigated as a technique for the
determination of Mg in aqueous samples. The influence of burner height will be
investigated, and some chemical interferences will be observed. Finally, a
sample of tap water will be analyzed for both Mg and Ca. In this analysis, Mg will
be determined by A.A.S., and Ca will be determined by F.E.S.
Question 1: (Preliminary) Both Ca and Mg are subject to ionization interference,
and both are subject to refractory compound formation in the presence of sulfate
or phosphate. What is refractory compound formation? What is ionization
interference, and how would it influence an emission or absorbance calibration
curve?
EXPERIMENTAL SECTION
A. Preparation of Solutions
1. Ensure that you have adequate quantities of the following standard
solutions at your station.
Standard 1 - 0.1 ppm Mg
Standard 2 - 0.2 ppm Mg
Standard 3 - 0.3 ppm Mg
Standard 4 - 0.4 ppm Mg
2. Obtain your unknown solution from your lab instructor and transfer the entire
solution quantitatively into a 100 ml volumetric flask. Dilute to the mark with
deionized water and mix thoroughly. Label this unknown solution as "stock
unknown". Using this stock unknown solution and the other stock solutions
provided (2,000 ppm Al, 10,000 ppm K, 10,000 ppm Sr and 10 ppm Mg),
prepare in 100 ml volumetric flasks the following solutions:
Solution A : 10 ml stock unknown
Solution B : 10 ml stock unknown + 10 ml 10,000 ppm K
Solution C : 10 ml stock unknown + 10 ml 10,000 ppm K + 10 ml 2000 ppm Al
Solution D : 10 ml stock unknown + 10 ml 10,000 ppm K + 10 ml 2,000 ppm Al
+ 10 ml 10,000 ppm Sr
Dilute all of the above solutions with deionized water and mix well.
6-3
3. For the final part of this experiment, you will also need to prepare the following
standard solutions in 100 mL volumetric flasks utilizing the 10 ppm Mg, 50
ppm Ca, 10,000 ppm La, and 10,000 ppm K stock solutions.
a. blank with 1000 ppm La and 1000 ppm K
b. Std1 - 0.2 ppm Mg, 1ppm Ca, 1000 ppm La and 1000 ppm K
c. Std2 - 0.5 ppm Mg, 3 ppm Ca, 1000 ppm La and 1000 ppm K
d. Std3 - 1.0 ppm Mg, 5 ppm Ca, 1000 ppm La and 1000 ppm K
4. Finally, prepare a tap water solution for analysis. Pipet 5 mL of tap water into
a 100 mL volumetric flask. Then pipet in 10 mL each of the 10,000 ppm La
and 10,000 ppm K stock solutions. Dilute to volume.
B. Method Development
The instrument used in this experiment is a model AA240FS from Varian
Instruments. It employs a long-path premix air-acetylene nebulizer-burner
assembly. Almost all functions of the instrument are under direct control of a
computer.
In order to carry out an analysis, a method must first be developed. The
method defines, within the computer software, all of the analytical and
instrumental parameters specific to the particular analysis being done. Once a
method has been developed, it is saved on the internal hard disk within the
computer. It may then be recalled at any later time to carry out the analysis
which has been defined. In this experiment, you will develop your own method
for the analysis of Mg by AAS, and subsequently use it to determine Mg in
several unknown samples.
1.
2.
3.
4.
5.
The main power switch on the spectrophotometer should already have been
turned on in order to allow adequate warm-up time (~1 hour) for the
instruments optical and electronic components. Ensure that this is the case.
(The green light directly below the AA240FS model number on the front of
the instrument should be on).
Turn on the power switches for the computer monitor, and for the computer
itself.
Once the computer has booted up, click on the SpectrAA icon appearing on
the monitor screen.
Click on the Worksheet icon. This allows creation and editing of methods.
Select "new" to indicate you wish to create a new method.
You will now need to enter a name for your method, such as Bob'sMgw09, in
the appropriate box that appears on the screen. It is not necessary to enter
anything in the Analyst or Comment box. Ensure that the Samples box
indicates 50 and click on Ok.
6-4
6.
7.
8.
9.
In the method development window that appears, the tab marked Develop
near the top of the screen should be selected. Now click on the Add
Methods icon.
In the new window that appears, select the Load from Cookbook radio
button, under Method Type select Flame, and then scroll down, select Mg,
and click on Ok.
Now click the Edit Methods Icon. The window that now appears has a
number of tabs near the top. Select the Type/Mode tab. Ensure the settings
in the resultant are as indicated:
Now move to the Measurement, Optical, Standards, and Calibration tabs,
ensuring the settings are as below:
6-5
6-6
6-7
6-8
10. When all of your settings have been checked and set as above, click on Ok
in the bottom of the window.
C. Optimization and Flame Ignition
1. Click on the Analysis Tab at the top of the window.
1. Click on the Optimize icon in the left of the window. Then click on okay in the
resulting pop up window. A new window appears instructing you to insert a
lamp in position 2. The lamp has already been installed, so click on Ok. This
should cause the lamp to turn on.
2. Click on optimize lamps. Open the lamp compartment and adjust the lamp
positioning controls to obtain maximum optical throughput as evidenced on
the green bar graph. Have your instructor show you how.
6-9
3, Once the lamp postition has been optimized, you may ignite the flame by
pressing and holding the ignite button on the front of the instrument.
4. Once the flame is lit, aspirate DI water from a graduated cylinder and use a
stop watch to verify that the aspiration rate is 4.0 to 4.5 mL/min.
5. Continuing to aspirate DI water, select optimize signal, and then Inst Zero.
6. Now aspirate your 0.4 ppm Mg standard, and optimize the horizontal and
vertical burner positions. Again, ask your instructor for assistance.
7. Now click cancel, and cancel again to exit optimization mode.
Question 2: Explain why, starting from very low burner height, as the burner
height is raised, the Mg absorbance at first increases, and then later decreases.
D. Calibration and Analyses
1. Click on Start. The instrument will ask you to prepare for Instrument Zero.
Aspirate DI water and click Ok.
2. After the instrument has completed its measurments of the blank, it will
ask you, in succession, to present your Standards. Aspirate each standard
and click on read.
3. When all of the standards have been run, the instrument will ask you to run
samples. In succession, run solutions A, B, C, and D.
Question 3: Report the apparent concentrations in each of the unknowns
determined above.
Question 4: Explain the effects of K , K + Al, and of K + Al + Sr on the apparent
Mg concentration.
Question 5: Experimentally, it is observed that the use of Mg standards with
concentrations higher than about 0.8 ppm results in deviation from linearity in
the resulting calibration curves. Explain why.
Question 6: Magnesium has another absorption line at 202.5 nm. The
absorbance at this wavelength is about 3 times weaker than the line you are
using at 285.2 nm. Would the 202.5 nm line be useful? Explain.
6-10
E Analysis of Ca and Mg in tapwater.
1. Click on the red stop light icon.
2. Under File, select load.
3. Select "new from", CaMg 515, and name a new file such as CaMg51501,
CaMg51502, etc. Then click Ok
4. The instrument will guide you through the calibration and analysis, beginning
with Present Top Standard, in which case you should aspirate Std 3. The
instrument uses this standard to first calibrate the analytical wavelength for
set the appropriate photomultiplier tube voltage. When it requests that you
remove "Top Standard", replace it with your blank solution, and proceed as
directed.
5. When all standards and your tap water unknown have been run, the
instrument indicates Autorun Complete. Click Ok.
6. You may record the results directly into your notebook. Results for Ca and Mg
are on separate pages. Toggle between these pages using the button with
the burner icons. The identity of the page is seen on the bottom left corner of
the page.
7. A printed copy of the results may be obtained by selecting Windows, and then
reports. Select the worksheet (eg. CaMg 515 01), click the report tab, and
then Print preview or print.
To shut down, under the file menu click Exit. Then shut down the computer, turn
off the Varian AA power, the power to the printer, and be sure to close the
valves to the air and acetylene tanks.
Question 7 (Preliminary): Show how you will prepare each of the solutions
indicated in A, step 3 above.
Question 8: (Preliminary): What is the purpose of the La and the K in the
solutions used for the Ca and Mg analyses?
Question 9: Report the individual concentrations of Mg and Ca in your tap water
sample. Water hardness is due to both Mg2+ and Ca2+ but is expressed as
mg/L of CaCO3 calculated from the total Mg2+ and Ca2+ content of the
6-11
sample determined, frequently, by EDTA titration. Calculate the water
hardness of your tap water sample.
Question 10: Many Atomic Absorption Spectrophotometers are equipped to
carry out correction for broad band background absorbance utilizing a
Deuterium arc lamp. Explain how this is accomplished, and indicate the
circumstances under which it might be useful.
REFERENCES
1. A. Walsh, Spectrochim. Acta, 7, 108 (1955). "The Application of Atomic
Absorption Spectra of Chemical Analysis".
2. W. Slavin, "Atomic Absorption Spectroscopy", Interscience, New York. (1968).
3. J. Ramirez-Munoz, "Atomic Absorption Spectroscopy", Elsevier, Amsterdam
(1968).
4. W.T. Elwell, J.A.F. Gidley, "Atomic-Absorption Spectrophotometry",
Pergamon, Oxford (1967).
5. J.D. Ingle, Jr., and S.R. Crouch, "Spectrochemical Analysis", Prentice Hall,
New Jersey (1988). (A copy is kept in the prep room).
6. A.C. West, W.D. Cooke, Anal. Chem., 32, 1471 (1960). Elimination of anion
interferences in flame spectroscopy--use of (ethylenedinitile) tetraacetic acid.
7. J.I. Dinnin, Anal. Chem., 32, 1475 (1960). Releasing effects in flame
photometry determination of calcium.
8. C.Th.J. Alkemade, Anal. Chem., 38, 1252 (1966). On the interpretation of
solute vapourization interference in flame photometry.
9. Analytical Flame Spectroscopy, ed. by Mavrodineanu, QD95 M38 1970.