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Atomic absorption _AA_ is the standard method for the analysis of .._2_

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									Science & Engineering Lab 2                                                         BKC 1721

                                         Experiment D1


    Atomic Absorption Spectrophotometer (AAS): Preparation of Working Curves


Objectives


     1. To prepare standard solution for different type of heavy metals.
     2. To determine working curve for each heavy metals using AAS.


Introduction


Atomic absorption (AA) is the standard method for the analysis of specific metals. It is
widely practiced in environmental analysis. The general aspects of the techniques are:


•    mg/L to μg/L detection is routine
•    analysis is generally very specific to a given element (although interferences are
     common)
•    some techniques offer multiple compound analysis for one injection
•    the techniques are fast and relatively inexpensive


Principle (some quantum chemistry)
When metal cations enter a flame (or high T furnace or plasma), metal is quickly reduced
to elemental (atomic) state.


for instance, Fe2+ + 2e- ---> Fe0


Kinetic energy from gaseous collisions in a flame excite outer electrons to higher energy
level. This excitation is a UV-visible transition. Light at a characteristic wavelength lA is
absorbed. A diagram for the energy states of Thallium during AA is shown as an example
in Figure 1.




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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Science & Engineering Lab 2                                                        BKC 1721




                              Figure 1. Thallium in flame (A, max = 378 nm)


Emission spectroscopy
As electrons "fall back" to ground state, photons are emitted for the transition to a
metastable energy state (*) and a portion of the deactivation is without radiation. Photons
are emitted at characteristic wavelengths (E) that are usually different than the
wavelengths for absorption. Only 1% or so of the atoms are involved in the transition for a
flame, and a more energetic source (such as argon plasma) is usually needed to take
advantage of emission for chemical analysis. An example energy diagram for Thallium is
shown in Figure 2.




                    Figure 2. Thallium in flame with emission (E, max = 535 nm)


Flame Atomic Absorption
A monochromatic beam of light is generated by a cathode ray tube. The lamp is selected
from a set of available lamps in order to match the wavelength range with the characteristic
wavelength (at maximum absorption) for the specific metal. A metal solution is aspirated
into a flame by a nebulizer and burner assembly, shown in Figure 3. A variety of gas
mixtures can be chosen to obtain the best flame temperature for the exitation of the metal.

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Science & Engineering Lab 2                                                          BKC 1721

In order to promote the highest number of transitions (more light absorbed), the light
should be aligned with the interconal region of the flame. A schematic of an air/acetylene
flame is shown in Figure 4.




Figure 3. Nebulizer and burner assembly for flame atomic absorption. Adapted from
Skoog (1984)




             Figure 4. Diagram of air/acetylene flame. Adapted from Skoog (1984)


Following Beer’s Law (see spectrophotometry notes), the absorbance A is directly
proportional to concentration (A = ebC). The molar absorptivity e is a function of the
wavelength and flame temperature. The other critical factor is the optical path length b.

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Science & Engineering Lab 2                                                          BKC 1721

This corresponds to the length of the light beam segment that is within the flame. The
burner head can be readjusted (see manufacturer’s notes) in order to get a different level of
sensitivity.


Calibration techniques follow closely with those used for spectrophotometers. However,
because the aspirator sometimes gets clogged, the rate of sample introduction into the
flame can be uneven, and standards should be checked often. One common aspect of the
methods (many times ignored by novice users) is filtering samples and keeping the
solution acidic. All samples, standards, and rinsing solutions should contain a background
acid concentration near 0.1 N (acid varies depending on the analysis). Also, the rinse
solution should be aspirated during the periods between samples. This way, most metal
ions are maintained in solution, rather than precipitating within the aspirator. Samples
containing moderate turbidity can also clog an aspirator.


Most modern Flame AA’s have automated optics controls and a computer interface. There
are a variety of manual checks that should be done, however, to assure data quality. These
steps include running external standards as samples, checking blanks often, running matrix
spikes (known additions), and checking the calibration curve. If a significant interference is
found, it is usually recommended to try an alternate wavelength.


Follow manufacturer’s guidelines (unless standard method specifies differently) for the
optical settings (wavelength, slit width, and lamp current) and flame conditions
(acetylene/air, nitrous oxide/acetylene). Manufacturers usually provide a tutorial and
methods manual for specific metals. Each analyst should get trained on the instrument by
an experienced user, especially with regard to safety protocol. Once the instrument settings
are well understood, the analyst should read and review Standard Methods 3110: "Metals
by flame atomic absorption spectrometry" or an EPA method (EPA 200.7, 206.2, 279.2)
before running routine analyses. Refer to Standard Methods Table 3111:I-III and/or Table
14.2, p. 174 in Csuros (1997) for more information on detection limits and precision
expected from the methods.


Furnace Atomic Absorption Spectrometry



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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Science & Engineering Lab 2                                                        BKC 1721

An electrothermal furnace may be used as the excitation source, and the optical methods
can be based on either absorption spectroscopy (Standard Methods 3113) or emission
spectroscopy. A furnace assembly is shown below in Figure 5. A small sample (μL) is
placed on a hot furnace platform in a graphite tube (1200 - 3000°C). The atoms are excited
and both absorption and emission are significant. There are two principle advantages of the
furnace.


First, the residence time of the metals within the light path is much higher than the flame,
and atomic absorption methods are about 20-1000 times more sensitive for furnace
methods (Standard Methods 3113). The other advantage is that a small sample volume is
needed. Two disadvantages of the furnace method is slower throughput and a higher level
of interferences. Specialized instruments and methods are also available that use magnetic
fields to greatly enhance selectivity and reduce interferences (Zeeman Effect).




Reagents and Equipment
1. 60% – 69% Nitric Acid (HNO3)
2. 1000 mg/L copper standard solution
3. 1000 mL beaker
4. 250 mL beaker
5. 100 mL volumetric flasks
6. 100µL micropipette
7. 1000µL micropipette
8. 500mL measuring cylinder

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Science & Engineering Lab 2                                                        BKC 1721

9. DI water
10. 1000 mg/L iron (Fe) standard solution


Method


1.        The flame atomic absorption spectrophotometer will be set up to measure Cu.


2.        An optimum working range of 0.1-24 mg/mL will be chosen. The wavelength will
          be set up at 327.4 nm and the slit width at 0.2 nm.


3.        A standard curve will be produced using the following standards: 1mg/L Cu,
          5mg/L Cu, 10 mg/L Cu, and 20 mg/L Cu.


4.        Standards will also be run during the sample analysis.


5.        Repeat step 3 to 4 by using the following standards: 1mg/L Fe, 5mg/L Fe, 10 mg/L
          Fe and 20mg/L Fe.


6.        Each group should record the response of a blank, a standard, and get other
          calibration and instrument information during the lab.


Questions


1.        What is the value of R (regression) from the working curve?


2.        What are the differences between flame and graphite method of AAS?


3.        What are the preparations should be taken before starting up the AAS equipment?
          List all the steps and procedures before the equipment can be run.




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Faculty of Chemical & Natural Resources Engineering, KUKTEM

								
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