Atomic absorption _AA_ is the standard method for the analysis of .._1_

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					Chemistry Lab 2                                                                     BKF 2721

                                          Modul 7
                                        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
Chemistry Lab 2                                                                    BKF 2721




                        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.
In order to promote the highest number of transitions (more light absorbed), the light

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                                      BKF 2721

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
Chemistry Lab 2                                                                      BKF 2721

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.




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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                                    BKF 2721

Furnace Atomic Absorption Spectrometry
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) (use only one droplet)
2. 1000 mg/L copper standard solution
3. 1000 mL beaker (for distilled water)
4. 100 mL volumetric flasks (8 units, 4 for standard cuprum and 4 for Nickel)
5. 25 mL measuring cylinder
6. 500 mL measuring cylinder (for distilled water)
9. Distilled water
10. 1000 mg/L iron (Ni) standard solution
11. 100 mL Beaker (4 unit to be filled with standard solution on AAS).

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                                   BKF 2721

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.


Follow this table for element method :
Item                             Nickel                           Cuprum
Lamp Current                     12 mA                            9 mA
Wavelength                       232.0 nm                         324.8nm
Slit                             0.2 nm                           1.3nm
Burner Head                      Standard type                    Standard type
Burner Height                    7.5 mm                           7.5 mm
Flame                            Air-acetylene                    Air-acetylene
Oxidant Gas Pressure             160 Kpa                          160 Kpa
Fuel gas flow rate               2.2 L/min                        2.2 L/min


Questions
1.       What is the function of AAS?
2.       What is the value of R (regression) from the working curve?
3.       What are the differences between flame and graphite method of AAS?
4.       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
Chemistry Lab 2                                                             BKF 2721

This Start up Procedures taken by only JP/PJP involved.
    1.        Make sure supply gas are on for :
               Acetylene (Air-C2H2)
               Compressed air
    2.        Switch on the computer
    3.        Switch on power supply
    4.        Switch on Canopy Hood.
    5.        Switch on Chiller
    6.        Switch on power AAS
    7.        Double Click on icon AAS at screen PC


AAS Method Setup
Caution : Before start with below procedure please click online to check and recheck
“lamp” 1 for Ni and 2 for Cu.
    1.        Analysis mode  Flame/Manual
    2.        Analysis name  Standard Analysis
    3.        Comment  e.g “my 1st attempt”
    4.        Element  Clear Element
    5.        Ok
    6.        Online “Check lamp”
    7.        Ready  open AAS cover
    8.        Element
    9.        Cadmium  check Hollow Cathod lamp
    10.       Edit Element
               Element : Ni (Nickel)
               Lamp position : 1
               Meas Order : 1
    11.       Edit Element 2
               Element : Cu (Cuprum)
               Lamp Position : 2
               Meas Order : 2
    12.       Next
    13.       Instrument  next

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                   BKF 2721

    14.        Analytical method  next
    15.        Working curve table
    16.        Edit work curve table
    17.        Calculation mode :
                w.curve
                order linear
                number of standard  4
                replicates : 1
                STD Unit : ppm
                Decimal place : 2
                        o STD 1 : 0
                        o STD 2 : 1
                        o STD 3 : 2
                        o STD 4 : 4
                OK
                Cu (copper)
                        o STD 1 : 0
                        o STD 2 : 1
                        o STD 3 : 2
                        o STD 4 : 4
          OK
    18.        Next
    19.        Sample table
    20.        Sample name  left blank
    21.        next
    22.        QC  next
    23.        Report format OK
    24.        File  save method as “BKF2721”
    25.        Verify  “yes”
    26.        “Guidance”  close *robot (Baseline Correction)
    27.        When “it show READY”
    28.        Click Flame on/off to assure the flame is work.
    29.        Click “auto zero”

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                                    BKF 2721

    Info : - before auto zero Flame on/off
    30.       Pressure is necessary
    31.       Auto flame “click” until spark produced
    32.       Auto zero “for DI water”
    33.       Click Ready and wait until icon change to ready (green)
    34.       Insert a tube into a sample (0) (distilled water)
    35.       When a graph achieve stability, Press Start
    36.       -------“progessing”-------
    37.       When the curve stable, insert a tube into a sample (2)
    38.       Line akan naik “curve terbentuk”
    39.       line Stable
    40.       Start
    41.       When “ready” take out a tube from sample & insert the tube into DI Water.
    42.       Repeat for all standard (0, 1, 2, 4 ppm)
    43.       Lastly Screen will show “unkown 1”insert a tube into sample  ready
              produce a Graph
    44.       End
    45.       reset
    46.       Screen will show next element “Cu”
Dilutions of Solutions (Percentage by Volume or ppm)
The most accurate formula for making dilutions of solutions is the following:
                  M1V1 = M2V2
                                           M1- Percentage you have
                                           M2- Percentage you want
                                           V1- Unknown volume
                                           V2- Volume wanted
How To prepare Stock Solution(1000ml & 100ppm) of “Aluminium Standard” from bottle
standard solution(1000ppm)
M1 = 1000ppm
M2 = 100ppm
V2 = 1000ml
M1V1 = M2V2
V1 = 100ml

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Faculty of Chemical & Natural Resources Engineering, KUKTEM
Chemistry Lab 2                                                                   BKF 2721

How to prepare :
Take 100ml Aluminium STD from standard solution bottle and put into connical
flask/beaker (1000ml). Add distilled water until the measurement reach 1000 mL.


The same step taken to prepare 0ppm,1ppm,2ppm & 4 ppm.
0 ppm (blank) = distlled water (100ml)
10ppm = 1ml Al from stock solution to be dilute into 100ml bikar
20ppm = 2ml Al from stock solution to be dilute into 100ml bikar
40ppm = 4ml Al from stock solution to be dilute into 100ml bikar


* After Std preparation, a drop of HNO3 (Nitric Acid) is added to STD
* 5-10 min, remain the standard.


Note : The same way taken to prepare 1000ml of copper 1001ppm . that’s why
micropipette 100µL,100mL,1000µL are needed in this experiment.




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Faculty of Chemical & Natural Resources Engineering, KUKTEM