EXPERIMENT 04:ATOMIC ABSORBTION SPECTROPHOTOMETER (AAS):

OBJECTIVE

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.

Emission spectroscopy

As electrons "fall back" to ground state, photons are emitted for the transition to a metastable energy state (x*) and a portion of the deactivation is without radiation. Photons are emitted at characteristic wavelengths (l_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.

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 should be aligned with the interconal region of the flame.

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.

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

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).

EQUIPMENT/APPARATUS

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

9. DI water

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

PROCEDURES

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.