Atomic Absorption Spectrometry (AAS)ICP CHEMISTRY A 2011

FMIPA UNM

2013

First Group

A. Khaerunnisa Hardyanti ArkiNur Hasanah Jusman

Nurhikmah

Sarce Siruru

Hasra Jalil

Contents

• What is AAS

• Theory

• Instrumentations

• Principle of AAS

• Applications

What is AAS ?

Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.

The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)

The application of atomic absorption spectra to chemical analysis

Concentrations range is in the low mg/L (ppm) range.

Theory

A much larger number of the gaseous metal atoms will normally remain in the ground state.

These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength.

If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed.

The extend of absorption will be proportional to the number of ground state atoms present in the flame.

the gaseous metal atoms

specific resonance wavelength

the extend of absorption vs the number of ground state atoms present in the flame.

extend of absorption

AAS Instrument

The simple diagram for the AAS

1. We set the instrument at

certain wavelength

suitable for a certain element

2. The element in the sample

will be atomized by

heat

3. A beam of UV light will be focused

on the sample

5. The monochromator

isolates the line of interest

4. The element in the sample will absorb

some of the light, thus reducing its intensity

6. The detector measures the

change in intensity

7. A computer data system converts the change in intensity into an absorbance

Processes occurring during atomization

Flame atomization

Nebuliser - burner

To convert the test solution to gaseous atoms

Nebuliser --- to produce a mist or aerosol of the test solution

Burner head --- The flame path is about 10 –12 cm

Vaporising chamber --- Fine mist is mixed with the fuel gas and the carrier gas

Larger droplets of liquid fall out from the gas stream and discharged to waste

1. Nebulizer:

1. mixes acetylene (the fuel) and oxidant

(air or nitrous oxide).

2. A negative pressure is formed at the end of the small diameter, plastic

nebulizer tube (aspiration).

3. The result is a heterogeneous mixture of gases (fuel + oxidant) and suspended aerosol (finely

dispersed sample).

4. The mixture flows immediately into the

burner head.

5. It burns as a smooth, laminar flame evenly distributed along a

narrow slot.

6. Liquid sample not flowing into the flame collects in the waste.

Note:When do we use NO2 ?

Elements that are highlighted in pink are detectable by AAS

Principle of Atomic Absorption Spectrophotometer

Atomized elements each absorb energy of a wavelength that is peculiar to that

element. The atomic absorption method uses as its light source a hollow cathode

lamp which emits light of a wavelength that is peculiar to each element. Elements

within a solution are heated in a flame or electrically (2000K to 3000K) and

subsequently determined using the fact that the degree of absorption will vary with

its concentration.

Light absorption process of atoms

Atomic Absorption Spectroscopy, AAS

Atomic Emission Spectroscopy, AES

Principle of Atomic Absorption Spectrophotometer

Ground state E0

Excited state E1

e

Absorption

Ground state E0

Excited state E1

e

Emission

ee

K0 - maximal absorption coefficientΔ - half width

0 -central wavelength

Characters of the atomic absorption spectrum

Profile of the absorption line

It = I0νe -Kνl

The relationship between absorbance The relationship between absorbance and the concentration of atomsand the concentration of atoms

A = log （ II0ν0ν/ / It ） = 0.4343 K l

Beer’s law

It - intensity of the transmitted light

Io – intensity of the incident light signal

l – the path length through the flame (cm)

DilutionDilute the sample with purified water, dilute acid, or organic solvents.

Examples: food products (e.g., dairy products), pharmaceuticals, and biological samples (e.g., blood, urine).

Types of Pretreatment

Dry DecompositionHeat the sample to a high temperature (400 to 500C), Decomposition is possible in a short time (a few hours) and operation is simple.Elements with low boiling points (e.g., Hg, As, Se, Te, and Sb) will vaporize

Wet DecompositionHeat the sample together with acid to a low temperature (approx. 300C). Suitable for volatile elements.A long time is required for the decomposition of organic substances.

Microwave DecompositionDecompose the sample at high pressure by heating it together with acid to a temperature in the range 100 to 200C in a sealed Teflon container.The decomposition process is sealed; there is little vaporization of elements with low boiling points; the decomposition time is short; there is little contamination from the operating environment and the reagent; and only a small amount of acid is required.Examples: Sediment, soil, dust, ceramics, living organisms, food products, etc.

Natural broadeningNatural broadening determined by the lifetime of the excited statedetermined by the lifetime of the excited state

and Heisenberg’s uncertainty principleand Heisenberg’s uncertainty principle （ 10-5 nm ）

Doppler Broadening Doppler Broadening （ 10-3 nm ）

results from the rapid motion of atoms as they emit or absorb radiation

Collisional BroadeningCollisional Broadening collisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral lines

Characters of the atomic absorption spectrum

Doppler Broadening Doppler Broadening （ 10-3 nm ）

results from the rapid motion of atoms as they emit or absorb radiation

Characters of the atomic absorption spectrum

Application of AAS

Pretreatment (dissolution) is required for solid samples.

AASAAS

Polished rice:0.118 ppm

Unpolished rice:0.070 ppm

0.1 ppm

Furnace method

Injected amount: 10 µL

Interference inhibitor: Pd 50ppm 5 µL

Ashing: 400C; Atomization: 1,800C

Results of Quantitative Analysis of Cd in Rice

Flame method

Air-C2H2

0.5 ppm

Polished rice : 0.118 ppm

Unpolished rice :0.073 ppm

The following 2 methods can be used to analyze unpolished and polished rice decomposed using acid:

AAS Interferences

AAS Advantages and Disadvantages'

Advantages

1. High selectivity and sensitivity

2. Fast and simple working

3. Doesn’t need metals separation

Disadvantages

1. Analysis doesn’t simultaneous

2. Fragment have to form ready measure solution

3. Limit types of cathode lamp (expensives)

THANK YOU

Questions

• How could atom’s collision

• Sample preparation of AAS

• The type of sample liquid or gas

• Function of AAS of few nonmetals examples

• What happen to the sample in flame