Chapter 4 atomic absorption spectroscopy

CChapter 4 hapter 4 Atomic Absorption Atomic Absorption Spectroscopy

4.1. Theoretical Concepts 4.2. Atomic Absorption Instrumentation 4.3. Graphite Furnace Atomic

Absorption 4.4. Control of Analytical Interferences

4.1. Theoretical Concepts

1. The Atomic Absorption Process

2. Theoretical Concepts

3. Quantitative Analysis

4. Characteristic Concentration

5. Detection Limits

1. The Atomic Absorption Process

Fig 4-3Grotrian diagrams for NaFig4-2 Grotrian diagrams for K

2. Theoretical Concepts

Integral Formula of AAS⑴

Fig 4-4 Typical Shape of A Atomic Absorption line

Integral Absorption FormulaBy atomic theory

0

2

fNdvk mce

v

Dv vkdvk

02ln0 21

02ln2

0

2

fNk mce

vD

By Line Shape Function ( the Nature distribute )

The Peak Absorption Coefficient k0 is:

(4.1)

(4.2)

(4.3)

⑵ Peak Absorption Theoryby Lambert-Beer’s Law:

lkvv

veII 0

dveIdvIP lkv

v

v

vv

0

0

0

dvIPv

v

0

00

(4.4)

(4.5)

(4.6)

dvI

dveI

PP

vv

lvkvvA

0

00

0

0lglg

lkdvI

dvIe

dvI

dveIA v

v

vv

lk

vv

lkvv

v

0

00

00

00

00

4343.0lglg0

002ln2 2

4343.0 KNflNA mce

vD

The Absorbance is:

When va>> ve, then kv≈k0,

(4.7)

(4.8)

(4.9)

3. Quantitative Analysis

KCA (4.10)

4. Characteristic Concentration

The ‘‘characteristic concentration’’ ( sometimes called ‘‘sensitivity’’) is a convention for defining the magnitude of the absorbance signal which will be produced by a given concentration of analyte. For flame atomic absorption, this term is expressed as the concentration of an element in milligrams per liter ( mg/L ) required to produce a 1% absorption ( 0.0044 absorbance ) signal.

(4.11)

5. Detection Limit

Having obtained the data, make the calculation as follows:1. Average the two blank readings taken immediately before

and after each standard and subtract from the standard reading.

2. Calculate the mean and standard deviation for the set of corrected high-standard readings. Do the same for the set of corrected low standard readings.

3. If the ratio of the means does not correspond to the ratio of the concentration prepared to within statistical error, reject the data.

4. If the data pass the ratio-of-the-means test, calculate theconcentration detection limit as follows:

(4.12)

4.2. Atomic Absorption Instrumentation

1. Photometers for AAS

A New Type Photometer for AAS

2. line source (Hollow Cathode Lamp )

Fig 4-13

Hollow Cathode Lamp Emission Process

Fig 4-14

3. Atomizer for AAS (Pre-Mix Burner System)

Fig 4-19 Tree Type Burner Head for different Type Flame

4. Control of Analytical Interferences

Ionization Interference

Matrix Interference

Chemical Interference

Background Interference

Ionization Interference

Matrix Interference

Chemical Interference

The Method Of Standard Additions

No.Sample

Added

ml

Standard

Added

ml

Concentr.of standard

mg/L

Last

Concentriation mg/L

1

Vx

0

Cs

Vx

VL

2 VsVx+CsVs

VL

3 2VsVx+2CsVs

VL

4 3VsVx+3CsVs

VL

Background Interference

AD2=Ab,

AHCL= Aa+ Ab

Aa = AHCL- AD2

(4.13)

(4.14)

(4.15)

4.3. Graphite Furnace Atomic Absorption

1. Graphite furnace atomizer components

2. The Graphite Furnace Power Supply and

Programmer

3. Quantitative analysis GFAAS 4. Effect of Matrix on Height and Area

1. Graphite furnace atomizer components

The Graphite Furnace Atomizer

A basic graphite furnace atomizer is comprised of the following components:

graphite tube

electrical contacts

enclosed water cooled housing

inert purge gas controls

THGA graphite tube

Fig 4-27

2. The Graphite Furnace

Power Supply and

Programmer

A Graphite Furnace Temperature Program

1) Drying

2) Pyrolysis

3) Cool Down(optional)

4) Atomization

5) Clean Out

6) Cool Down