Beers Law for a Single Component Sample

11bcAT

I0

A = Absorbance = - log10

I

I / I0

b = Optical path length

c = Solution Concentration (M/L)

ε = Molar Absorptivity (L/M cm)

b

I0 = Incident beam intensity

I = Transmitted beam intensity

Atomic Absorption Spectrometry

• Advantages over solution U.V./Vis spectrometry

1. More selective – narrow atomic lines, compared with broad molecular bands Only 5 spectral overlaps known

2. Lock-and-key match with HCL source and atoms in flame cell

3. Working ranges from 0.1 – 1,000 ppm

200250

300 350 400

Typical Molecular absorption band

Continuum source

Molecular band overlap area

Molecular band absorption

200250

300 350 400

Atomic line absorption

Hg 253.7 nm (HCL source)

Hg 253.7 nm (atoms in flame)

Atomic line absorption

Atomic line absorption

Helium Arc Lamp spectrum

Mercury Arc Lamp spectrum

Typical Atomic Absorption Spectrometer

Electrothermal Vaporization AAS

Electrothermal Vaporizer (ETV)

Graphite Tube

Electrode Terminals

Method Type of SamplePneumatic nebulization Solution or slurry

Ultrasonic nebulization Solution

Electrothermal vaporization Solid, liquid, solution

Hydride generation Solution of certain elements

Direct insertion Solid, powder

Laser ablation Solid, metal

Spark or arc ablation Conducting solid

Glow discharge sputtering Conducting solid

Methods of Sample Introduction in Atomic Spectroscopy

Types of Atomizers Used for Atomic Spectroscopy

Type of Atomizer Typical Atomization Temperature °C

Flame 1700 – 3150

Electrothermal vaporization 1200 – 3000

Inductively coupled argon plasma

4000 – 6000

Direct current argon plasma 4000 – 6000

Microwave-induced argon plasma

2000 – 3000

Glow discharge plasma Nonthermal

Electric arc 4000 – 6000

Electric spark 40,000 – ?

Laser induced breakdown tba

Processes that occur in flames

)()()( sdesolvate

aerosolnebulize

aq MXMXMX

hMXMXMX gemit

gexcite

gvolatilize )(

*)()(

hMMXM gemit

gexcite

gg )(*

)()()(

hXX gemit

gexcite )(

*)(

hMMM nemitng

exciteng g

)(

*)()(

atomize

ionize

Properties of Flames

Fuel Oxidant Temperatures °C

Methane Air 1700 – 1900

Methane Oxygen 2700 – 2800

Hydrogen Air 2000 – 2100

Hydrogen Oxygen 2550 – 2700

Acetylene Air 2100 – 2400

Acetylene Oxygen 3050 – 3150

Acetylene Nitrous Oxide 2600 – 2800

Degree of Ionization of Metals at Flame Temperatures

Element Ionization Potential, eV

Fraction Ionized at the Indicated Temperature and Pressure

P = 10^-4 atm P = 10^-6 atm2000 K 3500 K 2000 K 3500 K

Cs 3.893 0.01 0.86 0.11 >0.99

Rb 4.176 0.004 0.74 0.04 >0.99

K 4.339 0.003 0.66 0.03 0.99

Na 5.138 0.0003 0.26 0.003 0.90

Li 5.390 0.0001 0.18 0.001 0.82

Ba 5.210 0.0006 0.41 0.006 0.95

Sr 5.692 0.0001 0.21 0.001 0.87

Ca 6.111 0.00003 0.11 0.0003 0.67

Mg 7.644 0.0000004 0.01 0.000004 0.09