Lecture # 18 (April 1, 2008)

Chapter 21

Electromagnetic Spectrum

Lecture # 18 – Atomic Spectroscopy

Energy and Electromagnetic Radiation

Blackbody Radiation

Light is a particle = Photon E (energy of photon) = hv h (Planck’s constant) = 6.626 x 10-34 J • s c = v λ
 ⇒ E =

hc
 λ


v=c λ


=

~ hcv


~ v ≡ wavenumber

Jablonski Energy Diagram

Energy States and UV-Vis/Fluorescence Spectroscopy Timescale (Seconds)

Transition

Process

Rate Constant

S(0) => S(1) or S(n)

Absorption (Excitation)

Instantaneous

10-15

S(n) => S(1)

Internal Conversion

k(ic)

10-14 to 10-10

10-12 to 10-10

S(1) => S(1)

Vibrational Relaxation

k(vr)

S(1) => S(0)

Fluorescence

k(f) or G

10-9 to 10-7

S(1) => T(1)

Intersystem Crossing

k(pT)

10-10 to 10-8

S(1) => S(0)

Non-Radiative Relaxation Quenching

k(nr), k(q)

10-7 to 10-5

T(1) => S(0)

Phosphorescence

k(p)

10-3 to 100

T(1) => S(0)

Non-Radiative Relaxation Quenching

k(nr), k(qT)

10-3 to 100

1

Spectroscopy vs. Spectrometry Spectroscopy is “the study of the interaction between electromagnetic radiation and matter”

Spectrometry is “the measure of these interactions”

Atomic Spectroscopy

Atomic Spectroscopy

“determination of elemental composition”

“determination of elemental composition”

Atoms

Sample*

hv

Hollow-Cathode Lamp

hv

Monochromator

Detector hv

Atomic Emission Excited States

(2000 K +) hv

Atomic Absorption

hv (I) Po →T/A hv (I)

Atomic Fluorescence

Excited States

hv Ground State

Ground State

Atomic Absorption Hollow Cathode (made of element being analyzed)

Atomic Absorption Atomic Fluorescence

Laser

*Contains analyte in solvent

Atomization

Atomic Emission Po

Atomization 1.  Flame

Insulating Disk

Absorption Bandwidth

2.  Furnace (e.g. graphite furnace) 3.  Plasma (e.g. Inductively Coupled Plasma)

Intensity

Ne/Ar+

Anode Hollow-Cathode Lamp Bandwidth

Wavelength

2

Flame Atomization hv

Oxidant (e.g. air)

Fuel (e.g. acetylene)

“Outer Cone”

hv

“Blue Cone”

Nebulizatoin

Sample

Glass Bead

Nebulizer Drain

Premix Burner

Graphite Furnace

Heat hv

hv

(out)

(in) Sample (~1 µL)

Sample

Front View

Top View

Pre-Concentration (= multiple sample applications)

Graphite Furnace Time-Integrated Absorbance

Graphite Furnace Sample Volume: 1-100 µL Detection Limit: 0.02-0.06 ng/mL

Temperature (°C)

Atomize 1.  Pre-Concentration 2.  Automated Injection 3.  Transverse Heating

“Char” (pyrolysis)

4.  Matrix Modifiers - Matrix More Volatile

Dry

- Analyte Less Volatile 40

80

120

Time (s)

3

Analysis of Mercury by Cold Vapor Atomic Fluoresccence

Inductively Coupled Plasma (ICP) Plasma Gas (Ar) Coolant Gas (Ar)

Plasma Tube (ionized Ar collide with atoms; transfer energy) Quartz

Sample

T (K) 6,000 8,000 10,000

Induction Coil

Inductively Coupled Plasma (ICP)

Plasma

Charge Injection Device (CID)

Plasma

“can measure as many as ~70 elements simultaneously”

Sensitivity of Atomic Spectroscopy Comparison of Detection Limits for Ni+ Ion (at 231 nm)* Technique ICP/atomic emission (pneumatic nebulizer)

Detection Limits (ng/mL= pbb) 3-50

ICP/atomic emission (ultrasonic nebulizer)

0.3-4

Graphite Furnace/atomic absorption

0.02-0.06

ICP/mass spectrometry

0.001-0.2

*Note: Atomic Fluorescence (of Hg)

< 0.1 ppt

Polychromator

4

Boltzmann Distribution N* No

=

Effect of Energy Difference and Temperature on population of excited states

g* e-ΔE/kT go

N*/No = Relative Population of Excited/Ground States g*, go = “degeneracy” (number of states)

Absorption

Absorption

T = Temperature (K) k = Boltzmann Constant (1.381 x 10-23 J/K)

E* = Excited State (Ex: g = 3 states)

ΔE Eo = Ground State (Ex: go = 2 states)

“Bandwidth” in Spectroscopy

“Linewidth” in Atomic Spectroscopy Atomic Absorption Linewidth