19 장 Fundamentals of Spectrophotometry

• Fundamentals of Spectrophotometry– 19-1 Properties of Light– 19-2 Absorption of Light– 19-3 The Spectrophotometer– 19-4 Beer’s Law in Chemical Analysis– 19-5 What happens when a molecule absorbs

light?– 19-6 Luminescence

Spectrophotometry

• Any procedure that uses light to measure the concentration of a chemical species

• Light is composed of perpendicular, oscillating electric and magnetic fields

Electric fieldy

z

x (t)

Magnetic field

Fig. 19-1

Properties of Light

• Wavelength (): crest-to-crest (or trough-to-trough) distance between waves, generally measured in nm

• Light described as “waves”

y

x

Properties of Light

• Frequency (): number of complete oscillations the wave makes each second (1/s, s-1, Hz)

= c

c speed of light (in vacuum) = 2.998 x 108 m/s

• Units for above equation:

(s-1)(m) = (m/s)

Properties of Light

• Refractive index (n): measure of angle at which light is bent

• Speed of light in a medium other than a vacuum

c/n(n = 1 in a vacuum)

Properties of Light

• Photons (h): energetic particles of light

E = h E = hc/ = hc

h Planck’s constant = 6.626 x 10-34 Js

wavenumber (1/)

• Light described as “particles”

~

~

Electromagnetic Spectrum

• Compare energy of red and blue light:

E = hc/red > blue so Ered < Eblue

Cosmic -rays X-rays UV IR -wave Radio

Vis

ible

(Hz)

(m)

1020 1018 1016 1014 1012 108

10-12 10-11 10-8 10-6 10-3 10-1

(nm): 400 500 600 700 800

Fig. 19-2

19-2. Absorption of light

• Molecules absorb photons with energy (E)

E = h• Molecules gain that energy (E) when they

absorb photons

• M is promoted from the ground state to an excited state

Ground state

Excited states

Ene

rgy

Mh

Absorption of light

• Molecules gain energy when they absorb photons

• Molecules lose energy when they emit photons

Ground state

Excited states

Ene

rgy

Absorption

gain hEmission

lose h

Fig. 19-3

Types of electromagnetic radiation (light)

• X-ray light– promotes core e-s to higher energy orbitals– breaks chemical bonds and ionizes

molecules

• Ultraviolet and visible light (UV-VIS)– promotes valence e-s to higher energy

orbitals

• Infrared light (IR)– stimulates vibrations of molecules

• Microwaves– stimulates rotational motion of molecules

E

N

E

R

G

Y

Energy levels

• Quantized: discrete levels, not continuous

• Energy states are “quantized”

rotational levels

vibrational levels

v2

v1

S0

S1

E = h

electronic levels

Example Problem

• By how many kJ per mole is the energy of O2 increased when it absorbs UV radiation with a of 147 nm?

• New energy: E2

• Original energy: E1

E2 – E1 = energy of photon absorbed

hchE

Example Problem

hchE

m

nm10

nm147

)s/m10998.2)(sJ10626.6(E

9834

molecule/J1035.1 18

J1000

kJ1

mol

molecules10022.6

molecule

J1035.1 2318

mol/kJ814

Reminder

• Prefixes of SI units

pico nano micro milli centi kilo mega

p n m c k M

10-12 10-9 10-6 10-3 10-2 103 106

• Example:

nm10

m1or

nm

m109

9

Measuring Absorption of Light

selects that analyte

will absorb

sample

absorbs

radiation

bsource may

contain

many s

Light

SourceMonochromator DetectorSample

P0 P

detector

looks for

amount of

radiation

not

absorbed

Measuring Absorption of Light

• Detector measures P

• Amount of light transmitted through sample is what is measured

• Transmittance (T): fraction of original light that passes through sample– Absorbance is measured INDIRECTLY

0P

PT radiant power not absorbed by sample

incident radiant power=

T100T%

Sept. 19 – Ch. 19

• Fundamentals of Spectrophotometry19-1 Properties of Light– 19-2 Absorption of Light– 19-3 The Spectrophotometer– 19-4 Beer’s Law in Chemical Analysis– 19-5 What happens when a molecule absorbs

light?– 19-6 Luminescence

Ch. 19-2

• Fundamentals of Spectrophotometry19-1 Properties of Light– 19-2 Absorption of Light– 19-3 The Spectrophotometer– 19-4 Beer’s Law in Chemical Analysis– 19-5 What happens when a molecule absorbs

light?– 19-6 Luminescence

Measuring Absorption of Light

• Detector measures P

• Amount of light transmitted through sample is what is measured

• Transmittance (T): fraction of original light that passes through sample– Absorbance is measured INDIRECTLY

0P

PT radiant power not absorbed by sample

incident radiant power=

T100T%

Measuring Absorption of Light

0 < T < 1

0 < %T < 100

• Absorbance is related to T:

T

1log

P

PlogA 0

TlogA

Tlog1logA

Absorbance vs. Transmittance

• When P decreases, A increases– less radiant power (light) is reaching the

detector because the sample is absorbing light

• T and A are dimensionless (although some- times “absorbance units” are mentioned)

P/P0 %T A

1 100 00.1 10 10.01 1 2

Absorbance vs. Reflection• When a molecule absorbs different s of

white (visible) light, our eyes see the reflected s (the non-absorbed s)

of Max.Absorption

380-420420-440440-470470-500500-520520-550550-580580-620620-680680-780

ColorAbsorbed

violetviolet-blue

blueblue-green

greenyellow-green

yelloworange

redpurple

ColorObserved

green-yellowyelloworange

redpurpleviolet

violet-blueblue

blue-greengreen

Table

19-1

Blue Blocker Sunglasses

• The infomercial claims that “harmful blue light” is blocked from damaging your eyes.

• Why are they orange?

Measuring Absorption of Light

P(for absorption by sample) = P0 – P(reflected) – P( scattered)

• Reference blank: solution containing all components of a sample except analyte

A(analyte) = A(measured) – A(reference blank)

DetectorSampleP0

P

scattering

reflections

Absorbance• Spectrophotometry: Any procedure that uses

light to measure the concentration of a chemical species

• Beer’s Law:

A = bc

Molar absorptivity (or extinction coefficient)

b pathlength light travels through cuvet

c concentration of analyte

• Absorbance is directly proportional to concentration

Beer’s Law

• Best applied when c 0.01 M– when c > 0.01 M, solute molecules influence

one another because they are closer together– physical properties of molecules change when

they are close together

– physical property that will change relevant to

our discussion of Beer’s Law:

Beer’s Law

• Molar absorptivity (): characteristic of a substance that tells how much light is absorbed at a particular wavelength ( is -dependent AND analyte-dependent

because different analytes absorb different amounts of light at different s

• Pathlength (b): width of cuvet; dependent on instrumental setup

A = bc = (M-1 cm-1)(cm)(M)

A is dimensionless

Example Problem

• A 3.15 x 10-6 M solution of a colored complex exhibited an absorbance of 0.267 at 635 nm in a 1.000-cm cuvet. A blank solution had an absorbance of 0.019. Find the molar absorptivity of the colored complex.

bcA

)M1015.3)(cm000.1(

)019.0267.0(

bc

A6

114 cmM1087.7

Double-beam Scanning Spectrophotometer

• Alternately measuring P0 and P by diverting light beam through reference cuvet

lightsource

monochromator detector

amplifier

computer/recorder

samplecuvet

referencecuvet

mirrormirror/

beam chopper

mirrormirror

mirror/beam chopper

Fig 19-6

P0

P

P0’

Spectrophotometers• Single-beam spectrophotometer

– insert reference blank once at beginning of exp’t– only measure absorbance of EITHER sample OR

reference blank at a time

• Double-beam spectrophotometer– continuously checks reference blank to account

for changes in• source intensity (P0)

• detector response

– if absorbance measurement from reference blank changes, spectrophotometer corrects for that change to find true absorbance of analyte

Using a spectrophotometer

• Picking the (the source of the light)– Choose the at which the analyte absorbs the

most (max)

• measurement is most sensitive at max

• Keep samples clean and dust free

• Analyte solution should absorb in the range of 0.4 < A < 0.9– dilute solution if it is too concentrated

• reduce c to reduce A

– use cuvet with longer pathlength• increase b to increase A

Why should 0.4 < A < 0.9?

• Consider when A < 0.4– P is almost as large as P0

– difficult to see a small difference between two large numbers

• Consider when A > 0.9– P is very very small– difficult to see a small amount of light– stray light reaching the detector could compete

with P

• Adjust experimental parameters to keep A in an intermediate region

What happens when amolecule absorbs light?

• Consider Molecular Orbital (MO) Theory– describes the distribution of electrons in the

molecular orbitals of a molecule

• Electronic transition– e- from one MO moves to another MO

• Ex: Formaldehyde– 4 bonds– 1 bond– 1 lone pair (the other lone pair is mixed with the

bond orbitals)– refer to Figure 19-11

CHH

O

What happens when amolecule (Formaldehyde) absorbs light?

1

2

3

4

n

singlet

or

1

2

3

4

n

triplet

E

1

2

3

4

n

Ground

State

Electronic

States

Moleculeabsorbslight ()

n *transition

What happens when amolecule (Formaldehyde) absorbs light?

• Difference between 2 excited states is spin of e-

• Singlet– spin still opposite from

e- it was originally paired with

• Triplet– spin parallel with spin of

e- it was originally paired with

• In general, T1 < S1

1

2

3

4

n

singlet (S1)

or

1

2

3

4

n

triplet (T1)

Electronic

States

Processes that occur whena molecule absorbs light

• The above “state state” transitions are only examples– S T or T S: different spins– S S: same spins

Absorbance S0 S1

Fluorescence S1 S0

Phosphorescence T1 S0

Radiational

Transitions

Internal Conversion S1 S0

Intersystem Crossing T1 S0

Relaxations (within a state)

Radiationless

Transitions

Jablonski diagram

S1

S0

T1

A

Absorbance

F

Fluorescence

P

Phosphorescence

IC

Internal Conversion

ISC

ISC

Intersystem Crossing

R

Relaxation

Fig. 19-14

Processes that occur whena molecule absorbs light

• Internal conversions (IC) and intersystem crossings (ISC)– no gain or loss in energy– IC: S S– ISC: S T or T S

• Fluorescence (F) and phosphorescence (P) are rare processes– molecule emits photons (loss in energy)– F: S S– P: T S– examples of Luminescence (section 19-6)