chapter 17: fundamentals of...
TRANSCRIPT
Spectroscopy: the science that deals with “interactions of matter
with electromagnetic radiation or other forms energy”
Spectrophotometry: a more restrictive term,
- any procedure that uses light to measure chemical concentrations.
- the quantitative measurement of the intensity of electromagnetic
radiation at one or more wavelengths with photoelectric detector.
acoustic waves, beams of particles such as ions and electrons
Chapter 17:
Fundamentals of Spectrophotometry
17-1. Properties of Light
Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light
One linearly (or plane) polarized and consists of a single frequency, that is, is monochromatic.
17-1. Properties of Light
Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light
One linearly (or plane) polarized and consists of a single frequency, that is, is monochromatic.
Dual properties:
1) wave: perpendicular, oscillating electric and magnetic fields
explains diffraction, reflection
= C/ (: frequency, C; speed of light, ; wavelength)
2) particle (photon): absorption, emission
E = h• =h • C/ =hc
where E is the energy in joules (J)
h; plank constant ( 6x10-34J •s)
; wavenumber (1/ , cm-1 reciprocal centimeter ; Kayser)
17-2 Absorption of light
Absorption of light: increases the energy of molecule
(the molecule is promoted to an excited state)
Emission of light: decreases the energy of molecule
Ground state: lowest energy state of a molecule
M + h• υ M*
(life time: 10-6 ~10-9 S)
M* M + light (fluorescence, phosphorescence)
or M* M + heat
Excitation
Relaxation
Absorption
When light is absorbed by a sample
the radiant power of the beam of light is decreased
Radiant power (P): the energy per second per unit area of the light beam
Transmittance (T): T = P/Po (T = 0 ~ 1)
Absorbance (A), or optical density: A = log (Po/P) = -log T
(if 90% light is absorbed, 10% transmitted: T = 0.1Po/Po = 0.1, A= - log T=1)
Absorption spectrum: absorbance vs wavelength
Absorption: Beer’s Law
The part of molecule responsible for light absorption: chromophore
Absorbance is directly proportional to the concentration
Beer-Lambert law: A = εbc
ε : molar absorptivity (extinction coefficient)
characteristic of a substance that tells how much light is absorbed
at a particular wavelength
b: path length
c: concentration
Beer’s law works for monochromatic radiation passing through a dilute solution< 10 mM
Colorimetry: a procedure based on absorption of visible light
17-3 Measuring Absorbance
For visible and UV spectroscopy,
a liquid sample is contained in a cell called cuvet:
- fused silica: UV and VIS
- glass; suitable for VIS but not for UV spectroscopy (it absorbs UV)
For IR spectroscopy:
- cells of NaCl or KBr
17-4 Beer’s Law in Chemical Analysis
Determination of serum iron (Fe3+, from transferrin)
Fe2+ + 3 ferrozine2- (ferrozine)3Fe4-
purple complex (max = 562 nm)
Colorimetry: based on visible radiation
17-6 What happens when a molecule absorbs light ?
Ground state (planar)
Excited state (pyramidal)
electronic transition ( n π* (S1))e movement from one MO to another MO
Infra Red (IR)
Absorption of IR radiation: vibration
Absorption of microwave radiation: rotation
Nonlinear molecule with n atom: 3N-6
Linear molecule with n atom: 3N-5
(Formaldehyde: 3N-6 = 3x4 -6 = 6)
Electronic transitions involve simultaneous
vibrational and rotational transition
IR spectroscopy: good for structural information
T1: triplet excited state
S0: singlet ground state
S1: singlet excited state
IC: raditionless transition between states with the same quantum state (S2 S1, S1 S0)
ISC: raditionless transition between states with different quantum state (S1 T1)
What happens to absorbed energy?
Life time: 10-8-10-4 s Life time: 10-4-102 s
What happens to absorbed energy?
R: vibrational relaxation
Fluorescence: S1 S0
Phosphorescence: T1 S0 (very rare)
ISC (T1 S0)can occur before phosphorescence: cooling required
The relative rate of internal conversion, intersystem crossing, fluorescence, and phosphorescence depend on the molecule, the solvent, and conditions such as temperature and pressure.
Fluorescence vs Phosphorescence
Luminescence: 1) photoluminescence (fluorescence, phosphorescence)
2) chemiluminescence
3) bioluminescence
17-7. Luminescence
0 (abs) < 0 (emission)
The absorption and emission spectra will have an approximate mirror image if the spacings between vibrational levels are roughly equal and if the transition probabilities are similar
The o does not exactly overlap:
In the emission spectrum, o comes at slightly lower energy than
in the absorption spectrum
Relationship between absorption and emission spectra
Frank-Condon Principle:
Electronic transition are so fast, relative to nuclear motion
o (absorption) < o (emission)
Fluorescence Spectrometer
Luminescence is observed at 90o to the incident light
Emission spectrum: constant ex and variable em
Excitation spectrum: constant em and variable ex
Excitation and emission spectra
Emission spectrum: constant ex and variable em
Excitation spectrum: constant em and variable ex
An excitation spectrum looks very much like an absorption spectrum
Luminescence in Analytical Chemistry
Relation of emission intensity to concentration:
I = kPoC
I: emission intensity
Po: radiant power of incident light
C: concentration of emitting species
In FL: Higher radiation power higher intensity better detection
In Absorbance: Higher radiation power no change in absorbance
(Laser-induced fluorescence; LIF) good for the detection of trace amount
Emission intensity is not proportional to analyte concentration
at high concentration, or in the presence of significant amount of absorbing species
Self-absorption
Luminescence in Analytical Chemistry
Self-absorption
Luminescence in Analytical Chemistry
N
S
CH3CH3
OOCl
N
S
CH3CH3
OON H
CR
COOH
H
H
C
COOH
RNH2+
Dansyl Chloride Amino AcidDansyl Amino Acid
Fluorophore
ChromophoreNo FL, no absorbance
Most compounds are not fluorescent:
Fluorescent moiety (fluorophore) should be coupled to the compound.
Derivatization: the chemical alteration of analyte so that it can be detected easily or separated easily from other species
NH
NH
NH2 O
O
O-
O-
NH2 O
O
NH
NH
O
O
3-APA + ligh
N
OH-, H2O2, catalyst
(CH2)4
C2H2
3-aminophtalate*(3-APA*)
(a)
H2N
(b)
aminobutylethylisoluminol (ABEI)
*
Catalysts: Heme proteins (HRP, hemoglobin..)transition metal ions (Co2+, Cu2+, Fe2+, ...)
Application:- H2O2 detection at submicromolar concentration:
H2O2 producing substrates using oxidase enzymes
- Immunoassay using HRP, isoluminol, or ABEI as labels
- Amino acid detection using isoluminol or ABEI as labels
- Transition metal detection
- BOD sensor (HRP used)
- Forensic science (blood trace)
(425 nm blue emission)
Luminol Chemiluminescence
Luminol Chemiluminescence
Catalysts: Heme proteins (HRP, hemoglobin..)transition metal ions (Co2+, Cu2+, Fe2+, ...)
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