chapter 18: fundamentals of spectrophotometry · chapter 18: fundamentals of spectrophotometry i....
TRANSCRIPT
Chapter 18: Fundamentals of Spectrophotometry
I. Using light in Spectroscopy
A. Properties of light
-light is described as both particles and waves
Plane polarized light..
Visible: ~ 380-780 nm UV: 160 ~ 380nm UV/Vis: 160-780nm
(these cause transitions that we care about)
Electrical component: transitions we care about (absorption emission)
Magnetic component: Absorption of rf waves (NMR)
I. Using light in Spectroscopy
A. Properties of light
c=νλ where c (outside vacuum)= c/nRefractive index of solution
E= hν = hc/λ = hc ν
Wavenumber (cm-1) = 1/λ
B. Absorption of Light
Electronic transitions
may result in: rotational
vibrational changes
(increase in energy of molecule)
1. What happens when molecules absorb light?
B. Absorption of Light
1. What happens when molecules absorb light?
-MO diagram of formaldehyde
-electronic transitions near 397
or 355 (UV)
-is formaldehyde green-yellow?
a. Electronic transitions
Ground
planar
Excited
pyramidal
B. Absorption of Light
1. What happens when molecules absorb light?
a. Electronic transitions
-or if IR radiation, little E
no electronic transitions, only vibrational
-vibrational and rotational transitions
usually go together
-think microwave dinner
B. Absorption of Light
1. What happens when molecules absorb light?
b. Rotational and Vibrational Transitions
-As analytical chemists these aren’t
as important to us
B. Absorption of Light
1. What happens when molecules absorb light?
c. Electronic, Rotational and Vibrational Transitions
-Electronic transitions
usually involve
simultaneous vibrational
and rotational
-most transitions radiationless
(heat)
-UV/Vis detection is
based on absorption not
emission
Ground
Excited
F= Fluorescence P=Phosphorescence
-electronic absorption bands
usually broad
-many vibrational and rotational
levels avail. at slightly different
energies
Visible absorption spectra
c. Electronic, Rotational and Vibrational Transitions
-We want to detect at UV max to get best sensitivity
D. UV/Vis instrumentation- for Absorbance!
I. Using light in Spectroscopy
Light
Source
hν
Hg, Deuterium,
Xe for UV, or W for Vis
Filter (monochrometer)
Path length Detection
(flow cell)
-Choose λ max for analytes
-Want small detector volume so little
band broadening
-Common for LC
PincidentPath length
bPtransmitted
A= log Po/P = -log T
Beer’s Law
A=єbC
II. Atomic and Molecular Spectroscopy (see Chapter 21)
A. UV/VIS (looking at molecules)
- kinetics, molar concentration determination,
need a chromophore, look at molecules
B. Atomic Absorption (AA) (looking at atoms)
Lamp flame monochromator detector
-Use lamp made of the metal detecting- shoots λ at flame
-detection of metals in a flame 2000-300K (acetylene/air)
-sample atomized in flame (the flame is the cuvet)
-one metal at a time at its max λ
-need different lamp for each metal
-measure ground excited state absorbance
II. Atomic and Molecular Spectroscopy (see Chapter 21)
B. Atomic Absorbance (AA)
To detect Fe
it’s made of Fe
LOD 0.001 to 0.020 ppm
1 to 20 ng/mL (ppb)
II. Atomic and Molecular Spectroscopy
-Absorption and emission are mirror images
-emission is at lower energy than absorption
…geometry and solvation are factors
-Inductively Coupled Plasma (ICP)
II. Atomic and Molecular Spectroscopy (see Chapter 21)
C. Atomic Emission Spectroscopy (looking at atoms)
-plasma of Ar
-no light source!
-Measure emission
Excited Ground
Advantage:
measure >1 metal/run
-detect all metals!
-60 metals simultaneously!LOD 10 ppb or less
Let’s do some calculations for UV/Vis on the board…
I. Using light in Spectroscopy Revisited (Remix)
Ex1:a) What value of absorbance corresponds to 45.0 % T?
b) If a 0.0100 M soln. exhibits 45.0 % T, what will the %T be for a
0.0200M solution of the same compound at the same wavelength?
Ex2:
a) A 3.96 x 10-4 M solution of A had an absorbance of 0.624 at 238 nm in a 1.00
cm cuvet. A blank solution containing only solvent had an absorbance of 0.029 at
the same wavelength. Find the molar absorptivity of compound A.