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1.1 Range of molar absorptivity
Electronic excitation of outer valence (i.e. bonding) electron
How probable for this electronic excitation? (allowed transition, or forbidden)
judged by the range of = 104 -105 L mol-1 cm-1, strong absorption < 103 L mol-1 cm-1, low intensity
*MhM excitation
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1.2 Which electron get excited?1.2.1 Organic molecules
, (bonding) and n (non-bonding) orbitals*, * (anti-bonding) orbitals
* E large ( < 150 nm, out of range) = 10 -10,000 Lmol-1cm-1
n * E smaller ( = 150 - 250 nm) = 200-2000 Lmol-1cm-1
* n * E smallest ( = 200 - 700 nm) = 10-10,000 Lmol-1cm-1
Ideal for UV-Vis spectrometry of organic chromophore
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1.2.2 Inorganic moleculesMost transition metal ions are colored (absorption in Vis) due to d d electronic transition
Fig. 14-4 (p.371)Fig. 14-3 (p.370)
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1.2.3 Charge-transfer absorption
A: electron donor, metal ionsD: electron acceptor, ligand > 10,000
DAhAD excitation
Fig. 14-5 (p.371)
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2.1Approximation of T and A
A = -log T = log (P0/P) = ··b·c: molar absorptivity at one particular
wavelength (L·mol-1cm-1)b: path length of absorption (cm)c: molar concentration (mol·L-1)
Fig. 6-25 (p.158)
Fig. 13-1 (p.337)
Light loss due to reflection (17.3%), scattering, …
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PP
PPA
PP
PPT
solution
solvent
solvent
solution
0
0
loglog
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2.2 Application of Beer’s law to mixtures
Absorbance is additive
Atotal = A1 + A2 + … = 1bc1 + 2bc2
For a 2-component mixture, we measure the absorption at two different wavelength, respectively
A1 = 1,1·b·c1 + 2,1·b·c2
A2 = 1,2·b·c1 + 2,2·b·c2
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2.3 Limitations of Beer’s law
2.3.1 Real deviations
At low concentration
A = -log T = log (P0/P) = ··b·c
At c > 0.01 M solute-solute interaction, hydrogen-bond, …can alter the electronic absorption at a given wavelength
dilute the solution
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2.3.2 Chemical effects analyte associates, dissociates or reacts with a solvent to
give molecule with different
Example: acid-base equilibrium of an indicator
430 570 HIn 6.30 x102 7.12x103 (measured in HCl solution)In- 2.06 x 1049.61 x102 (measured in NaOH solution)
What’s the absorbance of unbuffered solution at c = 2 x 10-5M?
073.0][][
236.0][][
1088.0][
1012.1][
570,570,570
430,430,430
5
5
HInbInbA
HInbInbA
MHIn
MIn
HInIn
HInIn
5 a 102.1K InHHIn
][][
][][
1042.1][
]][[ 5
IncHIn
InHHInInHKa
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Fig. 13-3 (p.340)=[HIn] + [In-]
InHHIn
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2.3.3 Instrumental deviations due to polychromatic radiation
Beer’s law applies for monochromatic absorption only.
If a band of radiation consisting of two wavelength ’, and ”Assuming Beer’s law applies to each wavelength
bc"bcm
bc"
bc
PPPP
PPPP
A
PP"
PP
bcPP
A
"0
''0
"0
'0
"0
'0
"0
''0
'0
1010log
"'log
absorbance Measured
10"h wavelengtsecond For the
10'
''
log'
length first waveFor
Fig. 7-11 (p.176)
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2.3.3 Instrumental deviations due to polychromatic radiation
bc"bcm PPPP
PPPPA "
0'
0
"0
'0
"0
'0
1010log
"'log
absorbance Measured
Fig. 13-4 (p.341)
′″
′″
′″ Non-linear calibration curve
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How to avoid :
Select a wavelength band near its maximum absorption where the absorptivity changes little with wavelength
Fig. 13-5 (p.341)
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2.3.4 Other physical effects
stray light – the scattering, reflection radiation from the instrument, outside the nominal wavelength band chosen for measurement
mismatched cell for the sample and the blank
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3.1 Standard deviation of c
TTs
css
sTcbTT
c
Tb
c
Tc
TT
cc
Tc
log434.0
)(
434.0
log1
2
2
222
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3.2 Sources of instrumental noise Case I Limited readout resolution (31/2-digit displays 0.1% uncertainty from 0%T -100% T) Thermal noise in thermal detector, etc (particularly for IR and neat IR spectrophotometer)
Case IIShot noise in photon detector (random emission of photon from the light source orrandom emission of electrons from the cathode in a detector)
Case III Flicker noise,
Fail to position sample and blank cells reproducibly in replicate measurements (as a result, different sections of cell window are exposed to radiation, and reflection and scattering losses change)
1ksT
TTksT 22
TksT 3
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3.2 Sources of instrumental noise
Fig. 13-3 (p.344)
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4.1 Designsa. Single beam
b. Double-beam-in-space
c. Double-beam-in-time
Advantage of double beam configuration
• Compensate for fluctuation in the radiant output, drift in transducer, etc.
• Continuous recording of spectra
Fig. 13-13 (p.352)
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Shimadzu UV-2450 Spectrophotometer
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Wavelength Range
190 to 900nm (performance guaranteed range). Extendable to 1,100nm through the use of the optional photomultiplier. (The measurable range maybe restricted in the shorter wavelength side depending on the type of photomultipler used.)
Monochromator System
UV-2450: Single monochromator with a high-performance blazed holographic grating in the aberration corrected Czerny-Turner mounting.
Resolution 0.1nmSpectral Bandwidth 0.1, 0.2, 0.5, 1, 2 and 5nm
Wavelength Accuracy ±0.3nm
Wavelength Repeatability ±0.01nm
Wavelength Scanning Speed FAST, MEDIUM, SLOW, and SUPER SLOW
Light Source 50W halogen lamp (2,000 hours of life) and D2lamp (500 hours of life)Light Source lamp switching Selectable between 282nm and 393nm
Stray Light UV-2450: Less than 0.015% at 220nm and 340nmDetector Photomultiplier R-928Photometric System Double beam, direct ratio system with dynode feedback
Photometric Mode Absorbance (Abs.), transmittance (%), reflectance (%) and energy (E).
Photometric Range
Abosrbance: -4~5 Abs. (0.001 Abs. increments)Transmittance: 0~999.9% (0.01 increments)Reflectance: 0~999.9% (0.01% increments)
Photometric Accuracy
±0.002Abs(0~0.5Abs), ±0.004Abs(0.5~1Abs),±0.3T (0~100%T) (all determined with NIST 930D standard filter)
Photometric Repeatability 0.001Abs (0~0.5Abs), ±0.1%T
Baseline Correction Selectable with storage in firmware
Baseline Flatness Within ±0.001Abs. (excluding noise, 2nm slit width and SLOW wavelength scanning speed)
Drift Less than 0.0004 Abs. per hour (after 2 hours warm-up)Dimensions 570 (W) x 660 (D) x 275 (H) (mm)Weight 36kgPower Supply AC 100V/120V/220V/240V, 50/60Hz 250VA (swithc-selectable)
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Shimadzu UV-2450 Spectrophotometer
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d. Effects of monchromator exit slit width on spectraNarrow exit slit width improves the spectrum resolutionbut it also significantly reduce the radiant power
Trade-off between resolution and S/N ratio
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e. Multichannel spectrometerNo monochromator, but disperses transmitted light and measures “all wavelength at once”
No Scanning-simple and fastMore expensiveLimited resolution
Fig. 13-15 (p.353)
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5.1Quanlitative spectra
Solvent effects on the UV-Vis spectra
Polar solvents “blur” vibrational features
Polar solvents shift absorption maximan * blue shift * red shift -
UV-Vis not reliable for qualitative but excellent for quantitative analysis
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5.2 Quantitative analysis- Determining the relationship between A and c
External StandardsStandard-Addition
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Fig. 14-14 (p.382)
5.3 Spectrophotometric kinetics
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Fig. 14-16 (p.384)