fundamentals of modern uv-visible spectroscopy presentation materials
TRANSCRIPT
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Fundamentals of modern UV-visible spectroscopy
Presentation Materials
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Fundamentals of modern UV-visible spectroscopy2Figure :
The Electromagnetic Spectrum
= c / E = h
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Fundamentals of modern UV-visible spectroscopy3Figure :
Electronic Transitions in Formaldehyde
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Fundamentals of modern UV-visible spectroscopy4Figure :
Electronic Transitions and Spectra of Atoms
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Fundamentals of modern UV-visible spectroscopy5Figure :
Electronic Transitions and UV-visible Spectra in Molecules
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Fundamentals of modern UV-visible spectroscopy6Figure :
Derivative Spectra of a Gaussian Absorbance Band
1st Derivative:
2nd Derivative:
Absorbance:
)('
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dA
)(fA
)(''2
2
fd
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Fundamentals of modern UV-visible spectroscopy7Figure :
Resolution Enhancement
• Overlay of 2 Gaussian bands with a NBW of 40 nm separated by 30 nm
• Separated by 4th derivative
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Fundamentals of modern UV-visible spectroscopy8Figure :
Transmission and Color
The human eye sees the complementary color to that which is absorbed
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Fundamentals of modern UV-visible spectroscopy9Figure :
Absorbance and Complementary Colors
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Fundamentals of modern UV-visible spectroscopy10Figure :
Transmittance and Concentration
The Bouguer-Lambert Law
PathlengthConsteIIT 0/
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Fundamentals of modern UV-visible spectroscopy11Figure :
Transmittance and Path LengthBeer’s Law
ionConcentratConsteIIT 0/
Concentration
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Fundamentals of modern UV-visible spectroscopy12Figure :
The Beer-Bouguer-Lambert Law
cbIIIITA /log/loglog 00
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Fundamentals of modern UV-visible spectroscopy13Figure :
Two-Component Mixture
Example of a two-component mixture with little spectral overlap
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Fundamentals of modern UV-visible spectroscopy14Figure :
Two-Component Mixture
Example of a two-component mixture with significant spectral overlap
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Fundamentals of modern UV-visible spectroscopy15Figure :
Influence of 10% Random Error
Influence on the calculated concentrations• Little spectral overlap: 10% Error• Significant spectral overlap: Depends on similarity, can be much higher (e.g. 100%)
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Fundamentals of modern UV-visible spectroscopy16Figure :
Absorption Spectra of Hemoglobin Derivatives
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Fundamentals of modern UV-visible spectroscopy17Figure :
Intensity Spectrum of the Deuterium Arc Lamp
• Good intensity in UV range• Useful intensity in visible range• Low noise• Intensity decreases over lifetime
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Fundamentals of modern UV-visible spectroscopy18Figure :
Intensity Spectrum of the Tungsten-Halogen Lamp
• Weak intensity in UV range• Good intensity in visible range• Very low noise• Low drift
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Fundamentals of modern UV-visible spectroscopy19Figure :
Intensity Spectrum of the Xenon Lamp
• High intensity in UV range• High intensity in visible range• Medium noise
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Fundamentals of modern UV-visible spectroscopy20Figure :
Dispersion Devices
• Non-linear dispersion• Temperature sensitive
• Linear Dispersion• Different orders
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Fundamentals of modern UV-visible spectroscopy21Figure :
Photomultiplier Tube Detector
Anode
• High sensitivity at low light levels• Cathode material determines spectral sensitivity• Good signal/noise• Shock sensitive
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Fundamentals of modern UV-visible spectroscopy22Figure :
The Photodiode Detector
• Wide dynamic range• Very good signal/noise at high light levels• Solid-state device
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Fundamentals of modern UV-visible spectroscopy23Figure :
Schematic Diagram of a Photodiode Array
• Same characteristics as photodiodes• Solid-state device• Fast read-out cycles
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Fundamentals of modern UV-visible spectroscopy24Figure :
Conventional Spectrophotometer
Schematic of a conventional single-beam spectrophotometer
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Fundamentals of modern UV-visible spectroscopy25Figure :
Diode-Array Spectrophotometer
Schematic of a diode-array spectrophotometer
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Fundamentals of modern UV-visible spectroscopy26Figure :
Diode-Array Spectrophotometer
Optical diagram of the HP 8453 diode-array spectrophotometer
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Fundamentals of modern UV-visible spectroscopy27Figure :
Conventional Spectrophotometer
Optical system of a double-beam spectrophotometer
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Fundamentals of modern UV-visible spectroscopy28Figure :
Diode-Array Spectrophotometer
Optical system of the HP 8450A diode-array spectrophotometer
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Fundamentals of modern UV-visible spectroscopy29Figure :
Conventional Spectrophotometer
Optical system of a split-beam spectrophotometer
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Fundamentals of modern UV-visible spectroscopy30Figure :
Definition of Resolution
Spectral resolution is a measure of the ability of an instrument to differentiate between two adjacent wavelengths
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Fundamentals of modern UV-visible spectroscopy31Figure :
Instrumental Spectral Bandwidth
The SBW is defined as the width, at half the maximum intensity, of the band of light leaving the monochromator
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Fundamentals of modern UV-visible spectroscopy32Figure :
Natural Spectral Bandwidth
The NBW is the width of the sample absorption band at half the absorption maximum
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Fundamentals of modern UV-visible spectroscopy33Figure :
Effect of SBW on Band Shape
The SBW/NBW ratio should be 0.1 or better to yield an absorbance measurement with an accuracy of 99.5% or better
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Fundamentals of modern UV-visible spectroscopy34Figure :
Effect of Digital Sampling
The sampling interval used to digitize the spectrum for computer evaluation and storage also effects resolution
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Fundamentals of modern UV-visible spectroscopy35Figure :
Wavelength Resettability
Influence of wavelength resettability on measurements at the maximum and slope of an absorption band
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Fundamentals of modern UV-visible spectroscopy36Figure :
Effect of Stray Light
Effect of various levels of stray light on measured absorbance compared with actual absorbance
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Fundamentals of modern UV-visible spectroscopy37Figure :
Theoretical Absorbance Error
The total error at any absorbance is the sum of the errors due to stray light and noise (photon noise and electronic noise)
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Fundamentals of modern UV-visible spectroscopy38Figure :
Effect of Drift
Drift is a potential cause of photometric error and results from variations between the measurement of I0 and I
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Fundamentals of modern UV-visible spectroscopy39Figure :
Transmission Characteristics of Cell Materials
Note that all materials exhibit at least approximately 10% loss in transmittance at all wavelengths
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Fundamentals of modern UV-visible spectroscopy40Figure :
Open-topped rectangular standard cell (a) and apertured cell (b) for limited sample volume
Cell Types I
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Fundamentals of modern UV-visible spectroscopy41Figure :
Cell Types II
Micro cell (a) for very small volumes and flow-through cell (b) for automated applications
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Fundamentals of modern UV-visible spectroscopy42Figure :
Effect of Refractive Index
Changes in the refractive index of reference and sample measurement can cause wrong absorbance measurements
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Fundamentals of modern UV-visible spectroscopy43Figure :
Non-planar Sample Geometry
Some sample can act as an active optical component in the system and deviate or defocus the light beam
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Fundamentals of modern UV-visible spectroscopy44Figure :
Averaging of data points reduces noise by the square root of the number of points averaged
Effect of Integration Time
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Fundamentals of modern UV-visible spectroscopy45Figure :
• Wavelength averaging reduces also the noise (square root of data points)• Amplitude of the signal is affected
Effect of Wavelength Averaging
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Fundamentals of modern UV-visible spectroscopy46Figure :
Selection of a wavelength in the slope of a absorption band can increase the dynamic range and avoid sample preparation like dilution
Increasing Dynamic Range
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Fundamentals of modern UV-visible spectroscopy47Figure :
Scattering causes an apparent absorbance because less light reaches the detector
Scattering
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Fundamentals of modern UV-visible spectroscopy48Figure :
Scatter Spectra
• Rayleigh scattering: Particles small relative to wavelength• Tyndall scattering: Particles large relative to wavelength
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Fundamentals of modern UV-visible spectroscopy49Figure :
Absorbance at the reference wavelength must be equivalent to the interference at the analytical wavelength
Isoabsorbance Corrections
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Fundamentals of modern UV-visible spectroscopy50Figure :
Background modeling can be done if the interference is due to a physical process
Background Modeling
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Fundamentals of modern UV-visible spectroscopy51Figure :
Corrects for constant background absorbance over a range
Internal Referencing
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Fundamentals of modern UV-visible spectroscopy52Figure :
Three-Point Correction
• Uses two reference wavelengths• Corrects for sloped linear background absorbance
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Fundamentals of modern UV-visible spectroscopy53Figure :
Discrimination of Broad Bands
• Derivatives can eliminate background absorption• Derivatives discriminate against broad absorbance bands
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Fundamentals of modern UV-visible spectroscopy54Figure :
Scatter Correction by Derivative Spectroscopy
Scatter is discriminated like a broad-band absorbance band
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Fundamentals of modern UV-visible spectroscopy55Figure :
Effect of Fluorescence
The emitted light of a fluorescing sample causes an error in the absorbance measurement
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Fundamentals of modern UV-visible spectroscopy56Figure :
Acceptance Angles and Magnitude of Fluorescence Error
• Forward optics: Absorbance at the excitation wavelengths are too low• Reversed optics: Absorbance at the emission wavelengths are too low
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Fundamentals of modern UV-visible spectroscopy57Figure :
Inadequate Calibration
• Theoretically only one standard is required to calibrate• In practice, deviations from Beer’s law can cause wrong results
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Fundamentals of modern UV-visible spectroscopy58Figure :
Calibration Data Sets
• Forward optics: Absorbance at the excitation wavelengths are too low• Reversed optics: Absorbance at the emission wavelengths are too low
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Fundamentals of modern UV-visible spectroscopy59Figure :
Wavelength(s) for Best Linearity
• A linear calibration curve is calculated at each wavelength• The correlation coefficient gives an estimate on the linearity
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Fundamentals of modern UV-visible spectroscopy60Figure :
Wavelength(s) for Best Accuracy
• The quantification results are calculated at each wavelength• The calculated concentration are giving an estimate of the accuracy
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Fundamentals of modern UV-visible spectroscopy61Figure :
Precision of an Analysis
Precision of a method is the degree of agreement among individual test results when the procedure is applied repeatedly to multiple samplings
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Fundamentals of modern UV-visible spectroscopy62Figure :
Wavelength(s) for Best Sensitivity
• Calculation of relative standard deviation of the measured values at each wavelength• The wavelength with lowest %RSD likely will yield the best sensitivity
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Fundamentals of modern UV-visible spectroscopy63Figure :
Wavelength(s) for Best Selectivity
Selectivity is the ability of a method to quantify accurately and specifically the analyte or analytes in the presence of other compounds
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Fundamentals of modern UV-visible spectroscopy64Figure :
Ideal Absorbance and Wavelength Standards
• An ideal absorbance standard would have a constant absorbance at all wavelengths• An ideal wavelength standard would have very narrow, well-defined peaks
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Fundamentals of modern UV-visible spectroscopy65Figure :
Ideal Stray Light Filter
An ideal stray light filter would transmit all wavelengths except the wavelength used to measure the stray light
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Fundamentals of modern UV-visible spectroscopy66Figure :
Holmium Perchlorate Solution
The most common wavelength accuracy standard is a holmium perchlorate solution
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Fundamentals of modern UV-visible spectroscopy67Figure :
Potassium Dichromate Solution
The photometric accuracy standard required by several pharmacopoeias is a potassium dichromate solution
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Fundamentals of modern UV-visible spectroscopy68Figure :
Stray Light Standard Solutions
The most common stray light standard and the respectively used wavelengths
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Fundamentals of modern UV-visible spectroscopy69Figure :
Toluene in Hexane (0.02% v/v)
The resolution is estimated by taking the ratio of the absorbance of the maximum near 269 nm and minimum near 266 nm
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Fundamentals of modern UV-visible spectroscopy70Figure :
Confirmation Analysis
In confirmation analysis, the absorbance at one or more additional wavelengths are used to quantify a sample
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Fundamentals of modern UV-visible spectroscopy71Figure :
Spectral Similarity
Comparative plots of similar and dissimilar spectra
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Fundamentals of modern UV-visible spectroscopy72Figure :
Precision and Accuracy
Precision – Precision + Precision – Precision +
Accuracy – Accuracy – Accuracy + Accuracy +
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Fundamentals of modern UV-visible spectroscopy73Figure :
Hydrolysis of Sultone
Wavelength [nm]
Abso
rban
ce [
AU
]