UV-Vis Spectroscopy Flashcards
UV-Vis Transitions
bonding to antibonding
sigma to sigma*
pie to pie*
nonbonding to antibonding
n to sigma*
n to pie*
Forbidden UV-Vis Transitions
sigma to pie*
pie to sigma*
Lowest to highest energy
pie, sigma, sigma, pie, n
sigma, pie, n, pie, sigma
Functional groups that absorb in the UV and Vis regions
Chromophores
functional groups that do not absorb in the UV and Vis region but have effect of shifting chromophore peaks to different wavelengths
Auxochromes
absorbing species that shared pi and sigma electrons, unshared outer electrons
organic compounds
absorbing species which have unfilled d and f orbitals
inorganic species
Type of Shifts
Bathochromic
Hypsochromic
Hyperchromic
Hypochromic
shift in lamba max to longer wavelength
bathochromic shift
shift in lamba max to shorter wavelength
hypsochromic shift
increase in the intensity of UV maxima
hyperchromic shift
decrease in intensity of UV maxima
hypochromic shift
Basic Instrumentation (UV-Vis)
Radiation Source
Wavelength Selector
Sample Holder
Detector
*Computer for signal processing and output
Common Radiation Source for UV-Vis
Deuterium arc lamp
Tungsten halogen lamp
radiation source that emits in one direction only and had a half-life; common for UV
deuterium arc lamp
Radiation source common for Vis; polychromatic light is required; low noise
Tungsten Halogen Lamp
Device that uses filter as wavelength selector; for visible region; single wavelength
Photometer
Uses monochromator as wavelength selector; multiple wavelength; for UV-Vis region (occ for IR)
Spectrometer/ Spectrophotometer
small tube-like container with straight sides and a square cross-section
cuvette
distance of light that goes through the interior walls of a cuvette; inner distance from front to back window
Cuvette Path Length
Standard Cuvette Path Length
10mm
Lowest in cost cuvette; not resistant to all solvents; for Vis region only
acrylic plastic cells
Type of cell that is slightly more expensive than acrylic cells ; used in visible region only
Silicate glass cells
type of cuvette characterized for being very expensive; used for both UV and Vis region
Fused quartz cells
type of cell that can be used when sample volume is extremely limited (5microL)
microcells
The loss of sensitivity of microcells depends on the ____ and __
degree of aperturing and optical geometry
Do not touch the ____ with fingers, as oil can cause ___
optical surface; significant absorbance
If optical surfaces get mild contamination, the cell can be wiped with ___
photographic tissue
Remedy for serious contamination of optical surface
Using mild sulfonic detergent
Remedy for severe contamination of optical surfaces
Treatment with hydrochloric acid or nitric acid
Spectrophotometers usually contain either ___ or ___ detector
photomultiplier tube or photodiode detector
contain several photodiode detectors instead of a single one
photodiode array
Types of instruments (UV-Vis)
Single-beam
Double-beam in space
Double-beam in time
multichannel
type of instrument where there is only one light beam or optical path from source to detector
Single-beam
two beams are formed in space by a V-shape mirror called beamsplitter
simultaneously
double-beam in space
beams are separated in time by a rotating sector mirror that directs the entire beam from monochromator first through the reference cell and then the sample cell
alternately
double-beam in time
type of instrument that able to scan an entire spectrum in 0.1 seconds
multichannel
Problems in Measurement (UV-Vis)
Strong Absorbance
Weak Absorbance
Interference
Sample Decomposition
Remedy for strong Absorbance
dilute sample to an absorbance level within the linear dynamic range
Reducing the noise level will?
directly improves the precision of results
Increasing slit width will?
Allow more light, reduce the noise level, and improves the precision of results
Types of interference
Scattering
Fluorescence
Scattering effect can be reduced by?
placing the sample as close as possible to detector
Types of scattering
Rayleigh
Tyndall
occurs when the particles are small relative to the wavelength of light and is inversely proportional to the fourth power of wavelength
Rayleigh scattering
occurs when the particles are large relative to wavelength of light and is inversely proportional to the square of wavelength
Tyndall scattering
____ in the light can eliminate the error due to fluorescence
Placing a filter
How to prevent sample decomposition?
Placing a filter to eliminate low-wavelength UV light
Qualitative Analysis (UV-Vis) (4)
Detection of Impurities
Detection of Isomers
Detection of Functional Groups
Determination of MW
Quantitative Analysis
Determination of Concentration
Multicomponent Analysis
Spectrophotometric Titration
UV wavelength range
200-380 nm
Visible Light wavelength range
380 - 800 nm
