UV-Vis Spectroscopy Flashcards
What is a chromophore?
Part of a molecule which contains
*conjugated unsaturated systems (c=c)
*carbonyl groups (c=o)
What is a conjugated unsaturated system?
A molecule with two or more double/triple bonds each alternating with a single bond.
What happens when UV-Vis energy is absorbed?
*Energy promotes electrons from one molecular orbital to another
*conjugated systems contain pi bonding orbitals. Need energy to move electrons from pi->pi *orbital.
*for carbonyl groups, transition from non bonding oxygen with lone pair n->pi *orbital.
*conjugated system needs greater energy as higher jump.
*sigma bonds unaffected-molecule doesn’t fall apart
*greater length of conjugated system= more efficient and with less energy (longer wavelength).
How are chromophores extended?
Auxochromes-groups with lone pairs such as O or N atoms.
What is the beer lambert equations and what does each part mean?
A= e c l
A=absorbance
e=molar extinction coefficient
c=conc of analyte in moldm^-3
l=pathlength of cell cm
A= a b c
A=absorbance
a=the A(1%,1cm)
b=path length in cm
c=conc of analyte
What are e and A(1%,1cm)?
*measure of absorptivity at a given wavelength
When do we use A=ecl equation?
When the concentration is in moles
When do we use A=abc?
When concentration is in g/100ml
What is the rearranged beer lambert equation to calculate concentration?
c=A/e. l or c=A/a.b
What wavelength is appropriate for quantification?
Band maximum (gamma 1) because
*absorbance is high so instrument can measure it accurately
*reproducibility of absorbance high even with small instrument error in wavelength
How is a solid dosage form absorptivity determined?
*accurate mass of solid dissolved
*drug may be extracted into a solvent
*solution diluted to allow reliable readings (0.1<A<2)
*absorbance measured at appropriate wavelength
How do you prepare a calibration curve?
*prepare series of standards over a conc range
*plot absorbance as a function of conc
*plot straight line through origin
*normal absorbance range is 0.2-1.0
*report equation of line and correlation coefficient
*equation of line allows conc to be obtained for any absorbance in range studied
How do you know if the sample being studied is not made up of only a single component?
*extra band suggests another spectroscopically active compound present.
Describe reasons why the Beer Lambert law may not hold
*optical reasons-light scattering from particles
*changes in ionisation-particularly for amines and carboxylic acids
*if ionisable group is part of chromophore-nature of electronic transition
*weak organic acids and bases show large changes with dilution such as protonation/deprotonation equilibrium shifts.
How do you suppress protonation/deprotonation?
*Carry out analysis in strong acid or base or buffered media
*run spectra for both ionised and unionised forms-cross over on graph= isosbestic point (asborbance doesn’t depend on pH)
