UV VIS Flashcards
What wavelengths are the most energetic
Shorter wavelengths
What must a molecule contain to use UV VIS
a chromophore - part of a module that contains
Conjugated unsaturated system
And or
Carbonyl groups
What is UV VIS energy used for after absorption
Used to promotes electrons from one molecular orbital to another
Draw binding orbital diagram
How does energy jumping of electrons happen
The meoculcue needs to absorb enough energy corresponding to the difference between the binding and anti bonding molecule.
What transitions have energy that lies within the UV vis spectrum
Pie to pie *
N to pie *
What does a longer conjugated system mean
Uv vis absorption happens more efficiently and eigh less energy
What is an autxochrome
Groups with lone pairs eg o and N that help extend a chromophore
What is absorbance
Absorbance relates tp how much light passes through a sample at a particaour wavelets relative to the incident light
How to measure absorbance eqn
A= log Io/ I
A= absorbance
io= intensity of incident light
I= intensity of transmitter light
What is absorbance a linear function of
Pathlength (usually 1cm)
Conc of drug
What is absorbance independent of
The intensity of incident light
Beer lambert law
Notes
What is molar extinction coefficient
The absorbance of a 1M solutiom
What is A(1%1cm)
The absorbance of a 1% (w/v) solution (1g/100ml)
What is absorptivity a function of
Wavelength and nature of the Molecuke itself
What is absopritvity independent of
Pathlengyh and conc
What do simple methods of up vis spectrophotometer assume
There is no interference from excipients and there is no suspended matter , which causes scattering of incident light and may increase path length which may lead to an over estimation of conc of drug in sample
Why is band maximum an accurate wavelength for ecquatipms
Absorbance is high so instr7ment can measure accurately
Reproducibility of absorbance is high , as absorbance is similar if there is a small instrumental error in wavelength
Steps taken in lab to use uv vis spec
An accurate mass of solid is dissolved.
The drug may be extracted into a solvent if the excipients aren’t soluble.
The solution Is diluted accurately to allow reliable absorbance measurement ( typically 0.1<a></a>
Does beer Lambert law hold at all concentrations
No
What should you do if the reliability of experimental conditions is in doubt
Calibration curve should be prepared
In what point of calibrate curve does BL law hold
Straight line / linear region
Steps to prepare calibration curve
A series of standards are prepared over a conc range
The absorbance of each is plotted as a function of conc
Linearity is verified
A straight line through origin should be obtained
The normal absorbance range is 0.2-1
The method of least squares is used to givea line of best fit
The equation of the line and correlation coefficient (r2 , usally >0.99) are reported
The equation of the line allows conc to be obtained for any absorbance in the range studied.
How to ensure a solution being examined is truly single component
A spectrum of the pure substance is obtained.
This is compared to the spectrum of the sample to be analysed
If there is a general distortion , there may be a problem with the background , and analysis can be difficult
If there is an extra band , another spectroscopicslly active compound may be present
Reasons for deviations from BLL
Optical reasons - such as light scattering from particulates - increases path length
Chemical reasons -
Dimerisation at high concs - extends chromophore - impact on wl of absorbance
Changes in ionisation - ionisable groups - particular amines and carboxylic acids
If the ionisable group is part of the chromophore , the effect is strong - as the nature of electronic transition and therefore wavelength or energy associated with it , changes
Weak organic acids or bases will show large changes with dilution as protonation/ deprotonatiom equilibrium shifts
How can you suppress the protonation / deprotonationshift in weak acid bases
Carrying out analysis in
Strongly acidic
Strongly basic
Or buffered media
What is isobestic point
When a buffers and strongly acidic /basic media don’t be used
A spectrum is run at different ph values , there is a wavelength where absorption of ionised = ionised = isobestic point
Absorbance at this point does not depend on ph of solutiom . Reliable measurements can be used at this wavelength wihjtout ph control.
It’s not a max lambda - sensitivity may be lowered
What may background problems lead to
Featureless, unstructured absorbance
What does a background show on graph
Descending absorbance with increase wavelength and is approximately linear over short wavelength range
