is the UV and visible sections of the electromagnetic spectrum = 200-700nm
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Gamma rays have the
shortest WL , and the highest energy
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Radio waves have the
longest WL and the lowest energy
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Why light has a longer WL - red or blue?
red
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To abosrb in the UV vis region a ,molecule must be a
chromophore
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Chromophores usually have
conjugated unsaturated system, and or carbonyl groups
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conjugated unsaturated system
2 or mor double/triple bonds alternating with a single bond
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When UV vis energy is absorbed
electrons are promoted to higher energy levels. For UV vis we are only interested in the n-> pi* transition (carbonyl) and the pi->pi* transition
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The greater the length of the unsaturated conjugated system the ...
more effecient the energy transitions with less energy required
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Absorbance
the amount of light which passes thru the sample at a particular WL, relative to the incident light
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Absorbance log equation
A=Io/I
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A plastic cuvette will
absorb light of lower WL, but this is ok as long as sample doesnt absorb WL anywhere near this value
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Beer lamber law eq (2)
A= epsilon C l or A=abc
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pathlength of cuvette use
1cm tends to be used
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UV vis is a good
quantitative technique
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Uv vis is a bad
qualitative technique
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Assumptions
No interference by exipients, no suspended matter which would cause scattering and a higher A than expected
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Selection of WL
usually band max
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why band max
absorbance is high- allow more accurate measurement. Reproducibility is high
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If sample is solid
accurate mass dissolved. Drug extracted from solvent. Sol is diluted to appropriate conc to allow accurate A to be measured . A is measured at appropriate WL
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if sample is solution
it should not be diluted too much. if reliability of experimental condtions are doubtfull a calibration curve is used
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calibration curve
prepare series of standards of known concentrations. Plot their absorbances Vc concentrations. obtain eq of line. should *** origin. eq of line can be used to estimate concentrations from experiments absorbances
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To make sure analyte is only one compound we ..
compare to spectra of pure compound
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if general distortion
may be problem with background
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if extra band
may be another active compound present
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Deviations from Beer lambert law
WA and WB show large changes to spectra if diluted
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how can we overcome this?
ionisation may be suppresed by using either buffered. strongly acidic /strongly basic media
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If a moleucle has an ionisable group which contribute to chromophore
large changes in spectra
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if molecule has ionisable group which doesnt contribute to chromophore
minor changes to spectra
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alternative to buffer/acidic/basic media?
if spectra run at diff pH - there is a WL where absorbance is the same (intersection on graph) if spectra run at this specifc WL - then we do not need the specialied media
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isobestic point
WL at which both absorbances are the same at diff PH
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Non ideal systems
impurities present - even if impurities are not UV vis active they may contribute to the background
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How to correct background
figure out WL at which drug absorbance is negligible - allow spectra to run on roughly 50nm . extrapolate back to WL max and subtract from drug absorbance
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Other cards in this set
Card 2
Front
Gamma rays have the
Back
shortest WL , and the highest energy
Card 3
Front
Radio waves have the
Back
Card 4
Front
Why light has a longer WL - red or blue?
Back
Card 5
Front
To abosrb in the UV vis region a ,molecule must be a
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