UV VIS

Question 1

What wavelengths are the most energetic

Answer 1

Shorter wavelengths

Question 2

What must a molecule contain to use UV VIS

Answer 2

a chromophore - part of a module that contains

Conjugated unsaturated system
And or
Carbonyl groups

Question 3

What is UV VIS energy used for after absorption

Answer 3

Used to promotes electrons from one molecular orbital to another

Question 4

Draw binding orbital diagram

Question 5

How does energy jumping of electrons happen

Answer 5

The meoculcue needs to absorb enough energy corresponding to the difference between the binding and anti bonding molecule.

Question 6

What transitions have energy that lies within the UV vis spectrum

Answer 6

Pie to pie *
N to pie *

Question 7

What does a longer conjugated system mean

Answer 7

Uv vis absorption happens more efficiently and eigh less energy

Question 8

What is an autxochrome

Answer 8

Groups with lone pairs eg o and N that help extend a chromophore

Question 9

What is absorbance

Answer 9

Absorbance relates tp how much light passes through a sample at a particaour wavelets relative to the incident light

Question 10

How to measure absorbance eqn

Answer 10

A= log Io/ I

A= absorbance

io= intensity of incident light
I= intensity of transmitter light

Question 11

What is absorbance a linear function of

Answer 11

Pathlength (usually 1cm)
Conc of drug

Question 12

What is absorbance independent of

Answer 12

The intensity of incident light

Question 13

Beer lambert law

Answer 13

Notes

Question 14

What is molar extinction coefficient

Answer 14

The absorbance of a 1M solutiom

Question 15

What is A(1%1cm)

Answer 15

The absorbance of a 1% (w/v) solution (1g/100ml)

Question 16

What is absorptivity a function of

Answer 16

Wavelength and nature of the Molecuke itself

Question 17

What is absopritvity independent of

Answer 17

Pathlengyh and conc

Question 18

What do simple methods of up vis spectrophotometer assume

Answer 18

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

Question 19

Why is band maximum an accurate wavelength for ecquatipms

Answer 19

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

Question 20

Steps taken in lab to use uv vis spec

Answer 20

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>

Question 21

Does beer Lambert law hold at all concentrations

Answer 21

No

Question 22

What should you do if the reliability of experimental conditions is in doubt

Answer 22

Calibration curve should be prepared

Question 23

In what point of calibrate curve does BL law hold

Answer 23

Straight line / linear region

Question 24

Steps to prepare calibration curve

Answer 24

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.

Question 25

How to ensure a solution being examined is truly single component

Answer 25

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

Question 26

Reasons for deviations from BLL

Answer 26

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

Question 27

How can you suppress the protonation / deprotonationshift in weak acid bases

Answer 27

Carrying out analysis in
Strongly acidic
Strongly basic
Or buffered media

Question 28

What is isobestic point

Answer 28

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

Question 29

What may background problems lead to

Answer 29

Featureless, unstructured absorbance

Question 30

What does a background show on graph

Answer 30

Descending absorbance with increase wavelength and is approximately linear over short wavelength range