Medicine: EMR and UV-Vis Spectrophotometery

Question 1

What tends to shift absorption to longer wavelengths? What is this called?

Answer 1

Conjugated structure

This is called bathochromic shift

Question 2

How to calculate the concentration of a drug in grams/100ml?

Answer 2

C= A/A1%,1cm

where A1%,1cm=extraction coefficient

Question 3

What is Emission?

Answer 3

Electromagnetic radiation is produced when excited particles return to lower-energy levels or the ground state. This happens in florescence

Question 4

What does EMR spectroscopy involve?

Answer 4

Absorption, emission, or scattering of EMR by substances

Question 5

What are light waves made of?

Answer 5

Moving electric and magnetic fields vibrating 90 degrees to each other at the speed of light
The energy from these fields makes the wave move
Hence makes light waves electromagnetic

Question 6

What is an Auxochrome?

Answer 6

a functional group containing lone pair(s)
e.g. OH, NH2 of electrons that does not absorb appreciable amount of UV/visible light on its own but shifts peaks of molecules that it is attached with to longer λ (bathochromic shift) and makes the peak higher intensity (hyperchromic shift)

Question 7

What is a Chromophore?

Answer 7

A part of a molecule that absorbs UV or visible light (double bond, groups with lone pair atoms)

Question 8

UV spectrum absorption for drug analysis
Wavelength range?
Molecule must contain what?

Answer 8

> 200nm absorption
Molecule must contain unsaturated pi bonds or atoms with non binding orbitals (oxygen, nitrogen, halogen)
In reality UV absorption is mostly restricted to drug molecules with conjugated bonds (pi to pi*)

Question 9

Equation for the energy from each photon

Answer 9

E=hc/λ

E=energy
h= Plank’s constant
c= speed of light
λ= wavelength

Question 10

Which part of the UV spectrum is..

a) not much use in analysis
b) most commonly used for molecular analysis

Answer 10

a) <200nm absorption

b) >200nm absorption

Question 11

Using equations, if 100 photos enter a sample and only 10 leave, what is the absorbance value?

Answer 11

For 10% transmission
A=log(1/T)
1/T=10
thus A=log of 10=1

Question 12

What do these stand for?
HOMO
LUMO

Answer 12

Highest occupied molecular orbital

Lowest occupied molecular orbital

Question 13

where are n orbitals found?

Answer 13

n = non-bonding

e.g. in nitrogen, oxygen as lone pairs

Question 14

Wave and energy equation

Answer 14

c=fλ
where c=speed of light
f=frequency
λ= wavelength

Question 15

How is ε related to A1%, 1cm?

Answer 15

A1%,1cm = 10ε/Mr

where 1%=conc in g/100ml
1cm= path length in cm

Question 16

How are sigma molecular orbitals formed?

Answer 16

by electrons in 2S orbitals
e.g. CH3-CH3 (ethene)
forming single bonds.
Electron must be close to nucleus

Question 17

When photons hit something, EMR waves interact like…

Answer 17

other waves

Question 18

Why is the UV-visible spectrum made of broad rather than sharp lines?

Answer 18

Photons with slight difference in energy can still cause electronic transitions by exciting electrons from the many vibrational state that corresponds to its energy (wavelength)

Question 19

Total internal energy in a molecule=

Answer 19

sum of energy from
electrons
vibrations between the molecule’s own atoms
fluorescence emission - electrons returning back

Question 20

How are pi orbitals formed?

Answer 20

by electrons in p orbitals forming double or triple bonds

Question 21

Each energy level is discrete but has further vibrational and rotational levels which are also discrete, thus..

Answer 21

Excitation of a molecule can be electronic, vibration and rotational

Question 22

Deviations from Beer-Lambert Law - about absorption in a homogeneous solution

Answer 22

May not be true for turbid samples, causing scatter of light

Question 23

Classify the electromagnetic spectrum in order of decreasing wavelength

Answer 23

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, Gamma rays

Question 24

What are the “other factors” which are assumed if A is directly proportional to C?

Answer 24

Association, dissociation, photodegration, solvation, complexation or adsorption, or if sample emits fluorescence etc

Question 25

Deviations from Beer-Lambert Law - about independent absorption

Answer 25

Each molecule does not act as an independent absorbing species in solution, especially at high concentration

Question 26

Energy frequency equation

Answer 26

E=hf
E=energy
h= Plank’s constant
f= frequency

Question 27

If A(1%,1cm) is for most drugs, what can you deduce?

Answer 27

Knowing absorbance of a solution its concentration can be easily calculated.

Question 28

Name 3 uses of UV absorption

Answer 28

1) Identification of drugs - by comparing and interpreting spectra. Not absolute ID has overlapping of spectra can occur
2) Measuring reactions, ionisation (pKa determination., solubility, drug release - dependent on change in absorption spectrum λmax and/or intensity) can therefore determine the pka and the solubility
3) Quantification of drugs - most useful and widely used
(Beer Lambert Law)

Question 29

Deviations from Beer Lambert - why is incident radiation not always monochromatic?

Answer 29

due to:

• instrument calibration errors,
• stray light (without passing through sample) going directly to the detectors

Question 30

When a bonding orbital (HOMO) is created there is also created….

Answer 30

a corresponding anti-bonding orbital (LUMO) that is normally unoccupied but lies at higher (less stable) state

Question 31

Deviations from Beer-Lambet Law - about standards such as potassium dichromate?

Answer 31

-Stray light can cause difference against standard

Question 32

what happens when photons hit a molecule?

Answer 32

A molecule/atom changes its energy state by absorbing or emitting energy that is equal to the energy difference between ground E0 and excited E1 state
Absorption = change in energy is positive
emission = change in energy is negative

Question 33

Explain the relationship between concentration of sample, absorbance and transmission.

Answer 33

A more concentrated sample would absorb more and transmit less. However transmittance is not linear with increase in concentration, instead exponentially. To make this relationship linear it is best reported as absorbance which is the logarithm (base 10) of the reciprocal of the transmittance:
A= log(1/T) = log(I0/I)

Question 34

The more extensive the conjugated system… the 1 is the separation in energy levels and thus the 2 the wavelength of absorption

Answer 34

1= less
2=longer

the more delocalised and conjugated, the more easily excitable

Question 35

What is absorption? Describe it

Answer 35

• Selective removal of certain frequencies by transfer of energy to atoms or molecules
• electrons promoted from lower-energy (ground) states to higher energy (excited) states
• Energy of exciting photon must exactly match the energy difference between the ground state and one of the excited states of the absorbing species

Question 36

When does delocalisation (resonance stabilisation) extend?

Answer 36

• if benzene rings are present
• if alternating double and single bonds exist between C=C, C=O, C=N, N=N, N=O for example
• Lone pairs on substituent groups with nitrogen, oxygen or halogen become involved e.g. NO2 groups

Question 37

BP monographs usually report what for most drugs?

Answer 37

A(1%,1cm)

Question 38

Deviations from Beer-Lambert Law regarding florescence?

Answer 38

If sample emits fluorescence then positive or negative deviations of the Beer-Lambert law may occur

Question 39

The amount transmitted (T) when light enters and leaves a sample is given by

Answer 39

T= I/I0
or
%T=100xT

Question 40

Units of ε
units of molarity used, but in pharmaceutical analysis which units are preferred?
What is ε replaced by?

Answer 40

Grams
Here ε is replaced by the term A 1%, 1cm
Specific absorbance of a 1% (1g/100ml) solution using 1cm path length is at a given wavelength

Question 41

Why does conjugation shift wavelength to red?

Answer 41

In conjugated systems electrons are easily delocalised making the energy gaps between the various orbitals smaller, thus lowering the energy to excite them. This gives batochromic shift

Question 42

Describe Beer Lambert Law and give the equation

Answer 42

The amount of light absorbed (A) is dependent on the concentration of the sample
A=εbc
where ε= molar extinction coefficient (M-1cm-1) or molar absorpitivity (i.e. the absorbance value ofr a 1M solution)
It is constant for a particular compound at a given wavelength
b = path length of the cell in cm
c = concentration in moles/litre (molarity)

Question 43

The Beer lambert equation implies linear relationship, but in practice this is only true for up to A <1.5
Why?

Answer 43

Molecules too close to each other

causing saturation of the detector and hence a non-linear relationship after A>1.5

Question 44

BP standard set limits

Answer 44

Wavelength +/-1nm for standard at given wavelength
A269/A266 0.02% toluene resolution check - > or equal to 5

Stray light 1.2%KCL sol against water - A>2 at 198nm

Path length +/-0.005cm

Question 45

What is a conjugated system?

Answer 45

Having double bonds with single bonds

Question 46

ΔE=?

Answer 46

h x c
____
λ

Question 47

Most drugs absorb

Answer 47

UV light

apart from a few coloured ones

Question 48

Which parts of the electromagnetic spectrum are used in pharmaceutical analysis?

Answer 48

200-1000nm

Question 49

For absorbance to be directly proportional to c which assumptions apply?

Answer 49

• Incident radiation must be monochromatic
• Absorbance checks can be made against standards such as potassium dichromate, the absorbance must lie within specified range in BP
• each molecule in solution acts as an independent absorbing species in solution
• absorption takes place in a homogeneous solution
• Other factors… see other flashcard