UV-Vis and Mass Spectrometry

Question 1

UV/Vis Spectrophotometry

Answer 1

An instrument for irradiating a sample with photons of UV or vis light of particular wavelength/s

Must be run against a sample blank, (same solution but no drug)

Liquid sample dispensed into a cuvette (solvent not abs. above 200nm) UV – quartz; vis – plastic, polystyrene

Question 2

spectrophotometry measures either:

Answer 2

absorption - light not passing through sample transmission – light passing through sample

Question 3

Molecular Orbitals (MO)

Answer 3

The region in which there is the highest probability of finding an electron of a molecule

Question 4

Highest Occupied Molecular Orbital (HOMO):

Answer 4

Highest energy MO with any electrons in it

Question 5

Lowest Unoccupied Molecular Orbital (LUMO):

Answer 5

The next highest energy orbital. It will be empty and so is the lowest energy orbital in which to place an electron or excite one into.

Question 6

Ultraviolet Absorption Spectroscopy

Answer 6

Wavelengths of UV light absorbed by a molecule are determined by the electronic energy differences between the HOMO and LUMO.

Question 7

Absorption is only possible if the molecule contains ____ ________ levels separated by energy (Transition from bonding to anti-bonding orbitals, 𝜋 to 𝜋, 𝜋 to σ

Answer 7

two molecular

Question 8

Ultraviolet Absorption Spectroscopy - The molecule will now be in what state?

after energy will dissipate rapidly and return to what state?

Answer 8

excited

stable

Question 9

graph for UV energy

Answer 9

electronic excited states

AE = (E excited - E ground) = hv

Question 10

Lambhda max (A max) =

Answer 10

wavelength showing the greatest absorbance

Question 11

What is Beer’s Law?

Answer 11

states that the absorptive capacity of a dissolved substance is directly proportional to its concentration in a solution

Absorbance is what we measure and has arbitrary units
The molar absorption coefficient is a standard
The concentration is in moldm-3
The path length is the width of the cuvette (normally 1 cm)

Question 12

Beer’s Law equation

Answer 12

A = ecl

A - absorbance
e - molar absorption coefficient M-1cm-1
c - molar concentartion M
l - optical path length cm

Question 13

How to interpret UV-Vis Spectra?

Answer 13

1. Wavelength(s) of maximum absorbance
2. Molar absorptivity e at each maximum

Question 14

Explain the relationship between aromatic compounds and absorption of UV

Answer 14

highly conjugated
absorb strongly in UV

Question 15

UV-Vis = Active Compounds

Answer 15

Chromophores - structural features pf a molecule that absorb specific wavelengths ( e.g. conjugation)

Question 16

Auxochromes

Answer 16

auxiliary groups attached to a chromophore, unable to absorb, cause (often substantial) shifts

Question 17

resonance

Answer 17

different electronic configurations of same structure

Question 18

inductive effects

Answer 18

groups feeling in or pulling out e-

Question 19

example: Primaquine free base ( what colour)

Answer 19

yellow liquid

Question 20

primaquine diphosphate

Answer 20

red solid, due to bathochromic shift caused by auxochromes

Question 21

Coloured Organic Compounds

Answer 21

white object reflects all of the light

coloured object absorbs some wavelengths and reflects the rest of the spectrum

colour we see - complete visible - absorbed portion

Question 22

Mass Spectrometry

Answer 22

Provides Molecular Weight

Provides valuable information about the molecular formula

Does not use light (different from spectrometry)

Sample is destroyed

High-energy electrons fragment the molecules
Mass of the fragments is measured
Information used to “reconstruct” the molecule

Question 23

Mass Spectrometry - step by step

Answer 23

1. Negatively charged accelerator plate with narrow slit to allow ions to pass through
2. Charged particle passes through magnetic field (bends path) – Curvature of the bend depends on the m/z ratio (mass / charge)
3. Slit followed by detector – at any given magnetic field, only ions of one particular mass are bent exactly the right amount to pass through the detector. The detector signal is proportional to the number of ions striking it

Question 24

Peaks are typically very sharp - often vertical lines

Answer 24

masses rounded to the nearest whole-number mass unit

Question 25

Base peak does not necessarily correspond to the mass of the molecular ion (M+)

Answer 25

often very small peaks close to expected MW - isotope peaks

Question 26

molecular ions (M+)

Answer 26

usually the particles of highest MW in the spectrum

Question 27

m/z value - give the MW of the compound

Answer 27

if no M+ observed: use a gentle ionization

Question 28

Determination of Molecular Formula by MS

Answer 28

look at masses - exact mass

Question 29

molecular ion peaks (M+) what do they provide

Answer 29

information about the molecular formula

Question 30

most elements contain:

Answer 30

heavier isotopes in varying amount

Question 31

Heavier isotopes: small peaks at higher mass numbers than the M+ ion peak

Answer 31

M + 1 (ones mass unit heavier M+); M + 2 (two mass unit heavier M+)

Question 32

Determination of Molecular Formula by MS

Answer 32

use of HEAVIER isotope peaks - recognisable elements in the mass spectrum

Compound with no S, Cl or Br: M + 1 peak small and M+2 peak non visible

Question 33

Compound containing S: M+2 (34 S) larger than usual

Answer 33

(4% of M+ 33S)

Question 34

Compound containing Cl:L M+2 (37Cl) 1/3 as

Answer 34

large M+ (25Cl)

Question 35

Compund containing Br: M+2 (81 Br) as large as

Answer 35

M+ (79Br)

Question 36

Compound containing I:

Answer 36

I+ iodonium ion at 127; large gap

Question 37

Compound N:

Answer 37

odd M+ some even-numbered fragments

Question 38

Stable compounds (C H O )

Answer 38

even M+, odd numbered fragments

Question 39

Fragmentation pattern in MS

Answer 39

Fragmentation splitting out a small molecule

Question 40

loss of small stable molecules:

Answer 40

indicated by a fragment peak with even mass numbers