CH908
UNIVERSITY OF WARWICK
MASTER OF SCIENCE EXAMINATIONS:
MARCH 2014
MASS SPECTROMETRY
Time allowed 2 hours
Read carefully the instructions on the answer book and make sure that the particulars required are entered
on each answer book.
Percentages in square brackets are intended as a guide to the time candidates should spend in answering
the corresponding part of the question.
Answer ANY TWO QUESTIONS
Question 1
[50%]
Note: please see Tables 2 and 3 below for additional information.
A. Consider the electron impact Spectrum 1, below:
Spectrum 1. An electron impact mass spectrum.
1. What sub-structures are consistent with the peaks at:
i. 151 and 153
ii. 123 and 125
iii. 107 and 109
iv. 72
v. 55
vi. 44
vii. 27, 28, 29, 30
viii. 16, 17, 18
2. Is the peak at m/z 151 the molecular ion ? Why?
3. What are the best molecular structures consistent with Spectrum 1?
Report at most 3, if you have more structures, then circle the 3 best.
1
B. Consider the electron impact spectrum 2:
Spectrum 2. An electron impact mass spectrum
The molecule in Spectrum 2 generates a [M+H]+ ion at 128.1 from electrospray mass
spectrometry.
1. What sub-structures are consistent with the peaks at:
i. 127
ii. 100
iii. 87
iv. 84
v. 68 and 70
vi. 44
vii. 41
viii. 27, 29, 30, 31
ix. 14, 15, 16, 17, and 18
2. What are the best molecular structures consistent with Spectrum 2?
2
Question 2
[50%]
(a) The spectrum below shows a high resolution electrospray ionisation tandem mass spectrum of a
peptide called L3BBS using electron capture dissociation:
Note: please see tables 2 and 3 below for additional information.
(i) How does electron capture dissociation work?
(ii) Which peak represents the precursor ion peak?
(iii) This spectrum is for a peptide with the sequence, pQKLGNQWAVGHLM-NH2, where ‘pQ’ indicates a
‘cyclized’ pyroglutamic acid and ‘–NH2’ indicates an amidated c-terminus. Make a cleavage diagram and
mark all of the labelled peaks on it.
(iv) Are there any complementary fragment ions observed? If so, which?
(v) This peptide has modified endgroups. The c-terminius is amidated, and the n-terminus is cyclised to a
pyroglutamic acid. How would these modifications be detectable in the spectrum?
(vi) Draw the full structure of the c3 ion.
(vii) Draw the full structure of the z7 ion (sidechains can be defined as ‘RX’ where x is the single letter amino
acid code).
(b) (i) Outline the principles of Electrospray ionization.
(ii) Why is it necessary to ionize a molecule to measure its mass using a mass spectrometer?
(c) (i) How does a quadrupole mass spectrometer measure mass?
(ii) Sketch a Mathieu stability diagram and note its key features.
(iii) How is the low mass cut-off of a quadrupole mass analyzer running in the rf-only mode calculated?
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Table 1
Element
H
C
N
O
F
Cl
Br
Atomic mass
1.007825
2.014102
12.000000
13.003355
14.003074
15.000109
15.994915
16.999132
17.999160
18.998403
34.968853
36.9659
78.918338
80.916291
Stable
Isotopes
1
H
2
H
12
C
13
C
14
N
15
N
16
O
17
O
18
O
19
F
35
Cl
37
Cl
79
Br
81
Br
4
Isotope
Abundance (%)
99.99
0.01
98.9
1.1
99.6
0.4
99.76
0.04
0.20
100.0
68.0
32.0
50.7
49.3
Table 2
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