Mass Spectroscopy Problems
1. Explain by way of calculation the relative intensities for the lines for –Cl3 and –Cl4 are
as shown below.
Cl
Cl2
Cl3
Cl4
Relative Abundances
M+2
M+4
32.5
65
10.5
98
32
100
44
M
100
100
100
77
M+6
M+8
3.4
10.5
0.9
2. A mass spectrometer has a resolving power of 5000. To what value of ∆M is this
equivalent if M is,
a. 300?
b. 900?
c. 3000?
3. An isotope dilution analysis was done on a rock sample in order to assay for Rb. A
sample of rock weighing 0.350g was digested and to the sample was added a 29.45µg
spike of rubidium. The isotopic composition (number percent or mole percent) of the
spike was
87
Rb=95.4%,
85
Rb=4.6%,
The mass spectral results showed that the peak for 87Rb was 1.12 times the peak for 85Rb.
Assuming that both isotopes have exactly the same chemistries, what is the Rb content of
the rock in ppm (assuming both isotopes with normal isotope distribution)? The natural
abundances and atomic masses of the rubidium isotopes are shown in the following table.
Isotope
% Natural Abundance Atomic Mass (a.m.u.)
87
Rb
27.83
86.909
85
Rb
72.17
84.912
4. Mass spectra in Figures, I, II, III, IV, and V—are of important industrial chemicals that
are regulated in the workplace. Identify the molecular formula of each and the structure if
possible.
24
25
26
27
28
35
36
37
47
2.4
10.3
24.2
76.4
2.3
7.3
2.7
2.6
4.8
48
49
59
60
61
62
63
64
2.0
1.7
2.0
6.5
8.4
100.0
5.0
30.8
II
50
51
52
63
65
74
75
76
77
16.0
43.8
2.5
2.1
14.0
7.9
4.6
3.7
100.0
78
93
123
124
6.7
14.8
75.6
5.4
III
35
37
47
49
82
84
86
117
119
18.7
6.6
25.6
8.9
28.3
18.3
2.9
100.0
93.7
121
123
30.7
3.2
V
25
26
27
28
35
36
37
47
60
3.1
13.2
42.8
3.7
5.8
2.9
2.4
2.9
3.5
61
62
63
64
65
83
85
98
100
8.9
8.4
100.0
4.2
31.4
12.9
8.3
6.8
3.6
IV
25
26
27
28
35
36
47
48
49
3.0
13.1
39.8
3.3
4.2
2.6
2.1
2.2
24.9
51
60
61
62
63
64
65
98
100
7.8
3.2
8.8
100.0
12.5
31.1
3.2
7.3
4.5
5. The partial mass spectrum of a coal sample being analyzed for metal content is
shown below. The spectra amplified by different amounts are illustrated. All the
peaks are due to
positive ions of metals which were obtained by field desorption MS with a laserassisted vaporization. To separate the organic and anion components, the metals
were deposited on the emitter element by electro deposition. Identify the elements
present and the isotopes contributing to the peaks, taking into account the isotope
distributions of the elements present.
6. Below are illustrated the structures of four compounds with nominal asses of 194.
The accompanying table lists the mass spectra I, II, and III obtained with electron
bombardment ionization. Identify the compound from which each of the three spectra
came.
8. For the four compounds shown in problem 7.
a. What are their exact masses?
b. What resolution would be necessary to separate them in a mass spectrum?