JACK SKERRITT - STUDENT
Chromatography labs
Experiment : Introduction to GC. Use if an external standard. Examination of a mixture
of cyclohexane, cumene, cymene, ethyl benzene and toluene. Measurement of retention
time, adjusted retention time, N values, H(hetp), resolution and the % composition of an
unknown mixture of these compounds
Date 10/02/20
Lab partner: Abdul Momoh
Aims
To measure the retention times of the compounds and find the composition of the
unknown mixture of these compounds. Getting a knowledge of the GC machine and the
different operating temperatures available for use.
Procedure
As Per manual pg 5-9
Results
Table 1 Retention times and adjusted Retention times for all compounds at 95 celsius
tr
Compund
t𝜊
mm
min
25
1.25
Cyclohexane
198
9.9
Cymene
81
4.05
Cumene
58
2.9
Ethylbenzene
35
1.75
Toluene
Calculation of min retention times
mm
t'r= tr - t𝜊
min
0.35
0.35
0.35
0.35
0.35
7
7
7
7
7
mm
min
18
191
74
51
28
0.9
9.55
3.7
2.55
1.4
Chart speed = 20mm/min
Therefore for cyclohexane
25mm / 20mm/min= 1.25min
Table 2 : External standard at 95 degrees and the associated identities
tr
mm
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
t𝜊
min
21
33
54.3
77
143
mm
1.05
1.65
2.665
3.85
7.15
t'r= tr - t𝜊
min
7
7
7
7
7
mm
0.35
0.35
0.35
0.35
0.35
1
Identity
min
14
26
47.3
70
136
0.7
1.3
2.315
3.5
6.8
Cyclohexane
Toluene
Ethylbenzene
Cumene
Cymene
JACK SKERRITT - STUDENT
Table 3: calculation N and hetp from Program A
N
N1
N2
70.56
10
28.34
W
hetp
N3
144
11
13.89
N4
242.47
14
8.25
N5
561.33
13
3.56
Mean of N
484
300.472
26
14.8
4.13
11.634
Calculation of N
If we take Peak 1 as an example
Tr=21
W= 10
((4x21)/10)2
= 70.56
Calculation of hetp
Where l= the length of the column = 2000mm
Taking peak 1 again
N = 70.57
L = 2000
2000/70.56 = 28.34
Table 4 : resolution of the compounds at higher temperatures and the temperature
program
Resolution
95
1. /. 2.
15.60
2. /. 3.
17.63
3. /. 4.
14.46
4. /. 5.
22.31
Calculation of the resolution
105
17.83
17.31
27.00
35.76
115
12.83
13.22
19.18
25.65
T for peak 1 was 21
125 tempprogram
6.00
10.57
8.67
16.67
6.44
13.50
20.22
17.90
(2(33-21))/(10+11) = 15.6
T for peak 2 was 33
W1 = 10
W2 = 11
2
JACK SKERRITT - STUDENT
Table 5 : Determination of the % composition in the unknown mixture
Peak Area 1 Peak Area 2 Peak Area 3 Peak Area 4 Peak Area 5
External
Standard
Response
factor RF =
Area x /
Area 1
391961.594 439687.969 433141.549
1 1.12176289
412504.75 389890.813
1.1050612 1.05241115 0.99471688
Unknown
Mix
68921.945 119188.523 235054.906 153088.578
Adjusted
Area = Area
x / RF
68921.945 106251.084 212707.591 145464.611 152029.189 685374.421
151226 Total
Composition
%v/v
10.0561011 15.5026335 31.0352393 21.2241086 22.1819175
Discussion
The differences observed is down to the compounds structure. A stronger structure will
lead to a high boiling point which means at the higher temperatures it elutes a lot
quicker than the lower boiling points. You can see the retention times getting
consistently lower with each increase in temp.
The best program to use from looking at the chromatograms is the Temperature
program. This gives good separation and a quicker run time for each of the compounds.
This is due to each compound being able to elute at its preferred temperature instead of
the lower or higher ones. If you look at the lowest temp you can see that the peaks for
the last compound are very far down the chromatogram and are not very sharp. For the
next temp up we see the resolution is good bu the peaks are getting closer together anf
the last compound is still very far down the graph. At 115 we can see the last peaks
beginning to sharpen up bu the first peaks getting very close together. At 125 we can
see the final peak is closer and still separate but the other peaks are overlapping.
Cyclohexane
Toluene
Ethyl benzene
cumene
cymene
C6H12
C7H8
C8H10
C4H2
C10H14
110 Celsius
136 C
152C
177C
92.14
106.17
120.19
134.21
Boiling points
80 Celsius
MM g/mol
84.16
3
100