Basic Principles of Chromatography - Forensic Applications of Gas Chromatography - page 33

2.82 2.97

100
95

90
85
80
75
70
65
55
45

60
50

9.00

3.95

40
30
25

35

20
10
0

15
5

7.85

6.76

3.08 3.40

4.10 4.54 5.04 5.46 6.23 6.98 7.42

8.42 9.29

3 4 5 6

Time (min)

7 8 9

Figure 3.2 A chromatogram: a plot of signal (relative abundance; on the y-axis)
versus retention time (minutes; on the x-axis).

resultant output—the so-called chromatogram—represents the appearance
of the organic solvent and compounds (Figure 3.2). The chromatogram is
therefore a plot of the amount (concentration) of the compounds present as
a function of time.
Within the chromatogram it is possible to define some specific terms and
measurements (Figure 3.3); specifically, the following terms are identified:

• t o = the time of elution (minutes) of the unretained compound from
the point of sample injection; it is sometimes referred to as the col-
umn dead time. In practical terms this is often taken to be the time,
from sample injection, when the organic solvent appears in the chro-
matogram (see Figure 3.3).
• t r = the time of elution (minutes) of each compound from the point
of sample injection (or retention time) to the centre of the peak. In
the case of the example (see Figure 3.3), two compounds elute from
the column; therefore, we can refer to t r1 (the time of elution, from
the point of injection, of compound 1) and t r2 (the time of elution,
from the point of injection, of compound 2).
• h = the peak height (in units that are representative of the y-axis on
the chromatogram, e.g., microvolts). This is the height of the peak
measured from the baseline (i.e., the position with the chromato-
gram when no compound or solvent is present) to the highest point
that the compound attains in the vertical direction.
• A = the peak area (in units that are representative of the y-axis on
the chromatogram and the duration of the peak on the x-axis, i.e.,
time, e.g., μV.s).