Chemistry Reference
In-Depth Information
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Time (minutes)
20
Figure 4.1
Chromatogram for isocratic 20 min run at 50°C.
Starting with an isothermal method at 50°C for 20 min, the chromato-
gram produced (Figure 4.1) shows six prominent large peaks and two very
small peaks close to the baseline. The two small peaks are due to column deg-
radation and are not related to the sample, whereas the six prominent peaks
are. The very first peak is the solvent (methanol) peak, which means that only
five of the eight compounds are being separated with this method within the
20 min run time. This means that this temperature (i.e., 50°C) is too low for
all analytes to be detected. Either an increase in temperature or an extremely
long run time will be required for the other analytes to elute from the GC
column. (
Note:
50°C was chosen as the starting point since the boiling point
of methanol is approximately 65°C and, as has previously been mentioned,
normally a starting temperature at least 10°C lower than the lowest boiling
point of the analytes should be used.)
Figure 4.2 shows the resulting chromatogram when the temperature of
the isothermal method is altered from 50°C to 70°C. Eight peaks are now
present in the chromatogram. However, the first five peaks (after the solvent
peak) are sharp and well shaped (ignoring the small peaks on the baseline as
these are associated with column degradation). The last three peaks are not
as well shaped; they are resolved but a little wide at the baseline. Therefore,
an increase in temperature may resolve this issue. However, the first five
peaks may be too close to each other and poorly resolved. This is shown in
Figure 4.3(a), where the temperature was increased to 90°C, and (b) where
the temperature was increased to 120°C for comparison.
As can be seen from the resultant chromatograms in Figure 4.3, the iso-
cratic method at 90°C produces a chromatogram where the first few peaks
are too close together with no or little baseline resolution, but then the
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