The retention time for cyclohexane on a GC instrument is 0.75 minutes. The tempe

Question

The retention time for cyclohexane on a GC instrument is 0.75 minutes. The temperature of
the GC column is 70°C and the gas flow rate is 60 mL/minute.
1) Would the retention time be higher or lower if the column temperature is raised to
100°C?
2) Would the retention time be higher or lower if the flow rate is changed to 30
mL/minute?
3) Would the retention time be higher or lower if the column length was decreased to ½
the length?
B. A student at another university wants to duplicate a GC separation that you did in lab. What
information about the instrument and column conditions should you send the student?

Explanation / Answer

A. For the GC analysis, retention time for cyclohexane is 0.75 min. Temperature of GC is 70 0C and gas flow rate 60 ml/min.

1) If the column temperature is raised to 100 oC, the retention time would get lowered.

2) If the flow rate is reduced to 30 ml/min, the retention time would get increased.

3) If column length was decreased, the retention would remain unchanged.

B. For duplicate GC separation, column specification : type, length ; Method specification : injector temp., column temp., detector temp, flow rate, ramp rate, run time all must be known.

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