The retention time of a peak can be used as means of qualitative identification. The position of the target compound peak plays an important role in GC qualitative analysis. We call the length of time between injection and position of the target compound peak a retention time. On the other hand, the time difference between the peak of an unretained compound and a target compound is called the adjusted retention time. We call the retention time of a compound that is not retained by stationary phase the gas hold-up time. The hold-up time is directly measured as air peak for TCD measurement using a packed column, but air peak is not observed for a capillary column. The hold-up time can be indirectly estimated by using three peaks of n-alkane.