Question

The following data was collected from a CG experiment. Calculate the retention index for the unknown. NO error calculations necessary (Phew!)

		retention time (s)
unretained marker		100.7
butane		134.9
pentane		159.2
hexane		200.7
heptane		271.6
octane		392.7
nonane		599.6
unknown		163.9

Answer 1

First we have to adjust the retention time by subtracting the retention time for the unretained marker.

Compound	Retention Time(s)	Adjusted retention time(s)
unretained marker	100.7	0
butane	134.9	34.2
pentane	159.2	58.5
hexane	200.7	100
heptane	271.6	170.9
octane	392.7	292
nonane	599.6	498.9
unknown	163.9	63.2

The Retention Index in given by the following formula for a temperature programmed Gas chromatography data

Where, N is the alkane with highest number of carbon in the data. (In our case N=9 (Nonane))

n is the alkane with lowest number of carbon (n=4, butane)

All the t's are the adjusted retention times.

I in the retention index.

Hence, the retention index is