Recapitulation Of Basic Aspects:

In the operation of a gas chromatograph the, solutes in a mixture are completely vaporised in the injection port and they are moved through the column by a carrier gas under pressure. That is in the column whereas separation takes place. From the column, a separated solutes pass through a detector whereas they are sensed producing an electronic signal. The signal is then amplified and generally displayed on a strip chart recorder. The trace plotted on the recorder is called a "Chromatogram".  It is a plot of the detector response within minivolts as a function of time. Commonly, time is the abscissa and minivolts the ordinate.

Figure:  A typical chromatogram

From the chromatogram, several general observations can be made.

Under a provided set of experimental conditions, every peak has a features retention time (t_R) and the retention volume (V_R) which are meaningful in qualitative analysis of solutes. The retention time for solute A is depicted in the figure as the distance from the point of injection to the peak maximum (t_R) A. Same, (t_R)B and (t_R)C are the retention times for solutes B and C, correspondingly. If the flow rate of carrier gas, F_c is constant, then

V_R = t_R × F_c                                                                            ... (7.1)

whereas, VR is the retention volume. Therefore, (V_R)A, (V_R)B, and (V_R)C are the retention volumes of solutes A,B and C, correspondingly. In practice, one could use either retention time (t_R) or retention volume (V_R) for qualitative analysis

(b)

Figure (a):  Chromatogram of sample containing 'A' and 'C'; (b) Chromatogram of standard solution containing 'A', 'B' and 'C'