Extra Column Dispersion - GC Capillary Columns > Page 13

 

The sample isinjected into a heated tube where it is volatilized into the mobile phase stream that passes over the entrance to the capillary column and out to waste. An adjustable restriction is placed in the waste outlet to allow the necessary pressure to develop at the column inlet to force a flow (q ml/min) through the column. If the flow of mobile phase is Q ml/min then the waste flow will be (Q-q) ml/min.

If a mass (m) is injected into the splitter, then the proportion of the sample that enters the column (Dm) will be,

Thus, by adjusting the waste flow, the proportion of the sample entering the capillary column can be varied over a wide range of values and the necessary minimum permissible volume for the particular column in use can be selected for analysis. Unfortunately, the fraction taken in this way may not be representative of the original sample. This is due to the individual solutes in the mixture having different diffusivities and, thus, they distribute themselves across the sampling tube in an irregular manner. In general, the components with higher diffusivities (e.g., those solutes of lower molecular weight) will diffuse away from the bulk sample more quickly than those having lower diffusivities. As a consequence, the sample entering the column will have a disproportionally high amount of the higher molecular weight components relative to those with lower molecular weights.