Extra Column Dispersion - GC Capillary Columns > Page 15


This procedure involves using larger diameter columns that will allow a syringe needle to enter the column so that the sample can be discharged (using a micro syringe) directly into the column. Unfortunately, when the sample is injected into the column, it breaks up into separate parts, as bubbles form along the first part of the column. This causes the sample to be deposited at two or more positions along the tube as the solvent evaporates. When the separation is developed, each local concentration of sample can act as a separate injection and, as a result, a chromatogram containing very wide, double, or even multiple peaks may be produced. However, by removing the stationary phase from the first few centimeters of column and injecting the sample into this section of the column the sample will split and vaporize in the normal way. However, as there is no stationary phase present, the solutes will all travel at the speed of the mobile phase down the column until they reach a coated section of the column.

At this point they will be absorbed into the stationary phase and all the samplewill accumulate at that point. The temperature program is then started (initially from a fairly low temperature). This helps the solutes to accumulate at one point in the column (i.e., where the stationary phase coating begins). The solutes are eluted through the column in the normal way.

Figure 5. The Solute Focusing Method of Injection