Gas Chromatography - Injection Devices > Open Tubular Column Injection Systems > Page 16

The device has certain disadvantages due to component differentiation and the sample placed on the column may not be truly representative. The solutes with the higher diffusivities (low molecular weight) are lost preferentially to those with lower diffusivities (higher molecular weights). Consequently, quantitative analyses carried out using the high efficiency small diameter capillary columns may have limited accuracy and precision, depending on the nature of the sample.

This problem was partially solved by using larger diameter columns that would permit on-column injection. The columns are constructed to have an I.D. of about 0.056 in; which is slightly greater than the diameter of a certain hypodermic needles. This injection system is depicted in figure 9.

Figure 9 On-Column Injector for Large Bore Open Tubular Columns


However, there are also difficulties associated with this type of injector. On injection, the sample breaks up into separate portions, and bubbles form at the beginning of the column causing the sample to be deposited at different positions along the open tube as the solvent evaporates. On starting to develop the separation, each local concentration of sample acts as a separate injection. As a consequence, a chromatogram containing very wide or multiple peaks is produced. Procedures have been introduced in an attempt to eliminate sample splitting in this manner.