Capillary Chromatography - Injection Systems 2
If (m) g of sample is injected, then g of sample is placed on the column The split ratio can be changed by adjusting the portion of the carrier gas that flows to waste. This device is only used for small diameter capillary columns where the charge size is critical. Unfortunately, the device is not ideal due to component differentiation which results in the sample placed on the column not being truly representative of the actual material being analyzed. In general the solutes with the higher diffusivities (low molecular weight) are lost preferentially to those with lower diffusivities (higher molecular weights). this is an inherent problem with small bore capillary columns which is extremely difficult to obviate. In an attempt to overcome this problem, larger diameter open tubular columns have been employed that would permit on-column injection. The columns have an I.D. of about 0.056 in., which is slightly greater than the diameter of a specific hypodermic needle. The injection system is shown in figure 7.
Figure 7. Device for On-Column Injection in Large Bore Capillary Columns
Unhappily, this type of injector also is far from ideal, not so much from poor accuracy and precision but from its effect on column resolution. On injection, the sample breaks up into discrete segments, due to bubble formation in the first part of the column. As the solvent evaporates the sample is deposited at two or more locations along the column. When development commences, each local concentration of sample acts as a unique injection and a chromatogram containing very wide or multiple peaks is produced. There have been a number of procedures introduced in an attempt to eliminate the sample splitting on the column. The first solution was designated as the 'retention gap method' and is depicted in figure 8.