Topics - Split ratio
Capillary columns 100 micron or less in internal diameter, often require sample loads of less than a microgram for their effective use. It is extremely difficult (if not impossible) to place samples of a microgram or less directly onto such columns, even with a micro-syringe. Thus, to achieve such small sample loads in practice, a split injection system
is employed. The sample is injected into a heated glass tube (often called a ‘flash heater’) through which passes the stream of carrier gas, where it is rapidly vaporized. After vaporization the gas stream is split into two parts, a small portion passing into the column, the remainder passing out to waste. The ratio of the volume of gas passing to waste, to the volume of gas passing down the capillary column, is called the split ratio
. Commonly the split ratio is set at about 100 to 1 or more, so that only 1 % (or less) of the sample, passes into the capillary column. By adjusting the flow to waste, using a controllable restriction, split ratios of any desired value can be obtained. Due to the diffusivities of the individual solutes in a mixture being different, the mixture of solutes placed on the capillary column using a split injector may not necessarily be precisely representative of the original sample.