4.3.1 Basic Inlet Techniques

1. Split: The inlet is continuously purged with vent gas as some flow ratio to the column flow (at this lab, we use 60:1). This means the flow through the column is $\frac{1}{60}$ of the total flow. This results in most of the injected solution being vented rather than deposited on the column, which in turn gives a tight, spatially limited band of analyte on the column.
   • Split injection typically gives the best chromatography (highest theoretical plates).
   • Most of the sample is lost, so split injection is not used when absolute sensitivity is required. However, this does give 1-2 orders of magnitude of dynamic range in instrument response without changing any actual detector parameters (simply by splitting or not).
     

2. Splitless: The split vent is closed during the actual injection. Some time after the injection (for example, one minute), the split vent is opened to purge excess solvent. This technique allows a greater amount of the injected sample to be deposited on the column.
   • Splitless injection gives a greater response for a given solution than split since most of the sample is actually deposited (rather than vented).
   • Splitless injection gives the most precise quantitative results, but the chromatographic resolution may be less than with split injection.
   • The instrument parameters needed to produce that maximum resolution are compound specific. This means that each analysis must be optimized to achieve maximum resolution.
     

3. Pulsed Splitless: Pulsed splitless is similar to splitless injection, but during the vent closed portion of the timing cycle, the column flow is pulsed to a relatively high rate. This a relatively new technique that combines the advantages of split (better chromatographic resolution) and splitless (better quantitative results and greater response).