Inlet Systems
Split/splitless capillary inlet
Splitless sampling
For splitless operation, the dilute sample is vaporized inside the inlet insert. Most of the sample is then swept onto the column.
For full column efficiency, vaporized sample components must reconcentrate at the head of the column prior to separation; without reconcentration, peak widths of eluting components reflect inlet insert volume rather than column efficiency. Reconcentration occurs via two processes, the solvent effect or cold trapping ; these are both discussed on the next page.
Since the technique requires flow through the inlet insert to enter the column during injection, solvent vapor throughout the inlet continues to enter the column even after sample component reconcentration. This can create a long, substantial solvent tail, obscuring peaks of interest. Thus, after a short time interval, the inlet insert is purged of solvent vapor.
After the inlet insert is purged, oven temperature is increased, causing solvent trapped at the head of the column to volatilize and move through the column. Sample components are thus released to be separated.
Since sample residence time inside the inlet is longer than in split sampling, the inlet may be operated at a lower temperature: 150 to 200^C is adequate for most applications.
Solvent effect
One mode of sample reconcentration is the solvent effect which requires vaporized sample components to be retained by a region of stationary phase and absorbed solvent at the head of the column. In this region, the front of the sample plug undergoes stronger retention than does the rear: solvent•saturatedstationary phase acts as a barrier to sample components, thereby reducing their bandwidths.
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