The most popular GC injector features an outstanding design and thanks to the patented DFC provides excellent precision in split, splitless, and pulsed injection modes of operation. In addition, the post-injection carrier gas save mode is a valuable cost-saving feature. TU fulfill the user’s analytical needs a series of liner types are available, including liners suitable for SPME analysis.
Splitless injection uses the same hardware as split injection (Figure 10), but the split valve is initially closed. The sample is diluted in a volatile solvent (like hexane or methanol) and injected in the heated injection port. The sample is vaporized and slowly (flow rate of about 1 mL/min) carried onto a cold column where both sample and solvent are condensed. After about 45-60 seconds, the split valve is opened (flow rate of about 50 mL/min), and any residual vapors left in the injection port are rapidly swept out of the system. Septum purge of 2-3 ml/min is essential with splitless injections. In optimal conditions, relatively high sample volumes can be injected to achieve the sensitivity required by the analysis of traces.
In an electronically controlled inlet, a “pulsed pressure” mode can be used to improve the sample transfer in splitless injection: the column head pressure increases immediately upon injection, for example to 80 or 100 psi to reduce the sample vapor volume (PV = nRT). After one minute, when all sample vapors are in the column the inlet pressure is reduced back to normal operating pressure (for example 20-30 psi).
The column is now temperature programmed, and initially only the volatile solvent is vaporized and carried through the column. While this is happening, the sample analytes are being refocused into a narrow band at the head of the column. At some later time, these analytes are vaporized by the hotter column and pass through the system. High resolution of these higher boiling analytes is now observed.
The big advantage of splitless injection is the improved sensitivity over split. Typically 20- to 50-fold more sample enters the column and the result is improved trace analysis for environmental, pharmaceutical, or biomedical samples.
Splitless has several disadvantages, however; it is time-consuming; you must start with a cold column, and you must temperature program. You must also dilute the sample with a volatile solvent, and optimize both the initial column temperature and the time of opening the split valve. Finally, splitless injection is not well-suited for volatile and thermally labile compounds.