Splitless injection purge time

Figure 24-16 shows effects of operating parameters in split and splitless injections. Experiment A is a standard split injection with brisk flow through the split vent in Figure 24-15. The column was kept at 75"C. The injection liner was purged rapidly by carrier gas, and peaks are quite sharp. Experiment B shows the same sample injected in the same way, except the split vent was closed. Then the injection liner was purged slowly, and sample was applied to the column over a long time. Peaks are broad, and they tail badly because fresh carrier gas continuously mixes with vapor in the injector, making it more and more dilute but never completely flushing the sample from the injector. Peak areas in B are much greater than those in A because the entire sample reaches the column in B, whereas only a small fraction of sample reaches the column in A.

Splitless injection involves keeping the injector split vent closed during the time the sample is deposited on the column, after which the vent is reopened and the inlet purged with carrier gas. In splitless injection, the inlet temperature is elevated with respect to the column temperature. The sample is focused at the head of the column with the aid of the solvent effect. The solvent effect is the vaporization of sample and solvent matrix in the injection port, followed by trapping of the analyte in the condensing solvent at the head of the column. This trapping of the analyte serves to refocus the sample bandwidth and is only achieved after proper selection of the solvent, column and injector temperatures. Splitless injection techniques have been reviewed in References 29 and 30. 

Sample transfer to the column by splitless injection requires a comparatively long transfer time, from several seconds up to a few minutes, relying on cold trapping and/or solvent effects to refocus the sample at the column inlet. The sample transfer time is roughly equivalent to about twice the time required by the carrier gas to sweep out the volume of the vaporization chamber. Since the sample vapors are continually diluted with carrier gas, and some sample vapors accumulate in areas poorly swept by the carrier gas, complete sample transfer is difficult to achieve. At the end of the sample transfer period, the split flow is re-established to purge the inlet of remaining solvent vapors. 

Helium is applied as carrier gas for all instruments. The flow should be 0.8 mL/min, and injections should be performed in a splitless mode (split purge time 60s) at 280°C.The ion source and the GC/MS interface temperature should be 240 and 290°C, respectively. The temperature program for ID GC separations (on quadrupole and IT instruments) should be as follows the initial tempera-... 

Splitless injections are used for traee analyses or when the component concentration in the mixture of interest is about 200 ng (Fig. 3). The injected sample is vaporized and earried into the column by the carrier gas. At the moment of injeetion, the flow through the injeetor is the same (1-2 ml/min) as the eolumn flow. About 15-60 s after injeetion, additional carrier gas flow is introdueed into the injector. This extra gas purges the injector of any remaining sample that has not entered the column. The time at which the extra gas flow is introduced is called the purge activation time (or purge on). 

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