Purged splitless injection

Purged Splitless Injection. This term is given to a splitless injection (see Splitless injection) wherein the vent is open to allow the large volume of carrier gas to pass through the injector to remove any volatile materials that may be left on the column. Most splitless injections are purged splitless injections. 

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.

Experiment C is the same as B. but the split vent was opened after 30 s to rapidly purge all vapors from the injection liner. The bands in chromatogram C would be similar to those in B, but the bands are truncated after 30 s. Experiment D was the same as C, except that the column was initially cooled to 25 °C to trap solvent and solutes at the beginning of the column. This is the correct condition for splitless injection. Solute peaks are sharp because the solutes were applied to the column in a narrow band of trapped solvent. Detector response in D is different from A-C. Actual peak areas in D are greater than those in A because most of the sample is applied to the column in D. but only a small fraction is applied in A. To make experiment D a proper splitless injection, the sample would need to be much more dilute. 

Why is splitless injection used with purge and trap sample preparation ... 

Analysis of the analytes was done by gas chromatography on a HP 5890 equipped with 7673 auto-sampler and DOS series chemstation (Hewlett Packard Co., Avondale, PA). A 25m HP-5 column with 200 um internal diameter and 0.33 um film thickness was used for all separations reported herein. The injection mode was purged splitless (1 uL injection) and the temperature ramp used for all separations was 60°C for 1 minute ramping then to 300°C at 20°C/minute. 

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. 

Split injection, splitless injection, SPME, headspace SPME, gas valve injection, tenax trapping with thermal desorption, purge and trap sampling... 

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. 

For cold splitless injection, the sample is introduced into the vaporizing chamber at a temperature close to the solvent boiling point with the split vent closed. Shortly after sample introduction, the injector is rapidly heated to the temperature required to transfer the sample into the column, and the vaporization chamber purged of solvent residues by opening the split vent. Similar to hot splitless injection, cold trapping and solvent effects are employed as refocusing mechanisms. 

Splitless injection uses the same hardware as spUt injection (Fig. 6.9), but the split valve is initially closed. The sample is diluted in a volatile solvent (like hexane or methanol) and 1 to 5 /iL is 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 45 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 is essential with splitless injections. 

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). 

Splitless injections are used to introduce dilute solutions. The sample is injected into the heated injection port, which is in the "purge off"mode. In this... 

The flow and purge valve settings for splitless injection are relatively straightforward to optimize. The column flow should be set as high as possible during the... 

Another technique is the so-called splitless or combined split/splitless injection [29], In this case the split hne is first closed. After the desired amount of sample is loaded onto the column, the split line opens (typically lO-KX) s after injection and evaporation of the sample) and the remaining (superfluous) sample is purged from the injector. 

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