Injection Techniques used in GC: Names and What are we actually doing with each Technique?

When traveling around I experience a lot of confusion on the naming of Injection techniques in Gas Chromatography.  The challenge can be the “mode” settings of the GC. Often the GC does not use the same name as what the actual technique is what we are using.

Split injection

The sample is introduced in a hot liner where only a percentage of the sample enters the column(the sample amount is split).  The rest goes out via the split vent.  Amount entering the column depend on the actual volumetric flow that passes the split-point (=column inlet). Typical split ratios of 1:5 up to 1:500 are used which allows concentrations that can range from 2ppm to percent levels of sample. Sample transfer from the injection port to the column occurs quickly and the best way to assure even vaporization is by using wool. Often precision liners are used with wool to aid in sample evaporation.

GC setup done in split-mode

Splitless injection

Is used in trace analysis and the majority of the sample is transferred onto the column.  Transfer times are slower and peaks are broader when compared to split injection. Solvent focusing or analyte focusing are used to get a narrow band at the head of the column. In splitless injection focusing is essential.  It focuses (read: concentrates) the components at the inlet of the capillary. This focusing can be realized by using retention (for higher boiling components or using thicker film stationary phases), or by using the solvent effect. This last technique is very powerful and allows focusing of components that elute just a little later then the solvent itself, as a sharp band. (see fig.1)

Figure 1 With splitless injection, the solvent must look like the trunk of a tree.. Note that the peaks that elute immediate after the solvent are already very sharp, indicating the focusing

General –rule of thumb-setting is that during the splitless injection time, the oven is set at a temperature 20°C below the (atmospheric)boiling point of the solvent. The injection time is the time needed to empty the liner volume, which usually is between 60 and 90 seconds. After the injection time has passed, the split-vent (or purge valve) is opened and the liner is flushed (this will take out the last molecules of solvent, generating a very sharp solvent peak, see fig 1. At the same time the oven is programmed and the separation starts. For details on injection time, you can also use the EZGC Flow calculator.

Splitless injection only works for components that elute later then the solvent. Single gooseneck/single tapered (Restek Premium)-liners with wool on the bottom are recommended.

The GC setup done in splitless mode

Direct injection using a split/splitless inlet system

In a direct injection, all the sample is transferred into the column. There is no splitting done. Special liners are developed for the direct injection, we call the “uniliner”.

The uniliner have the tapered part in the bottom, allowing to make a “Press-Tight” type connection, (see fig. 2).

Figure 2: position of column in a Uniliner. Note there is a seal formed by the polyimide outer coating

Uniliners for Agilent GC also have a “side hole”, which is required to make the EFC work correctly.  As most GC’s do not have a separate “direct injection mode” to choose from, the GC is setup  in “splitless” mode. This assures that all of the sample enters the column. Here is where the confusion starts, as we are really performing a “Direct” injection.

Uniliners are mostly recommended for low level analysis and we cannot use the splitless technique. For instance if the analytes of interest elute before the solvent peak.

As all sample in the liner is transferred into the column, and the chromatographic separation starts immediate after injection, often the solvent peak will show some broadening and tailing and early eluting peaks after the solvent may elute on a skimmed baseline.  The best results are obtained by injection fast and use of 0.53mm ID columns.

Direct injection with a PTV inlet system

One can also do a Direct injection using the PTV (programmed Temperature Vaporizer). This technique is often used for High temp. simdist. The sample is introduced in a cold liner using the taper at the bottom, which is rapidly heated to high temperature. As all sample transfers, it’s direct injection. In a PTV configuration the software allows for the injection port to track the oven. Generally the injection port should always be kept 10°C above the oven temperature.

GC  software is configured for PTV, but be aware, it goes by many names…

Direct injection using Valves

In petrochemical methods, often valves are used for injection of the sample. The sample size is determined by the sample loop or by the internal volume of the rotor. This is also a direct injection as all the sample is transferred into the column. To make transfer quantitative, often the valves are heated or are positioned in the oven or a heating-box. The GC software allows for valve times for sample transfer.

GC setup is done by the valve settings in the software…

Sometimes a valve is used before a split inlet. In this case we use a splitted injection setup.

Cold On-Column injection

Here the sample is injected into the column as a liquid. The needle actually gets inside the column and introduces the sample. Injection temperature must be low, to prevent flash, usually 20C below the BP of solvent. For good on column injection, retention gaps are required to correct for the injection error. Mostly a 0.53mm ID retention gap is used, which allows most easy entrance with needle.

To make on column work possible, we have to choose the on-column mode of the GC settings; Total flows are very low, and as everything is transported to the column, one must be assure there are no leaks.

Cold on-Column using a PTV inlet system

Fig. 3 Inside the PTV the syringe needle is guided by the tapered liner, to realize an on-column injection

When using a PTV, one can also use this system in an “on column mode”. Using a tapered liner, but positioning the taper on top, allows the needle to be inserted inside the column.

Also here retention gaps are recommended and before starting the oven program, first the PTV must be programmed to transfer the analytes from the first 5 - 6 cm column inside the PTV;

GC must be setup in on-column mode.

 

With special thanks to Chris English for practical recommendations on GC settings