Polishing Your Injection Technique

Ninety percent of gas chromatography (GC) problems that users experience are due to injection technique and conditions. That’s what 36 years of teaching GC has taught me. Users generally discover problems when reviewing the results or the chromatogram. Like fingerprints, chromatograms tell a story.

Many things can go wrong. The user needs to know what is happening in the injection port and also understand the most critical parts of the equipment/parameters for efficient and effective GC, especially for troubleshooting the technique. And that’s not limited to choosing the correct liner diameter. The user also needs to understand its various components and the conditions occurring within it. A few areas of concern include the use of wool and its position in the liner, injection volume, contamination, septa type, temperature, seals, O-rings, column position, and so on.

Once the user understands the purpose (and useful life expectancy) of all parts of the injection port, they can move onto the next challenge: choosing an appropriate liner for the application and optimizing injection conditions to introduce the sample as a narrow band. A user will generally choose the liners they are familiar with: “I know! I’ll use the same liner I used before for this application.” I think that’s why vendor catalogues list an assortment of liners based on what customers have bought previously. Indeed, I know from experience that you just need four types of liner to perform 90 percent of injections using split and splitless methods. Here, I share my liner secrets.

Split injection using liquid samples is rapid. It needs fast evaporation and mixing, and you must minimize the temperature drop in the liner when the solvent evaporates, because splitting occurs at the same time and a change in temperature will cause significant discrimination.

A 4mm internal diameter precision liner is always my first choice for split injection. The deactivated wool is positioned at the top of this type of liner, and the injection is done just into the wool. The wool delivers the heat capacity, minimizing the temperature drop when the solvent evaporates. Such liners will work in most split applications and will give under one percent relative standard deviation. An additional advantage is that septum particles will accumulate on top of the wool. In fact, I only use a different type of liner for split injection if sample components or matrix show reactivity or an unwanted interaction with the wool. If any of these happen, I’ll choose a liner without wool like a cyclo liner. You can also use a hot-needle injection to facilitate correct evaporation.

Splitless injection is used for trace analysis and requires you to inject a larger amount of sample than you would with split injection, so you need a focusing mechanism to minimize injection bandwidth. Typically, the default choice of liner is a 4 mm single bottom-taper liner, containing wool. These kinds of liners are also suitable for large volume injections up to 100 µl. Sample injection takes 20-80 seconds and – to facilitate focusing – the initial oven temperature is set at 20 °C below boiling point, which allows some condensation in the first section of the capillary. After the sample is injected completely, the split purge valve is opened and the oven is programmed. Again, I only use a different liner if the matrix or components show unwanted interaction with the wool. In such cases, I’ll use a single bottom taper (gooseneck) liner.

Special liner designs are available for direct injection or on-column injection using programmed temperature vaporization (PTV). And for headspace and solid phase micro extraction, I prefer to use liners with under 1mm ID, usually used with high retentive stationary phases.

So, only four liner designs for most applications. Not so difficult after all?