GC Injection Techniques

GC Injection Techniques

When it comes to which injection technique is the best to use, there is no one correct answer due to the variety of samples and analyses required. No one method is optimal for all analyses, but in today's article we will look at some of the most common injection techniques.

Split injection is used for volatile to semi-volatile compounds and is an easy injection technique to use. With this technique, the flow of carrier gas is split between the capillary column and atmosphere via the "split vent" (see Figure 1). Generally split ratios are between 50:1 and 200:1 as this helps maintain accuracy and reproducibility. It is a robust method due to high carrier gas flows, but sensitivity is reduced due to splitting of the sample.

Figure 1
Splitless injection, as the name suggests is where the carrier gas is not split to atmosphere. This is not true splitless since it is only in this state for a period of time (0.3-1.0 minutes) and then the valve is opened to a split mode (see Figure 2 for the gas flowpath). This technique is used for trace analysis due to increased sensitivity when compared to split injection. Greater sensitivity is achieved because most of the injected sample is transferred to the capillary column. One drawback of this technique however is due to low carrier gas flow, there is a need for some type of peak focussing method.

 

Figure 2
Headspace technique is used when analysing volatile compounds in solid and liquid samples. In this technique the sample and solvent are mixed in a sealed vial where the volatile components are released into the "headspace". This headspace gas is then injected into the GC, providing a clean sample and reducing the potential of non-volatile material entering the column and reducing its lifetime. Concentrations of analytes using this technique are in the ppm range.
Purge and Trap (with Thermal Desorption) is a more sensitive technique than headspace, therefore allowing analysis of analytes in the ppb range. The samples are adsorbed onto a suitable resin such as Tenax A and then desorbed in the injector allowing the analytes to be separated in the capillary column. Negatives of this method are that it needs greater sample prep time and this technique is usually unable to be automated, therefore less sample throughput.
Large Volume Injection (LVI) has two techniques available. They are On-column and Programmed Temperature Vaporising (PTV). Both techniques allow detection limits in the part per trillion (ppt) level.

1. On-column injection with a retention gap is where the sample is injected onto a pre-column and then the solvent is vented, leaving the analytes only to transfer onto the analytical column for separation.
2. Programmed Temperature Vaporising (PTV) vaporises the solvent at low temperature in a packed chamber and then removes this solvent through the split vent. This leaves the solutes on the packing or inlet liner wall. Once the injection port is heated, the analytes are transferred onto the column. This technique has many parameters that need to be optimised therefore more time consuming than on-column injection.

The last technique we are going to look at is Solid Phase Micro Extraction (SPME), which is a technique whereby the analytes are adsorbed on a fibre and released into an injection port. The material generally used is a short length of fused silica fibre (approx. 10mm) and is coated with a stationary phase which is generally bonded to the tip of the plunger in a plunger-in-needle syringe. The technique works by exposing this fibre to a sample matrix, for example drinking water, and the organic analytes present will extract from the matrix onto the fibre. Once equilibrium is reached, the fibre is then removed from the sample and exposed into the heated inlet of the GC where the analytes are desorbed and then move onto the capillary column for analysis.