Kinesis Solutions Techtips

First, lets look at the five attributes of an effective inlet liner:

1. the design should minimise mass discrimination by ensuring complete vaporisation of the sample before it reaches the column entrance
2. the volume of the inlet liner must be larger than the volume of vaporised sample
3. adding quartz wool will dramatically increase the vaporisation surface area and promote mixing
4. the liner must not react with the sample. This is especially important for polar solutes where the liner should be deactivated
5. the glass wool should be in an optimum position

The simplest form of liner is the Straight-through Liner, which is ideal for gaseous samples. However, when injecting liquid samples, not all of the sample will vaporise instantly so droplets of liquid can hit the metal seal at the bottom of the injector. Remember that even at an injector temperature of 250’C, the liquid sample will not vaporise until it hits a hot surface. Even then, the droplets may only partially vaporise like droplets of water skating about on a hot frying pan.

The other problem with Straight-through liners is that when the sample hits the bottom of the injection port and vaporises, it absorbs heat from the metal base seal instead of the glass in the liner. Because metal has less thermal mass than glass and does not contain as much energy as glass at the same temperature, the metal base seal will cool down more when the liquid sample hits it. This leaves the high boiling point compounds with less energy to use when they vaporise. Because of this, straight-through liners can discriminate against high boiling point compounds.

Using a Single Tapered Liner with the taper at the bottom, overcomes both the above problems by preventing small droplets from hitting the metal surface at the bottom of the injector. The difference in mass discrimination between these two liner designs is shown in Figure 1.

 

Figure 1

 

The second aspect of liner design I want to consider is the internal diameter. As we have seen in an earlier issue, having a liner with too small an internal volume can cause flashback. So it is important that the inside diameter of the liner not be too small. However, for some injection techniques such as Splitless injection (where the overall flow rate through the liner is low) and SPME, reducing the inside diameter of the liner can greatly reduce the peak widths and, therefore, improve resolution. Figure 2 shows the splitless injection of a pentane sample run on almost identical set-ups - the only difference being the ID of the liner (4 mm vs. 2.3 mm). The improvement in the widths of the early eluting peaks is quite dramatic. For SPME injection, liners with an ID of 0.75 - 0.8 mm provide the narrowest peaks and best resolution.

 

Figure 2