Kinesis Solutions Techtips
Choosing the right liner for the job
Welcome to the first of latest techtip from Kinesis intended to address the important considerations for inlet liner choice. If you have further questions on this or any other technical point, please contact us through our website www.kinesis-solutions.co.uk.
Quartz wool or no Quartz wool.
The use of quartz wool has many advantages. Firstly, quartz wool acts as a filter for the expensive analytical column and minimises the chances of any particulate or non-volatile material from reaching the column. Secondly, the wool captures the liquid dispensed from the syringe and promotes vaporisation thus preventing the liquid hitting the bottom of the injector. Even though the quartz wool used in SGE liners is fully deactivated in situ, it is still recommended not to use wool if analysing low level pesticides such as DDT or Endrin.
Taper at the bottom
The taper at the bottom of the liner acts as a feed-in for the capillary column and again minimises the dispensed liquid from hitting the bottom of the injector (Figure 1). If the quartz wool is packed loosely, i.e. not the FocusLiner, then the taper prevents the wool moving out of the column which might happen during high pressure scenarios such as pulsed splitless injection.
Restrictions, baffles, cups, complex forms, etc. in the liner without quartz wool
All these complex glass forming of the liner can promote vaporisation and mixing of the formed vapours which will minimise mass discrimination. One of the key functions of the inlet liner is for representative transfer of the sample from the syringe into the capillary column. If less of the high molecular weight component of the sample is transferred then this is known as high molecular weight mass discrimination (Figure 2). Liner design and quartz wool can both promote mixing and minimise mass discrimination.
Taper at the top
The taper at the top can be used to minimise the effect known as Flashback. This occurs when an excessive amount of liquid is injected into the liner and the volume of the vaporised gas is larger than the volume of the liner (Figure 3). The gas can then escape back out of the liner into the inlet lines and cause contamination. The taper at the top reduces this effect by acting as a partial lid on the liner.
Inner diameter
The inner diameter will obviously determine the volume capacity of the liner. Therefore for a liquid volume greater than 2µl, the liner ID should be as large as possible. Remember, when the liner ID is halved, the subsequent liner volume is reduced to one quarter of the original. The other important consideration for liner ID is the velocity of the carrier gas through the liner. A smaller ID will result in a higher velocity of gas and this means faster analyte transfer and therefore sharper peaks, especially for the early eluting component. The transfer rate is more critical in splitless injection because the liner flow is equal to the column flow and is low. A smaller ID can have a dramatic effect of peak shape as shown in Figure 4.
Deactivation
Deactivation is more critical during splitless injection than split injection. In splitless injection, the split vent is usually closed for about 1 minute and this results in a low liner gas flow. A low gas flow will result in slow transfer and the residence time of analytes within the liner is increased. Thus for thermally labile and heat sensitive compounds, the interaction time of the analyte with the inner surface of the glass liner is increased and this enhances breakdown. The effect is not nearly as pronounced in split injection because the residence time in the liner is very short (Figure 5).
