Liner Selection Guide

The geometry of the liners is usually designed for a specific application:

Liner Geometry		Injection Technique	Sample Types	Function
Straight Liner		Split* SPME Headspace P&T	General purpose Concentrated samples Gaseous samples Dirty samples (only if wool is present)	A higher split flow is facilitated, ideal for fast GC and gaseous samples
Tapered / Gooseneck		Splitless*	Trace level Active compounds	It focuses the sample onto the head of the column and minimizes degradation due to contact with the hot metal parts of the injector
Focus Liner		Split Splitless	Dirty samples	Ensures that the wool remains in the correct position, critical for into-wool injections
Direct Tapered Liner		Direct On-column WBI / WBC SINJ	Trace level Active compounds	It connects the liner directly to the column and has the same function as tapered liners

*Most split and splitless liners can be used for both split and splitless applications. However, they perform best for the specified application.  

Purpose and advantage of using wool

The liner wool is a vaporization support that offers several advantages:

It increases the surface area for vaporization, which in turn enables a uniform and fast sample vaporization and helps prevent boiling point discrimination
It protects the column against non-volatile impurities
However, wool is not suitable for active samples since it can increase the number of active sites that can interact with the sample. This can lead to an increased degradation of active compounds, which could translate to peak tailing or the appearance of ghost peaks. 

 
Wool Position

The position of the wool in the liner can have a significant effect in the chromatographic results and may also vary by injector models:

Near the top - "Into wool"		Especially suitable for autosampler split injections, where high flows are used, and the residence time of the sample is short. The sample is injected into a hotter region of the injector to facilitate vaporization. Advantages: -The needle gets wiped during injection -Complete sample transfer and increased reproducibility Disadvantages: -Active sites are created when the needle enters the wool
Near the middle - "Onto wool"		Especially suitable for autosampler split injections, where high flows are used, and the residence time of the sample is short. The sample is injected into a hotter region of the injector to facilitate vaporization. Advantages: -Higher inertness Disadvantages: -The syringe is not wiped during injection
Near the bottom		Especially suitable for autosampler splitless injections, where the residence time of the sample is long. It allows the vaporizing sample to use more liner volume (as the total gas flow in splitless injections is quite slow).

When setting up the method, and especially if some parameters are critical, like redproducibility or LODS/LOQs, it is recommended to try wool in different positions.

 
Types of Wool

The first type of wool that comes to mind after hearing “wool” is the textile fibre typically obtained from sheep. However, that is not the type of wool used to fill liners, instead either silica/quartz or glass “wool” is used. Alternatively, frits or baffles can also be used for active compounds.

If needed, wool can be bought separately: 

201-47616-01    - 2 g non-deactivated silica/quartz wool
221-48600         - 2 g deactivated glass wool
However, when manually filling the liners with wool it is almost impossible to do so in a reproducible manner. The amount, density and position of the wool can therefore easily vary from liner to liner. Furthermore, the liner deactivation can become damaged in the process. Therefore, it is strongly recommended to buy pre-filled liners with wool, like the ones offered by Shimadzu.

Liner with Wool		Mainly used for liquid injections
Liner without Wool		Mainly used for headspace injections
Liner with a frit ot baffle		Mainly used for extremely active compounds

 
All wool liners are first packed with wool, and then deactivated (see next section), to avoid creating active sites in the liner.

Some analytes can react with or adsorb to the liner surface, leading to decreased peak intensity and poor peak shape (tailing). To prevent that, most liners are treated with a process called deactivation, which creates an inert surface by covering active sites like silanols.
There are different types of deactivations for different types of analytes:

Standard Deactivation

Suitable for a broad range of polar and non-polar compounds
Tested with EPA 8081B method
Temperature stable up to 400 °C
If not stated otherwise all liners are "standard deactivated"
 
Base Deactivation

Better recovery of basic compounds
Application examples: amines, basic drugs
Temperature stable up to 350 °C
 
Siltek Deactivation

Excellent inertness for highly active compounds
Not suitable for bases
Ideal for chlorinated pesticides
Temperature stable up to 380 °C
 
Silanization

Ideal for pesticides and polar compounds
Suitable for acidic and basic compounds
Not suitable for aqueous samples
Temperature stable up to 320 °C

When injecting a sample into the GC, it is important to be aware of the volume injected in relation to the volume of the liner used. If the injected volume is too large, so called “backflash” can occur, leading to several problems:

Contamination of the injection port
Asymmetrical peaks
Appearance of ghost peaks
Not reproducible chromatographic results
To avoid those problems, it is important to inject a lower volume than the liner volume. Typical injection volumes are:

Liquid samples: Approx. 1 - 2 µL    

Gas samples: Approx. 0.2 - 1 mL

Typical Liner Volume

Type		ID [mm] 5.0 mm OD x 95 mm Length	Volume [µl]
Splitless: single taper with wool		3.5	670
Split: straight with middle restriction and wool		3.5	800
Split: straight with middle restriction		3.5	860
HS / SPME / P&T: straight through		0.75	40
		2.6	500
		3.4	860

However, especially for liquid samples, the solvent in which the analytes are dissolved is crucial. When the samples are vaporized, and depending on the temperature and pressure of the injector, 1 – 2 µL of injected sample can expand to several hundreds to thousands of microliters!

In table 1, the expansion volume of some solvents calculated for typical injector conditions can be seen.

Table 1. Vaporization Volumes of 1 µL of Various Solvents at an Injection Unit Temperature of 250 °C and a Pressure of 140 kPa.

The GC inlet functions as the interface between the syringe and the GC capillary column, where the sample is introduced, vaporized, mixed with carrier gas, and transferred to the column. Shimadzu offers several types of injectors, such as SPL, PTV or OCI, and the corresponding liners for each of them.

There are different aspects that should be considered when choosing the perfect liner:

- Liner geometry
- Evaporation auxiliary /Packing material (wool/frit)
- Liner deactivation
- Injection volume