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Varian GC Liners

Injection and vaporization in gas chromatography are influenced by a number of factors. The
most important ones being sample composition, injector type, injector heat capacity, heat
transfer, liner design and volume and adsorption effects. The type of injection is a key factor in liner choice. It is determined mainly by the concentration range of the compounds of interest.

Three major types of injection based on the flash evaporization principle can be distinguished:
Direct injection, Split injection and Split/Splitless injection. Each type of injection will require a dedicated liner design depending on sample characteristics and other demands. A critical parameter which needs careful attention is the liner volume in relation to the injected volume
and sample solvent. For an efficient and rapid vaporization of the sample a sufficient injector
heat capacity is needed. Injector heat capacity is linked to heat transfer, the process which determines to a large extent the speed of vaporization. It is because of this process that
glass or quartz wool
plays such a significant role in the injector. Glass wool also is significant
for minimizing discrimination effects.

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Direct injection

After flash vaporazation the entire sample is transferred to the column. The sample transfer speed in the injector is determined by column flow and volume of the injector liner. This injection type is most commonly applied for 0.53mm ID columns or packed columns. Injection volumes usually do not exceed 1µl. This maximum in injection volume is related to the internal volume of the liner. Direct injection can be applied for ppm to percent level concentration ranges.

Liners for direct injections

Straight tube

straighttube

This type of liner is more suited for packed columns than it is for 0.53mm ID columns. The wide-bore column should be positioned at the bottom of the liner just piercing through the ferrule. This will minimize the effects of possible dead volume during injections although injector parts which are swept poorly with carriergas will result in poor injection profiles.

Gooseneck

gooseneck

This is a different type of gooseneck liner where the gooseneck holds the wide-bore column in position. This design effectively eliminates any dead volume in the injector and provides the best possible injection profile for wide-bore columns. In reversed position it can also be used for on-column injections on 0.53mm columns.

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Split injection

After flash vaporazation the sample is divided into two streams. One directed towards the column, the other to the split vent. Here, the sample transfer speed in the injector is mainly determined by the flow of the split vent and the liner volume. The column flow plays an insignificant role. A high transfer speed of the sample is critical for the quick introduction of the compounds into the capillary column maintaining the columns separation efficiency. Split injections are used for all capillary columns with ID's ranging from 100µm to 530µm. The maximum injection volume for split injections is about 1µl. Split injections are used for concentrations from 100ppm upto the percentage level.

Liners for split injections

Frit Splitter

Frit Splitter

The ceramic frit will prevent dirt and non-volatiles from reaching the column. The frit can become active. The gooseneck prevents backflash.

Inverted -or Laminar Cup Splitter

Inverted -or Laminar Cup Splitter

The cup will promote evaporation and mixing of the sample with the carrier gas. It is the best splitter for higher molecular weight compounds. Slightly larger injection volumes are possible because the cup will hold the sample liquid until it vaporizes. It is difficult to clean and is relatively expensive.

Baffle Splitter

Baffle Splitter

The baffle creates a turbulent flow which results in more reproducible injections. Not well suited for higher boiling compounds because of incomplete vaporization and thus subject to discrimination.

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Split/Splitless

During the initial splitless period the vaporized sample is transferred to the column. The speed and efficiency of this process is determined by the capillary column flow and the liner volume. Larger injection volumes are possible for splitless type injections, volumes upto 5µl are not uncommon. These large injection volumes can never be injected at once because of possible liner overload but must be carefully introduced at a rate of about 1µl/sec. The splitless technique is used for the ppb to ppm level concentration range.

Liners for splitless injections

Straight liner

straight liner

Low cost liner for samples with a narrow boiling point range and little risk of thermal decomposition. Can be packed with quartz wool to aid evaporation, limit discrimination and trap non-volatile materials. Do not use for high boiling samples. Comes usually in 2 or 4mm ID. Use the 4mm version for sample volumes > 2µl to prevent liner overload.


Gooseneck

gooseneck

Minimizes breakdown of active compounds and improves the splitless efficiency. The gooseneck prevents the sample from reaching metal injector parts which could cause decomposition of sensitive compounds. The gooseneck is positioned at the top of the injector. It is generally regarded as the best splitless liner.

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Liner volume

The sample gas volume which at one time resides in the injector should in all cases be smaller than the internal volume of the liner. If not, the liner is overloaded and backflash of the sample into various (metal) parts of the injector may occur. This phenomenon will cause poor injection profiles, bad peakshapes, adsorption and possible sample carry over. Many liners used for direct and split injection have dimensions of about 80mm x 4mm resulting in an internal volume of ca. 1ml. The effective volume available for the sample is however significantly lower as part of the liner volume will always be filled with carrier gas. The list below indicates the solvent expansion volume per µl at 250°C and 50kPa.
Methanol 0.65 ml
n-hexane 0.2 ml
Isopropanol 0.34 ml
Water 1.46 ml
Dichloromethane 0.41 ml

It is clear from the above that water and methanol in particular quickly can cause liner overload and some care must taken if these solvents are used. For these solvents smaller injection volumes than 1µl are to be recommended.

Heat capacity and transfer

The efficient and quick evaporation of the sample is influenced by the following factors:

-sample volume
-type of solvent and sample components
-injector temperature
-injector and liner geometry and design
-presence of quartz wool or other liner fillings

The last three determine the heat capacity of the injector and its ability for heat transfer to the sample. The below list compairs the amount of energy needed for evaporation of methanol and water relative to cyclohexane.
Water 5.22
Methanol 3.29
Cyclohexane 1.00

Solvents like methanol and water require much more energy than other common solvents for their evaporation therefor they need a larger heat capacity and better heat transfer system for a quick evaporation.
This can be accomplished by using higher injector temperatures (> 275°C) and increased liner surface areas by the use of glass wool.

Glass or quartz

wool is often used to increase the injectors heat transfer capacity. The evaporation of higher boiling compounds also requires more energy and in those cases quartz wool is often used. It also collects non-volatile heavy molecular weight residues which might harm the column. Besides these benefits glass or quartz wool also has its disadvantages. The wool can become adsorptive especially if some fibers are broken or when it has become dirty. It should be exchanged at a regular basis to prevent chromatographic problems. Avoid the use of glass wool when it is not advantageous.

Discrimination effects

The main aim and challenge of injections is to maintain sample integrity and sample composition during its transition from the liquid into the gaseous state and also during the sample transfer from injector to the column. The evaporation process of low boiling compounds is always quicker than that of high molecular weight ones. This difference accounts for the often mentioned discrimination effects caused by injections. In general, the quicker and more complete the evaporation of the sample, the less discrimination effects will be observed. Higher injector temperatures, special liner design and quartz wool will help to minimize these effects. Inconsistent discrimination profiles will result in poorer repeatability.
 
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