Choosing the right GC Injection Technique

Version 2

    A vast number of injection techniques are being used in gas chromatography today. Some have been around since the early days of GC while the popularity of others is sometimes of more recent date. This article attempts to briefly characterize the most commonly used techniques, their applications and their limitations.

    The analysis in GC starts with the introduction of the sample onto the column. It is also by far the most critical step in the entire analysis. The liquid sample is vaporized either inside the column itself or in a vaporization chamber positioned just in front of the column.


    The injection system should fulfill the following requirements:

    • The injected amount should not overload the column
    • The injection bandwidth should be small compared to the band broadening effects in the capillary column.

     

    Other requirements, which could be applied, are:

    • The column should be able to achieve its optimum separation efficiency.
    • Sample composition should remain intact. No discrimination should occur on basis of boiling point, polarity, concentration or thermal stability.
    • Applicable to trace analysis as well as high concentration samples.
    • The injected amount should be reproducible

     

    No injection technique will meet all of the above demands. Some requirements will have to be sacrificed in order to meet the others. The detection of traces and higher concentrations both at the same time is often difficult to combine within one single injection technique. The choice of injection technique is often determined by the concentration levels, which have to be reached:

    • Direct Injection
    • Split Injection
    • Splitless Injection
    • Cool on-column
    • Programmed Temperature Vaporizer (PTV)

     

    Direct Injection

    Description

    Sample is vaporized in the hot insert. The entire sample enters the column

    Volume(s)

    0.1 µL – 1 µL

    Concentration Range

    1 ppm - %-levels

    Application

    Applicable to packed and 0.53 mm ID capillary columns
    No refocusing on the column is needed

     

    Direct injection is the most commonly used injection technique for packed columns and 0.53 mm ID columns. Its major advantages are its ease of use and the wide concentration range that it can cover. It is especially suitable for the lower concentrations in the ppm range, which are below the scope of the split injection. Higher gas flow rates are needed for an efficient and quick sample transfer from the injector liner to the column. The minimum gas flow for direct injection is about 5 mL/min. Direct injection can therefore only be applied to columns, which can cater for higher column flow rates such as, packed columns or 0.53 mm ID columns. These columns can also handle the higher concentration levels because of their increased sample capacity. Direct injection will produce poor peak shapes if too large injection volumes (> 1 µL) are applied. Discrimination effects are low compared to split injections. Direct injection is sometimes confused with the splitless type injection described further below.

     

    Split Injection

    Description

    Sample is vaporized in the hot insert. Only a fraction of sample enters the column. The majority is vented of through the split vent

    Volume(s)

    0.1µL – 1 µL

    Concentration Range

    50 ppm - %-level

    Remark

    Applicable to all capillary columns, usually no refocusing on the column is needed

     

    Beyond doubt the most widely used injection technique for capillary columns. This is despite its tendency for discrimination of compounds with higher boiling points. The reproducibility of the injection is strongly dependent on liner geometry and heat capacity. Sample discrimination during the evaporation from the syringe also occurs due to differences in component volatility. The difference in sample capacity of the various capillary columns (varying internal diameters and film thickness) can easily be addressed by changing the split flow. The relatively high split flow takes care of a quick and efficient sample introduction onto the column. Split injections can exploit the full separation power of the capillary column because of this quick sample introduction. The following table gives some general guidelines; which split-flows should be applied to which column diameter. Split flows and split ratios are related to column sample capacity and the minimum flow needed in the injector to minimize band-broadening effects.

     

    Column ID

    Split ratio

    Minimum split flow

    530 µm

    1:5 to 1:15

    15 mL/min

    320 µm

    1:20 to 1:250

    25 mL/min

    250 µm

    1:50 to 1:250

    40 mL/min

    150 µm

    1:150 to 1:500

    75 mL/min

    100 µm

    > 1:500

    150 mL/min

     

    The suggested minimum split ratios and split flows can result in band broadening inside the injector liner and therefore loss of separation efficiency is likely.

     

    Splitless Injection

    Description

    Sample evaporates in the hot injector. The majority of the sample enters the column during the first 1 – 2 min of the analysis

    Volume(s)

    0.5 µL – 2 µL

    Concentration Range

    0.5 ppm – 50 ppm

    Remark

    Refocusing on the column needs to be supported by retention gaps and low oven temperatures

     

    The most widely used technique for low ppm level samples. Splitless is often confused with direct injection. Sample is being introduced on the column during the entire splitless time. Due to this 1 – 2 min period there is a large initial injection bandwidth and refocusing of the analytes on the column is essential in order to obtain good symmetrical narrow peak shapes. This is done in two ways:

     

    1. Cold trapping. The large temperature drop between injector (250 °C) and a low initial oven temperature (50 °C) effectively reduces the mobility of higher boiling compounds to virtually zero. These compounds freeze in a narrow band and only start to migrate again during the temperature program. This cold trapping effect combines both focusing due to thermal condensation as well as focusing as a result of a strong retention in the columns stationary phase.
    2. Solvent focusing. Re-concentration of lower boiling components (close to the boiling point of the solvent) takes place by the solvent effect. Low oven temperatures will allow the solvent to condense in the column together with the low boiling sample components. The liquid film formed by the solvent will start to evaporate and the sample components will concentrate in a continuously smaller liquid film, resulting in a narrow band of concentrated sample components. Whether this process really takes place as described here is still a point of discussion among chromatographers. Fact is that both choices of solvent and initial oven temperatures are important in obtaining narrow peak shapes. The table below provides some general guidelines.

     

    Solvent

    Boiling Point
    [°C]

    Suggested Initial Oven Temperature
    [°C]

    n-Pentane

    36

    10 to ambient

    Dichloromethane

    36

    10 to ambient

    Carbon disulfide

    40

    10 to ambient

    Chloroform

    46

    10 to ambient

    Methanol

    61

    25

    n-Hexane

    65

    35

    Ethyl acetate

    69

    40

    Acetonitrile

    77

    45

    n-Heptane

    82

    50

    Isooctane

    98

    70

     

    Column flow and the volume of the liner determine the speed and effectiveness of the sample transfer from injector to column. Very low column flows prevent the use of splitless and it is mainly for this reason that 100 µm are not suited of splitless type of injections. 150 µm columns can be used but only if the splitless time period is long enough and the liner volume limited to 250 µL to permit adequate sample transfer.

     

    Splitless is not very suitable for samples where the components of interest elute closely to the solvent either in front or behind. Insufficient focusing of the analytes will produce poor peak shapes. In those cases a direct injection on a 0.53 mm ID column is often preferable.

    Cool on-column

    Description

    Sample passes from the syringe into the column. The column is kept cool during injection and is subsequently heated. The injector is kept cool during sample introduction

    Volume(s)

    0.1 µL – 2 µL

    Concentration Range

    0.25 ppm – 50 ppm

    Remark

    Refocusing on the column needs to be supported by retention gaps and low oven temperatures

    Cool on-column is perhaps the most “ideal” injection technique. It eliminates inlet-related discrimination and alteration and combines this with a high analytical precision. Syringe discrimination, as experienced with split and splitless, is absent. Components eluting just in front of the solvent however are difficult to focus and therefore difficult to determine. Cool on column follows closely the splitless injection type with respect to refocusing aspects like oven temperatures and the use of retention gaps. The market acceptance of the cool on-column injector has always been rather limited. This is mainly due to the instrumental difficulties during the early days of its introduction. Its competitor splitless is far easier to automate and this is one of the reasons for its wide acceptance. Automated on column injectors require a 0.53 mm ID piece of deactivated fused silica or retention gap as inlet piece because of the gauge 26 syringes used in auto samplers.

    Programmed Temperature Vaporizer (PTV)

    Description

    Sample passes as a liquid from the syringe into a cooled inlet. The sample is subsequently heated to vaporize the sample. Split or splitless injection can be applied.

    Volume(s)

    0.1 µL – 250 µL

    Concentration Range

    1 ppb – 50 ppm

    Remark

    Refocusing on the column needs to be supported by retention gaps and low oven temperatures.

     

    The PTV injector can arguably be considered as the most universal of injector, capable of handling a wide variety of sample types, concentrations and volumes. In essence, it is designed as a split/splitless injector that can be rapidly heated (and cooled. The syringe introduces the sample in it liquid state into a cold injector insert. Discrimination of the sample compounds by additional evaporation of the more volatile constituents is absent. Evaporation of components starts as the insert is being heated. The PTV can operate in various modes:

                  

    Cold Split Injection… is comparable with a split injection but with a cold transfer from syringe to insert. This provides less discrimination effects and therefore more accurate results. Refocusing using retention gaps and low oven temperatures may be necessary for lower boiling compounds.
    Cold Splitless Injection… is similar to hot splitless. It has the same advantages as cold split with respect to the cold transfer from the syringe. The classic hot splitless technique can result in thermal degradation due to the long residence time of components in the hot insert. The PTV in cold splitless mode also avoids this phenomenon. Refocusing using retention gaps and low oven temperatures may be necessary for lower boiling compounds.
    Solvent Venting… permits the injection of large sample volumes (250 µL). It enables detection limits in the ppb range. The majority of the solvent is removed through the split vent. The components are retained in the liner by cold trapping, aided in some cases by packed supports. At the end of the vent period the split vent is closed and the insert is rapidly heated. This technique requires a slow sample introduction in order to avoid overloading the insert. Lower boiling compounds will unavoidably be partly vented of together with the solvent depending on insert temperatures, type of packed support and vent times. In general, light boiling solvents are preferred for better results. Some column refocusing on the column is supported by retention gaps.