Choosing the right GC Injection Technique

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: