Agilent Community
Agilent Community
  • User
  • Site
  • Search the Community
  • User
Consumables
  • Technical Areas
Consumables
Wiki Choosing the right GC Injection Technique
  • Announcements
  • Forum
  • Files
  • Wiki
  • More
  • Cancel
  • Consumables
  • A Beginner’s Guide to Hydrophobic Interaction Chromatography
  • A simple tip to protect your columns
  • A Tip for Preparing Robust and Consistent Mobile Phases
  • Achieve accurate and repeatable gas flow meter measurements
  • AdvanceBio Columns Blog Series
  • Agilent 123 Meter Sodium probe maintenance
  • Agilent Collection of Columns, Supplies, and Standards Resources
  • Analysis of microplastics in the environment
  • Analyzing ADCs by HIC
  • Avoiding downtime in the lab: top tips for GC/MS success
  • Best Practices for Aqueous Mobile Phases
  • Best Practices for Making Good Connections
  • Bio LC Column User Guides
  • Bursting Tubing and Columns (GC and HPLC)
  • Calculate the Maximum Allowable Working Pressure for Tubing
  • Calculating Column Volume
  • Cannabis Potency Testing: a Reliable, Cost-Effective Method
  • Carbohydrate Analyses in LC
  • Checking Your Intuition - Sub 2 µm vs Superficially Porous
  • Choosing the right GC Injection Technique
  • Choosing the right pore size for size exclusion chromatography
  • Columns, Supplies, and Standards Knowledgebase
  • Consumables Applications and Workflows
  • Consumables Recommended Supplies Lists for Agilent.com
  • Custom Product Request
  • Extreme Makeover – Derivatizations in Chromatography – Part 1 GC
  • Extreme Makeover – Derivatizations in Chromatography – Part 2 LC
  • Fake It Until You Make It: When BioInert Isn’t an Option
  • Flipping Amino Acid Analysis on Its Head
  • Glycans at a glance:  Analyzing therapeutic glycoproteins
  • Handle and Care of Syringes
  • Help! My Peaks Look Strange - Fronting and Tailing in GC
  • Help! My Peaks Look Strange - Saddle Points - LC/GC Troubleshooting
  • How do I select a Split/Splitless liner?
  • Hydrophobic Interaction Chromatography of Proteins and mAbs
  • Importance of Silica Particle Strength for Sub-2 µm SEC Columns
  • KB: Ferrules recommended for GC self-tightening column nut
  • LC and LC/MS Columns - USP Designations
  • LC Column User Guides
  • LC Method Translation - the Dwell Volume
  • Minimize spectroscopy workflow disruptions
  • Minimizing Metals for Best HILIC Results
  • More than just a drink: Analyzing the elemental composition of beer
  • Multi-Attribute Methods – Peptide Mapping Part IV
  • Must See Webinars
  • Nomenclature of CFC's/Freons/Halons/Coolants
  • Oligonucleotide Analysis - Unexpected Details Matter
  • Optimizing Bonding Chemistry for Sub-2 µm SEC Particles
  • Pass the Salt, Please – Mobile Phase Preparation for HIC
  • Pesticides and their stability during GC analyses
  • Pre-Columns - the forgotten art of using retention gaps
  • Problematic polar analytes? Hello HILIC…
  • Protecting your laboratory productivity
  • Recommended Reading
  • Sample Prep Pointers - Peptide Mapping Part I
  • Save your results with sample filtration
  • Simplified cone inspection with the new Agilent LED measuring magnifier
  • Software - Supported Method Development - The Scanview Application
  • Software tool for the ADM Flow Meter (G6691A)
  • Stay Safe: A Win-Win for Solvent Storage
  • Streamline your sample processing
  • The importance of chemical composition for vial performance
  • Tips & Tricks for Amino Acid Analysis – Part I
  • Tips & Tricks for Amino Acid Analysis – Part II
  • Tips & Tricks for Amino Acid Analysis – Part III
  • Tips & Tricks for Amino Acid Analysis – Part IV
  • Tips for Smooth Sailing with HIC
  • Troubleshooting HPLC autosamplers
  • Troubleshooting HPLC degassers
  • Troubleshooting Sequence Coverage – Peptide Mapping Part III
  • UltiMetal Plus Flexible Metal Ferrule
  • UV, MS, TFA, and Formic Acid – What to use? Peptide Mapping Part II
  • What are the typical % Gain or EHT values for hollow cathode lamps?
  • You Need Lamps or Chemical Standards for Atomic Absorption Single-Element Analyses?
Still Need Help?

Post your question in our User Forum or Contact Support.

Choosing the right GC Injection Technique

Created by Dr. Know Dr. Know over 2 years ago

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.
  • injection
  • injector
  • Syringes and Sample Introduction
  • General
  • direct
  • Inlets and Liners
  • PTV
  • GC
  • splitless
  • split
  • Share
  • History
  • More
  • Cancel
Anonymous
Related

Agilent Community Feedback

Agilent Community Feedback

×
We are glad this was helpful! We are sorry this was not helpful. If you still need assistance please create a community post or contact support. To help us improve, please provide any additional feedback. For full details of how we will treat your information, please view our privacy policy.
Submit Cancel
Submit Cancel
Recommended
Privacy Statement
Terms of Use
Contact Us
Site Help