Gas Chromatography

by Warangkana Punrattanasin and Christine Spada


Table of contents

  1. Introduction
  2. Basic Components of a GC
  3. Theory of Gas Chromatography
  4. Example Chromatograms
  5. EPA 500 Series Methods
  6. Key to Terminology
  7. Pop Quiz
  8. Web Links to Related Topics
  9. References

Introduction


Gas chromatography (GC) is an analytical technique for separating compounds based primarily on their volatilities. Gas chromatography provides both qualitative and quantitative information for individual compounds present in a sample. Compounds move through a GC column as gases, either because the compounds are normally gases or they can be heated and vaporized into a gaseous state. The compounds partition between a stationary phase, which can be either solid or liquid, and a mobile phase (gas). The differential partitioning into the stationary phase allows the compounds to be separated in time and space.




Figure 1. Gas Chromatographic System





Figure 2. Schematic of a Gas Chromatographic System






Basic Components of a GC


Gas supply or Carrier Gas


Figure 3. Gas Supply

The carrier gas is usually helium, hydrogen, or nitrogen. This serves as the mobile phase that moves the sample through the column. The carrier gas flow can be quantified by either linear velocity, expressed in cm/sec, or volumetric flow rate, expressed in mL/min. The linear velocity is independent of the column diameter while the flow rate is dependent on the column diameter.




Injector


Figure 4. Auto Sampler Injection System (Hewlett-Packard Model No. 7673)

The injector is a hollow, heated, glass-lined cylinder where the sample is introduced into the GC. The temperature of the injector is controlled so that all components in the sample will be vaporized. The glass liner is about 4 inches long and 4 mm internal diameter.


Column


Figure 5. Capillary GC Column (Supelco ® PAG)

The GC column is the heart of the system. It is coated with a stationary phase which greatly influences the separation of the compounds. The structure of the stationary phase affects the amount of time the compounds take to move through the column. Typical stationary phases are large molecular weight polysiloxane, polyethylene glycol, or polyester polymers of 0.1 to 2.5 micrometer film thickness. Columns are available in many stationary phases sizes. A typical capillary column is 15 to 60 meters in length and 0.25 to 0.32 mm ID. A typical packed column is 6 to 12 feet long and 2.2 mm ID.


Oven


Figure 6. GC Oven with column in place

The column is placed in an oven where the temperature can be controlled very accurately over a wide range of temperatures. Typically, GC oven temperatures range from room temperature to 300C, but cryogenic conditions can be used to operate at temperatures from about -20C to 20C.


Detector


Figure 7. Electron Capture Detector (ECD) and Flame Ionization Detector (FID) (Shown actual size)

As compounds come off the column, they enter a detector. The compound and detector interact to generate a signal. The size of the signal corresponds to the amount the compound present in the sample. There are several different types of detectors that can be employed, depending on the compounds to be analyzed. These detectors can measure from 10-15 to 10-6 gram of a single component.


Data Recorder System


Figure 8. Data Recorder

The data recorder plots the signal from the detector over time. This plot is called a chromatogram. The retention time, which is when the component elutes from the GC system, is qualitatively indicative of the type of compound. The data recorder also has an integrator component to calculate the area under the peaks or the height of the peak. The area or height is indicative of the amount of each component.




Theory of Gas Chromatography


Retention Time (tR)

The retention time is the total time that a compound spends in both the mobile phase and stationary phase. Retention time is generally reported in minutes.

Dead Time (tm)

The dead time is the time a non-retained compound spends in the mobile phase which is also the amount of time the non-retained compound spends in the column. Dead time is generally reported in minutes.

Adjusted Retention Time (tR')

The adjusted retention time is the time a compound spends in the stationary phase. The adjusted retention time is the difference between the dead time and the retention time for a compound.



Capacity Factor (or Partition Ratio) (k')

The capacity factor is the ratio of the mass of the compound in the stationary phase relative to the mass of the compound in the mobile phase. The capacity factor is a unitless measure of the column's retention of a compound.



Phase Ratio (ß)

The phase ratio relates the column diameter and film thickness of the stationary phase. The phase ratio is unitless and constant for a particular column and represent the volume ratioß.



Distribution Constant (KD)

The distribution constant is a ratio of the concentration of a compound in the stationary phase relative to the concentration of the compound in the mobile phase. The distribution constant is constant for a certain compound, stationary phase, and column temperature.



Selectivity (or Separation Factor) (alpha)

The selectivity is a ratio of the capacity factors of two peaks. The selectivity is always equal to or greater than one. If the selectivity equals one the two compounds cannot be separated. The higher the selectivity, the more separation between two compounds or peaks.



Linear Velocity (u)

The linear velocity is the speed at which the carrier gas or mobile phase travels through the column. The linear velocity is generally expressed in centimeters per second.



Efficiency

The efficiency is related to the number of compounds that can separated by the column. The efficiency is expressed as the number of theoretical plates (N, unitless) or as the height equivalent to a theoretical plate (HETP, generally in millimeters). The efficiency increases as the height equivalent to a theoretical plate decreases, thus more compounds can be separated by the column. The efficiency increases as the number of theoretical plates increases, thus the column's ability to separate two closely eluting peaks increases.





Example Chromatograms


Sample contains 6 aromatic hydrocarbons dissolved in a solvent (methanol). The compounds' properties are summarized in Table 2.



The compounds were separated on an nonpolar, 95% methyl, 5% phenylpolysiloxane column, 30 m long, 0.25 mm ID, and 0.25 micrometer film thickness. About 1 microliter of the hydrocarbon sample were injected. Approximately 5 nanograms (ng) of each component was injected per 1 microliter. A flame ionization detector (FID) was used.


Temperature Programming Effects


Figure 9.

  • Temperature Program: 50C (min) - 10C/min - 100C
  • Head Pressure: 12 psi
  • Split Ratio: 1/50
  • This chromatogram shows an ideal temperature program for separation of the 6 aromatic compounds on this column. The first peak is the solvent, methanol. The compounds elute in order of increasing boiling point, that is, compounds with higher boiling points are more retained by the stationary phase. Note that para-xylene and meta-xylene cannot be separated on this column; the peak (#5) containing these compounds is broad at the baseline and shows a distinct shoulder.




    Figure 10.

  • Temperature Program: 60C Isothermal
  • Head Pressure: 12 psi
  • Split Ratio: 1/50
  • This chromatogram shows the effects of an isothermal* temperature program at 60C. The result is an increase in the retention time of all compounds. The heights of the later eluting peaks are reduced and the peak widths increased because they are more affected by the lower temperature program used. (*isothermal means a constant oven temperature was used throughout the run.)





    Flow Rate Effects


    Figure 11.

  • Temperature Program: 50C (1 min) - 10C/min - 100C
  • Head Pressure: 9 psi
  • Split Ratio: 1/50
  • This chromatogram shows the effects of a reduced head pressure while using the ideal temperature program. The flow rate was reduced by decreasing the head pressure. The retention time is slightly increased due to the low flow rate used. All of the peak heights were reduced and the peak widths are increased.




    Figure 12.

  • Temperature Program: 50C (1 min) - 10C/min - 100C
  • Head Pressure: 15 psi
  • Split Ratio: 1/50
  • This chromatogram shows the effects of a higher head pressure while using the ideal temperature program. The flow rate was increased by increasing the head pressure. The retention time was reduced and all of the peak heights were increased.





    Split Ratio Effects


    Figure 13.

  • Temperature Program: 50C (1 min) - 10C/min - 100C
  • Head Pressure: 12 psi
  • Split Ratio: 1/25
  • This chromatogram shows the effects of a low split ratio while using the ideal temperature program. All of the peak heights were increased due to the greater amount of the sample introduced into the column.




    Figure 14.

  • Temperature Program: 50C (1 min) - 10C/min - 100C
  • Head Pressure: 12 psi
  • Split Ratio: 1/75
  • This chromatogram shows the effects of a high split ratio while using the ideal temperature program. All of the peak heights were reduced due to the smaller amount of the sample introduced into the column.





    Effect of Stationary Phase on Separation of Para-Xylene and Meta-Xylene


    Figure 15.

    This chromatogram shows the separation of benzene, toluene, para-xylene, meta-xylene and ortho-xylene. The first peak is the solvent, hexane. A polyalkylene glycol fused silica capillary column 30 m long, 0.25 mm ID, and 0.25 micrometer film thickness was used for separation. Para-xylene (peak #4) and meta-xylene (peak #5) can be separated on this column. This illustrates the matching of the stationary phase with the desired compounds to be separated.







    EPA 500 Series Methods





    Key to Terminology








    Pop Quiz



    1. What is the component that most influences the separation of compounds?

    a) injector
    b) mobile phase
    c) stationary phase

    2. Given napthalene (boiling point = 218C), phenol (boiling point = 181.7C), and toluene (boiling point = 110.6C). Which compound will elute first on a nonpolar column?

    a) naphthalene
    b) phenol
    c) toluene

    3. What will happen to the retention time if the flow rate is increased?

    a) increase
    b) decrease
    c) no change

    4. If a variable temperature program is used rather than an isothermal temperature program, what parameter will not be affected?

    a) order in which compounds elute
    b) retention time
    c) peak height

    5. What will happen to the peak height if the split ratio is decreased?

    a) increase
    b) decrease
    c) no change




    Web Links to Related Topics







    References







    Sampling & Monitoring Primer Table of Contents

    Previous Topic

    Next Topic

    Send comments or suggestions to:
    Student Authors: Warangkana Punrattanasin, pum@vt.edu and Christine Spada, cspada@vt.edu
    Faculty Advisor: Andrea Dietrich, andread@vt.edu
    Copyright © 1997 Daniel Gallagher
    Last Modified: 09-10-1997