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Basic Gas Chromatography PowerPoint PPT Presentation

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Basic Gas Chromatography. Prepared by: Mina S. Buenafe. Gas Chromatography Chromatography – A Very Brief History Definitions / Terminologies in GC Instrumentation Overview System Modules Mobile Phase (Carrier Gas) Inlets Stationary Phase(s) Columns (Packed and Capillary) Detector(s)

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Basic Gas Chromatography

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Basic Gas Chromatography

Prepared by: Mina S. Buenafe

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Gas Chromatography

  • Chromatography – A Very Brief History

  • Definitions / Terminologies in GC

  • Instrumentation Overview

  • System Modules

    • Mobile Phase (Carrier Gas)

    • Inlets

    • Stationary Phase(s)

    • Columns (Packed and Capillary)

    • Detector(s)

  • Troubleshooting

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Chromatography – A (Very) Brief History


M. Tswett published his work on separation of plant pigments. He coined the term chromatography (literally translated as color writing) and scientifically described the process – earning him the title “Father of Chromatography”

W. Ramsey published his work on separation of mixture of gases and vapors on adsorbents like charcoal.


A. Martin and R. Synge first suggested the possibilities of gas chromatography in a paper published in Biochem. J., v.35, 1358, (1941). Martin won a Nobel Prize for his work in Partition chromatography.


A. Martin and A. James published the epic paper describing the first gas chromatograph

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Definition of Terms


A physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary while the other moves in a definite direction

“Official” IUPAC definition

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Definition of Terms


It is the output signal from the detector of the instrument.

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Definition of Terms

Distribution Constant (KC)

It is the tendency of a given component to be attracted to the stationary phase. This can be expressed in chemical terms as an equilibrium constant. Also called the partition coefficient (KP) or the distribution coefficient (KD)

KC = [A]S/[A]M

Mathematically, it is defined as the concentration of solute A in the stationary phase divided by its concentration in the mobile phase.

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Definition of Terms

The attraction to the stationary phase can also be classified according to the type of sorption by the solute.

Adsorption: sorption on the surface of the stationary phase

Absorption: sorption into the bulk of the stationary phase (usually called ‘partition’ by chromatographers)

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Definition of Terms

Retention Volume (VR)

It is usually defined as the distance between the point of injection to the peak maximum. It is the volume of the carrier gas necessary to elute the solute of interest.

Mathematically: VR = FC x tR

Where FC is the constant flow rate

tR is the retention time

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Definition of Terms

Phase Ratio (b)

For packed columns:

  • b = Mobile Phase Volume

  • Stationary Phase Volume

  • For capillary columns:

  • b = rc/2df

  • Where rc is the radius of the column

  • df is the thickness of the film

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Definition of Terms

Retention Factor (k)

It is the ratio of the amount of the solute (NOT concentration) in the stationary phase to the amount in the mobile phase. It is also called capacity factor (k’), capacity ratio, or partition ratio


k = (WA)S/(WA)M = KC/b

Also k = (tR - t0) = time in stationary phase

t0 time in carrier gas

k is temperature and flow dependent. Best separations occur when k is between 5 and 7

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Definition of Terms

Theoretical Plates (N)

This is the most common measure of column efficiency in chromatography

N = 16(tR/Wb)2 = 5.54(tR/ Wh)2

Where Wb is thepeak width at the base

Wh is the peak width at half-height

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Definition of Terms

Height Equivalent to a Theoretical Plate (H)

This is a related parameter that also defines column efficiency. Also identified as HETP


H = L/N

Where L is the Column Length

(An efficient column will have a large N and a small H)

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Definition of Terms

Separation Factor (a)

It is a measure of relative distribution constants. Also known as selectivity and/or solvent efficiency.

Mathematically: a = k2/k1 = (KC)2/(KC)1

It is dependent on:

  • Chemical composition of the phase

  • Partitioning between the two phases

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Definition of Terms

Resolution (Rs)

It is the degree to which adjacent peaks are separated.


Rs = (tR)B – (tR)A

[(Wb)B + (Wb)A]/2

Also Rs =—L/H x k/(k+1) x a-1/a

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Instrumentation Overview

Schematic of a Typical GasChromatograph

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System Modules

Main purpose: carries the sample through the column

Carrier Gas

Secondary purpose: provides a suitable matrix for the detector to measure the sample component.

Carrier gases should be of high purity (minimum of 99.995%).

  • Oxygen & water impurities can chemically attack the liquid phase of the column and destroy it.

  • Trace water content can desorb other column contaminants and produce high detector background or ‘ghost peaks’.

  • Trace hydrocarbon contents can cause high detector background with FID’s and limit detectability.

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System Modules

Flow Measurements and Control:

  • Essential for column efficiency and qualitative analysis (e.g. reproducibility of retention times)

    Average linear flow velocity (ū) in OT columns:

    ū = L/tm

    where L is column length in cm

    tm is the retention time of an unretained peak (e.g. methane) in sec

    To convert linear flow velocity to flow rate (Fc) in mL/min:

    Fc = ū x Pr2 x 60sec/min

Carrier Gas

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System Modules

Carrier Gas

Effect of mobile phase (carrier gas) density on column efficiency. Van Deemter plots for the 3 common carrier gases for a column of capacity factor k’ = 7.90. The low density gases (H2 & He) have optimum efficiency at slightly higher flow rates than N2. The much lower slopes of H2 and He curves allow them to be used at higher flow rates (compared to N2) with very little loss of separation efficiency.

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System Modules


Inlets are the points of sample introduction

Ideal Sample Inlets for Column Type:

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System Modules


Split Injector

The oldest, simplest, and easiest injection technique.

Cross section of a typical split injector

Advantages to Split Injection:

  • High resolution separations

  • Neat samples can be introduced.

  • Dirty samples can be introduced by putting a deactivated glass wool plug in the liner to trap non-volatile components


  • Trace analysis is limited

  • Process sometimes discriminates between high molecular weight solutes so that the sample entering the column is not representative of the sample injected.

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System Modules


Splitless Injector

Samples have to be diluted in a volatile solvent and 1-5mL is injected in the heated injection port. Septum purge is essential in splitless injections.

Cross section of a typical splitless injector

Advantages to Splitless Injection:

  • Improved sensitivity over a split injector


  • Time consuming

  • Initial temperature and time of opening the split valve needs to be optimized.

  • Not well suited for volatile compounds (boiling points of peaks of interest have to be about 30oC higher than solvent.

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System Modules


Other Types of Inlets:

  • Direct Injection: involves injecting a small sample into a glass liner where vapors are carried directly into the column.

  • On-Column Injection: inserting the precisely aligned needle into the capillary column and making injections inside the column.

  • Flash Vaporization: involves heating the injection port to a temperature well above the boiling point to ensure rapid volatilization

  • Static Headspace: concentrates the vapors over a solid or liquid sample (best for residual solvent analysis)

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System Modules

Stationary Phase

Sub-classification of GC Techniques

  • GSC: gas solid chromatography - stationary phase is solid

  • GLC: gas liquid chromatography -stationary phase is liquid

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System Modules

Gas Solid Chromatography (GSC)

  • Solids used are traditionally run in packed columns

  • These solids should have small and uniform particle sizes (e.g. 80/100 mesh range)

Stationary Phase

Some of these solids have been coated on the inside walls of capillary columns and are called “Support Coated Open Tubular” or SCOT columns.

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System Modules

Stationary Phase

One major application of Packed Column GSC is in Gas Analysis.


  • Adsorbents provide high surface areas for maximum interaction with gases that may be difficult to retain on liquid stationary phase.

  • Large samples can be accommodated, providing lower absolute detection limits.

  • Some packed column GC’s can be configured to run below ambient temperature which will also increase the retention of the gaseous solutes.

  • Unique combinations of multiple columns and/or valving make it possible to optimize analysis of a particular sample.

Packed Columns also provide the flexibility of allowing mixed packings for special applications (e.g. 5% Fluorcol on Carbopack B® for analysis of Freons)

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System Modules

Stationary Phase

Gas Liquid Chromatography (GLC)

To use liquid as stationary phase,techniques were applied to hold the liquid in a column.

  • For packed columns: liquid is coated onto a solid support, chosen for its high surface area and inertness. The coated support is then dry-packed into a column as tightly as possible.

  • For capillary or open tubular (OT) columns: liquid is coated on the inside of the capillary. To make it adhere better, the liquid phase is often extensively cross-linked and sometimes chemically bonded to the fused silica surface.

Schematic representation of (a) packed column and (b) capillary column

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System Modules

Stationary Phase

Gas Liquid Chromatography (GLC)

Requirements for the stationary liquid phase:

  • Low vapor pressure

  • Thermal stability

  • (if possible) Low viscosity (for fast mass transfer)

  • Should interact with the components of the sample to be analyzed (“Like dissolves like”)

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System Modules

Stationary Phase

Gas LiquidChromatography (GLC)

Types of Capillary Columns (OT)

WCOT: Wall-coated open tubular column (provides the highest resolution of all OT’s – i.d.’s range from 0.1mm to 0.53mm and film thickness from 0.1 – 5.0m)

PLOT: Porous layer open tubular column (less than 5% of all GC use these days)

SCOT: Surface-coated open tubular column (no longer available in fused silica)

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System Modules


The part of the system that ‘senses’ the effluents from the column and provides a record of the analysis in the form of a chromatogram. The signals are proportional to the quantity of each analyte.

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System Modules

FID: Flame Ionization Detector

The most common GC detector used.

The column effluent is burned in a small oxy-hydrogen flame producing some ions in the process. These ions are collected and form a small current that becomes the signals. When no sample is being burned, there should be little ionization, the small current is produced from impurities from the from the hydrogen and air supplies.

Hydrogen flow rate is commonly set to 40 – 45mL/min, Air, 350- 450mL/min, and for OT columns (with flows of about 1 mL/min), Make-Up gases is added to carrier gas (to make up the flow to 30mL/min)


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System Modules


TCD: Thermal Conductivity Detector

This is a differential detector that measures the thermal conductivity of the analyte in the carrier gas compared to the thermal conductivity of the pure gas. At least two cavities are required. These cavities are drilled into a metal block and each contain a hot wire or filament. The filaments are incorporated into a Wheatstone Bridge Circuit (for resistance measurements).

The choice of carrier gas will depend on the thermal conductivity of the analyte (H2 and He have highest TC’s, N2 gives rise to unusual peak shapes)

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System Modules


NPD: Nitrogen Phosphorus Detector

A bead of Rb or Cs is electrically heated when flame ionization occurs. The detector shows enhanced detectability for nitrogen-, phosphorus-, or halogen- containing samples.

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System Modules


MSD: Mass Spectrometric Detector

Analyte molecules are first ionized in order to be attracted or repelled by the proper magnetic or electrical fields.

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Common GC Problems:

Retention Time Problems

Resolution Problems

Baseline Problems

Peak Problems

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Retention Time Problems

Retention Time Shift

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Retention Time Problems

Retention Time Shift

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Resolution Problems

Decrease in separation

Loss of Resolution

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Resolution Problems

Increase in peak width

Loss of Resolution

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Baseline Problems

Excessive Column Bleed

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Baseline Problems

  • Erratic Baseline (drift, wander)

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Baseline Problems

  • Erratic Baseline (drift, wander)

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Baseline Problems

·Noisy Baseline

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Baseline Problems

·Noisy Baseline

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Baseline Problems

·Ghost Peaks

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Peak Problems

·Fronting Peaks

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Peak Problems

·Tailing Peaks

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Peak Problems

·Tailing Peaks

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Peak Problems

·Tailing Peaks

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Peak Problems

·Split Peaks

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Peak Problems

·Split Peaks

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Peak Problems

·Changes in Peak Size

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Peak Problems

·Changes in Peak Size

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Peak Problems

·Changes in Peak Size

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