Chapter 3
Gas Chromatography (GC)
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Introduction
Basic Principles of GC
Instrumentations for GC
Carrier gas
Sample injection port
Column
Column temperature
Detector
Applications of GC
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The Separation Process in Gas
Chromatography
• In gas chromatography
gaseous analyte is transported through the
column by a gaseous mobile phase, called the carrier gas.
• In gas-liquid partition chromatography, the stationary phase is a
nonvolatile liquid bonded to the inside of the column or to a fine solid
support.
• In gas-solid adsorption chromatography, analyte is adsorbed directly on
solid particles of stationary phase.
• In the schematic gas chromatograph, volatile liquid or gaseous sample is
injected through a septum (a rubber disk) into a heated port, in which it
rapidly evaporates.
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• Vapor is swept through the column by He, N2, or H2 carrier gas, and
separated analytes flow through a detector whose response is displayed on
a computer.
• The column must be hot enough to provide sufficient vapor pressure for
analytes to be eluted in a reasonable time.
• The detector is maintained at a higher temperature than the column so
analytes will be gaseous.
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Figure 1. GC modes showing interaction between the mobile phase and the
stationary phases.
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Instrumentation in Gas Chromatography
• The basic components of a typical, modern gas chromatographic system are
indicated in figure 2.
 The main components of typical GC include: (1) carrier gas, (2) flow
control, (3) sample inlet and sampling devices, (4) columns, (5) controlled
temperature zones (ovens), (6) detectors, and (7) data systems.
1. CARRIER GAS
 The main purpose of the carrier gas is to carry the sample through the
column.
 It is the mobile phase and it is inert and does not interact chemically with
the sample.
 A secondary purpose is to provide a suitable matrix for the detector to
measure the sample components.
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Figure 2. Schematic of a typical gas chromatograph
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• Below are the carrier gases preferred for various detectors
• For the thermal conductivity detector, helium is the most popular.
• While hydrogen is commonly used in some parts of the world (where
helium is very expensive), it is not recommended because of the potential
for fire and explosions.
• With the flame ionization detector, either nitrogen or helium may be
used.
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• Nitrogen provides slightly more sensitivity, but a slower analysis, than
helium.
• For the electron capture detector, very dry, oxygen - free nitrogen is
recommended.
• Purity of Carrier Gas
• The carrier gas should have high purity because impurities such as oxygen
and water can chemically attack the liquid phase in the column and
destroy it.
• Polyester, polyglycol, and polyamide columns are particularly susceptible.
• Trace amounts of water can also desorb other column contaminants
and produce a high detector background or even “ ghost peaks. ”
• Trace hydrocarbons in the carrier gas cause a high background with most
ionization detectors and thus limit their detectability.
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2. FLOW CONTROL
• The measurement and control of carrier gas flow is essential for both
column efficiency and for qualitative analysis.
• Column efficiency depends on the proper linear gas velocity, which can be
easily determined by changing the flow rate until the maximum plate
number is achieved.
• For qualitative analysis, it is essential to have a constant and reproducible
flow rate so that retention times can be reproduced.
• Obviously, good flow control is essential method of identification.
• Two-stage pressure regulator to regulate pressure or flow control .
• Soap bubble meter are used to control flow
• Molecular sieve to remove water and other impurities
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3. SAMPLE INLETS AND SAMPLING DEVICES
• The sample inlet should handle a wide variety of samples including gases,
liquids, and solids, and permit them to be rapidly and quantitatively
introduced into the carrier gas stream.
• Different column types require different types of sample inlets.
• Injection into open tubular columns:
• Split: routine for introducing small sample volume into open tubular
column.
• Splitless: best for trace levels of high-boiling solutes in low-boiling solvents
• On-column: best for thermally unstable solutes and high-boiling solvents;
best for quantitative analysis.
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Split
• And as the image in figure 3 shows the gas flow passes through the septum
purge and the split vent. This
• configuration is called the split mode because some of the gas in the
injector exits though the split vent.
• This means that some of the sample injected into the injector by the sample
syringe will get vaporized and escape through the split vent.
• The pink color in the injector is meant to be vaporized sample.
• That lost sample will not go on the column (but to waste) and so is split
away.
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Figure 3. Representative injection conditions for split into an open tubular
column.
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Splitless injection
• Splitless injection is required for very dilute solutions.
• Splitless is better than split injection for compounds of moderate thermal
stability because the injection temperature is lower.
• Splitless injection introduces sample onto the column slowly, so solvent
trapping or cold trapping is required and temperature programming is
necessary.
• Samples containing less than 100 ppm of each analyte can be analyzed with
a column film thickness of <1 mm and splitless injection.
• Samples containing 100–1 000 ppm of each analyte require a column film
thickness ≥1 mm.
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Figure 4. Representative injection conditions for split into an open tubular
column.
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On-column injection
• With on-column injection a liquid sample is introduced directly into the
column with a thin injection needle.
• During the course of the temperature program the vapour pressure of the
solutes increases and the chromatographic process begins.
• With this injection technique no evaporation in a heated space takes place.
• By using an initial temperature below the boiling point of the solvent,
selective evaporation and, hence, discrimination is precluded.
• This makes on-column injection the method of choice for all samples
containing high-boiling components that would not be quantitatively
transferred to the column in split- and splitless injection.
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4. Column
• Two general types of columns are used in GC, packed columns and
capillary columns.
• In the past, the vast majority of gas chromatographic analyses used packed
columns.
• But currently, packed columns have been replaced by the more efficient
capillary columns.
Packed Columns
• Because the first commercial instruments accepted only packed columns,
all initial studies of GC were performed on packed columns.
• Packed columns are typically made of stainless steel and have an outer
diameter of 0.64 or 0.32 cm and lengths of 0.61–3.05 m.
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• Alternative to steel inert materials have also been used, including glass, nickel,
fluorocarbon polymer (Teflon), and steel covered with glass or Teflon.
• The packing is an inert support impregnated with 5–20% stationary phase.
• The solid support holds the liquid stationary phase and should have a large
surface area (0.5–5 m2/g), be chemically inert.
• Diatomaceous earth, composed of hydrous silica with impurities, has been
used as a solid support.
Capillary Columns
• Capillary chromatographic columns are not filled with packing material;
instead, a thin film of liquid phase coats the inner wall.
• Because the tube is open, its resistance to flow is very low, and it is thus
referred to as an open tubular column.
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Figure 5. Examples of gas chromatography packed columns: (a) stainless steel
column, (b) Pyrex glass column packed with a dark stationary phase, and (c)
Pyrex glass column packed with a white stationary phase.
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• Open tubular columns can be divided into three groups
I. Porous Layer Open Tubular Column (PLOT)
• PLOT columns contain a porous layer of a solid adsorbent such as alumina,
molecular sieves.
• PLOT columns are well suited for the analysis of light, fixed gases, and
other volatile compounds.
• PLOT columns, have solid-phase particles attached to the column wall, for
adsorption chromatography.
• Particles of alumina or porous polymers (molecular sieves) are typically
used.
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II. Wall-Coated Open Tubular Column (WCOT)
• WCOT columns has a 100-fold or higher increase in efficiency relative to
packed columns.
• In WCOT columns, the wall is directly coated with the stationary-phase
layer at a film thickness of 0.05–3 μm.
• The walls are coated by slowly passing a dilute solution of the liquid phase
through the columns.
• WCOT typically have 5000 plates/m. So a 50-m column will have 250,000
plates.
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III. Support Coated Open Tubular (SCOT)
• SCOT columns, solid microparticles coated with the stationary phase
(much like in packed columns) are attached to the walls of the capillary.
• These have higher surface area and have greater capacity than WCOT
columns.
• The tubing diameter of these columns is 0.5 to 1.5 mm, larger than WCOT.
• The advantages of low pressure drop and long columns is maintained, but
capacity of the columns approaches that of packed columns.
• Flow rates are faster and dead volume connections at the inlet and detector
are less critical.
• Sample splitting is not required in many cases.
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Figure 6. The three subtypes OT column
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Summery of Packed and Capillary Columns
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STATIONARY PHASES
• Stationary phases are selected based on their polarity, keeping in mind that
“like dissolve like.”
• That is, a polar stationary phase will interact more with polar compounds,
and vice versa.
• Non-polar liquid phase are nonselective so separations tend to follow the
order of the boiling points of analytes.
• Polysiloxanes are the most common stationary phases for capillary GC.
• The polysiloxanes have the backbone shown below and the R group
determine the polarity
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Fused silica columns
• It is the new type of WCOT columns that have much thinner walls than
glass capillary columns and are given strength by polyimide coating
• In it the majority of separations can be done with fewer 10 bonded liquid
stationary phases of varying polarity.
• This is because with their very high resolving power; selectivity of the
stationary phase is less critical.
• The stationary phases are high-molecular-weight, thermally stable
polymers that are liquids or gums.
• The walls of these columns are coated with liquid stationary phase and
column is drawn from pure silica and is much thinner (as small as 0.1mm)
than glass columns and lengths up to 100 m.
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4. OVEN
• GC column is contained in oven, temperature of which is precisely
controlled electronically.
• Minimal temperatures give good resolution but increase elution times.
• The higher the column temperature, the faster the sample moves through
column the less it interacts with the stationary phase, the less analytes are
separated.
• If sample has wide boiling range, then temperature programming can be
useful.
• Column temperature is increased (either continuously or in steps) as
separation proceeds.
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5. Gas Chromatography Detectors
Thermal Conductivity Detector (TCD)
• The original GC detector was the thermal conductivity detector (TCD), or
hot wire, detector are inexpensive and exhibit universal response, but they
are not very sensitive.
• As a gas is passed over a heated filament wire, the temperature and thus the
resistance of the wire will vary according to the thermal conductivity of the
gas.
• The pure carrier is passed over one filament, and the effluent gas containing
the sample constituents is passed over another.
• These filaments are in opposite arms of a circuit so long as there is
only carrier gas in the effluent, the resistance of the wires will be the same.
• But whenever a sample component elutes, a small resistance change will
occur in the effluent arm.
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Flame Ionization Detector (FID)
• The flame ionization detection is both general and sensitive and it is also the
most commonly used general detector.
• Most organic compounds form ions in a flame, generally cations such as
CHO+.
• This property is the basis of the flame ionization detector (FID).
• The ions are measured (collected) by a pair of oppositely charged electrodes.
• The number of ions collected depends on the number of carbon atoms in
the sample and on the oxidation state of the carbon.
• Those atoms that are completely oxidized do not ionize, and the compounds
with the greatest number of low oxidation state carbons produce the largest
signals.
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Flame Photometric Detectors (FPD)
• It is selective and highly sensitivity detector for phosphorus (P) compounds,
and sulfur (S) compounds.
• When sulfur and phosphorus compounds are burned in an FID-type flame,
chemiluminescent species are produced that produce light at 393 nm (sulfur)
and 526 nm (phosphorous).
• It is highly selective as it detects element-specific light emitted within
hydrogen flame
• Its main application will be in analysis of phosphorus pesticides, analysis of
sulfur based malodors and food odor components, and analysis of organic
tin in marine products.
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Flame Thermionic Detector (FTD)
• The flame thermionic detector is essentially a two-stage flame ionization
detector designed to give an increased specific response for nitrogen- and
phosphorus containing substances.
• A second flame ionization detector is mounted above the first,
with the flame gases from the first passing into the second flame.
• The two stages are divided by a wire mesh screen coated with an alkali salt or
base such as sodium hydroxide.
•
This detector is also known as a nitrogen–phosphorous detector (NPD).
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• Quantitative Measurements
• The concentrations of eluted solutes are proportional to the areas under the
recorded peaks.
• Electronic integrations in GC instruments report the areas of peaks, and the
retention times of peaks are also generally given.
• It is also possible to measure peak height to construct a calibration curve.
• The linearity of a calibration curve should always be established.
• The method of standard additions is a useful technique for calibrating,
especially for occasional samples.
• One or more aliquots of the sample are spiked with a known concentration of
standard, and the increase in peak area is proportional to the added standard.
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Application of GC
• Substances that vaporize below about 300 °C can be measured quantitatively
by GC.
• Samples are also required to be salt-free; they should not contain ions
• GC is very accurate if used properly and can measure pico moles of a
substance in a 1 mL liquid sample, or ppb concentrations in gaseous samples.
• Components that can be analyzed with GC have following three main
features:
• Compounds with boiling point up to 400 °C.
• Compounds that are not decomposed at their vaporization temperature.
• Compounds that decompose at their vaporization temperature, but always by
same amount.
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• Compounds that cannot be analyzed by GC are
• Compounds that do not vaporize inorganic metals, ions, and salts
• Highly
reactive
compounds
and
chemically
unstable
compounds
hydrofluoric acid and other strong acids, ozone, NOx, and other highly
reactive compounds.
• Compounds that are difficult to analyze by GC are
• Highly adsorptive compounds
• Compounds containing a carboxyl group, hydroxyl group, amino group, or
sulfur
• Compounds for which standard samples are difficult to obtain
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• Information obtained from the analysis results of GC.
• tr is characteristic value of each component
• Investigating tr under given analysis conditions makes it possible to
determine what component is (qualitative analysis)
• Additionally, size of component peak, in other words its area and
height, makes it possible to determine how much of component there is
(quantitative analysis).
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• Qualitative analysis
• Elution time when analyzed under given conditions is characteristic of
each component.
• When same component is analyzed under same conditions, peak is
confirmed at same time.
• Imagine an unknown sample known to contain component A and
component B.
• Chromatogram obtained from unknown sample looks as follows.
• However, It is not possible to know which peak is component A, and
which peak is component B.
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• If standard samples of A and B are prepared, and are analyzed under same
conditions, tr for A and B become evident.
• By comparing these chromatograms, peaks for A and B in chromatogram
of unknown sample can be determined.
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• Quantitative Analysis
• In GC chromatogram, size and area (height) of component peak are
proportional to amount of component reaching detector.
• Assume first, an of unknown sample is analyzed, and area of peak for
component A in chromatogram obtained has count of 700.
• Next, standard sample is prepared with concentration of component A of
100 ppm is analyzed under same conditions, and has count of 1000,
• Peak area is proportional to amount of component, so if 100 ppm
concentration has count of 1000, 700 count means 70 ppm concentration.
•
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•
N
A
N
N
A
A
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Quantitative Methods in Chromatographic Techniques
• Percentage peak area method uses area of target component (component A)
peak as proportion of total area of all detected peaks to analyze quantity.
• This method is used to determine changes in concentration of known sample
mixture, or to determine approximate concentration of sample mixture.
• Advantages: simple analysis since no standard sample is used.
• Disadvantages: reduced quantitation accuracy due to effect of relative
component sensitivity.
• Component A concentration is: [A] = (Peak Area of A)/(Total Peak Area) =
1000/4500 = 0.22
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Corrected percentage peak area method
• It is percentage peak area method with compensation for relative sensitivities
of each component.
• Advantages: performs quantitative analysis by percentage peak area method
but with compensation for relative component sensitivity.
• Disadvantages: requires standard sample containing all components in known
concentrations.
• Component A concentration is: [A] = (Corrected Peak Area of A)/(Corrected
Total Peak Area) = 500/2417 = 20.7 %
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Absolute calibration curve method (external standard method)
• This method uses standard sample of known concentration to prepare
calibration curve.
• Analysis can be relatively simple since only target component needs to
be detected to determine quantity - most popular method of quantitative
analysis.
• Advantages: quantitative analysis requires only separation and detection of
target component.
•
Disadvantages: sample
injection volume errors carry
over as errors in
quantitative results.
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Standard addition method
• The analyzes unknown sample and same unknown sample spiked with known
amount of target component, then uses difference between detected peak
areas (peak height) to determine quantity.
• This quantitative method is often used to analyze samples containing target
component affected by concentration of other components in sample, such as
odor component analysis and headspace analysis.
• Advantages: other components in sample (matrix) can mitigate effect (matrix
effect) of changes in sample composition when introduced to gas
chromatograph
• Disadvantages: extra work is required to add target component to unknown
sample. Because a target component is added to the unknown sample
(sometimes multiple quantities), rare samples cannot be used
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• concentration (ppm) of component A in unknown sample is shown by
absolute value at intersection with horizontal axis (quantity added)
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