Chemistry 331
Chapter 26 & 27
Introduction to Chromatographic Separations and Gas Chromatography
In gas chromatography (GC), the sample is vaporized and injected onto the head of a
chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase.
In contrast to most other types of chromatography, the mobile phase does not interact with
molecules of the analyte; its only function is to transport the analyte through the column.
Gas liquid chromatography is based upon the partition of analyte between a gaseous mobile
phase and a liquid phase immobilized on the surface of an inert solid. The concept of gas-liquid
chromatography was first enunciated in 1941 by Martin and Synge.
Principles of Gas-Liquid Chromatography
Retention Volumes:
To take into account the effects of pressure and temperature in gas chromatography, it useful to
use the retention volumes. The specific retention volume, Vg is defined as follows:
where, J = the pressure drop factor, F = average flow rate, tr and tm = retention times W = mass
of stationary phase and Tc = column temperature in Kelvin.
Instruments for GC
The schematic of a gas chromatograph is shown below:
Carrier Gas Supply
Carrier gases, which must be chemically inert, include helium, argon, nitrogen, carbon dioxide,
and hydrogen. The choice of gases is often dictated by the detector used. Associated with the gas
supply are pressure regulators, gauges, and flow meters. In addition, the carrier gas system often
contains a molecular sieve to remove water or other impurities.
Flow rates are normally controlled by a two-stage pressure regulator at the gas cylinder and some
sort of pressure regulator or flow regulator mounted in the chromatograph. Inlet pressures
usually range from 10 to 50 psi, which lead to flow rates of 25 to 150 mL/min with packed
columns and 1 to 25 mL/min for open-tubular columns. Flow rates can be established using a
soap-bubble meter.
Sample Injection System:
Column efficiency requires that the sample be of suitable size and be introduced as a plug of
vapor; slow injection of oversized samples causes band spreading and poor resolution. The most
common method of injection involves the use of a micro-syringe to inject a liquid or gaseous
sample through a silicone rubber diaphragm or septum into a flash vaporizer port located at the
head of the column. For ordinary analytical columns, sample sizes vary from a few tenths of a
microliter to 20microliters. Capillary columns require much smaller samples (~0.001microliters)
Column Configurations and Column Ovens:
Two general types of columns are encountered in gas chromatography, packed and open tubular,
or capillary. To date, the vast majority of gas chromatography has been carried out on packed
columns. Chromatographic columns vary in length from less than 2m to 50m or more. They are
constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for
thermostating, they are usually formed as coils having diameters of 10 to 30cm.
Column temperature is an important variable that must be controlled to a few tenths of a degree
for precise work, so the column is ordinarily housed in a thermostatted oven.
The optimum column temperature depends upon the boiling point of the sample and the degree
of separation required. Roughly, a temperature equal to or slightly above the average boiling
point of a sample results in a reasonable elution time. For samples with a broad boiling range, it
is often desirable to employ temperature programming, whereby the column temperature is
increased either continuously or in steps as separation proceeds. The figure below shows the
improvement brought about by temperature programming. In general, optimum resolution is
associated with minimal temperature; the cost of lowered temperature, however, is an increase in
elution time and therefore the time required to complete an analysis.
Detectors
Characteristics Of The Ideal Detector:
1. Adequate sensitivity.
2. Good stability and reproducibility.
3. A linear response to analyses that extends over
several orders of magnitude.
4. A temperature range from room temperature to
at least 400degC.
5. A short response time that is independent of flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all analyses.
8. Nondestructive of sample.
Flame Ionization Detector:
The flame ionization detector (FID) is one of the most widely used and generally applicable
detectors for gas chromatography. The effluent from the column is mixed with hydrogen and air
and then ignited electrically. Most organic compounds, when pyrolyzed at the temperature of a
hydrogen/air flame, produce ions and electrons that can conduct electricity through the flame. A
potential of a few hundred volts is applied across the burner tip and a collector electrode is
located above the flame. The resulting current is then directed into a high impedance operational
amplifier for measurement. Functional groups such as carbonyl, alcohol, halogen, and amine,
yield fewer ions or none at all in a flame. In addition, the detector is insensitive toward
noncombustible gases such as H2O, CO2, SO2, and NOx. These properties make the flame
ionization detector a most useful general detector for the analysis of most organic samples,
including those that are contaminated with water and oxides of nitrogen and sulfur. The flame
ionization detector exhibits a high sensitivity, large linear response range, and low noise. It is
generally rugged and easy to use. A disadvantage of this detector is that it destroys the sample. A
diagram of a typical FID is shown in below.
Thermal Conductivity Detector:
A very early detector for gas chromatography is based upon changes
in the thermal conductivity of the gas stream brought about by the presence of analyte molecules.
This device is sometimes called a katharometer. The sensing element of a katharometer is an
electrically heated element whose temperature at constant electrical power depends upon the
thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold or
tungsten wire or, alternatively, a semiconducting resistor. The advantage of the thermal
conductivity detector is its simplicity, its large linear range, its general response to both organic
and inorganic species, and its nondestructive character. A limitation of the katharometer is its
relatively low sensitivity.
Thermoionic Detector:
The thermoionic detector (TID) is selective toward organic compounds containing phosphorous
and nitrogen. It is similar in structure to the flame detector.
Electron-Capture Detector:
Electron-capture detector (ECD) operates in much the
same way as a proportional counter for measurement of X-radiation. Here the effluent from the
column passes over a beta-emitter, such as nickel-63 or tritium (adsorbed on platinum or
titanium foil). An electron from the emitter causes ionization of the carrier gas (often nitrogen)
and the production of a burst of electrons. In the absence of organic species, a constant standing
current between a pair of electrodes results from this ionization process. The current decreases
however in the presence of those organic molecules that tend to capture electrons. The electroncapture detector is selective in its response, being highly sensitive toward molecules that contain
electronegative functional groups such as halogens, peroxides, quinones, and nitro groups.
Electron-capture detectors are highly sensitive and possesses the advantage of not altering the
sample significantly. On the other hand, their linear response range is usually limited to about
two orders of magnitude.
Atomic Emission Detector (AED):
The newest commercially available gaschromatographic detector is based upon atomic emission. In this device, the eluent is introduced
into a microwave-energized helium plasma that is coupled to a diode-array optical emission
spectrometer. The plasma is sufficiently energetic to atomize all of the elements in a sample and
to excite their characteristic atomic emission spectra.
GC COLUMNS AND STATIONARY PHASE
Packed Columns:
Present day packed columns are fabricated from glass, metal (stainless steel, copper, aluminum),
or Teflon tubes that typically have lengths of 2 to 3 m and inside diameters of 2 to 4mm. These
tubes are densely packed with a uniform, finely divided packing material, or solid support, that is
coated with a thin layer of stationary liquid phase. The tubes are formed as coils having
diameters of roughly 15cm.
Solid Support Materials:
The solid support in a packed column serves to hold the stationary phase in place so that as large
a surface area as possible is exposed to the mobile phase.
The ideal support consists of small, uniform spherical particles with good mechanical strength
and a specific surface area of at least 1m/g. In addition, the material should be inert at elevated
temperatures and be uniformly wetted by the liquid phase.
Particle Size Supports:
The efficiency of a gas-chromatographic column increases rapidly with decreasing particle
diameter of the packing. The pressure difference required to maintain a given flow rate of carrier
gas, however, varies inversely as the square of the particle diameter; the latter relationship has
placed lower limits on the size of particles employed in gas chromatography, because it is not
convenient to use pressure differences that are greater than about 50psi.
Open Tubular Columns:
Open tubular, or capillary, columns are of two basic types, namely, wall-coated open tubular
(WCOT) and support-coated open tubular(SCOT). WCOT columns are simply capillary tubes
coated with a thin layer of the stationary phase. In SCOT columns the inner surface of the
capillary is lined with a thin film of a support material, such as diatomaceous earth. This type of
column holds several times as much stationary phase as does a wall-coated column and thus has
a greater sample capacity.
ADSORPTION ON COLUMN PACKINGS OR CAPILLARY WALLS
A problem that has plagued GC has been the physical adsorption of polar or polarizable analyte
species such as alcohols or aromatic hydrocarbons, on the silicate surfaces of column packings or
capillary walls. Adsorption results in distorted peaks, which are broadened and often exhibit a
tail. It has been established that adsorption is the consequence of silanol groups that form on the
surface of silicates by reaction with moisture. The SiOH groups on the support surface have a
strong affinity for polar organic molecules and tend to retain them by adsorption. Support
materials can be deactivated by silanization with dimethyl chlorosilane (DMCS).
STATIONARY PHASE
Desirable properties for the immobilized liquid phase in a gas liquid chromatographic column
include:
1. low volatility
2. thermal stability
3. chemical inertness
4. solvent characteristics such that k’ and alpha values for
the solutes to be resolved fall within a suitable range.
Generally, the polarity of the stationary phase should match that of the sample components.
Commonly used stationary phases are listed in below.
Stationary Phase
Common
Trade Name
Maximum
Temperature ( C)
Polydimethyl siloxane
OV-1, SE-30
350
General-purpose nonpolar phase;
hydrocarbons; polynuclear
aromatics; drugs; steriods; PCBs
Poly(phenylmethyldimet
hyl) siloxane (10%
phenyl)
OV-3, SE-52
350
Fatty acid methyl esters; alkaloids;
drugs; halogenated compounds
OV-17
250
Drugs; steriods; pesticides; glycols
Poly(phenylmethyl)
siloxane (50% phenyl)
Common Applications
Poly(trifluoropropyldim
ethyl) siloxane
Polyethylene glycol
Poly(dicyanoallyldimeth
yl) siloxane
OV-210
200
Chlorinated aromatics;
nitroaromatics; alkyl-substituted
benzenes
Carbowax
20M
250
Free acid; alcohol; ethers; essential
oils; glycols
OV-275
240
Polyunsaturated fatty acids; rosin
acids; free acids; alcohols
Bonded and Cross-linked Stationary Phases:
The purpose of bonding and cross-linking is to provide a longer-lasting stationary phase that can
be rinsed with a solvent when the film becomes contaminated. With use, untreated columns
slowly lose their stationary phase due to bleeding in which a small amount of immobilized liquid
is carried out of the column during the elution process. Cross-linking is carried out in situ after a
column is coated with one of the polymers listed in above
Chiral Stationary Phases:
There has been recent developments of stationary phases which can separate chiral compounds.
One method is to form a derivative of the analyte with an optically active reagent that forms a
pair of diasteromers that can be separated on an achiral column. The alternative method is to use
a chiral liquid as the stationary phase.
Film Thickness:
Film thickness primarily affect the retentive character and the capacity of a column. Thick films
are used with highly volatile analytes, because such films retain solutes for a longer time and
thus provide a greater time for separation to take place. Thin films are useful for separating
species of low volatility in a reasonable time.
APPLICATIONS OF GC
Qualitative Analysis
Gas chromatographs are widely used as criteria for establishing the purity of organic compounds.
Contaminants, if present, are revealed by the appearance of additional peaks.
The Retention Index:
The retention index, I, was first proposed by Kovats in 1958 as a parameter for identifying
solutes from chromatograms. The retention index for any given solute can be derived from a
chromatogram of a mixture of that solute with at least two normal alkanes having retention times
that bracket that of the solute. That is, normal alkanes are the standards upon which the retention
index scale is based. By definition, the retention index for a normal alkane is equal to 100 times
the number of carbons in the compound regardless of the column packing, the temperature, or
other chromatographic conditions. The retention index system has the advantage of being based
upon readily available reference materials that cover a wide boiling range.
Quantitative Analysis:
The relative concentration of an analyte is proportional to the peak area obtained on the gas
chromatogram. Thus the GC can be calibrated with several standards and a calibration curve is
obtained, then the concentration of the unknown analyte can be determined using the peak area.
INTERFACING GC WITH SPECTROSCOPIC METHODS
When GC is coupled with other separation methods, both serve as a powerful tool for identifying
the components of complex mixtures. A popular combination is GC/MS. This interface have
been used for the identification of hundreds of components that are present in natural and
biological systems. For example, these procedures have permitted characterization of the odor
and flavor components of foods, identification of water pollutants, medical diagnosis based on
breath components, and studies of drug metabolites. Another popular interface is that of
GC/FTIR. Below is a diagram of GC/MS a instrument.
GAS-SOLID CHROMATOGRAPHY
Gas-solid chromatography is based upon absorption of gaseous substances on solid surfaces.
Distribution coefficients are generally much larger than those for gas-liquid chromatography.
Consequently, gas -solid chromatography is useful for the separation of species that are not
retained by gas-liquid columns, such as the components of air, hydrogen sulfide, carbon
disulfide, nitrogen oxides, and rare gases. Gas-solid chromatography is performed with both
packed and open tubular columns. For the latter, a thin layer of adsorbent is affixed to the inner
walls of the capillary. Such columns are sometimes called porous layer open tubular columns, or
PLOT columns. Two types of adsorbents are molecular sieves and porous polymers.
Molecular Sieves:
Molecular sieves are aluminum silicate ion exchangers, whose pore size depends upon the kind
of cation present. The sieves are classified according to the maximum diameter of molecules that
can enter the pores. Commercial molecular sieves come in pore sizes of 4, 5, 10, and 13
angstroms. Molecular sieves can be used to separate small molecules from large.
Porous Polymers:
Porous polymer beads of uniform size are manufactured from styrene cross-linked with
divinylbenzene. The pore size of these beads is uniform and is controlled by the amount of crosslinking. Porous polymers have found considerable use in the separation of gaseous polar species
such as hydrogen sulfide, oxides of nitrogen, water, carbon dioxide, methanol, and vinyl
chloride.