SUBJECT FORENSIC SCIENCE

Paper No. and Title PAPER No.4: Instrumental Methods and Analysis

Module No. and Title MODULE No.9: Gas Chromatography

Module Tag FSC_P4_M9

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 TABLE OF CONTENTS

1. Learning Outcomes
2. Introduction
3. Advantages of Gas Chromatography
4. Principle of Gas chromatography
5. Gas- Liquid Chromatography
6. Instrumentation
6.1 Carrier gas
6.2 Sample injection system
6.3 Separation system
6.4 Detectors
6.5 Thermostat chambers
6.6 Recorder system
7. Applications of GC-MS
8. Summary

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 1. Learning Outcomes

After studying this module, you shall be able to know about-

 Gas Chromatography
 Advantages of Gas Chromatography
 Principle of Gas Chromatography
 Instrumentation required for Gas Chromatography
 Applications of gas chromatography

2. Introduction
3.
Gas chromatography (GC) is probably the most utilized of all the chromatographic
techniques. Since early 1950s, when the technique was first used for separation of amino
acids, gas chromatography now finds thousands of applications in virtually all spheres of
chemistry and biochemistry. The technique is used in the separation and identification of
constituents of the atmosphere, drugs, foodstuff, petrochemicals, pesticides etc.
.
Gas chromatography is similar to column chromatography, except that the gas is used as
the mobile phase instead of a liquid.

In gas – liquid chromatography (GLC), the stationary phase is a thin layer of a non-
volatile liquid bound to a solid support. Separation takes place by the process of partition.
On the other hand, gas-solid chromatography (GSC), utilizes a solid adsorbent as the
stationary phase and separation takes place by adsorption process.

The gas-liquid chromatography is more popular than gas-solid chromatography and has
relatively more applications. Gas-Liquid chromatography was developed by A.P.J.
Martin in 1951, together with A.T. James. In 1952, Martin and Synge were awarded the
Nobel Prize in chemistry for their work on the development of partition chromatography.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 3. Advantages of Gas Chromatography

The main advantages of gas chromatography are given below:

 The technique has good resolving power and even complex mixture can be
separated into constituents.
 It is a sensitive technique with few mg of sample being sufficient for analysis.
 It provides good precision and accuracy.
 The analysis can be completed in a short time.
 The cost of instrument is relatively low with a long life span.
 The technique is suitable for routine analysis as the operation of a gas
chromatograph and related calculations do not require highly skilled operators.

Gas chromatography is at present the most widely used and powerful tool available for
separations. The major factors for this are the speed, good resolving power and extreme
sensitivity of the technique. For example, using this technique, it is possible to separate
ten isomers of Heptane in less than ten seconds. Detectors are available with detection
12 14
limits as low as 10¯ to 10¯ g.

4. Principle of Gas Chromatography

When a gas or vapor comes in contact with an adsorbent, certain amount of it gets
absorbed on the solid surface. The phenomenon takes place according to the well-known
1/n
laws of Freundlich, i.e. x/m= Kc or Langmuir, i.e. x/m=K1c + K2c, where x is the mass
of the gas or vapor sorbed in mass m of the sorbent and c is the vapor concentration in
the gas phase and K, K1 and K2 are constants.

Similarly, if the vapor or gas comes in contact with a liquid, a fixed quantity of gas is
dissolved in the liquid. The phenomenon takes place according to Henry’s law of
partition, i.e., x/m=Kc. Now both the phenomena are selective and there are different K-
values for different vapor-sorbent pairs.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 The principles of gas chromatography can be explained in terms of the following
experiments. A gas that is flowing smoothly at the rate of 3 ft/min down an empty tube
that is 6ft long takes 6/3 = 2 min to flow from one end of the tube to the other. If such a
tube were filled with sand, the gas would flow through it more slowly. If the rate at which
the gas flows in the sand-filled tube is 2ft/min, it will take the gas 6/2 = 3 min to traverse
the tube. The sand-filled tube in this example has some properties of gas chromatography
column. The gas is the moving mobile phase. The sand is the stationary phase. The gas
emerges after it has passed through the column is called the eluent. In practice the mobile
phase should be relatively insoluble in the stationary phase; otherwise the stationary
phase becomes overloaded.

5. Gas-Liquid Chromatography

Gas-liquid chromatography consists of a mobile gas phase and a stationary liquid phase
which is coated on with either a solid matrix (e.g., diatomaceous earth) or the wall of a
capillary tube. Typically, the stationary phase has a sufficiently low vapor pressure (mm)
at the column temperature so that it can be considered as non-volatile. The sample
mixture in gaseous form is run through the column along with the carrier gas. Separation
can be achieved by the differences in the distribution ratios of the components of the
mixture at different rates and with different retention times. After elution, the sample
apparatus can be detected by a suitable detector at the exit.

6. Instrumentation

Basically all gas chromatographs, whether GLC or GSC, consist of six basic components:

1. A carrier gas: An inert gas is supplied at a high pressure and is passed to the
instrument at a rapid and reproducible flow rate
2. A sample injection system
3. The separation column
4. One or more Detectors
5. Thermostat chambers for temperature regulation of column and detectors
6. An amplification and recorder system

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 Figure 1: Instrumentation for Gas Chromatography

The gas chromatographic separation is conceded out in a column made of glass, metal or
teflon. In this column a sorbent is filled as stationary phase. The adsorbents are packed in
the variety of fine size graded powder, whereas the liquids are either coated as fine film
on the column wall or first covered over an inert graded porous support such as firebrick
powder followed by packing in the column.

An inert gas as mobile phase flows continuously through the column. It is known as the
carrier gas and serves to transport sample components through the column. The sample is
introduced in the vapor form at the carrier gas entrance end of the column called injector
port. Different constituents of the sample are adsorbed on the stationary phase to different
extent depending upon their distribution coefficients. The portion of each constituent in
the gas phase is swept along by the carrier gas.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 To maintain the K-value, a portion of the sorbed amount will go into the sorbent at the
next point in the column again. This sorption-desorption process goes on consecutively
and endlessly. The entire band for each constituent moves further in the column having
the shape of, more or less Gaussian distribution. It should be kept in mind that the gas is
the driving force for the movement of zones through the column and the solid for GSC or
liquid for GLC provides the selective retarding force. The detector can be regarded as the
brain of the chromatograph and the column as its heart.

The detail of the various components of a gas chromatograph is given below:

(a) Carrier Gas: - The carrier gas plays a vital role, and varies as per application.
Carrier gas should be dry, free of oxygen and chemically inert. The most extensively
used carrier gases are hydrogen, helium, nitrogen and air. Hydrogen has more
advantage as carrier gas compared to other gases but is dangerous to use. Helium is
second best but is expensive. Nitrogen is inexpensive but gives reduced sensitivity
and air is used only when the oxygen in the air is useful for the detection or
separation. Helium is generally used because of its exceptional thermal conductivity,
inertness, low density and it allows grater flow rates. Hydrogen has a superior
thermal conductivity and lower density but has the disadvantage that it may react
with unsaturated compounds and explode. The purity range for all carrier gases
should be in the range of 99.995% - 99.9995% and should also contain less than 0.5
ppm of total hydrocarbons and oxygen present in the tank. A molecular sieve is
present in the carrier gas system which helps in removing water and other impurities.
Another option used for the removal of traces of water and other contaminants is to
use traps. A two step pressure regulation is required to minimize the pressure surges
and to monitor the flow rate of the gas.

A carrier gas is required in GC system to flow through the injector and push the
gaseous constituents of the sample onto the GC column, which leads to the detector.
For selecting a carrier gas following considerations should be taken into account.
The carrier gas should be:

(i) Inert: It implies that the carrier gas should not react with the stationary
phase, sample and contacted hardware.
(ii) Suitable for the detector employed and the type of sample analyzed.
(iii) Readily accessible in high purity and be cheap.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis
MODULE No.9: Gas Chromatography
 (iv) Best column performance consistent with required speed of the analysis.
(v) Not cause the risk of fire or explosion hazard.

(b) Sample Introduction System: - The sample introduction system is very vital
as very small amount of the sample is injected into the column. This system must
introduce the sample in a reproducible manner and must vaporize it instantaneously so
that the sample will enter the column as a single plug.

Figure 2: Sample Introduction system

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 Liquid samples are commonly introduced by a hypodermic syringe through a self-sealing
rubber septum into small inlet chamber, which may be heated to cause flash evaporation.
The temperature must not be so high that it decomposes the sample. Solid samples must
be dissolved in volatile liquids for introduction or may be introduced directly if it can be
liquefied. Gas samples require special gas sampling valves for introduction into the
carrier gas stream.

(c) The Separation Columns:-

Open Tubular Columns and Packed Columns: Open tubular columns, which are also
called as capillary columns, come in two basic forms. The first type is the WCOT or
Wall-coated tubular column and the second one is the SCOT or Support-coated tubular
column. WCOT columns are capillary tubes that have a thin layer of the stationary phase
coated along the column walls. In support-coated tubular columns, the wall of the column
is coated with a thin layer of about 30µm thickness of the adsorbent solid, for example
diatomaceous earth, a material which comprises of uni-cellular sea plant skeletons.
Treatment of the adsorbent solid is then carried out with the liquid stationary phase.
While SCOT columns are capable of holding a larger volume of stationary phase than a
WCOT column due to its larger sample capacity, WCOT columns have better column
efficiencies.

One of the most popular types of capillary columns is a special WCOT column called the
fused-silica wall-coated (FSWC) open tubular column. In fused silica columns, the walls
are drawn from purified silica consisting of minimal amount of metal oxides. These
columns are a lot thinner than glass columns, with diameters as small as 0.1 mm and
lengths up to 100 m. To protect the column, a polyimide coating is applied to the outside
of the tubing and bent into coils to fit inside the thermo-stated oven of the gas
chromatography unit. The FSWC columns are replacing older columns due to increased
chemical inertness, greater column efficiency and smaller sampling size requirements.

Packed columns are made of a glass or metal tubing which is densely packed with a solid
support like diatomaceous earth. Due to the complexity of packing the tubing uniformly,
these types of columns have a larger diameter than open tubular columns and have a
limited range of length. As a result, packed columns can only achieve about 50% of the
efficiency of a comparable WCOT column.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 Furthermore, the diatomaceous earth packing is deactivated over time due to the semi-
permanent adsorption of impurities within the column. In contrast, FSWC open tubular
columns are manufactured to be virtually free of these adsorption problems.

Figure 3: Types of columns

(d) Detection Systems: The detector is a device located at the end of the column
which provides a quantitative measurement of the constituents of the mixture as they
elute in combination with the carrier gas. In theory, any property of the gaseous mixture
that is different from the carrier gas can be used as a detection method. These detection
properties fall into two categories: bulk properties and specific properties. Bulk
properties, also known as general properties, are properties that both the carrier gas and
analyte possess but to different degrees. Specific properties such as nitrogen-phosphorous
contents have only limited applications but compensate for this by their
increased sensitivity of the detector.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 Each detector has two main parts which when used together serve as transducers to
convert the detected property changes into an electrical signal that is recorded as a
chromatogram. The first part of the detector is the sensor which is placed as close to the
column exit as possible in order to optimize detection. The second is the electronic
equipment used to digitize the analog signal so that a computer may analyze the acquired
chromatogram. The sooner the analog signal is converted into a digital signal, the greater
the signal-to-noise ratio becomes, as analog signal are easily susceptible to many types of
interferences.

Common Types of Gas Chromatography Detectors:

i) Mass Spectrometry Detectors

Mass Spectrometer (MS) detectors are most powerful of all gas chromatography
detectors. In a Gas chromatography - Mass spectroscopy system the masses are
continuously scanned throughout the separation process with the help of mass
spectrometer. When the sample exits the chromatography column, it is passed through a
transfer line into the inlet of the mass spectrometer. After this step, with the help of an
electron-impact ion source, the sample is then ionized and fragmented. During this
process, the sample is bombarded by energetic electrons which ionize the molecule by
causing them to lose an electron due to electrostatic repulsion. Furthermore, bombarding
causes the ions to fragment. These ions are then passed into a mass analyzer wherein they
get sorted in accordance with their mass-to-charge ratio or m/z ratio. Most of the ions are
singly charged only.

ii) Flame Ionization Detectors

Flame ionization detectors (FID) are the most extensively used detectors. In this type of
detector, the sample after exiting from the column is aimed at an air-hydrogen flame. At
the high temperature of the air-hydrogen flame, the sample undergoes pyrolysis, or
chemical disintegration. Ions and electrons are released from the pyrolized hydrocarbons
which help in carrying of current. Hence, elution of the sample is readily studied by a
high-impedance pico-ammeter which measures this current.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 iii) Thermal Conductivity Detectors

TCD or Thermal conductivity detectors were the first detectors to be developed for use in
conjunction with the technique of gas chromatography. It works by measuring the change
in carrier gas thermal conductivity caused by the presence of the sample, which has a
different thermal conductivity from that of the carrier gas. Their design is relatively
simple, and consists of an electrically heated source that is maintained at constant
power. The source which is generally a thin wire is composed up of gold or platinum.
The source temperature is dependent on the thermal conductivities of the surrounding
gases. The resistance within the wire depends upon temperature, which is dependent
upon the thermal conductivity of the gas.

iv) Electron-Capture Detectors

Electron-capture detectors (ECD) are highly selective detectors frequently used for
detecting environmental samples. The device selectively detects organic compounds
with moieties such as halogens, peroxides, quinones and nitro groups and gives little to
no response for all other compounds. Therefore, this method is best matched for
applications where trace quantities of chemicals such as pesticides are to be detected and
other chromatographic methods are unfeasible.

v) Atomic Emission Detectors

Atomic emission detectors (AED) are the latest embellishments to the gas
chromatographer's arsenal. They are element-selective detectors that utilize plasma,
which is a partially ionized gas, to atomize all of the elements of a sample and excite their
characteristic atomic emission spectra. There are three ways of generating plasma:
microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current
plasma (DCP). Microwave-induced plasma is the most frequently employed type which
is used in conjunction with a diode array such that the atomic emission spectra of various
elements can be simultaneously monitored. AED is an extremely powerful detector that
has wide applicability.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 Type of Detector Applicable Samples Detection Limit
Mass Spectrometer (MS) Tunable for any sample .25 to 100 pg
Flame Ionization (FID) Hydrocarbons 1 pg/s
Thermal Conductivity (TCD) Universal 500 pg/ml
Electron-Capture (ECD) Halogenated hydrocarbons 5 fg/s

Atomic Emission (AED) Element-selective 1 pg

Figure 4: Typical Gas chromatography detectors and their detection limits.

(e) Thermo-stated Chambers for the Temperature Regulation of the

Column and Detectors:

The column(s) in a GC are contained in an oven; the temperature being specifically

controlled electronically. The rate at which a sample passes through the column is
directly proportional to the temperature of the column. The higher the column
temperature, the faster the sample moves through the column. Nevertheless, the sample
will interact less with the stationary phase when it is moving faster through the column
thereby resulting in incomplete separation of the analytes.

“Isothermal mode” is the technique wherein for the entire analysis the column is
maintained at the same temperature. Most methods, however, increase the column
temperature at a certain rate during the analysis. Programming of the initial temperature,
rate of temperature increase (the temperature "ramp"), and final temperature are called
"temperature programming". This method helps the analytes to elute out early so that
the separation takes place adequately, while lessening the time taken for the late eluting
analytes to easily pass through the column.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 (f) Amplification and recorder systems:

These are the final components of the Gas chromatographic instrumentation setup which
records the signals from the detector systems. They use certain electronic circuits to
process and amplify these signals so as to display in an understandable graphical format,
representing several peaks of the constituents of the sample in analysis.

7. Applications of GC-MS

Gas Chromatography (GC-MS) is a sophisticated separation technique that cannot be

compared with other modern analytical equipments but can be complemented by mass
spectrophotometer to achieve GC-MS/MS. It has wide variety of applications that caters
to academic research, quality control as well as industrial applications. Its concise,
efficient, automated system gives fast, reproducible and effective results that serve a key
role in advancement of Science and Technology. This resourceful analytical technique
could be explored for better prospects in future. It has established applications in various
fields such as :

 Environmental monitoring
 Food, beverage, flavor and fragrance analysis
 Biological and pesticides detections
 Security and chemical warfare agent detection
 Chemistry and Geo chemical Research
 Medicine and Pharmaceutical Applications
 Petrochemical and hydrocarbons analysis
 Doping of drugs
 Recent applications of GC in the forensic sciences, including those in forensic
toxicology, which include alcohol and drugs in drivers, markers of alcohol abuse,
volatiles and anesthetics, carbon monoxide (CO) poisoning, other poisons, drug-
facilitated sexual assault (DFSA) and drug profiling, and more general application
to criminalistics in areas such as fire debris, car paints, explosives, toners, fibers,
inks, and fingermark analysis.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
  Forensic and criminal cases such as analysis of ethanol in blood, prohibition cases
for the percent of ethanol in alcohol, analysis of drugs, mixture for one or more
component, chemical identification in individual substances, etc.
 The gases most commonly encountered by the forensic chemist are carbon
monoxide, carbon dioxide, light hydrocarbons, cyclopropane, and ether. Carbon
monoxide and carbon dioxide are usually absorbed by blood from an environment
of partial combustion. All these gases are analyzed using GC.
 With respect to the emerging role of forensic science for arson investigation, a
low cost and promising onsite detection method for ignitable liquids is desirable.
Gas chromatography- differential mobility spectrometry (GC-DMS) was
investigated as a toll for analysis of ignitable liquids from fire debris. Flammables
are the number one tool of the arsonist. Fortunately, evidence of their use is not
completely destroyed even for flammable material of high vapor pressure, or
when the environment has been elevated at temperature for several hours. Types
of materials to be expected include gasoline, paint thinner, charcoal lighter,
turpentine, paint remover, fuel oil, kerosene and the like. None of these materials
are single compounds but definite mixtures, and for this reason identification may
be established by preparing a chromatogram.
 In addition to the applications mentioned in this paper, such materials as
perfumes, coatings, plastics, oils, solvents, and poisons readily lend themselves to
chromatographic analyses.

8. Summary

1. Gas chromatography (GC) is probably the most utilized of all the

chromatographic techniques.
2. In gas – liquid chromatography (GLC) the mobile phase is a gas and the
stationary phase is a thin layer of a non-volatile liquid bound to a solid support.
Separation takes place by the process of partition.
3. On the other hand, gas-solid chromatography (GSC) utilizes a solid adsorbent
as the stationary phase and separation takes place by adsorption process.
4. Gas chromatography is similar to column chromatography, except that the gas is
used as the mobile phase instead of a liquid.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography
 5. Gas chromatography is at present the most widely used and powerful tool
available for separations.
6. The instrumentation required for gas chromatography are-
o A carrier gas
o Sample injection system
o Separation column
o Detectors
o Thermostat chambers
o Amplification and recorder system.
7. Gas chromatography can be used in various fields like –
o Environmental monitoring
o Food, beverage, flavor and fragrance analysis
o Forensic and criminal cases
o Doping for drugs
o Medicine and Pharmaceutical applications
o Security and chemical warfare agent detection
o Forensic applications such as in cases of arson, blood alcohol, etc.

FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis

MODULE No.9: Gas Chromatography