Gc ms (Revised)

Presented by – ANAMIKA BANERJEE (RESEARCH SCHOLAR)

Gas chromatography coupled to mass spectrometry is a
versatile tool to separate, quantify and identify unknown
(volatile) organic compounds and permanent gases. By
combining sensitivity and a high resolving power, complex
mixtures can be analyzed. The information obtained can be
used for detection of impurities, contamination control and
improvement of, for example, semiconductor manufacturing
processes

FUNDAMENTAL OF GC
Gas Chromatography (GC) uses a carrier gas to transport sample
components through either packed columns or hollow capillary columns
containing the stationary phase. In most cases, GC columns have smaller
internal diameter and are longer than HPLC columns.
In Gas Chromatography, the mobile phase is a gas and the stationary
phase is either:
• A solid, commonly termed “Gas solid chromatography (GSC)”.
• An immobilised polymeric liquid, referred as “Gas Liquid
Chromatography (GLC)”.
6

• Carrier – (H2, He, N2 – helium is most usual with MS detection)
• Make-up gas– (H2, He, N2 - if using an FID detector in parallel to the
MS detector)
• Detector gas – (H2 & Air – if using an FID detector in parallel to the
MS detector)
1. Gas Supply: Gas is fed from cylinders through regulators and
tubing to the instrument. It is usual to filter gases to ensure high gas
purity and the gas supply pressure. Required gases might include:
2. Interface: After separation in the GC system, analyte species have
to be transported to the mass spectrometer to be ionised, mass filtered
and detected. The interface in modern instruments is heated to
prevent analyte condensation and in some instruments will contain
a device to remove carrier gas molecules to allow analyte pre-
concentration.

5. Column: Gas Chromatography uses a gaseous mobile phase to
transport sample components through columns either packed with coated
silica particles or hollow capillary columns containing the stationary
phase coated onto the inner wall. Capillary GC columns
are usually several meters long (10 – 120m is typical) with an internal
diameter of 0.10 – 0.50mm, whilst packed GC columns tend be 1 – 5
meters in length with either 2 or 4mm internal diameter.
6. Injector: Here the sample is volatilised and the resulting gas
entrained into the carrier
3. Pneumatic controls: The gas supply is regulated to the correct pressure
(or flow) and then fed to the required part of the instrument. Modern GC
instruments have Electronic Pneumatic pressure controllers –older
instruments may have manual pressure control via regulators.
4. Oven: Gas chromatography have ovens that are temperature
programmable, the temperature of the gas chromatographic ovens
typically range from 5oC to 400oC but can go as low as -25oC with
cryogenic cooling.

Weaknesses of GC:
• Requires the analyte to have significant vapour
pressure between about 30atm and 300oC.
• Presents a lack of definitive proof of the nature of
detected compounds. The identification process is
based on retention time matching, which may be
inaccurate or at worst misleading.

INSTRUMENT FUNDAMENTALS –MS
The mass spectrometer is an instrument designed to
separate gas phase ions according to their mass to charge
ratio (m/z) value.
mass to charge ratio (m/z):
This represents the mass of a given particle (Da) to the
number (z) of electrostatic charges (e) that the particle
carries. The term m/z is measured in Da/e.

1. Control Electronics: The MS parameters can be selected and
controlled from this panel. Modern instruments will also allow to
control MS parameters from a computer by using specially
designed software.
2. Detector: The ion beam that emerges from the mass analyzer, have
to be detected and transformed into a usable signal.
The detector is an important element of the mass spectrometer that
generates a signal from incident ions by either generating secondary
electrons, which are further amplified, or by inducing a current
(generated by moving charges).

3. Vacuum System: Mass analysers require high levels of
vacuum in order to operate in a predictable and efficient way.
The vacuum systems of most modern GC-MS systems consist of
a differentially pumped system, usually with a foreline pump
establishing a ‘rough’ vacuum and a high vacuum pump or
pumps situated on the analyser body to establish the high levels of
vacuum required for effective mass to charge ratio measurement.
4. Mass Analyser: There are several very popular types of mass
analyser associated with routine GC-MS analysis and all differ
in the fundamental way in which they separate species on a
mass-to-charge basis.
5. Ion Source: In the ion source, the products are ionized prior to
analysis in the mass spectrometer.

The Coupling of GC/MS
GC-MS has the advantages of higher chromatographic resolution and
higher peak capacity, a single mobile phase (helium), fewer issues with
solubility and separations can be adjusted by electronic controls such
as temperature and flow programming.
Today with the advent of capillary columns using relatively low carrier
gas flow rates, the need for vapour concentrator devices has been
eliminated. The mass spectrometer pumping system can easily deal
with gas flows for direct injection into the mass spectrometer.

The Bieman Concentrator
The Bieman concentrator device is used with packed columns
or with wide bore capillary columns at higher flow rates. In this
device the carrier gas is removed in preference to the analyte
and hence analyte enrichment occurs.

Direct Introduction
Direct introduction is typically used with capillary GC columns
and most modern instruments can easily cope with flow rates up
to 2 mL/min

Ionisation
Mass spectrometry involves the separation of charged species which are
produced by a variety of ionisation methods in GC-MS. These include:
• Electron impact Ionisation (EI) - where analyte molecules are directly
ionised through collision with a bombarding electron stream resulting in the
removal of an electron to form a radical cation species.
• Chemical Ionisation (CI) - where analyte molecules are charged through
reaction processes with a charged reagent gas plasma producing either anion
or cation species depending upon the analyte and analyser polarity.

Electron Impact (EI)
In the Electron Impact (EI) process, electrons are emitted from a
heated filament (usually made of tungsten or rhenium) and are
accelerated across the source by using an appropriate potential
(5-100V) to achieve the required electron energy (sufficient to
ionize the molecule).
M (g) + e- M+ + 2e-

Chemical Ionisation (CI)
Chemical ionisation involves the ionisation of a reagent gas,
such as methane at relatively high pressure (~1 mbar) in a
simple electron impact source. Once produced, the reagent gas
ions collide with the analyte molecules producing ions through
gas phase reaction processes such as proton transfer.
nNH3 + e- NH4+ + 2e-
NH4
+ + M NH3 + [M+ H]+
NH4
+ + M [M+ NH4]+

GC-MS Process
To achieve the identification of different compounds,
three steps can be distinguished in a GC-MS system:
• Sample injection
• Separation.
• Detection

INJECTION
Depending on the sample (gas, liquid or solid), compounds in a
mixture need to be volatilized or extracted from the matrix
INJECTION TECHNIQUE USED FOR
Split High concentration samples (liquid/gas)
Only small amounts of sample required
Split-less Trace analysis (liquid/gas)
Thermal Desorption Trace analysis (gases) and solid samples
Desorption of volatile compounds in a solid
matrix
Headspace Volatile compounds in difficult matrices
For collection of vapors above dirty samples
Solid Phase Micro Extraction
(SPME) Volatile compounds in difficult matrices
with pre- concentration

SEPARATION
After injection of a mixture, separation is achieved in the
capillary column. This column is coated with a fluid or a
solid support, the stationary phase. An inert gas, also
called the mobile phase, is flowing through the column.
Depending on the phase equilibrium between the
stationary and mobile phase, compounds travel with
different velocities through the column. The mixture
becomes separated, and as a result, individual
compounds reach the detector with a different retention
time. By choosing a column, which separates on boiling
point, polarity, size or stereochemistry, a wide range of
compounds can be separated.

DETECTION
Many different detectors can be used for detection of the separated
compounds. The mass spectrometer combines a high sensitivity with the
unique property of being able to determine the molecular composition.
DETECTOR USED F OR
Flame Ionization Detector (FID) Compounds containing Carbon
Thermal Conductivity Detector (TCD) Universal detection for gases without C
Nitrogen Phosphorous Detector (NPD) Selective detection of N and P containing
organic compounds
Electron Capture Detector (ECD) Selective detection of halogen containing
compounds
Atomic Emission Detector (AED) Selective detection of elemental composition

APPLICATIONS
The use of GC-MS in many application areas within analytical
science continues to grow almost exponentially. Listed below are
some pointers as to the applicability of both GC as a separative
technique and MS as a means of detecting analyte species
GC analysis of urine sample know to contain cocaine

GC Separations
• Produce fast analyses because of the highly efficient nature of
the separations achieved
• By using a combination of oven temperature and stationary
phase very difficult separations may also be undertaken
• Excellent for quantitative analysis with a range of sensitive and
linear detectors to choose from (including the MS detector)
• A practical upper temperature limit for conventional GC columns
is around 350-380oC.
In GC analysis, analyte boiling points rarely exceed 400oC and
the upper molecular weight is usually around 500 Da

MS Detection
• Allows specific compound identification. Structural
elucidation via spectral interpretation combined with
elemental composition from accurate mass analysers is
possible.
• Allows high sensitivity detection - femto-gram amounts
have been detected by certain mass analyzer types.
• Is highly selective, certain analyzer and experiment
combinations can lead to highly selective and sensitive
analysis of a wide range of analyte types.

Cleanroom air
The presence of airborne molecular contamination (AMC) in cleanroom air
can have a negative effect on the health. To assess the level of AMC in a
cleanroom several samples are taken using adsorption tubes and sent to the
laboratory for analysis. At the laboratory, the adsorption tubes are desorbed
at elevated temperature and the organic contamination present is analyzed
using GC-MS. Typically, over 100 different compounds are detected,
including compounds like cleanroom solvents (acetone, ethyl acetate etc.)
and large hydrocarbons. Quantification of the obtained data yields the
exact composition of the gas/cleanroom air.
Chromatogram of
cleanroom air.
Some of the characteristic
peaks are indicated:
1= ethanol
2= acetone
3= ethylacetate
4= cyclohexanone
5= dodecane

REFERENCES
• Chromacademy – e- learning for analytical chemistry
community : Fundamentals of GC-MS
• Material analysis by GC-Ms : www.innovatechlabs.com
› analytical services
• Google Search engine: Wikipedia

Gc ms (Revised)

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