Gas Chromatography Mass Spectrometry

1. GAS CHROMATOGRAPHY
MASS SPECTROMETRY
Principles, Working, Applications and Critical Analysis

2. DESIRED CHARACTERISTICS OF SAMPLE
• To be suitable for GC analysis, a compound must have sufficient volatility and thermal stability
• Works on the principle that mixture will separate into constituents when heated
• Organic compounds must be in solution with a volatile organic solvent for injection into the
chromatograph
Gas
Chromatograp
h

3. PRINCIPLE
• GC is the separator
• MS is the analyser
• GS has a mobile phase travelling through a column coated with a mobile phase
• The component more soluble in the stationary phase travels slower and is eluted (removed) later
• This is governed by their partition coefficients
• The MS separates the ionised sample based on their m/z ratio
• The analysation relies on every compound having a different fragmentation pattern.

4. SCHEMATIC OF THE INSTRUMENT

5. MASS SPECTROMETER

6. GAS
CHROMATOGRAPH
Gas chromatography (GC) is a common
type of chromatography used in
analytical chemistry for separating and
analyzing compounds that
can be vaporized without decomposition.
Typical uses of GC include testing the
purity of a particular substance, or
separating the different components of a
mixture

7. INSTRUMENTAL COMPONENTS
Carrier gas
• The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon,
and carbon dioxide. The choice of carrier gas is often dependent upon the type of detector which is
used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
Sample injection port
• For optimum column efficiency, the sample should not be too large, and should be introduced onto
the column as a "plug" of vapor - slow injection of large samples causes band broadening and loss
of resolution. The most common injection method is where a micro syringe. The temperature of the
sample port is usually about 50C higher than the boiling point of the least volatile component of
the sample. The injector can be used in one of two modes; split or spitless. The injector contains a
heated chamber containing a glass liner into which the sample is injected through the septum.

8. SCHEMATIC DIAGRAM OF THE GC
THE INJECTOR

9. Columns
• There are two general types of column, packed and capillary (also known as open tubular).
Packed columns contain a finely divided, inert, solid support material (commonly based
on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 -
10m in length and have an internal diameter of 2 - 4mm.
Capillary columns can be one of two types;
wall-coated open tubular (WCOT) or support-coated open tubular (SCOT).
Detectors
• A non-selective detector responds to all compounds except the carrier gas, a selective
detector responds to a range of compounds with a common physical or chemical property
and a specific detector responds to a single chemical compound. Detectors can also be
grouped into concentration dependent detectors and mass flow dependent detectors. The
signal from a concentration dependent detector is related to the concentration of solute in
the detector, and does not usually destroy the sample Dilution of with make-up gas will
the detectors response. Mass flow dependent detectors usually destroy the sample, and the
signal is related to the rate at which solute molecules enter the detector. The response of a
mass flow dependent detector is unaffected by make-up gas.

10. DETECTOR (FLAME IONISATION
DETECTOR)
GC COLUMN

11. MASS
SECTROMETRY
Mass spectrometry (MS) is an
analytical technique that measures
the mass-to-charge ratio of ions. The
results are typically presented as
a mass spectrum, a plot of intensity as
a function of the mass-to-charge
ratio.

12. A mass spectrometer generates
multiple ions from the sample under
investigation, it then separates them
according to their specific mass-to-
charge ratio (m/z), and then records
the relative abundance of each ion
type.

13. COMPONENTS
The instrument consists of three major components:
• Ion Source: For producing gaseous ions from the substance being studied.
• Analyzer: For resolving the ions into their characteristics mass components according to
their mass-to-charge ratio.
• Detector System: For detecting the ions and recording the relative abundance of each of
the resolved ionic species.
In addition, a sample introduction system is necessary to admit the samples to be studied to
the ion source while maintaining the high vacuum requirements (~10-6 to 10-8 mm of
mercury) of the technique; and a computer is required to control the instrument, acquire
and manipulate data, and compare spectra to reference libraries.

14. FLOW CHART SHOWING THE PROCESS
AND COMPONENTS

15. Stage 1:
Ionization
Stage 2:
Acceleration
Stage 3:
Deflection
Stage 4:
Detection

16. GC
Volatilising the
sample
Pump it through the
stationary medium
Elution at different
time

17. MS
Convert neutral
molecule to an ion
[(+ve)]
Separating charged
molecule
Weighing them to
determine m:z ratio

18. Retention Time
m/zRatio
Abundance PEAK CHART

19. APPLICATIONS
• The information obtained from a GCMS can be used to detect impurities, control
contaminations and identify toxic compounds
• Petrochemical analysis
• Forensics
• Increasingly used for narcotics analysis

20. ANALYSIS
• Long start-up time due to slow vacuuming process
• The machine has auto-sampler to process many samples in a single cycle
• 3D graph can be obtained between elution separation and mass to charge ratio for accurate
inference
• Limited to materials that can be volatilized (30 atm, 300℃)
• Chemically derivatised components can be identified in most cases
• GC has efficient separation compared to LC
• The GC works based on retention time matching which isn’t definitive proof of nature of detected
compounds
• The spectrometer gets random peaks by stray gas so care should be taken to keep the conditions
dry.