GC-MS (GAS CHROMATOGRAPHY – MASS SPECTROMETRY) DETAILED NOTES
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1. INTRODUCTION
GC-MS is a hyphenated analytical technique combining:
Gas Chromatography (GC): Separates volatile compounds in a mixture.
Mass Spectrometry (MS): Identifies and quantifies compounds based on mass-to-charge ratio (m/z).
Developed in the 1950s–1970s, GC-MS is now widely used in forensic, pharmaceutical, environmental, and food analysis.
Importance:
Identifies unknown compounds
Detects trace contaminants
Provides both qualitative and quantitative information
Example: Detection of pesticides in water, drug analysis in biological fluids, environmental pollutant detection.
2. PRINCIPLE
GC-MS principle is based on two steps:
A. Separation (GC)
Sample is vaporized and carried by an inert gas (helium, nitrogen) through a capillary column.
Compounds separate based on:
Volatility
Boiling point
Interaction with column stationary phase
Result: Different compounds elute at different retention times.
B. Detection (MS)
Eluted compounds are ionized (usually by electron impact ionization, EI).
Ions are separated based on mass-to-charge ratio (m/z) in the mass analyzer.
Mass spectrum is generated, showing molecular ion peak (M⁺) and fragment ions.
Compound identification is done by comparing with mass spectral library.
3. COMPONENTS OF GC-MS
Gas Chromatograph (GC)
Injector: Introduces sample into column; usually heated for vaporization.
Carrier Gas: Inert gas (He, N₂, Ar) transports vaporized sample.
Column: Fused silica or metal capillary column coated with stationary phase.
Oven: Maintains controlled temperature; temperature programming can be used.
Interface
Connects GC to MS
Maintains vacuum in MS while transferring analytes
Often called transfer line
Mass Spectrometer (MS)
Ion Source: Commonly Electron Impact (EI) or Chemical Ionization (CI)
Mass Analyzer: Quadrupole, TOF, or Ion Trap separates ions by m/z
Detector: Measures ion abundance; usually electron multiplier
Data System: Records chromatogram (signal vs time) and mass spectrum (m/z vs intensity)
4. PROCEDURE OF GC-MS
Step-by-step process:
Step 1: Sample Preparation
Must be volatile or derivatized to volatile form (e.g., silylation of sugars).
Filter or centrifuge if liquid sample contains particles.
Avoid moisture if column is moisture-sensitive.
Step 2: Sample Injection
Introduce 1–2 µL of sample using microsyringe or autosampler.
Sample vaporized in heated injector (250–300°C).
Step 3: Chromatographic Separation (GC)
Carrier gas flows through column at constant flow or pressure.
Oven programmed to increase temperature gradually (temperature programming) for better separation.
Components elute at specific retention times.
Step 4: Ionization (MS)
Molecules are bombarded by high-energy electrons (70 eV in EI).
Electron impact generates positive ions and fragment ions.
Chemical Ionization (CI) produces softer ionization for molecular ion detection.
Step 5: Mass Analysis
Ions separated in mass analyzer according to m/z ratio.
Quadrupole or TOF commonly used.
Step 6: Detection
Detector measures ion current for each m/z value.
Data recorded as mass spectrum (molecular and fragment peaks).
Step 7: Data Analysis
Chromatogram shows retention time vs signal intensity.
Mass spectrum compared with spectral libraries for compound identification.
Quantification by peak area or internal standards.
Step 8: System Maintenance
Flush column if necessary
Replace septa and liners in injector
Maintain vacuum in MS system
5. ADVANTAGES OF GC-MS
High sensitivity (ng to pg levels)
High specificity: Can distinguish compounds with similar properties
Provides structural information via fragmentation
Rapid and automated analysis
Can analyze complex mixtures
Trace detection of contaminants possible
6. LIMITATIONS OF GC-MS
Only suitable for volatile or derivatized compounds
Thermally labile compounds may degrade
High cost of instrument and maintenance
Requires skilled operation
Moisture and non-volatile salts can damage column or interface
7. APPLICATIONS OF GC-MS
Environmental Analysis
Pesticides, herbicides, pollutants, VOCs
Forensic Science
Drug detection, toxicology, arson investigation
Pharmaceuticals
Drug purity, metabolites, pharmacokinetics
Food Industry
Flavor analysis, contaminants, additives
Clinical Diagnostics
Hormone analysis, metabolic disorders
Petrochemical Industry
Hydrocarbon profiling, fuel analysis
FLOW DIAGRAM OF GC-MS
Sample Prep → Sample Injection → GC Separation → Ionization → Mass Analysis → Detection → Data Analysis → System Maintenance
GC-MS combines:
A) Gas Chromatography and Mass Spectrometry
B) Gel Chromatography and Mass Spectrometry
C) Gas Chromatography and UV Spectroscopy
D) Gas Chromatography and NMR
Answer: A
The main purpose of GC in GC-MS is:
A) Ionize the sample
B) Separate volatile components
C) Detect ions
D) Fragment molecules
Answer: B
The main purpose of MS in GC-MS is:
A) Separate molecules
B) Detect and identify ions
C) Vaporize sample
D) Prepare mobile phase
Answer: B
GC-MS is suitable for:
A) Non-volatile compounds
B) Volatile or derivatized compounds
C) Solid metals
D) Liquids only
Answer: B
m/z in mass spectrometry stands for:
A) Mass multiplied by charge
B) Mass divided by charge
C) Mass plus charge
D) Mass minus charge
Answer: B
SAMPLE PREPARATION & INJECTION
Sample injection in GC-MS is usually:
A) 1–2 µL
B) 10–20 mL
C) 100–200 µL
D) 0.1–0.2 mL
Answer: A
Samples must be:
A) Non-volatile
B) Moisture-free and volatile
C) Solid only
D) High molecular weight proteins
Answer: B
Filtered samples prevent:
A) Detector saturation
B) Column blockage
C) MS ionization
D) Oven heating
Answer: B
Derivatization is used to:
A) Increase polarity
B) Reduce volatility
C) Make compounds volatile
D) Increase ion suppression
Answer: C
Autosamplers improve:
A) Manual injection
B) Reproducibility
C) Mass spectrum resolution
D) Ionization efficiency
Answer: B
CARRIER GAS & COLUMN
Common carrier gas in GC-MS:
A) Helium
B) Oxygen
C) Hydrogen peroxide
D) Nitrogen dioxide
Answer: A
Carrier gas functions:
A) Heat the column
B) Transport sample through column
C) Ionize analyte
D) Detect fragment ions
Answer: B
GC columns are usually:
A) Stainless steel or fused silica
B) Glass only
C) Plastic
D) Aluminum foil
Answer: A
Stationary phase in GC columns determines:
A) Oven temperature
B) Separation efficiency
C) Ionization mode
D) Carrier gas type
Answer: B
Oven programming in GC:
A) Maintains constant pressure
B) Gradually increases temperature
C) Ionizes the sample
D) Detects fragments
Answer: B
IONIZATION TECHNIQUES
Most common ionization method in GC-MS:
A) Electrospray ionization (ESI)
B) Electron impact (EI)
C) Atmospheric pressure chemical ionization (APCI)
D) Matrix-assisted laser desorption (MALDI)
Answer: B
EI produces:
A) Positive ions and fragment ions
B) Neutral molecules only
C) Negative ions exclusively
D) Solid particles
Answer: A
CI (Chemical Ionization) is used for:
A) Soft ionization, preserving molecular ion
B) High-energy fragmentation
C) Only volatile metals
D) Solid derivatization
Answer: A
Molecular ion (M⁺) represents:
A) Most abundant fragment
B) Unfragmented molecule with charge
C) Protonated solvent
D) Neutral analyte
Answer: B
Fragment ions are used for:
A) Solvent selection
B) Structural elucidation
C) Column cleaning
D) Carrier gas calibration
Answer: B
MASS ANALYZERS
Common mass analyzers in GC-MS:
A) Quadrupole, TOF, Ion Trap
B) Orbitrap only
C) NMR coil
D) UV detector
Answer: A
TOF analyzer separates ions based on:
A) Mass-to-charge ratio and flight time
B) Color
C) Pressure
D) Solubility
Answer: A
Quadrupole mass analyzer uses:
A) Magnetic field
B) Radio frequency + DC electric fields
C) Laser pulse
D) High pressure
Answer: B
Ion Trap MS can perform:
A) Single-stage analysis only
B) Tandem MS (MS/MS)
C) UV absorption
D) Ionization only
Answer: B
FT-ICR MS is known for:
A) Low sensitivity
B) Ultra-high resolution
C) Large sample volumes only
D) UV detection
Answer: B
DETECTION & DATA ANALYSIS
GC-MS detector commonly:
A) Electron multiplier
B) UV spectrophotometer
C) pH meter
D) Flame photometer
Answer: A
Mass spectrum plots:
A) m/z vs intensity
B) Time vs wavelength
C) Retention time vs wavelength
D) Temperature vs flow
Answer: A
Chromatogram plots:
A) Retention time vs detector signal
B) m/z vs intensity
C) Temperature vs pressure
D) Flow rate vs column length
Answer: A
Peak area is used for:
A) Column cleaning
B) Quantitative analysis
C) Ionization efficiency
D) Gas flow adjustment
Answer: B
Retention time helps in:
A) Identifying compound
B) Ionization
C) Fragmentation
D) Solvent selection
Answer: A
APPLICATIONS
GC-MS is used for:
A) Pesticide residue detection
B) Protein sequencing
C) DNA sequencing
D) Sugar quantification in solids only
Answer: A
Forensic applications include:
A) Drug detection
B) Flame tests
C) Density measurement
D) Melting point determination
Answer: A
Pharmaceutical applications include:
A) Drug purity and metabolite profiling
B) Color analysis
C) Solvent density measurement
D) Flame photometry
Answer: A
Environmental applications include:
A) VOC detection
B) Hardness testing
C) pH determination
D) Density measurement
Answer: A
Food industry applications:
A) Flavor and contaminant analysis
B) Electrical conductivity
C) Water content only
D) UV absorption
Answer: A
ADVANTAGES
GC-MS is highly sensitive to:
A) ng–pg levels of compounds
B) g–kg levels only
C) Only solids
D) Only liquids
Answer: A
Structural information is obtained from:
A) Fragment ions
B) Retention time only
C) Solvent polarity
D) Carrier gas type
Answer: A
GC-MS can separate:
A) Complex mixtures
B) Pure compounds only
C) Non-volatile salts
D) Solid metals
Answer: A
Automated analysis improves:
A) Throughput and reproducibility
B) Column size
C) Detector type
D) Solvent choice
Answer: A
GC-MS is faster than:
A) TLC for identification and quantification
B) NMR for retention time
C) HPLC for non-volatile compounds
D) Flame photometry
Answer: A
LIMITATIONS
GC-MS cannot analyze:
A) Non-volatile or thermally labile compounds
B) Volatile compounds
C) Small organic molecules
D) Solvent molecules
Answer: A
Thermally unstable compounds may:
A) Decompose in injector
B) Produce clean spectra
C) Increase retention time
D) Ionize better
Answer: A
High cost is a:
A) Limitation
B) Advantage
C) Neutral factor
D) Optional
Answer: A
Moisture in sample can:
A) Damage column and interfere with MS
B) Enhance ionization
C) Reduce flow rate
D) Help separation
Answer: A
Requires:
A) Skilled personnel
B) No training
C) Only visual observation
D) Manual plotting
Answer: A
ADVANCED / MISCELLANEOUS
EI is considered a:
A) Hard ionization technique
B) Soft ionization technique
C) Non-ionizing method
D) Solvent detector
Answer: A
CI is considered:
A) Soft ionization
B) Hard ionization
C) Non-ionizing
D) Inert
Answer: A
Molecular ion peak is:
A) Unfragmented analyte ion
B) Fragment peak
C) Carrier gas peak
D) Column contaminant
Answer: A
Fragmentation pattern helps in:
A) Structural elucidation
B) Retention time calculation
C) Gas flow measurement
D) Column selection
Answer: A
GC-MS provides:
A) Qualitative and quantitative data
B) Only qualitative data
C) Only quantitative data
D) Only retention time
Answer: A
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