Gas Chromatography Mass Spectroscopy
Gas Chromatography-Mass Spectrometry (GC-MS) is a highly sensitive analytical method that couples the specificity of gas chromatography with mass spectrometry for quantitation and identification of the components of multi-component mixtures. GC-MS finds extensive applications in environmental analysis, pharmaceuticals, food technology, and forensic chemistry because it is highly sensitive and specific.
Principle
The working of GC-MS consists of the gas chromatographic separation of volatile components of the mixture with identification in terms of mass-to-charge ratios by mass spectrometry. Every compound has a characteristic mass spectrum that serves as a molecular fingerprint.
Working Mechanism
1. GC:
- The sample is injected into the chromatographic column in the vapor phase.
- An inert carrier gas (usually helium) helps in moving the sample components through the column.
- The components are separated depending on their boiling points and interactions with the stationary phase.
2. MS:
- The separated components are introduced into the instrument.
- The components are ionized and fragmented usually by electron impact, and then detected quantitatively.
- The mass analyzer separates the ions on the basis of their mass-to-charge ratio.
- A detector collects the information and generates a mass spectrum for each component detected.
Sample Preparation
Proper sample preparation is the key to successful and accurate GC-MS analysis. Sample preparation is intended for the separation of the target analytes from interferences and preparation of the sample in a state that is optimal for separation with the use of gas chromatography.
Typically, these include:
- Extraction: Soxhlet extraction or Solid-Phase Extraction (SPE) is carried out for solids. Another approach is protein precipitation or liquid-liquid extraction (LLE) for liquids. Such techniques are used to separate target compounds from complex matrices.
- Filtration and cleanup: The solution is filtered to take out the suspended particles that could cause clogging within the GC column and interfere with analysis.
- Concentration: The sample concentration could be achieved by nitrogen drying or evaporation to detect some analytes that are present at low levels.
- Derivatisation (if applicable): Certain compounds are chemically modified to enhance their detection or, in some cases, thermal stability or volatility (e.g., silylation for acids and alcohols).
- Final dissolution: The prepared samples are dissolved in volatile solvents such as hexane, methanol, or acetonitrile before injections into the GC-MS apparatus.
Forensic Application
Drug Analysis
- Detection of illegal drugs (e.g., cocaine, heroin, amphetamines)
- Quantitation of controlled drugs in seized evidence
- Drugs in blood, urine, and hair samples (toxicology)
Toxicology and Poison Detection
- Detection of poisons such as cyanide, organophosphates, or pesticides in body fluids
- Analysis of post-mortem specimens to diagnose cause of death
Arson Investigation
- Identification of accelerants (gasoline, kerosene) in residue from fire
- Separation of petroleum products from natural combustion residue
Explosives Analysis
- Identification of organic explosive residues (TNT, RDX, PETN)
- Identification of post-blast residues to determine the used explosive
Forensic Toxicology in Driving Under the Influence (DUI) Cases
- Identification of alcohol, narcotics, or other drugs from blood or breath samples
- Determination of levels of intoxication for legal proceedings
Body Fluid and Tissue Analysis
- Chemical exposure or contamination evaluation in forensic pathology
- Volatile compound detection in cases of decomposition
- Determination of postmortem interval based on decomposition chemicals
Forensic Entomology
Strengths
- Highly sensitive and specific
- Precise qualitative and quantitative analysis
- Numerous compounds detectable
- Huge spectral libraries to recognize
Weaknesses
- Applicable only for volatile and thermally stable compounds
- Labour-intensive sample preparation
- Expensive instrumentation and maintenance
- Skilled operators and interpretation of data are needed
References
- get_pdf.cfm
- A review on gas chromatography-mass spectrometry (gc-ms)
- GC-MS Principle, Instrument and Analyses and GC-MS/MS | Technology Networks
- ch31.pdf
- Forensic Mass Spectrometry: Scientific and Legal Precedents - PMC
- New Applications of Gas Chromatography and Gas Chromatography-Mass Spectrometry for Novel Sample Matrices in the Forensic Sciences: A Literature Review
- GC×GC–MS for Forensic Analysis
- e-PGPathshala
