GCMS 2024

Gas Chromatography/Mass Spectroscopy (GC-MS)

Overview

  • GC-MS combines two analytical instruments: Gas Chromatography (GC) and Mass Spectrometry (MS).

  • The GC separates compounds and the MS identifies them.

Operation of GC-MS

  • GC: A long glass tube separates complex mixtures by passing them through the column. Each compound exits the column at different times, akin to a race.

  • MS: Identifies compounds by fragmenting them after they are fed from the GC.

Advantages of GC

  • Requires very small sample sizes (microliters).

  • High separation and resolution capability.

  • Short analysis time (1 to 100 minutes).

  • Greater sensitivity for low concentrations.

  • Relatively simple operational equipment.

Applications of Chromatography

  • Analysis: Examine mixtures and their components.

  • Identification: Determine identity of components based on known substances.

  • Purification: Isolate specific components for further study.

  • Quantification: Measure amounts of components in a sample.

Practical Uses of Gas Chromatography

  • Pharmaceutical Companies: Analyze chemical compositions.

  • Hospitals: Detect blood/alcohol levels.

  • Law Enforcement: Compare samples from crime scenes.

  • Environmental Agencies: Measure pollutants in water supplies.

  • Manufacturing Plants: Purify chemicals for products.

  • Autosampler: Allows for continuous operation by preparing samples during the day for overnight runs.

Gas Chromatography Equipment

  • Components:

    • Mobile Phase: Generally helium (inert, small, fast).

    • GC Oven: Maintains precise temperatures (up to 450 ºC).

    • Column: Long glass tubes (15 to 60 meters), coated internally with stationary phase.

    • Detector: Identifies compounds passing through.

Stationary Phase

  • Coats the interior of the capillary tube.

  • Types include extremely polar, polar, nonpolar, and chiral.

  • Affects retention time based on interaction with sample.

Retention Time

  • Time taken for a sample to travel from injection to detection.

  • Influenced by volatility; less volatile compounds retain longer.

Identifying Unknowns

  • GC can identify compounds based on known retention times.

  • Consistent retention time indicates compound identity.

Lists of Compounds

  • Gasoline Compounds:

    • Includes isobutane, n-pentane, toluene, etc.

  • Abused Drugs:

    • Includes amphetamines, cocaine, heroin, etc.

Mass Spectrometry Functions

  • Ionization: Converts sample molecules into ions.

  • Separation: Ions separated by mass-to-charge ratio.

  • Detection: Identified and quantified by the spectrometer.

Electron Ionization (EI)

  • Creates distinct fragmentation patterns used for compound identification (e.g., cocaine).

Applications in Forensics

  • Arson Analysis: Detect accelerants using GC-MS.

  • Explosive Analysis: Analyze residual explosives in investigations.