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Gas Chromatography in the detection of Volatile Organic Compounds

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  1. Gas Chromatography in the detection of Volatile Organic Compounds

  2. Voltatile Organic Compounds • Volatile organic compounds (VOCs) are gases emitted from a variety of products • Paints • Pesticides • Aerosol sprays and cleaners • Exposure to these chemicals can affect health. • Eye and respiratory tract irritation • Headaches • Dizziness • Visual disorders • Cause cancer in animals

  3. Common VOCs Methylene Chloride Animal carcinogen (converted to carbon monoxide in the body) Formaldehyde Most common Benzene Human carcinogen Perchloroethylene Used in dry cleaning

  4. Voltatile Organic Compounds • EPA has set the Permissible Exposure Level of 0.75 ppm and an action level of 0.5 ppm. • VOC levels are two to five times higher inside than outside • Exposure can be reduced by increasing ventilation and storing materials with VOCs in closed containers • VOCs can be detected by gas chromatography

  5. Gas Chromatography • Separate and detect compounds in gas phase • Can analyze liquids if they can be volatilized without decomposition

  6. Sampling • Solid-phase Microextraction (SPME) • Gas sample exposed to a fiber coated with extraction phase (commonly silica) • Fiber placed in inlet and heated to desorb analyte • Static Headspace Sampling • Gas sample is sealed into a vessel, warmed, and injected directly into injection port

  7. Gas Chromatography • Mobile Phase: inert carrier gas nitrogen • Stationary Phase: column coated with active material • Packed column: fine silica beads coated with liquid or solid active material • Capillary column: inner column coated with active material; long column wound into a small coil • Retention time: time it takes for analyte to exit the column • Analyte components that are adsorbed more by the stationary phase have longer retention times

  8. Detection • Flame Ionization Detection (FID): • Sample hydrocarbon is combusted with oxygen and ionized, which releases electrons • Current measured is proportional to amount of analyte • Good general detector for many compounds • Electron Capture Detection (ECD): • Emitted beta electrons collide with nitrogen carrier gas removing an electron • Analyte captures an electron reducing detected current • Analyte concentration is proportional to degree of electron capture • Best for highly electronegative halogenated samples

  9. Detection • Photoionization Detection (PID) • Shine UV light on sample, causes it to emit an electron • Current measured is proportional to concentration if extent of ionization is the same for all analytes • Good for hydrocarbon samples • Mass Spectrometry • Interface with GC to ionize sample after it elutes from column • Detect m/z ratio of compounds • Gives better information about chemical identity

  10. Results Gas Chromatograph of an air sample taken from a synthetic organic chemistry lab

  11. Portable GC • Goal: Take instrument to places of interest to monitor air quality • Conduct indoor air sampling studies • Monitoring on-site emission levels • Challenge: Miniaturize components • Minimize power consumption for outdoor uses

  12. Experimental Results Pentane Hexane Heptane Octane Nonane Decane Gas chromatograph for a sample of indoor air collected by SPME analyzed for n-alkanes