Gas Chromatography in the detection of Volatile Organic Compounds

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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

Common Volatile Organic Compounds


Formaldehyde
Most common
          

Methylene Chloride
Animal carcinogen  (converted to carbon
monoxide in the body


Benzene
Human carcinogen
                                                                   

Perchloroethylene
Used in dry cleaning

EPA has set the permissible exposure level FOR VOCs 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

Gas Chromatography

Separate and detect compounds in gas phase
Can analyze liquids if they can be volatilized without decomposition

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

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

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

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

Results

Gas Chromatograph of an air sample taken from a synthetic organic chemistry lab (Chai and Pawliszyn 1995)

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

Experimental Results

Gas chromatograph for a sample of indoor air collected by SPME analyzed for n-alkanes (Jia et. al. 2000)

References

Chai, M and Pawliszyn, J.  “Analysis of Environmental Air Samples by Solid-Phase Microextraction and Gas Chromatography/Ion Trap Mass Spectrometry.”  Ennvironmental Science and Technology (1995): 693-701. 

Jia, M., Jacek, K., and Pawliszyn, J.  “Fast Field Sampling/Sample Preparation and Quantification of Volatile Organic Compouns in Infoor Air by Solid-Phase Microextraction and Portable Gas Chromatography.”  Field Analytical Chemistry and Technology (2000): 73-84.

http://www.cdsanalytical.com/instruments/headspace/why_headspace.html

http://www.chromatography-online.org/topics/electron/capture/detector.html

http://www.epa.gov/apti/course422/ce4b3.html

http://www.epa.gov/iaq/voc.html

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm