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