Transcript Field Methods of Monitoring Atmospheric Systems

Field Methods of Monitoring
Aquatic Systems
Unit 11 – Organics
Copyright © 2006 by DBS
Organic Trace Pollutants
Include
• Naturally occurring compounds from decomposition of OM
• Anthropogenic pollutants
• Degradation and inter-reaction products of pollutants
• Substances derived from sewage treatment
Typical analysis:
• Individual compounds or groups of compounds
• Total analysis of all organic components
• Field screening for specific pollutants prior to lab analysis
• Qualitative identification of trade products in spills and
discharges
Question
Compounds causing widespread environmental problems
include toxicity, resistance to biodegradation and
bioaccumulation. List the types of organic compounds which fall
under these catagories
Pesticides, chlorinated solvents, PCBs. dioxins, edochrine
disruptors
Techniques
GC
• Gas Chromatography – organic compounds have significant
volatility
HPLC
• High Performance Liquid Chromatography – increasing in use
UV-Vis Spec
• Ultraviolet-Visible Spectrometry – useful for groups of
compounds since absorptivities vary little between similar
compounds
Storage Considerations
(a)
(b)
(c)
(d)
(e)
Volatility – containers should be completely filled and kept at
sub-ambient temperatures
Microbial degradation – store at < 0 °C to lower further
Photolysis – store in the dark (e.g. organochlorines)
Contamination – glass should be used (no plastics)
Adsorption – analyze ASAP (low-solubility organics)
Volume Considerations
• Depends on concentration of analyte
• Analysis may require microliters
• But…sample may need several extractions
Precautions - Eliminating Contamination
(i) Perform analysis in lab free from analyte
(ii) Solvents should be kept away from work area
(iii) Samples and standards should be kept well away
from stock solvents
(iv) Pesticide-free grade solvents should be used for
analysis
(v) Glassware must be cleaned thoroughly
Extraction Methods for GC
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Extract into organic solvent
Separates unwanted components
Separates minor components which may interfere
Concentrates the components of interest
Method depends on:
(i) Properties of compounds determined and potential interference
(ii) Choice of GC or LC as separation method
(iii) Possibility of solvent-free methods
(no contamination/health effects/waste)
(iv) No. of samples
(v) Field extractions
Extraction Methods
(a) Solvent extraction, (b) solid-phase extraction – cartridge,
(c) solid-phase extraction – disc, (d) head-space analysis,
(e) purge and trap, (f) solid-phase microextraction – direct,
(g) solid-phase microextraction – head-space
(a) Solvent Extraction (liquid-liquid)
• Sample is shaken with immiscible organic solvent
• Hexane or petroleum ether (most common)
• pH may be altered to favor acidic or basic components
RNH2 + HCl
RNH3+ Cl-
amine (soluble in np solvent)
amine hydrochloride (less soluble in np solvent)
RCO2H + NaOH
RCO2- Na+
carboxylic acid (soluble np solvent)
carboxylic acid salt (less soluble np solvent)
(b + c) Solid-phase Extraction
Disposable column 100-500 mg
adsorbent material (e.g. Florisil)
– Column first conditioned with
methanol
– Sample gravity or vacuum
filtered through column
– Wash with water etc. to remove
interfering compounds
– Dry by passing air through
column
– Elute with a few mLs of suitable
organic solvent (concentrates x
100)
e.g. hexane used for non-polar
pestcides
Question
What are the advantages of solid-phase extraction over liquidliquid extraction?
Rapid process
High concentration factors
Less solvent used
May have problems with high solid content or OM content
which block the column
(d) Head-space Analysis
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Water sample in container with septum seal
Air sample with volatile components is injected into
GC
No solvent interference
Favors low molecular-mass neutral species
(e) Purge and Trap
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Volatile organic content are extracted
using N2 gas and collected in a tube of
adsorbent material (TENAX resin)
Trap tube is flash-heated releasing
organics
May be second liquid N2 trap
(f + g) Solid-Phase Microextraction
(SPME)
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Fused silica fiber coated in non-volatile polymeric coating
(polydimethylsiloxane or polyacrylate)
Dissolved components partition onto fiber
Fiber retracts into syringe for storage
Thermally desorbed into GC
Useful technique for polar organics (e.g. phenols)
Pros:
– 100-700 x lower detection limits
– Solvent free
Cons:
– Equilibration texhnique
– Each compound extracts differently
– Changes in sample composition may
affect extraction equilibria
Source: http://www.science.uwaterloo.ca/chemistry/pawliszyn/Research/SPME/spme.html
SPME
http://www.sigmaaldrich.com/Brands/Supelco_Home/Spotlights/SPME_central.html#spmeanim
Gas Chromatography
• Chromatographic separation
by differential partition of
components between
stationary phase (liquid
adsorbed or bonded to a
solid) and mobile phase
(gas)
• High separation efficiency
• Highly sensitive detectors
Detectors
Detector
Applications
Thermal Conductivity
Universal detector for organics
Flame Ionization
Universal detector for organics
Electron Capture
Specific to atoms with high e- affinity e.g. Cl
e.g. chlorinated pesticides, PCBs
Hall electrolytic conductivity
Specific for halogens, N and S
e.g. pesticides, trihalomethanes
Thermionic
Element-specific for compounds containing N and P
e.g. pesticides
Flame photometric
Element-specific for compounds containing NS and P
e.g. pesticides
Photo-ionization
Specific tocompounds containing aromatic rings or double bons
e.g. solvents
Mass spectrometric
All organic compounds
Columns and Stationary Phases
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Narrow-bore Capillary Columns
Wide-bore Capillary columns
Packed Columns
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Compounds of interest have high M and low volatilities, require high
temperatures
Best separation when stationary phase has same polarity as analyte
– Fuel oils on non-polar columns
– Pesticides and chlorinated solvents on medium-polarity columns
– Dioxins on highly polar columns
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Decreasing
separation efficiency
Question
How would you confirm that a peak is due to a single
component rather than two components with identical retention
times?
Chromatogram should be made on two columns of differing
polarities
Many procedures specifiy the use of a second confirmational
column
Example Procedures
• Most involve extraction prior to analysis
– Separate interferences
– Concentrates
– GC would get blocked if injected with water
• For analysi of individual components expected at low
concentrations (e.g. pesticides) further pre-treatment may be
necessary
e.g. DDT
p,p’-DDT 70-80%
o, p’-DDT 15-20 %
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DDT is a mixture
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Environmental scamples
would also contain metabolic
breakdown products
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p, p’-DDE
p, p’-DDD
Multi-component mixture
even without other species
expected in water!!!
Possible interference from
PCBs, similar pesticides etc.
o,p’-DDD
p,p’-DDD 1-4 %
DDT - Pretreatment
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Hexane extraction into 3 aliquots of 30 mL of solvent
Drying extract in a 5 g column Na2SO4
Concentration to 1 mL using an evaporator
Clean-up of extract by colum chromatography
– Al2O3 (removes polar components)
– AgNO3 (retains unsaturated C compounds)
– Na2SO4
• Concentration to 1 mL using an evaporator
• Overall C.F. x 100
detection limit ng L-1
Question
Clean-up of the extract simplifies the chromatogram. What is a
further advantage?
Protection of the column and detector from contamination
Quantification
• External standards
– Compare the peak area of unknown with areas of a series of
solutions used to form a calibration curve (library)
– Not ideal as injection volumes may be different
• Internal standards
– Produces peak close to but resolved from unknown species
– Variation in volume shows up as change in peak area of
internal standard
– Results are normalized to internal standard peak area
– Also useful to quantify losses during pretreatment and
calculate %-age recovery
HPLC
• Fluorescence detection of
PAHs
• Typical extracts would
contain up to 70 PAH’s ~ 1
μg L-1
• Require preconcentation via
solid-phase extraction
• Some pestcides may be
analyzed via post-column
derivatization
• Pestcides may be detcted
via less sensitive UV
Text Books
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Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil.
Wiley-VCH.
Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker.
Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods.
CRC Press.
Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis
Publishers.
Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books.
Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative
methods. Taylor & Francis.
Reeve, R.N. (2002) Introduction to environmental analysis. Wiley.
Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the
Examination of Water and Wastewater, 20th Edition. Published by American Public Health
Association, American Water Works Association and Water Environment Federation.