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) Volatility – containers should be completely filled and kept at sub-ambient temperatures

(b) Microbial degradation – store at < 0 °C to lower further(c) Photolysis – store in the dark (e.g. organochlorines)(d) Contamination – glass should be used (no plastics)(e) 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

• 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 liquid-liquid 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

• 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

• 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)

• 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. Cle.g. chlorinated pesticides, PCBs

Hall electrolytic conductivity Specific for halogens, N and Se.g. pesticides, trihalomethanes

Thermionic Element-specific for compounds containing N and Pe.g. pesticides

Flame photometric Element-specific for compounds containing NS and Pe.g. pesticides

Photo-ionization Specific tocompounds containing aromatic rings or double bonse.g. solvents

Mass spectrometric All organic compounds

Columns and Stationary Phases

• Narrow-bore Capillary Columns

• Wide-bore Capillary columns

• Packed Columns

• 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

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

• DDT is a mixture

• Environmental scamples would also contain metabolic breakdown products

• Multi-component mixture even without other species expected in water!!!

• Possible interference from PCBs, similar pesticides etc.

p,p’-DDD 1-4 %

o, p’-DDT 15-20 %

p,p’-DDT 70-80%

o,p’-DDD

p, p’-DDD

p, p’-DDE

DDT - Pretreatment

• 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

• 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.