Chap 4. Water Analysis-Trace Pollutants

Analysis of the constituents of water found at ug/L

(Organic chemicals and Metal)

• Organic trace pollutants

- Naturally occurring compounds from decaying organic material

- Pollutants discharged into the environment

- Degradation and inter-reaction products of the pollutants

-substances introduced during sewage treatment

• types of organic compounds

- pesticides, PCBs, dioxin etc

General Guidelines of Environmental Sampling Techniques

Sample Preservation and Storage Purpose – minimize physical, chemical and biological changes 3 approaches:

Refrigeration, Use of proper sample container, Addition of preserving chemicals

Storage of samples- The volatility of organic compounds ; filled and kept at sub-ambient T- Microbial degradation : storage below 0oC- Photolytic decomposition : store in dark- Contamination from the container : glass bottle- Maximum Holding Time (MHT) is the length of time a sample can be stored after collection and prior to analysis (refer to Korean standard method)

- Experiment in a clean laboratory (free from the analytes)

- Prevent contamination of stock solvent,

- prevent contamination of samples and working standards

- Use pesticides free grade solvent

- Clean the glassware

QA/QC

Need Method blank

Field blank ; this undergoes the full handling and shipping process of an actual samples. to detect sample contamination that can occur during field operation or during shipment.prepare with certified clean water or clean sand or soil in the field

What is GC? GC ; gas chromatography (GLC ; gas-liquid chromatography)

-Mobile phase ; gas -Stationary phase ; microscopic layer of liquid or polymer on an inert

solid support

Chromatography ; The collective term for a family of laboratory techniquesfor the separation of mixtures

• Gas Chromatograph ; Equipment

Schematic diagram of GC

Carrier gas- inert gas like nitrogen, helium, hydrogen etc

Naphthalene-d8 (IS)

NaphthaleneAcenaphthylene

Indeno[1,2,3-cd]pyrene

(M.W., 128) (M.W., 152) (M.W., 276)

• Gas Chromatography

Carrier gas injector column detector

• Separation ; differential retention of the components between stationaryphase and a mobile phase

-Volatility of compound: Low boiling (volatile) components will travel faster through the column than will high boiling components

-Polarity of compounds: Polar compounds will move more slowly, especiallyif the column is polar.

-Column temperature: Raising the column temperature speeds up all the compounds in a mixture

-Column packing polarity: Usually, all compounds will move slower on polar columns, but polar compounds will show a larger effect

He, N, H2

-Flow rate of the gas through the column: Speeding up the carrier gas flowincreases the speed with which all compounds move through the column

-Length of the column: The longer the column, the longer it will take all compoundsto elute. Longer columns are employed to obtain better separation

• Columns and Stationary phases- Consider stationary phase and column dimension

; affect separation efficiency - Column types

1. narrow-bore capillary columns ; 30~60m, 0.2mm id, 0.4ml/min

2. Wide-bore capillary column ; 15~30m, 0.53mm id, 25ml/min

3. Packed column ; 2m, 2mm id, 20ml/min

The greatest detection sensitivity, suitable for mass detectorDirect injection is not possible (overload), need splitting device

Large sample capacity, direct syringe injection

Less affected by contamination from non-volatile componentsin the sample so suitable for highly contaminated samples

• Column bleeding ; background signal generated by the columnstationary phase

* Use two columns of different polarities for resolving compounds

Two dimensional GC (GC X GC)

• some compounds are needed a derivatization for analysisPolar N-H and O-H groups on which give hydrogen bonding may be converted to relatively nonpolar groups on a relatively nonvolatile compound. The resultant product may be less polar, thus more volatile, allowing analysis by GC. (ex ; alkylation)

• Detectors

-FID ; micro hydrogen/oxygen burner that continuously maintain a hydrogen flame sensitive universal detector for organic compounds(suitable for hydrocarbon) nearly universal, destructive, sensitive with dynamic range

-Thermal Conductivity Detector

-TCD ; universal, non-destructive, insensitive, limited dynamic rangeMeasurement of major constituents of air (H2O, CO, CO2, H2)

- ECD ; highly sensitive specific detector responding to atoms with a high electronaffinity like halogen, nitro and some other oxygen containing functional groupsvery selective and sensitive, limited dynamic range

Electron capture detector Nitrogen phosphorus detector

-NPD ; selective for organic compounds with N or P atoms, destructive, less dynamic range than the FID

• Detectors

-FID ; micro hydrogen/oxygen burner that continuously maintain a hydrogen flame sensitive universal detector for organic compounds

-ECD ; highly sensitive specific detector responding to atoms with a high electronaffinity like chlorine

-TCD ; universal detector

-Hall electrolytic conductivity detector (EICD); highly sensitive specific detector for halogens, nitrogen and sulpher

- Thermionic detector ; element specific detector for nitrogen and phosphorus

- Flame photometric detector ; sulphur and phosphorus

- photo-ionisation detector ; aromatic rings or double bonds

-mass spectrometric detector ; highly specific and sensitive detection for all organiccompounds

Comparison of GC and LC

All gases and volatile compounds can be analyzed via GC not by HPLC

Nonvolatiles and thermally unstable compounds cannot be analyzed via GC unless their structures are changed through derivatization

For some semivolatile compounds (e.g. PAHs, nitroaromatics and explosives) both GC and HPLC can be used

HPLC is preferred when direct analysis of aqueous sample is needed to avoid time consuming extractions

In other cases, GC is instrument of choice due to variety of detection methods

• Extraction procedures1. GC columns; Incompatible with water2. Selective towards the analytes

3. Direct injection can cause a deposition of non-volatile solids on column4. Lower detection limit

• Solvent extraction ; hexane and light petroleum etc• Headspace analysis ; remove solvent interference, sensitivity depends

on volatility of compounds (increase the T)• Purge and trap ; extract VOCs by using purge gas stream• solid phase extraction

• Analysis of Individual Trace Pollutants

1. extraction2. Clean-up to remove interfering components3. Concentration of extract

• Analysis of DDT- Technical DDT ; p-p’ DDT(70~80%), o,p’-DDT (15~20%), p,p’-DDD(1~4%)- Decomposition products ; p-p’-DDE, p-p’-DDD

1) Extraction with hexane(increase 40 fold conc by extraction of 2L samples into 50ml solvent)

2) Drying the solvent using a column containing Na2SO43) Concentration of the extract to 1ml4) clean-up of extracts

• Clean-up of extract(a) Alumina (Al2O3)column- Alumina is polar column material and retain polar compounds - AgNO3 ; retain compounds containing unsaturated C-C bond

(b) Silica gel (SiO2) column- Less polar column than the alumina, separate non-polar interference- Hexane eluent ; remove PCBs, DDTs elute with more polar solvent

• Clean up process; simplify the subsequent chromatogram,protect column and detector from contaminants

• Metal ions - Found in the ppb to ppm range (mostly Na, K. Ca, Mg)

Fe, Mn, Zn ; reach towards the ppm, the others ; lower end of this range

- Occur naturally from leaching of ore deposits, also from anthropogenic sources

• Analysis techniques - Flame atomic absorption spectrometry (Flame AAS)- Flameless atomic absorption spectrometry (Flameless AAS)- Inductively coupled plasma optical emission spectrometry (ICP-OES)-Inductively coupled plasma mass spectrometry (ICP-MS)

• Storage of metal samples-Polyethylene bottle are better than glass bottle (except Hg ; glass bottle)

- Acidified to minimise precipitation of metal ions( addition hydrochloric acid)

- Cleaning bottle ; acid washing

• Pretreatment-Evaporation to dryness, dissolution of the residue in acid, partial evaporation with acid, digestion with acid etc ; to dissolve suspended material and make free metal ions

- For modern techniques ; need extraction/concentration Formation of neutral complex and extraction this with organic solventUse of chelating or ion-exchange columnsUse SPE

Atomic Spectroscopy for Metal Analysis

Flame and Flameless Atomic Absorption

Basic instrument components:

Element specificHallow-cathod lamp

Flame

(burner)monochromator detector

SampleAcetylene + air/nitrous oxide

-Hollow cathode lamp ; the most common radiation source, tungsten anodeand a hollow cylindrical cathode made of the element to be determined each element has its own unique lamp.

-Flame ; atomise sample (ground state), absorbing radiation from the lamp

Monochromator: Isolates photons of various wavelengths that pass through the flame or furnace.Similar to the monochromator in UV-VIS spectroscopy in that it uses slits, lenses, mirrors and gratings/prisms

Flame and Flameless Atomic Absorption

Basic instrument components:Nebulizer and atomizer: In a flame system (a), the nebulizer sucks up the liquid sample, creates a

fine aerosol, mixes the aerosol with fuel/air. Flame creates vaporized atoms.

In a flameless graphite furnace system (b) both liquid and solid samples are deposited into a graphite boat using a syringe inserted through a cavity. Graphite furnace can hold an atomized sample in

the optical path for several seconds– results in higher sensitivity of the GFAA compared to FAA

• Flameless atomic absorption spectrometry (graphite furnace AAS)

- One major source of error ; background interference due to light scatteringby solid particles within the beam

AAS ; determine one element at one time, limited the linear range

ICP-OES(optical emission spectrometry), ICP-MS

• ICP-OES- ICP (plasma) is an ionized gas at extremely high temperature

- atomise in a plasma flame at 6000 ~ 10000K.- Sample is nebulized and entrained in the flow of plasma support gas (Ar)- ICP can theoretically analyze almost all elements- ICP can also measure multiple elements simultaneously- some problem of spectral overlap from different elements- sensitivity ; varied with element types, between those of flameand graphite furnace AAS

- wide linear range (about 105)

• ICP-MS- separate according to their mass/charge ratios- sensitivity is lower than that of graphite furnace (below 1ppb)

http://www.chemiasoft.com/chemd/node/52

Cost:FAAS < GFAAS > ICP-OES << ICP-MS

• Anodic Stripping Voltammetry

- Particular use in environmental analysis (at least #19 metals)- Consist of three electrodes (working, reference, counter)

M2+ + 2e- M (working electrode)(the quantity of metal deposited on electrode conc of metal)M M2+ + 2e- (at specific potential)

Bubbled N or Ar to remove dissolve oxygen

-Method is slow (30s ~ 30min for one sample)measure only free metal ion and some loosely associated complexes sample pretreatment is necessary to analyze total metal

www.shinhantech.co.kr

reduction

oxidation

counter

• Metal Speciation-Different physical and chemical forms of a substances(ex ; free metal ions, colloidal forms, adsorbed or suspended particles etc)

-Transport of each in environment is different - Have a different toxicological properties

(ex; metal complex << free metal ion, Cr2O72- >> Cr3+ )

• Analysis technique-AA ; all the metal species-Visible absorption spectrometry ; free metal ion + ions released from

complex by colour forming agent-ASV ; free metal ion + ions released from complex-Chromatography ; Non-labile species can sometimes be determined

separately

• alternative simple method -Organic solvent soluble (neutral complex, organometallic species)- Organic solvent insoluble (charged complex, free ions)