va samplepreparation

7/28/2019 VA SamplePreparation

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V o l t a m m e t r y

Metrohm Ltd.CH-9100 Herisau Switzerland

Sample preparationfor Voltammetry


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V


Trace Analysis

Analysis Procedure

SamplingSample StorageSample Preparation

MeasurementEvaluation


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Sampling

Analysis is only as good as the sample

Make sure sample is representative

Be aware of contamination risk


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Sample Storageand Sample Preparation

dependent on analytical problem

total metal concentrationspeciationanalysis of anions or organic substances

following sample preparation methods onlysuitable for determination of total metal concentration


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Sample Storage

Sample Container:

bottles from high-pressure (low density)polyethylenePTFE

Liquid samples:acidifying to pH 2.0 or lower prevents loss of metals due to adsorption on wall of samplecontainer (not good for speciation studies)

Store at max. 4C


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Requirements for Analyte Solution

for successful and correct analysis

Analyte must be in ionic form:Cd 2+ or Pb 2+

The solutions have to be free of interferingsubstances


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Interferences in VA

1. substances producing interferingvoltammetric signals:

oxygen, H 2O 2 organic compounds (e.g. nitrocompounds)anions (nitrite, sulphite, etc.)

2. substances complexing the analyte ionscomplexing agents (hydroxy carboxylicacids, humic acids, sulphide, EDTA, etc.)

3. substances blocking the electrodesurface, i.e. in AdSV

surfactants



Interferences in VA

most interferences are caused by organic

compounds

decomposition of the organic matter bysample preparation techniques due tooxidation processes

by oxygenby oxidising reagents (acids, peroxides, ...)

by radicals induces by irradiation



Requirements for decomposition

Complete liberation of the analytes

destruction of metal complexes andorganometallic compounds

Complete removal of compounds whichinterfere voltammetrically

usually by oxidation



Requirements for decomposition

Avoid sample contamination from

decomposition reagents and vesselsuse of ultrapure reagents and water, min.suprapur

No loss of analytes

some metals are volatile at temperatures



Classification of samples

Inorganic samplesOres, salts, acids, lyes

Organic samplesPlant tissue: leaves, seedsanimal tissue: blood, etc.products from chemical industry

Water samples, soils:treated as organic samples when organiccontaminations are present, e.g. waste

water



Sample Preparationfor Inorganic Samples

no destruction of organic matter necessary

usually chemical dissolution of the sample



Sample Preparationfor Inorganic Samples

Dissolutionin water

in acids or basesin complexing agents

Extractionalternative method for hardly soluble

samples (soils)only mobile metals are determined Fusion

Alkaline: NaOH, KOH, Na 2CO 3 Acidic: KHSO 4



Digestion for organic samples

Dry ash ing

High temperature Muffle furnaceOxygen flask (Schniger, Wickbold)

Low temperature

Plasma ashing (60C - 150C)



Dry ashing methods

Muffle furnacesample with low water contenteasily oxidisable samplese.g. plants leaves not for volatile elements, e.g. Hg, As

Oxygen flask (Schniger, Wickbold)combustible materialpure organic substances

Plasma ashing

previously dried material, i.e. freeze-dried



Digestion for organic samples

Wet ash ing

Low temperature UV IrradiationHigh temperature

Oxidising acid mixtures:

HNO 3/H2SO 4, H 2SO 4/H2O 2



705 UV Digester

Elimination of low to

moderate amounts of dissolved organic matter (DOM)Photolytic generation of

OH-radicals705 UV Digester:

wet partcontrol unit



Advantages of UV Digestion

Up to 16 samples in one run

Very low amount of chemicals used (H 2O 2 and HCl)Very low contamination risk

Main reagent: UV irradiation (nocontamination !)

All kind of waters digested in 90 minalso suitable for volatile metals, e.g. Hg, As



Wet ashing at high temperature

Pressure :

Open system (atmospheric pressure)for non volatile metalsClosed system (up to 350 bar)

for volatile elements, e.g. Hg, As

Heating :Conventional heatingHeating with microwaves microwave digestion



Most importantdigestion methods in voltammetry

Digestion with 705 UV Digester for all kind

of water samplesPressure ashing with microwave or conventional heating in closed vesselWet acid digestion with H 2SO 4/H2O 2 inopen vessel



Have a look at ...

Metrohm Monograph

Sam ple Preparation Tech niq ues in Vol tam m etr ic Trace An alys is

va samplepreparation

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