topic 1.2 analytical process

8/20/2019 Topic 1.2 Analytical Process

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INTRODUCTION TO ANALYTICAL CHEMISTRY

(PART 2)

THE ANALYTICAL PROCESS

THE STEPS IN

A CHEMICAL ANALYSIS

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Steps of the Analytical Process6 basic main steps in chemical analysis

1. Problem definition - Identify the problem andformulate questions

2. Method selection - Select method of analysis

3. Sampling - Obtain bulk sample and extractsmaller laboratory sample from bulk

4. Sample Preparation - Prepare sample foranalysis

5. Analysis – Analytical measurement of the

analyte in question6. Calculation and reporting - Calculate results,

interpret/present data and draw conclusion

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(1) Problem Definition

Need to translate general questions into specificquestions - to be answered by chemical measurement

Example: Why did Singapore Gov. reject our cropshipments last year?

Formulate the Question: How much pesticide residue is

left on the crop■ What information is needed?

□ What type of sample to be analysed?

□ How sensitive must the method be?

□

What degree of accuracy andprecision required?

□ How are interferences eliminated?

The answers will help determine the specifictechniques to be adopted

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Selection of method depends on anumber of factors:

□ Sample type, sample size & preparationrequired

□ Skill and training of analyst□ Tools/instruments available□ Selectivity, precision, sensitivity required□ Cost (budget) and speed

□ Time required/target deadlines□ Availability of methods or ‘standardmethods’ (chemical literature: Books, journals, manuals, etc)

(2) Method Selection

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‘Standard methods’ of analysis are available inpublished materials such as the following:

Journals• Analytical Chimica Acta Analytical Abstracts• Analytical Chemistry Analyst• Analytical Communications Talanta

• J. of Association of Official Analytical Chemists (AOAC)• Journal of Chromatography Journal of Chromatographic Science• Trends in Analytical Chemistry

Books

ASTM Book of Standards

Official Methods of Analysis of the AOAC

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• Sampling is the process to obtain a smallrepresentative and homogeneous sample- Representative - content of analytical sample

reflects content of bulk sample- Homogeneous – content is the same throughout

the whole sample• Sampling is the most critical step because it can

limit the accuracy of measurements

• Requires storage and preservation steps

• Sampling methods depends on the samples’ - type, size & homogeneity- physical state (solid, liquid, gas)- chemical state

(3) Sampling

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Sampling Step/Designing A

Sampling Plan Sampling plan is a plan that ensures that

a representative sample is collected.

(1) Identify the population to be studied(2) Collect a gross representative sample

from the population

(3) Reduce the gross sample to a laboratory

sample suitable for analysis

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Gross sample – representative portions of the material to betested.

Laboratory sample- a small portions of gross sample andmade homogeneous

Analysis sample- small portionof lab sample that actuallyanalysed

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Sampling Steps

Common sampling methods:Grab/Random Sample – A portion

of sample removed randomly from

the populationComposite Sample - Several grab

samples combined to form a singlesample

In-situ Sampling - Sampling donewithin the population (on-site/without removal) -

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‘Standard methods’ for sampling are available inreference materials such as:

ASTM (American Society for Testing andMaterial)

APHA (American Public Health Association)

AOAC (Association of Official Analytical Chemists

International)

General guideline:- Homogeneous parent samples – Simple ‘grab

sample’ approach taken at random and assumedrepresentative

- Heterogeneous parent samples - Severalsamples have to be taken

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SAMPLING SOLIDS

Problem: Solid materials areheterogenous making sampling difficult

The larger the particle size, the larger

the gross sample should be Best to take 1/50 to 1/100 of the total

bulk or total population

Special sampling techniques are

required to obtain a representativegross sample

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Homogenization of Solid Samples

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The smaller the particle size of the sample, the lowerthe error in analysis

Crushing Pulverizing

Grinding Rendering the sample into a thoroughly

mixed powder

Sample mixtures

Homogeneous samples may becomeinhomogeneous upon standingIt should be thoroughly mixed before analiquot is taken for analysis


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Example sampling techniques to obtainrepresentative gross sample

Stockpile of cereals: take increment fromsurface and work into the interior

Compact solids (metals and alloys): randomdrilling or sawing across the metal at randomintervals and collect the `sawdust’

Bulky material (ore, grain, coal): obtain arandom sample while the material is in motion(eg conveyor belt), and periodically transferportion into a sample container

Shipment/cargo of bagged material (bags ofgrain, cement, etc): obtain a small sample fromeach bag or every 10th bag, and combinesamples

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This is a common method used to reduce thegross sample to a smaller laboratory sample

The sample is crushed and mixed to form aconical pile

The pile is flattened and cut into equal quarters,and two opposite quarters are collected atrandom

The quartering process is repeated until thedesired sample size is obtained

‘Cone and Quarter Method’

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Sediment sampling


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Van Veen grab

sampler Ponar type grab sampler

Bottom Sediment Grab Samplers

These samplers are designed to

collect an accurate representative

sample of the sediment bottom.

Sediment Sampler


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SAMPLING LIQUIDS

Liquid samples are homogeneous, thuseasier to sample

The gross sample can be relatively small Sampling techniques will depend on the

types of liquid

Examples of liquid sampling techniques:• Small quantities of non homogeneous liquid

sample is shaken and sampled immediately

• Large volume of liquids are sampled (i) after

a transfer or (ii) during discharge or (iii) if ina pipe, after passing through a pump whenthey have undergone thorough mixing

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• Large stationary liquids (eg lakes, rivers) aresampled at different depths using a ‘thiefsampler’ (a special device for obtainingaliquots at different level)

• The separate aliquots of liquids can beanalyzed individually or can be combinedinto one gross sample (composite sample)

• For biological fluids, the timing of sampling

is very important (eg, Blood sample iscollected after the patient has fasted for anumber of hours to analyze for sugar)

Continued….

Examples of liquid sampling techniques

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Gases tend to be homogeneous

and a large volume of sample isrequired (b’cause of low density)

Examples- Air analysis: Use `Hi- Vol’

sampler that contain filters tocollect particulates- Liquid displacement method:

Sample must be slightlysoluble in the liquid & does not

react with it- Breath sample: Subject blowsinto an evacuated bag

SAMPLING GASES Devices for gassampling

Air Sampling Filters

Air/Hi VolSamplers

Dust Sampler

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Hi Volume sampler


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SAMPLE STORAGE AND PRESERVATION

• Gross sample must be transported from thesampling point to the analytical laboratorywithout a physical or chemical change in itscharacteristics

• Preservation is can be carried out right at thesampling point or in the laboratory

• Sample preservation help minimize:- physical changes such as adsorption, diffusion,

volatilization- chemical changes such as oxidation and

microbiological degradation

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Method Description

• Keep in sealedcontainers orunder vacuum ornitrogen

• Refrigerate orfreeze or protectfrom light

- Prevent decomposition ofbiological samples frombacterial action orsensitive samples from

oxidation- Protect from light

- Avoid thermal degredationof thermal labile samples

- Prevent loss of water fromhygroscopic sample

- Prevent loss of volatileanalytes from sample

Methods of Preservation & Storage


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Method Description

• Add chemicalstabilizers:antibacterials orantioxidants

- Prevent sampledecomposition bybacteria

- Prevent sample oxidation

• Adsorption on asolid phase

- To immobilize orstabilize the analyte

• Store inappropriatecontainers

- Teflon (PTFE) for ionicanalyte

- Glass for organic analyte

• Acidify (Add 10%HNO3 uponcollection, pH


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Sample preparation (sample pretreatment) is a step

in chemical analysis where the sample is broughtinto the correct size or form for analysis

General principles: Determine amount (Weight or Volume) Prepare ‘replicate’ samples for statistics- Replicates – samples analyzed in the same way, same

size, at the same time

Bring the analyte into the best chemical form forassay method used – Dissolve in solution

Bring analyte into the best concentration rangefor the chosen method

Pretreat, separate, pre-concentrate, eliminateinterferences

(4) Sample Preparation

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Maximize RECOVERY During Sample Preparation

Recovery:

A measure of the amount of analyte in the assayrelative to the amount of analyte in the sample

Percentage Recovery or % recovery= (analyte concentration in assay) x 100

(analyte concentration in sample)How analytes can be lost from the sample duringsample preparation?

- Absorption (eg. metal ions can absorb on the glass

surface, organic compounds can absorbed on plasticcontainers)- Decomposition (eg. organic compounds can

decompose by oxidation)

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Sample Preparation for Solids

(1) Grind and sieve to a suitable size to obtainrequired homogenous size

Homogenize

- Solid sample is placed in a blender (solvent

may be added) and sample is homogenized tofinely divided particles

- Useful for plant and animal tissue, food, andenvironmental samples

- Aqueous or organic solvent can be used- Finely divided particles promote moreefficient extraction


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Sample• Inorganic sample• Common organic

sample• Biological sample• Hygroscopic sample

or oxidizable sample• Heat sensitive

sample

Drying Conditions

- Heat at 110oC- Depends on sample

(removes vapors)- Heat at


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(3) Solubilize the sample in solution (aka dissolution)

(i) Simple dissolution - solid is dissolved in solventwithout chemical change(ii) Wet Digestion (aka acid treatment) - Heat

sample with mineral acids in open/closedcontainer or in a microwave digester

(iii) Fusion Technique - Heat with flux or acid untilmolten state(iv) Dry Ashing - Oxidize by slow heating in oxygen

at very high temperature (400-700oC in furnace)

Destructive - harsh methods that destroys the

sample matrix (for inorganic analyte or analytethat can be converted to inorganic derivative)Nondestructive/partially destructive - mild or

non evasive dissolution (for organic analytes)

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Wet digestion/Acid Treatment

Low cost but may lose analyte by volatilization

HCl : Carbonates, phosphates, oxides H2SO4 : Organic material at 300 C HNO3 : Any metals not dissolve by HCl HClO4 : Steel

HF : Silica Aqua Regia (3:1, HCl:HNO3) for most inorganic HNO3+HCl+HF (5:15:3) for alloys

Grades of acids Very High Purity Chemicals eg Ultra-Pure (NBS) Analytical Reagents eg Certified AR TM (Fisher) Chemically Pure (CP) eg CPTM (Sigma) Practical Grade eg PurifiedTM (Sigma) Commercial or Technical Grade

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Microwave DigestionRapid and efficient acid decomposition using

microwave energy in a specially designed oven

Advantages- Sample contained within the digestion vessel- Highly efficient and rapid (5-10 min)- Volatile elements are retained in reaction

vessel- Easy to automate: a computer controls the

pressure and the temperatureDisadvantages

- Inability to add reagents during the digestion- Limited amount of sample (typically 1 g or less)- Safety concerns due to the use of high

pressures and corrosive reagents

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Fusion Techniques

1. Inorganic samples are mixed with largeexcess of alkali metal salts known as flux(at a ratio of 1:10-20, sample:salts) in acrucible and heated at a high temperatureuntil the substance fuse together in a

molten state2. The melt is allowed to cool at room

temperature and dissolved in dilute acidor water

(Used when acids fail to dissolve sample,eg. silica, mineral oxides, steel)

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Types of flux

Base flux: Na carbonates, hydroxides,borate for alkaline : pyrosulfates, boricoxide, fluoride acids

Oxidizing Flux: Sodium peroxide or

nitrate/alkaline metal + Sodium CarbonateDisadvantages: Contamination by fluxmaterial, high salt content may complicateanalysis, high temperature result in loss of

analyte (evaporation), sample containermay react with flux material

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Several sources of error are encounteredin the sample dissolution step

Incomplete dissolution of the analyte

Losses of analyte by the volatilization

(evaporation) Introduction of external analyte from a

solvent (solvent contamination)

Contamination from the reaction of the

solvent with vessel walls

Errors In Sample Dissolution

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a) Use a selective analytical technique (egion selective eletrode)

b) Use selective derivatization that converts

the analyte into another chemical speciesthat can be measured more easily

c) Use Standard Solution of high-puritystandard materials

d) Remove the analyte from the samplematrix by a separation or extractionprocess

To Measure Analyte containing

Interfering species


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Separation of analyte from matrix -

Eliminate interferences from samples* Provide suitable selectivity Required if many analytes are present Cause preconcentration of analyte - good for

more sensitive or accurate measurementPreconcentration -

Required if amounts of analyte(s) too small inbulk material (eg trace element analysis)

Extraction + preconcentration (10-100 times)- Improves sensitivity- Eliminate interferences from the sample matrix*

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Separation and Preconcentration


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Soxhlet extraction Liquid-liquid extraction

Chromatography : Gas chromatography,liquid chromatography, gel permeation

chromatography, TLC, Ion exchangechromatography, Paperchromatography

Electrophoresis : liquid/solid

Dialysis

Precipitation : liquid/solid Addition of Masking agent

Summary of Separation Techniques


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Soxhlet Extraction

Classical extraction method(Apparatus named after developer)

• Place sample in porousthimble

• Exhaustive reflux for

up to 1 - 2 days• Solution of analyte(s)

in volatile solvent(eg. CH2Cl2 , CHCl3 etc)

• Evaporate to dryness

or suitableconcentration forseparation/analysis

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Dilution and Matrix Matching

After dissolution and separation, the sample

may not be suitable for direct measurement

(1) The instrument or method requires a particulartype of sample form, concentration, solvent,oxidation state, etc

(1) Some samples require modifications:- pH adjustment with acid/base/buffer- Adjustment of oxidation state using anappropriate oxidising agent

- Dissolving in acid (trace metal analysis byspectroscopic techniques often requresnitric acid to avoid mass-spectralinterferences with chloride solution)


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Measurement is often the simpleststage of the analytical process

Use reagents of high purity (eg. analyticalgrade, reagent grade, etc)

A ‘blank’ measurement must be performedfor trace analysis (The results of the blank willbe subsequently subtracted from the rawanalytical measurement)

Choice of Analytical Measurements:- Classical methods- Instrumental methods

(5) Analytical Measurement

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Classical methods (gravimetric, volumetric)- Accurate and precise to approx. 0.1% but

require large (mmol, mg) amount of analyte

Instrumental method (Spectroscopic, HPLC,GC, etc)

- More selective and sensitive than classical

methods but less precise, accurate toapprox. 1%

- Based on measurement of the physical orchemical property of an analyte directlyrelated to the concentration

- Rapid, may be automated and may be usedfor the determination of multiple analytesat a time

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(1) Determine the concentration of theanalyte in the analytical sample solution(2) Use results to calculate the amount of

analyte in the original (bulk) sample Evaluate the results

- Requires appropriate use of statistics- Must be reasonable, reliable and related to theproblem as originally stated

(6) Calculating Results and Reporting

Data presentation must be understood,conclusions clearly shown

Report results with accuracy & precision(include standard deviation, mean value, etc) Verify reports (Professional/charted chemist)

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ILLUSTRATIONS ANDPHOTOGRAPHS

OF SAMPLINGTECHNIQUES

AND APPARATUS

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Consider the different

flowrates, mixing andturbulance (eg rainy anddry season)

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GOOGLE EARTH

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In-situ testing(Parameters such as pH, DO, turbidity)

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Composite WaterSampler (with

rechargable battery, ableto adjust to desiredsample: 25 mL to 600mL, time 5 min to 4 hrs)

Grab Sampler(Van Dorn Sampler)

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Water Sampling(How much water do you need?)

Depends on parametersto be analyzed

Chlorophyll and TSS oftenrequire the greatestvolume (> 1L)

Better be safe to havetoo much water ratherthan too little

Depends on the system

and how practical it isto carry out largevolumes of water

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Water sampling(Microbes)

Grab samples takenwith sterile containers

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Examples of Gas Samplers

Underground gassampler

(sampling gas leaks,methane decomposition

in landfills, etc)

Hi-vol sampler[Particulate (PM10) and

microbes on filters]

Gascanister

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