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STEPS IN A QUANTITATIVE ANALYSIS

Define the Problem Select a Method

Select a method

Sample Preparation

Sampling

Eliminate

Interferences

Calculate the Results

and Report Writing

Perform the

Measurement

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DEFINITION OF THE PROBLEM

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To determine the best method of analysisthe analyst should ask the following questions.

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SELECT A METHOD

Factors to consider include:

Accuracy Detection limit

Time Cost/number of analyses

Complexity of sample - selectivity Equipment Technical expertise

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LIST OF JOURNALS

Analytical Chimica Acta Analytical Abstracts Journal of Analytical Chemistry Analytical Communications Analyst Journal of the Association of Official Analytical

Chemists

Journal of Chromatography Journal of Chromatographic Science Talanta Trends in Analytical Chemistry

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LIST OF BOOKS

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SELECTING AN ANALYTICAL METHOD

Performance CharacteristicsCriteria used to compare which of severalinstrumental methods would be the best for a

particular analysis.e.g. Limit of Detection.

Figures of MeritQuantitative (numerical) measures of

performance characteristics.e.g. LOD of Pb (AAS) = 15 ppb

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LOD COMPARISON

(g/L) / ppb

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VALIDATION OF ANALYTICAL METHOD

(METHOD VALIDATION)

Analysis of Standard Samples (SRM)Analysis by Other Methods

Standard Addition to the Sample

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VALIDATION OF ANALYTICALMETHOD

Analysis of Standard Samples- A sample whose analyte concentration is

known.

- The standard reference material (SRM)

can be obtained from The National InstituteOf Standard and Technology (NIST).

- The analyte concentration in the SRMhas been certified by the institute.

- Compare the data obtained from the

method with the certified value.

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Analysis by Other Methods- The result of the analytical method can be

evaluated by comparison with data obtainedfrom a different method.

Standard Addition to the Sample- The known amount of the analyte is added

to the sample and then analyzed by the

proposed method. The effectiveness of themethod can be established by evaluating

the recovery of the added quantity.

- The standard addition method will revealerrors arising from the way the sample was

treated or from the presence of the other

compounds in the matrix.

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SAMPLING

Sampling is the process to get arepresentativeand homogeneoussample.

Representativemeans that content ofanalytical sample reflects content of bulksample.

Homogeneousmeans that the analyticalsample has the same content throughout.

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OBTAIN A SAMPLE DEPEND ON

The size of the bulkto be sampled. The physical stateof the fraction to be

analyzed (solid, liquid, gas) The chemistry of the materialto be assayed.

(Nothing can be done that would destroy oralter the identity or quantity of the analyte)

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SAMPLING BULK MATERIALS

Identifythe population from which the sampleis to be obtained.

Collect a gross sample that is trulyrepresentative of the population being

sampled.

Reducethe gross sample to a laboratorysample that is suitable for analysis.

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SAMPLING METHODS

Methods of sampling are given in standard referencebooks.ASTM (American Society for Testing and Material)

APHA (American Public Health Association)

AOAC (Association of Official Analytical ChemistsInternational)

Homogeneous samples present no problem, a simplegrab sample approach taken at random and

assumed to be representative.

Several samples have to be taken if parent sample isheterogeneous.

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SAMPLING SOLID

Inhomogeneityof the material, make sampling ofsolids more difficult.

The easiest way to sample a material is grabsample,the sample taken at randomand assumed

to be representative. For reliable results, it is best to take 1/50 to 1/100of

the total bulk. The larger the particle size, thelarger the gross sample should be.

The gross sample must be reduced in sizeto obtaina laboratory sample.

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EXAMPLES

Stockpile of cereals:take increment from surface andinterior.

Compact solids (metals and alloys):obtained byrandom drilling or by sawing across the metal atrandom intervals and collecting the `sawdust as thesample.

Obtaining a random sample from a bulky material(ore, grain, coal)can be achieved while the materialin motion (conveyor belt). Periodically transferportion into a sample container.

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SAMPLING SOLID

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SAMPLING LIQUIDS

Liquid samples are homogeneousand aremuch easier to sample.

The gross sample can be relatively small. If liquid samples are not homogeneous, and

have only small quantity, they can be

shaken and sampled immediately.

Sampling techniques will depend on thetypes of liquid.

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EXAMPLES

Large volume of liquids (impossible to mix)- Sampled after transfer (during discharge)- If in a pipe, sampled after passing through a pumpor at different points in pipe system.

Large stationary liquids (lakes, rivers)- Sampled at different depths using a sample thief-Sample thiefisa bottle that can be opened and

filled at any desired location in the solution.

Biological fluids- The timing of sampling is very important

e. g. before meal or after meal.

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SAMPLING GASES

Tend to be homogeneous. Large volume of samples is required because

of their low density.

Examples:- Air analysis: Use a `Hi-Vol sampler that is

containing filters to collect particulates.

-Liquid displacement method: The sample musthas little solubility in the liquid and does not reactwith the liquid

- Breath sample: The subject could blow intoevacuated bag.

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SAMPLING GASES

Air Samplung Pump Dust Sampler

Air Sampling Filters

Hi-VolHi-Vol

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SAMPLE STORAGE ANDPRESERVATION

Samples are preserved to prevent from:

Decompositionof biological samples through theaction of bacteria. Refrigerated after collection until

the time of analysis.

Precipitationof metals from water samples. Acidified(10% HNO3) immediately upon collection.

Loss of waterfrom hygroscopic material. Loss of volatile analytesfrom water samples.

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SAMPLE PREPARATION

Sample preparation or sample treatment is a step inchemical analysis where the sample is brought intothe correct size form for analysis.

Sample preparation:

Most laborious Time-consuming Error-prone steps

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Distribution of Time Analysts Spend on

Sample Analysis

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GENERAL PRINCIPLES FOR SAMPLE

PREPARATION

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PREPARING A LAB SAMPLE

Converting the sample to a useful form:

Solids are usually groundto a suitable particulatesize to get a homogeneous sample.

Dry the samples to get rid of absorption water.(Drying at 110 to 120C for 1 hour and cooled indessicator before weighing).

Solid samples must be dissolvedin solution.

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DEFINING REPLICATE SAMPLES

Replicate samples are always performedunless the quantity of the analyte, expense

or other factors prohibit.

Replicate samples are portion of a material ofapproximately the same sizethat is carried

through an analytical procedure at the same

time and thesame way.

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PREPARING SOLUTION

Most analyses are performed on solutions. A solvent is chosen that dissolves the whole sample

without decomposing the analyte.

Digestion should be performed in Teflon or quartzcontainers to reduce contamination.

Several sources of error are encountered in thesample dissolution step:

- Incomplete dissolution of the analyte.

- Losses of analyte by the volatilization.

- Introduction of analyte as a solvent contamination.- Contamination from the reaction of the solvent with

vessel walls.

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DECOMPOSITION AND DISSOLUTION

Simple Dissolution Acid Treatment/Wet Digestion Fusion TechniquesOrganic Solid

Dry Ashing Wet Digestion

Inorganic Solid

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SIMPLE DISSOLUTION

Dissolution by water.

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WET DIGESTION

Dissolution in various strong mineral acids.

HCl Carbonates, phosphates, oxides H2SO4 Organic material at 300C HNO

3

Any metals not dissolve by HCl

HClO4 Steel HF Silica Aqua Regia (HCl:HNO3, 3:1) Not stable HNO3:HCl:HF (5:15:3) Alloys

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GRADE OF ACIDS

Very High Purity Chemicals- ARISTARTM(BDH), Ultra-Pure (NBS)

Analytical Reagents- Certified ARTM(Fisher) and PurissTM(Fluka)

Chemically Pure (CP)- CPTM(Sigma), GPRTM(BDH)

Practical Grade- Purified

TM

(Sigma), LRTM

(BDH) Commercial or Technical Grade

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WET DIGESTION

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MICROWAVE DECOMPOSITION

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FUSION TECHNIQUES

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Contamination by flux material High salt content

High temparature, loss of analyte throughevaporation

Sample container may react with flux material

DISADVANTAGES OF FUSION

TECHNIQUES

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DESTRUCTION OF ORGANIC MATERIAL

FOR INORGANIC ANALYSIS

For organic materials (animal and plant tissue,biological fluid ) containing inorganic analytes suchas trace metals, the organic matrix is destroyed via :

Dry AshingInvolves slow combustion at 400-700

C , which

leaves behind the inorganic residue which is solublein dilute acid.

Wet DigestionHeat organic with oxidising acids (HNO3/H2SO4

mixture), inorganic residue left behind.

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ELIMINATING INTERFERENCES

Interferences are substances that preventdirect measurement of the analyte and must

be removed.

Techniques- Separation

- Standard Addition

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MEASUREMENT

The analytical measurement is often the simplest stageof the analytical process.

All reagents used must be of high purity (reagent grade).For trace analysis a blank measurement must beperformed.

Analytical measurements are divided into two types:classical (gravimetric, volumetric) and instrumental.

The physical or chemical propertyproportional to theanalyte concentrationis measured.

Suitable standards must be measured to determine therelationship between analyte quantity and the physical/

chemical property being measured (i.e., calibration).

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Classical Methods (Gravimetric, Volumetric)Accurate and precise to approx. 0.1% but require large

amount of analyte (mmol, mg).

Instrumental Methods (Spectroscopy,Chromatography)

More selective and sensitive than classical methods but

less precise.Accurate to !1%

Measure of physical/chemical property of an analyterelated to concentration.

Rapid/may be automated/may be used for determination

of multiple analytes at a time.

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REPORT

Report results with limitation/accuracyinformation.

A professional chemist/charted chemistshould verify the report.

Test Report an example Professional Chemist