lecture 3- sample preparation(0.5h)

7/27/2019 Lecture 3- Sample Preparation(0.5h)

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Lecture 3: Sample preparation( D. Harvey; Chap.7)

USTH

Analytical chemistryFall, 2013



Key notes• Representative sample: is one that truly

reflects the composition of the material to beanalyzed within the context of a definedanalytical problem.

• Sample storage: to avoid undesirable losses,

contamination or other changes that couldaffect the results of the analysis.

• Sample pe- treatment: involve a separation orconcentration of the analytes or the removal of

matrix components that would otherwiseinterfere with the analysis.

• Sample preparation: involve dissolution,grinding, fabricating into a specific size and

shape, pelletizing or mounting in a sampleholder.



3.1. Representative sample



3.2. Sample storage● The following effects should be considered:

- increases in temperature leading to the loss ofvolatile analytes, thermal or biologicaldegradation, or increased chemical reactivity;

- decreases in temperature that lead to the

formation of deposits or the precipi-tation ofanalytes with low solubilities;

- changes in humidity that affect the moisturecontent of hygroscopic solids and liquids or

induce hydrolysis reactions;-UV radiation, particularly from direct sunlight,

that induces photochemical reactions,photodecomposition or polymerization;



3.2. Sample storage

- air-induced oxidation;- physical separation of the sampleinto layers of different density orchanges in crystallinity.

• A particular problem associated withsamples having very low (trace andultra-trace) levels of analytes in

solution is the possibility of losses byadsorp-tion onto the walls of thecontainer or contamination bysubstances being leached from the

container by the sample solvent.



3.3.Sample pre-treatment

Examples of pre-treatments ● drying at 100°C to 120°C to eliminate

the effect of a variable moisture content;

● weighing before and after dryingenables the water content to becalculated or it can be established bythermogravimetric analysis

● separating the analytes into groups withcommon characteristics by dis-tillation,filtration, centrifugation, solvent or solidphase extraction



3.3.Sample pre-treatment

● removing or reducing the level of matrixcomponentsthat are known to causeinterferencewith measurements of theanalytes;

● concentrating the analytes if they arebelow the oncentration range of theanalytical method to be used by

evaporation, distillation, co-precipitation,ion exchange, solvent or solid phaseextraction or electrolysis.

Sample cleanup if neccessary



3.4. Sample preparation

Note: An internal standard for reference purposes in quantitative analysis issometimes added before adjust-ment to the final prescribed volume



Example of preparing samples

lecture 3- sample preparation(0.5h)

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