Sample Preparation

Practical Considerations

Sample processing may demand as much skill and experience as the chromatographic analysis itself. Sample processing can range widely in complexity from simple dissolution in an appropriate solvent to a complicated extraction procedure, followed by derivatization and further extraction. Most samples will require the addition, or preparation, of an internal or external standard. Likewise all these operations must be accomplished quantitatively, with no material loss, so the integrity of the final analysis is not prejudiced.

Extraction Techniques

The determination of trace materials in bulk solids and liquids, involves extraction and concentration procedures. The chosen techniques are determined by the nature of the sample. There are three basic extraction methods, solvent extraction, solid phase extraction and supercritical liquid extraction. The use of these techniques, however, often entails the use of precipitation procedures, crude separation processes and subsequent concentration methods. Typically, pre-extraction procedures might include, centrifugation precipitation, dialysis etc., that are often provoked by the presence of bulk materials in the sample that interfere with the separation and measurement of the substances of interest.

Pre-Extraction Practices

The more common methods and procedures of pre-extraction are as follows.

Ultra Centrifugation

Ultra centrifugation can concentrate large, high molecular weight materials (e.g. proteins) by centrifugal forces developed at extremely high rotational speeds. In some samples, proteins are interfering materials and it is the supernatant liquid that contains the materials of interest. The ultracentrifuge can also be employed to determine the molecular weight of the proteins from their sedimentation rate under centrifugal forces. The sedimentation rate, . where ((r) is the radius and (t) the time) is given by,

(9)

where (NA)	is Avogadro's Number,
(r)	is the solution density,
(n)	is the reciprocal of the protein density,
(f)	is the frictional coefficient of the protein molecule

Ultra centrifugation is often employed as a last resort when other techniques have proved ineffective. After centrifugation the concentration of the low molecular weight solutes are evenly distributed throughout the now, heterogeneous, sample and the supernatant liquid provides a representative sample of those materials to be assayed. A sample of the supernatant liquid is then taken for subsequent examination