Conversely, sample solutions containing organic solvents and other organic
liquids should be stored in glass containers because the base plastic or additives
such as plasticizers and antioxidants may be leached from the walls of plastic
containers.Samples arriving in an analytical laboratory come in a very wide assortment of
sizes, conditions and physical forms and can contain analytes from major
constituents down to ultra-trace levels. They can have a variable moisture content
and the matrix components of samples submitted for determinations of the same
analyte(s) may also vary widely. A preliminary, or pre-treatment, is often used to
conditionthem in readiness for the application of a specific method of analysis or
to pre-concentrate(enrich) analytes present at very low levels. Examples of pre-
treatments are:● drying at 100°C to 120°C to eliminate the effect of a variable moisture content;
● weighing before and after drying enables the water content to be calculated or
it can be established by thermogravimetric analysis (Topic G1);
● separating the analytes into groups with common characteristics by dis-
tillation, filtration, centrifugation, solvent or solid phase extraction (Topic
D1);
● removing or reducing the level of matrix componentsthat are known to cause
interferencewith measurements of the analytes;
● concentrating the analytes if they are below the concentration range of the
analytical method to be used by evaporation, distillation, co-precipitation, ion
exchange, solvent or solid phase extraction or electrolysis.Sample clean-upin relation to matrix interferenceand to protect special-
ized analytical equipment such as chromatographic columns and detection
systems from high levels of matrix components is widely practised using solid
phase extraction(SPE) cartridges (Topic D1). Substances such as lipids, fats,
proteins, pigments, polymeric and tarry substances are particularly detri-
mental.A laboratory sample generally needs to be prepared for analytical measurement
by treatment with reagents that convert the analyte(s) into an appropriate chem-
ical form for the selected technique and method, although in some instances it is
examined directly as receivedor mounted in a sample holder for surface
analysis. If the material is readily soluble in aqueous or organic solvents, a simple
dissolution step may suffice. However, many samples need first to be decom-
posed to release the analyte(s) and facilitate specific reactions in solution. Sample
solutions may need to be diluted or concentrated by enrichment so that analytes
are in an optimum concentration range for the method. The stabilization of solu-
tions with respect to pH, ionic strength and solvent composition, and the removal
or maskingof interfering matrix components not accounted for in any pre-treat-
ment may also be necessary. An internal standardfor reference purposes in
quantitative analysis (Topic A5 and Section B) is sometimes added before adjust-
ment to the final prescribed volume. Some common methods of decomposition
and dissolution are given in Table 1.Sample
preparation
Sample pre-
treatment
A4 – Sampling and sample handling 13