Sample Preparation Methodologies

Although chromatography is a separating technique it can not be expected to completely separate all components of a complex sample. Very often it is necessary to prepare the sample, filtrate, concentrate, clean up, etc. before chromatography. These steps vary depending on the nature of the sample. In this section a brief but recent view of these techniques is shown together with some specific applications to environmental analysis.

One important and general step in sample treatment, especially when liquid chromatography is to be used, is filtration of the solution. A first filtration may be necessary to separate large particulate matter from solvent since this may physically interfere with extractions in later stages. The final filtration before chromatography injection uses 0.45 pm to 0.20 ^m or smaller disposable filters to prevent small particulate matter getting into the chromatography column and damaging it. Different types of filters can be used including filter membranes, centrifugal filters, and syringe filters. The nature of the filters depends on the type of solvent used. The commercially available materials are: cellulose acetate (aqueous solvents), polypropylene (aqueous samples), nylon (aqueous and most solvent based samples) and polytetrafluoroethylene (organic based, highly acidic or basic solvents) among others. When commercial syringe filters are used, different diameter sizes are offered depending on the volume of sample to filter, which is important when small samples have to be filtered.

An alternative to normal filtration is ultrafiltration. In this case, pressure is applied to a membrane and molecules smaller than the molecular weight cutoff can pass through while larger molecules are retained. Ultrafiltration can be used for sample concentration or to eliminate higher molecular weight compounds.

A. Liquid Sample Preparation

Depending on the nature of analyte(s) of interest, sample preparation processes will be quite different. In liquid sample preparation, as for water and wastewater analysis, the analytes from matrices can be separated in two different ways:

• by extracting the analytes into a liquid phase as in LLE, purge and trap technique, membrane extraction, and single drop extraction.

• or by trapping the analytes in a solid phase such as SPE, SPME, and stir-bar extraction.

The classical LLE method is still in use due to the simplicity of the instrumentation, just a separation funnel, and also because of its extensive implementation in official methods (U.S.-EPA methodology, EEC standard methods). As shown in Table 2.1 nearly all U.S.-EPA (United States Environmental Protection Agency) methods for nonvolatile and semivolatile analytes in environmental samples apply LLE even though there is a trend to change this.

When doing a LLE, a given volume of the sample, for example water, is shaken with a given volume of a suitable organic solvent so that the organic micropollutants migrate from the aqueous to the organic phase. Sometimes it is advisable to add a small quantity of sodium chloride to avoid foam formation and so obtain a better separation. Then the organic solvent with the analytes is separated and evaporated to concentrate the sample to a precise volume. LLE can be done in different ways:

• Discontinuous liquid extraction. This is the most traditional extraction method which can be carried out in one or multiple steps.

• Continuous LLE. This is applied when the distribution constant is low or when the sample volume is large.

• Countercurrent extraction. This is advisable when complex samples with analytes of similar distribution are to be extracted.

• Online LLE. This is a dynamic process which allows the extraction of low volume samples and reduces the organic solvent consumption but it has the drawback of instrument complexity.

LLE has several disadvantages such as large volumes of generally toxic organic solvents. With some samples, the initial solvent extraction step results in the formation of an emulsion and hence prolongs the extraction process. A loss of sample frequently occurs during the concentration step and so reduces analyte recovery. To avoid these limitations a considerable interest in developing alternative sample preparation methods has been increasing in the last few years.

2. Liquid Membrane Extraction

Membrane LLE (MLLE) or sorptive membrane extraction (SME) is an alternative to LLE, and is an extension of the LLE principles. A membrane is used as a selective filter of the analytes, limiting diffusion between two solutions or as an active membrane in which its chemical

Was this article helpful?

0 0

Post a comment