Selection of Basic Analytical Equipment

In starting a residue analysis in foods, the choice of proper vials for sample preparation is very important. Available vials are made of either glass or polymeric materials such as polyethylene, polypropylene, or polytetrafluoroethylene. The choice of the proper material depends strongly on the physicochemical properties of the analyte. For a number of compounds that have the tendency to irreversible adsorption onto glass surfaces, the polymer-based vials are obviously the best choice. However, the surface of the polymer-based vials may contain phthalates or plasticizers that can dissolve in certain solvents and may interfere with the identification of analytes. When using dichloromethane, for example, phthalates may be the reason for the appearance of a series of unexpected peaks in the mass spectra of the samples. Plasticizers, on the other hand, fluoresce and may interfere with the detection of fluorescence analytes. Thus, for handling of troublesome analytes, use of vials made of polytetrafluoroethylene is recommended. This material does not contain any plasticizers or organic acids, can withstand temperatures up to 500 K, and lacks active sites that could adsorb polar compounds on its surface. 

The color of the vial wall may also be of importance when photolabile compounds are analyzed. In these circumstances, brown vials are generally used, although in some special cases, green, blue, or red vials may be more appropriate. However, the dyes in the colored vials may sometimes interfere with the analysis. In such cases, transparent vials should be used photodegradation of the samples can also be avoided by wrapping the vials in aluminum foil. 

The type of pipette used for addition or transferring of solvents is another issue of importance. Automated micro- or macropipettes in the volume range of 

000 1 with disposable wetting polypropylene tips are commonly used for this purpose. These pipettes have some advantages over the classic glass pipettes. 

Proper sampling requires knowledge of the purpose for which sampling is required. The precise method of sampling depends heavily on the particular food commodity and the nature of the tested analyte. For example, fresh meat should be separated as completely as possible from any bone to be further passed three times through food chopper, and mixed thoroughly after each grinding (1). 

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