The Need For Sample Preparation
Conventional Sample Preparation
Preliminary Considerations
Fluids
Pumps
Extraction Vessel
Restrictor
Summary




The analytical chemist usually employs the following steps for the characteriza­tion of complex samples:

Sample preparation Sample analysis Data interpretation Report generation

Over the past several decades, considerable time and effort have been devoted to improving the speed and specificity of analyses as well as developing and im­proving instrumentation, data handling, and report-generating software. In con­trast, sample preparation has not received a great deal of attention.Yet, if one is to achieve successful detection-identification and quantification in a highly re­producible manner, some degree of isolation and/or concentration of the analyte is usually necessary. A recent survey of analytical chemists indicates that ap­proximately 60% of the analysis time is spent on sample preparation (Fig. 22). If the mixture is heterogeneous, mechanical procedures such as filtration and cen-trifugation may be necessary to separate the various phases. To separate the corn ponents of a homogeneous or intractable mixture one must take advantage of the differing physical-chemical properties of the individual components. Such methods as distillation, absorption, adsorption, crystallization, and extraction are then applied.

 

Fig. 22. Survey results for the distribution of time that analytical chemists spend on sample analysis.

Not only is the majority of analysis time spent on sample preparation, but this step may be the source of many errors. In the survey cited earlier, it was reported that sample preparation typically accounts for at least 30% of the errors encoun­tered in the performance of analytical methods. Operator error, on the other hand, was approximately 20%. Clearly, sample preparation is the most error-prone, least glamorous, and most labor-intensive task in the analytical laboratory. Techniques that minimize (1) the use of toxic and carcinogenic organic solvents in sample preparation and (2) the problems of solvent waste disposal would be welcomed. If the procedure could also be faster and easy to automate, thereby in­creasing sample throughput, its value would be enhanced even more.

Extraction with SFs has enabled the analytical chemist to achieve these goals with both nonpolar and polar analytes contained in a variety of solid and semi-solid sample matrices. Recent studies have demonstrated that SFE has definite advantages over conventional extraction methods. These reports have enhanced interest in the further development of SFE as an alternative to extraction using liquid solvents. An ideal extraction method should afford total recovery and high purity of the isolated analyte. Naturally, the original composition of the compo­nents should remain unchanged during the extraction. Since most spectrometric and chromatographic methods utilize liquid introduction, the extraction proce­dure should also yield a solution that requires no (or minimal) cleanup or con­centration.

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