Gas Chromatography - Quantitative Analysis > Page 52

Quantitative Analysis

There are three important stages in a GC analysis,

1. The preparation of the sample.

2. The development of the separation and the production of the chromatogram

3. The processing of the data and the presentation of the results.

Each stage is equally important and if not carried out correctly the results will be neither precise nor accurate. Sample preparation can be very simple involving no more that diluting a known weight of sample with mobile phase or be much more complex including an extraction procedure followed by derivatization and then dilution. For some samples the preparation can be the most time consuming and difficult part of the whole analysis. Details of sample preparation is the subject of Book 18 but an example of one of the more complex sample preparation methods will be given to illustrate some of the procedures that may be necessary.

 

Liquid extraction is a clumsy procedure, particularly when used on the micro scale which is often necessary in sample preparation. An alernative procedure is solid phase extraction. The procedure is relatively simple and involves the use of a short tube packed with an appropriate adsorbent such as silica, reversed phase silica or, for some applications, macro porous polymer beads. The adsorbent must be capable of removing the substances of interest from the liquid medium. Extracting trace materials from water (e.g., pollution analysis) a reversed phase would be appropriate. Then the substances could be displaced into solvents such as n-hexane, methylene dichloride etc. A diagram of a simple solid phase extraction tube is shown in figure 34.