Gas chromatography and high performance liquid chromatography are common techniques to analyse many type of samples. Sample preparation is the first step and play an important role in the process of quantitative analysis. Sample preparation can have a major effect in the overall accuracy and reliability of the results. The aim of this step is to provide a sample aliquot that will not damage the column or instrument, have appropriate concentration, and is compatible with the intended analytical method. In the process of sample preparation and analysis, different errors may influence the results. Internal standards are usually used to limit these errors. The data in report I and II shows that the internal standard has helped to reduce errors due to sample preparation and the factors affecting results such as variations in injection volumes, evaporation of solvent and instability in detection.