Chromatographic methods, fine-tuning

Gas chromatographic methods can be modified and fine-tuned so that they are suitable for measurement of specific petroleum products or group types. These... 

Normal-phase chromatography is a very powerful separations tool because of the wide range of solvents available that can be used to fine-tune the selectivity of a separation. However, it has fallen in disfavor with many chromatographers because of some of the complexities involved. Under some circumstances, lengthy equilibration times or reproducibility problems may be encountered, which are due largely to the sensitivity of the technique to the presence of small concentrations of polar contaminants in the mobile phase. If these problems are controlled, the technique typically gives chromatograms superior to reversed-phase methods, due to the low viscosity of the commonly us solvents. 

---