By: Sayeed Mohammad
Problem: It was recently observed that the gas chromatograph (GC) analyses times were quite long at the FRI unit. Since the GC provided good data over the years, no method updates were planned this year. However, recent discussions with the operators and samplers at the unit revealed that the long analysis time from the GC was indeed a bottleneck. Specifically, when a larger number of samples is required on certain tests, the longer analysis time from the GC impacted the operational efficiency at the FRI unit. It became clear that the removal of this bottleneck can lead to improved operational efficiency at the FRI unit. An effort began for GC method optimization to shorten the analysis time while maintaining or improving the GC resolution. FRI routinely uses three test systems – Isobutane/n-Butane, Cyclohexane/n-Heptane and O-Xylene/P-Xylene. Of the three test systems, two are isomers with very similar physical properties and boiling points. Much like distillation, the separation of these isomeric components is quite challenging on a GC column. A careful review of the molecular properties showed that O-Xylene possessed a significant dipole moment, whereas P-Xylene did not exhibit a dipole moment. This difference in molecular polarity is advantageous to improve resolution (separation) and obtain much faster analysis times from the GC for this test system when an appropriate stationary phase is selected. Further, an appropriate selection of the capillary column dimensions can optimize the GC method for the other two test systems as well.
Actions Undertaken: A combination of theoretical principles, experimental data from published chromatographic libraries and validated chromatographic modeling tools was used to screen a large number and variety of candidate stationary phases for the GC capillary column. Based on a careful review, a stationary phase that is highly selective towards dipole-containing compounds was chosen for the GC. The GC method was then optimized for this new stationary phase and capillary column. In addition, new calibrations and validations were conducted to ensure optimal and reliable GC performance. The new GC method provided improved resolution, sharper peaks, and faster analysis times for all three test systems.
Solution: The GC method optimization has provided much faster analysis times. Specifically, the analysis times for all three test systems were halved. FRI Team was successful in removing this bottleneck and improve the operational efficiency at the FRI unit. This is another example of why it is important for engineers to communicate closely with operators at the unit and/or plant. Such communication led to addressing an important operational bottleneck that may have otherwise gone unnoticed.
Before GC Method Optimization for the P/O Xylene System
After GC Method Optimization for the P/O Xylene System (Note the Two-times Reduction in Retention Times)