Lab 4BA
Gas Chromatography of Distillation Fractions
Reading: Zubrick, pages 250-259 (skip “Electronic Interlude” section)
Introduction: Gas chromatography is an instrumental technique that is used to separate volatile organic compounds. A gas chromatograph consists of a flowing mobile phase, an injection port, a separation column containing the stationary phase, a detector, and a data recording system. The organic compounds are separated due to differences in their partitioning between the mobile gas phase and the stationary phase in the column. Efficient separation of compounds by GC is dependent on the compounds traveling through the column at different rates. The rate at which a compound travels through a particular GC system depends on the volatility and polarity of the compound, the temperature of the column, the polarity of the stationary phase, the flow rate of the mobile phase and the length of the column. However, the number one factor to consider in separation of compounds in most modern gas chromatographic applications is the boiling points of the different components
In this lab you will perform gas chromatographic analysis on your cyclohexane-toluene distillation fractions saved from the distillation lab. You will “shoot” your samples then calculate the relative amounts of cyclohexane and toluene in each distillation fraction.
Procedure:
Short of us providing a small book on GC use and application here, you will get a significant lecture by your instructor on what is required for the day’s activities. The GC’s will be set up for you prior to the start of lab. They will be at the proper temperature, flow rate, etc. You will be instructed on the proper use of the GC, proper injection techniques and how to make the required calculations from the integrator output…So, take good notes!
Results:
As part of your results you will need to put your chromatographic data into an easy to read table. This should include the names of compounds of interest, their retention times, their raw “peak areas”, and the normalized peak areas that you calculated (i.e. removed the air peak contribution). Your table should also include the data from the standards run by your instructor. Make sure you label all your chromatograms and identify all peaks of interest.
Discussion:
Based on your GC results, discuss which distillation technique was more effective.
Safety Information:
The GC injector ports are HOT.
Again, avoid skin contact, breathing, etc. for any of the organic solvents (toluene, cyclohexane, and dichloromethane (the wash solvent)).
Waste Disposal:
Any left over toluene or cyclohexane from the distillations goes in the non-halogenated waste.