Distillation and GC

Understand the procedure for conducting a GC run (including sample preparation); understand the purpose of each step.

1. analyze fractions #1,3,6,and 8 collected. Place these vials in a beaker labeled "simple" or "fractional". 2. Check out GC kit from stockroom, and take kit and beaker of samples to the instrument room. Avoid bending syringe needles or the plunger during the course of injection. DO NOT CLEAN THE SYRINGE WITH ACETONE BETWEEN SAMPLE INJECTIONS, this would interfere with your analysis. 3. Rinse syringe 3X with your first sample (fraction) by drawing up some liquid and then ejecting the liquid onto a Kimwipe. 4. To fill syringe, draw 1 uL of your first sample into the barrel by pulling up plunger slowly to avoid drawing in air. 5. Insert needle straight into the rubber injection septum as far as it will go, push plunger all the way down in with one quick motion, and start the data collection software. Remove syringe from septum while holding the plunger in place. If sample is injected too slowly, it will be spread along the column and the peaks will be broadened. 6. Repeat technique with remaining fractions. Small peaks on GC ( or no peaks) may be caused by touching the syringe needle to the hot injection port, vaporizing the sample out the back of the syringe or by leaking the sample into the injection port if the plunger is depressed too slowly. 7. Once finished, place the syringe back in the box and return it to the stockroom.

4 elements in GC instrument

1. carrier gas 2. injector port for introducing the sample 3. column for separating the sample (coil in pic) 4. detector at end of column

The purpose of Simple and Fractional Distillation

Distillation: The process of heating a liquid to the boiling point, condensing the heated vapor by cooling and returning either a portion of, or none of, the condensed vapors to the distillation vessel. *purpose of Distillation is to PURIFY A LIQUID* *Simple Distillation* is effective to isolate the various components, if the difference between the boiling points of each substance is greater than 40 C. This technique allows separation of only a single volatile component of the mixture to distilled -simpler setup than fractional -faster distillation times -consumes less energy than fractional distillation *Fractional Distillation* is effective to give a separation equivalent to SEVERAL successive simple distillations. This technique is equivalent to repeated simple distillations. This repetitive process is equivalent to performing a number of simple distillations -much better separation between liquids than simple distillation -can more readily purify complex mixtures than simple distillation -Fractional distillation Increasing the surface area in contact with the vapor phase can increase the theoretical plates in the distillation column

Be able to describe what occurs in a GC run

In a GC run, it partitions the components of a mixture between a mobile (gaseous) phase and a stationary (liquid) phase. In GC, the sample is injected into a heated chamber where it is vaporized. Mobile phase = mixture of vaporized sample and an inert carrier gas (usually helium or nitrogen) Stationary phase = (a packed column) is a film of a viscous, high-boiling liquid adsorbed on a finely divided solid support *an equilibrium exists in the distribution of the sample as a liquid on the stationary phase and a vapor in the mobile phase. Components that have higher affinity for stationary phase are held more tightly to the column and hence migrate more slowly Components that have a higher affinity for the mobile phase move through the column more quickly. The fundamental data collected in a GC run are periods of time (RETENTION TIME). Retention time is a physical property independent of the presence or absence of other components in the sample mixture and is inversely proportional to the vapor pressure of the sample in the mobile phase.

Be able to determine whether simple or fractional distillation is the appropriate method for a given experiment

Simple: -If there is a large difference in the boiling points (>70 C) between the two liquids then simple distillation is probably the best option Fractional: -fractional distillation gives better separation between the liquids - if there is only a small temperature difference between the two liquids a fractional distillation is the preferable option.

Recognize the correct set up for both Simple and Fractional distillation, and make sure you understand the purpose of each step.

Simple: -Attach 3-arm distillation head to the distilling flask. No clamp is needed on the distillation head. Make sure the entire bulb of the thermometer is just below the side arm of the 3-way adapter. -Start Variac setting at ~ 50 and slowly increase to achieve distillation rate of approximately 1-2 mL / minute. Wrap top of flask and 3-arm distillation head with aluminum foil to prevent heat loss. Fractional: -Attach fractionating column to distilling flask -Attach 3 arm distillation head to the top of the fractionating column. No clamp is necessary on the distillation head. -Start Variac setting at ~ 80 and slowly increase head to distillation rate of 1-2 mL/minute. Wrap top of flask, fractionating column, and 3-arm distillation head with aluminum foil to prevent heat loss. -

Understand the purpose of GC; know when this technique is appropriate

TRUE for GC: -GC can be used as a quantitative tool for determining composition of a mixture -GC may be used to separate compounds with limited thermal stability -GC requires that compounds have reasonable volatility -GC can separate compounds with close boiling points if their functionality is different Distillation is most commonly used for purifying liquids. Gas Chromatography is most often used to separate a mixture of volatile compounds; with this technique it is feasible to separate even compounds whose boiling points differ only be a few degrees. Using a detector on the GC = you can obtain infor on the amounts of two or more different compounds in a mixture. Principle of partitioning the components of a mixture between a mobile gaseous phase and a stationary liquid phase.

Be able to analyze GC data (i.e., be able to do calculations similar to what you did in your lab report; be able to identify which peak is which based on either BP or structural information)

The compound with the lowest BP will have the shortest retention time.

Be able to predict results from a given simple or fractional distillation (i.e., using graphs similar to those included in you lab report).

To find % composition: Area of curve/total area

Mobile Phase

gaseous

Stationary Phase

liquid