GC Alcohols

8/7/2019 GC Alcohols

1/3

S '08 v2 M. Hauser (Survival Manual 7e) Gas Chromatography of Alcohols

QUANTITATION OF AN ALCOHOL BY

GAS CHROMATOGRAPHY

PURPOSE

Utilize gas chromatography to separate butanol from methanol and analyze for the

volume percent composition of butanol in an unknown sample.

TECHNIQUES

Gas chromatography

Preparation and use of a standardized curve

THEORY - GAS CHROMATOGRAPHY

Gas chromatography (GC) is an analytical technique that allows volatile sample mixtures

to be separated and detected as they pass through a coiled column of packing material.An inert carrier gas (often helium) serves as the mobile phase to push the now gaseous

substances through the porous column packing (stationary phase). Substances that spendmore time on the stationary phase will take longer to travel through the column. They

will have a longerretention time than substances that move quickly through the column.It is often seen that lower boiling point components travel more rapidly while substances

with higher boiling points will stay on the column longer. A detector can measure howmuch of a component exits the column at any given time. By comparing the "area counts"

of each component, it is often possible to quantitate the amount of each substance.

BACKGROUND STANDARDIZED CURVE

The detectors used with gas chromatographs do not always respond equally to chemicals

of different structures. A series of samples of known concentrations can be carefullyprepared and analyzed by GC. By plotting the detector response to these knownquantities of a particular chemical, a standardized curve can be created. This curve can

then be used to predict the concentration of a substance based on how the detectorresponds to a sample of unknown concentration.

PROCEDURE

1) To cut down on the time it takes to analyze samples, it is best if you work in a group

of three students. While two of your group starts to prepare your standards, one of youshould approach the instructor and ask to have an unknown alcohol mixture injected. You

and your teammates should record the unknown code. The printed chromatogram will begiven to your group when it is ready.

2) Use the burets preloaded with methanol or butanol to quantitatively prepare the

following series of standards. Each solution should be mixed in a small labeled flask andstoppered shut. You should be able to easily calculate the "volume percent of butanol in

each standard."(continued on other side)


2/3


Identity Volume Butanol

(mL)

Volume Methanol

(mL)

Volume Percent

Butanol

Standard 1 2 8

Standard 2 5 5

Standard 3 8 2

GAS CHROMATOGRAPHY

3) Your instructor will make a random draw of groups to determine the order that yourset of standard samples will be injected. Take the standards to the instructor at the gas

chromatograph when your turn is announced. He will instruct you on the use of the GCand your samples will be analyzed. You should record the gas chromatograph parameters

in your lab notebook.

4) When your chromatograms are returned to you, you should be able to logically

determine the identity of each of the two major peaks.

CALCULATIONS

5) From the data for each standard, identify the peaks corresponding to butanol and

methanol. Record the area counts associated with each of these two peaks. Divide thebutanol area count by the methanol area count to get the BuOH/ MeOH ratio for each

standard. Put your data in a table similar to this one:

Standard Vol % ofStd.

BuOHarea counts

MeOHarea counts

BuOH/MeOHratio

12

3

6) Each of you should use graph paper to prepare a graph. The vertical axis shouldindicate the BuOH/MeOH ratio for each standard, while the horizontal axis should show

volume % butanol. Plot the three standards and draw the best-fit straight line.

7) Analyze the chromatogram for your unknown sample to get the BuOH/MeoH ratio asbefore. Use your newly generated standardized curve to predict the volume percent of

butanol in your unknown. The samples may be discarded in the "nonhalogenated waste."

RESULTS

In your notebookConclusion, list the typical retention time of each alcohol. Report thevolume percent of butanol from your unknown sample (dont forget the unknown code).

State what physical property you could use to tell which of your major peaks was butanoland which was methanol. Tape your Standardized Curve into your notebook. A member


3/3


of your group should staple and submit the chromatograms to the instructor; write eachgroup member's name on top of this packet.

QUESTIONS

1) Which one of the following gases do you think would be unsuitable for use as a GCcarrier gas- nitrogen, argon, or oxygen? EXPLAIN your answer.

2) If the GC oven temperature was accidentally set too low, how would this impact

retention times?

3) Suppose a student left the cap off of her prepared standard sample and the sample wasnot analyzed immediately. How might this affect the GC results?

4) Suppose a student accidentally splashed a small amount of water into his sample and

then injected this contaminated sample onto a GC operating with a Flame IonizationDetector. Would this incident impact the gas chromatogram printout? If so, how would

the chromatogram be changed?

5) When you did the GC area calculations in this lab, you made a broad assumption thateach peak we observed was not superimposed on top of another peak. This would make a

peak show a higher area count than it actually had. [Suppose another chemical in ourunknown sample had the same retention time as butanol but it was hidden from sight.

Would this make the % butanol number we report higher or lower than it actually was?](You need rewrite only the portion in brackets in your notebook before providing an

answer.)

GC column

Documents

GC Alcohols