4. A gas chromatogram of the organic components of a sample of beer using a colu

Question

                    4. A gas chromatogram of the organic components of a sample of beer                     using a column that separates compounds on the basis of their relative boiling points provides a GLC trace with several peaks. Two of the smaller peaks, with                     retention times of 9.56 and 16.23 minutes, are believed to be ethyl acetate and ethyl butyrate.                 

                    a. From the above information, which component of the sample, ethyl                     acetate or ethyl butyrate, elutes faster? What are the reported boiling points of these two substances?                 

                    c. The major component of the sample elutes first. What organic                     compound present in beer do you suspect is responsible for this peak? Is your speculation consistent with the reported boiling point of the compound you are                     proposing? Explain.                 

Explanation / Answer

You can confirm the identity of the peaks is by the use of standards.

To do this, you will need a pure sample of ethyl acetate and a pure

sample of ethyl butyrate.

Firstly, you can determine the retention times of each of the pure

samples. If these match the retention times you have already

determined, then this is a reasonably good confirmation that the peaks

in the beer sample are due to ethyl acetate and ethyl butyrate.

However, retention times can vary from run to run. Therefore, it

might be necessary to obtain further confirmation by using your pure

samples as internal standards to spike the beer sample. Take one

volume of the beer sample and add a volume of ethyl acetate to it.

Using the same GC, obtain a gas chromatogram of the mixture. If the

peak at 9.56 minutes increases in size and percent composition

compared to the chromatograph for beer alone, it is quite likely that

it is due to ethyl acetate. Repeat the same procedure, this time

adding a volume of ethyl butyrate to a volume of beer. If the peak at

16.23 minutes increases in size and percent composition, it is quite

likely that it is due to ethyl butyrate.

However, many compounds have similar retention times. A better

confirmation can be obtained if you use the GC separate out the two

components (making use of what you know about their retention times

and elution rates) and then use another technique to identify them.

For example, you could do mass spectroscopy on each of your separated

components and each of your pure samples and see if there is a match

in the results. This is why GC is so often coupled with mass

spectroscopy (GC/MS) as an analytical technique.

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