Hello again greatone100,
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.
Sources:
“To support your hypothesis that the peak with a retention time of
2.27 is octane, first obtain a true sample of pure octane to use as a
standard… If the standard shows a peak at a retention time of 2.27,
you can be pretty certain that compound B is octane. If the standard
octane shows a peak with a retention time of as low as 2.0 or as high
as 2.5, it could still mean that compound B is octane, since the GCs
vary in retention time measurement from run to run. You should try
running an internal standard. Take a milliliter of the mixture and add
a milliliter of octane to it. Run a GC of this mixture + standard
octane; if the slower moving peak (RT might vary from 2.0 - 2.5)
increases in size and percent composition, it is quite likely that
compound B is indeed octane. (Since other compounds might also run
with the same RT, it does not absolutely identify the compound.)”
Answers to Study Problems in Gas Chromatography, CU Boulder Organic
Chemistry Undergraduate Course
http://orgchem.colorado.edu/hndbksupport/GC/GCsqans.html - the
questions are at http://orgchem.colorado.edu/hndbksupport/GC/GCsq.html
“Retention time and retention volume are properties of a solute when
variables such as temperature and flow rate are controlled. This
implies that retention behavior could be used to identify a compound.
However, because many compounds have similar retention times,
techniques such as mass spectroscopy, UV, or FTIR can be used to
confirm the identity of a separated component of a complex mixture. If
the sample contains a limited number of species whose identities are
known, comparison of retention times with those of known standards may
confirm the identities of some of the species.” From “Gas
Chromatography of n-alcohols”
http://www.geocities.com/CapeCanaveral/Lab/3975/Chem125/125GCAlc.htm
“What is GC/MS?
GC/MS is a highly effective analytical technique. In this technique,
the power of gas chromatography (GC) to separate compounds from a
complex mixture is coupled with the ability of mass spectroscopy (MS)
to identify those compounds (qualitative analysis), and accurately
determine the amounts present (quantitative analysis).”
Company web site of Thermo Finnigan
http://www.thermo.com/eThermo/CDA/Technology/Technology_Listing/0,1214,1000000010073-113-113,00.html |
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