Author Question: Suppose that, on a GC column, methyl hexanoate eluted in 1.87 minutes, methyl heptanoate eluted in ... (Read 100 times)

vinney12 · **on:** Aug 23, 2018

Suppose that, on a GC column, methyl hexanoate eluted in 1.87 minutes, methyl heptanoate
eluted in 2.35 minutes, and methyl octanoate eluted in 3.0 minutes. What is the structure of an
unknown methyl ester, known to belong to the same homologous series, that elutes at 4.7 minutes?

Question 2

Helium is the carrier gas of choice for a gas chromatograph containing a thermal conductivity
detector, and nitrogen gas is preferred for a gas chromatograph having a flame ionization
detector. Suggest a reason for this.

at · **Reply #1 on:** Aug 23, 2018

Answer to Question 1

Construct a graph of the log of the retention time versus the number of carbons for the known
compounds. From the graph, read the number of carbons for the unknown methy ester, which,
in this case is methyl decanoate.

Answer to Question 2

Helium has a very high thermal conductivity, thus maximizing the response when compounds
of low thermal conductivity pass the detector. In addition, all organic compounds show a peak
response that rises above (rather than falls below) the baseline. Nitrogen has a lower thermal
conductivity than helium, but is much less expensive. In a flame ionization detector, thermal
conductivity is of no consequence; therefore, nitrogen is preferred.