I need help with this instrumental analysis question; it’s about gas chromatogra
ID: 915534 • Letter: I
Question
I need help with this instrumental analysis question; it’s about gas chromatographic separations, please help with each part(a,b,c,d), thank you!
1. Consider the gas chromatographic separation of the esters methyl acetate, methyl propionate,
and methyl butyrate on a column containing a stationary phase of intermediate polarity (relatively
polar).
(a)What would be the retention order on this column? Explain.
(b)Would this retention order change if a non-polar stationary phase had been used instead? Explain.
(c)Would the retention times change on the non-polar column? Explain.
(d) In preparing a benzene/acetone gradient for a silica gel liquid chromatographic column (note,
this is silica gel and not a bonded-phase material), is it desirable to increase or decrease the
proportion of benzene as the column is eluted? Explain your answer.
Explanation / Answer
For the above given three esters,
(a) the retetntion order on GC would be in the following order : First eluted would be methylacetate, then comes off methylpropionate and the last would be methylbutyrate.
(b) the retention order would remain reverse If a non-polar stationary phase is employed. Longer chain hydrocarbon ester methylbutyrate is most non-polar among the three and would be eluted last whereas, methylacetate is the most polar and would be eluted first in this case.
(c) the retention time would change due to the difference in polarity of the three compounds in the decreasing order : methylacetate > methylpropionate > methylbutyrate. the most polar would elute first and the least polar would stick to column for longer period.
(d) It is desirable to decrease the proportion of benzene in the benzene as the column is eluted. This would ensure th non-polar comes off of the column first and polar at the end as the acetone portion increases.