This experiment involves the use of gas chromatography to determine the concentr

Question

This experiment involves the use of gas chromatography to determine the concentration of decane, dodecane and tetradecane in an unknown sample using internal standardisation where the internal standard is undecane. Answer the following questions:

A. The stationary phase in the column that was used in the experiment is a polydimethylsiloxane which is chemically bonded onto the inner surface of the silica capillary. Is this phase non-polar, medium polarity or high polarity? And why?

B. Give reasons why it would be the stationary phase of choice in the separation of hydrocarbons.

C. Give two advantages of capillary GC columns over packed GC columns. Explain your answer.

D. When do you think that external standardisation would be used as the quantitative method of choice in GC analysis? Why?

E. Give two characteristics that an internal standard for a gas-chromatographic analysis must possess and why.

F. Single point calibration was used in the procedure for this experiment but what does this mean and what assumptions would need to be made if the analytical results obtained using the calibration method are valid?

Explanation / Answer

A. The stationary phase in the column that was used in the experiment is a polydimethylsiloxane which is chemically bonded onto the inner surface of the silica capillar is non polar. The general reprentation of PDMS is (C2H6OSi)n and the major backbone has bonding between atoms without much electronegativity difference. Therefpre it makes it non polar.

B. The dispersive interactions between non polar part ie., C-H bonds (alkanes) of analyte molecules are low energy vanderwaals forces and so this plays a major role in selecting the stationary phase because majority of phase polymeric backbone is non polar.

C. Packed columns have lower resolution efficienceies and larger column bleed than capillary columns. capillary column withstand higher column oven temperature compared to packed column because the use of bonded phases permits higher oven temperatures .

D. we can use an external standard when the method is stable and the recovery is sufficient because in this method the  the absolute analyte response is plotted against the analyte concentration to create the calibration curve.

E. The internal standard for GC analysis must possess chemical and physical properties very similar to the analyte and it should be in pure form and unreactive towards the sample. if it doesnt possess the above mentioned characteristics it will disturb the peaks given by the detector therefore effecting the experiment in a negative way.

F. A single point calibration is a fast way of checking the calibration of a system when there is no doubt about the linearity of the calibration function and the system is unbiased.

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