Question number 5?? Oor Calevlationeel Derived Quantities Linear mobile ptase nd

Question

Question number 5??

Oor Calevlationeel Derived Quantities Linear mobile ptase ndociny Volume of mobile phase Diutribution constant Selectivity factor Number of plates Plate beight Symbol of Experimental Quantity Migration time, unretained specics Retention time, species A and B Adjusted retention time for A Peak widths for A and B Length of column packing Volumetric flow rate Linear flow velocity Stationary phase volume Concentration of analyte in mobile and stationary phases 16 5. In the process of investigating an unknown mixture by gas chromatography, three peaks of interest were found to be partially merged with one another. Among the options available to improve resolution of these sample peaks from one another are to: a) Slow the chart recorder speed b) Change the mobile phase flow rate c) Inject more sample d) Inject less sample e) Alter the temperature Change columns (eg, Change stationary phase) Explain the effect each of these options would have on the chromatographic process. Which of these options would be most likely to produce the desired effect? Defend your answer.

Explanation / Answer

a) Slow the chart recorder speed –

Three peaks were partially merged with one another. Standard chart recorder speed is 1cm/min.

Slowing recorder speed would be the easiest step to get more resolution. We can try by decreasing speed to 0.5 cm/min. As speed is decreased by half we may get double the resolution/separation and may get distinct peaks.

b) Change the mobile phase flow rate –

Mobile phase nothing but carrier gas. It is inert gas such as helium, argon or nitrogen.

We need to decrease the flow rate of carrier gas. Maybe partially merged peaks are result of higher flow rate of mobile phase. This can be great solution to solve the problem above.

But decreased flow rate increases retention time. It will take more time to complete the experiment.

c) Inject more sample –

If too much sample is injected, the peaks will show a significant tailing, which in turn result in poorer separation. This is not the solution to our problem.

d) Inject less sample –

There won’t seem a reason to decrease the amount of sample. Peaks are just partially merged with one another.

e) Alter the temperature –

High temperatures may cause very short retention time; we can try to decrease temperature to get more retention time. Also much less temperature is not useful for us because we need moderate high temperature for compound to react with stationary phase. The best separations are usually observed for temperature gradients.

f) Change columns

Separation of compounds depend on their interaction with stationary phase. If three peaks are merged, it may be the result of non-interaction with stationary phase. We can reconfirm the polarity of compounds and use appropriate column. E.g modified cyclodextrin column can be used for separation if compounds are enantiomeric ones.

As per points discussed above, slowing the chart recorder speed and changing mobile phase flow rate would get us desired effect. As only peaks of interest were merged and overall GC experiment was not a failure. Also these two choices are easy to try. They will not take much time as compared to changing temperature or column itself.

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