Question 12 (12 marks) Explain the following statements noting that the efficien

Question

Question 12 (12 marks) Explain the following statements noting that the efficiency of chromatographic analysis can be optimised by minimising the factors contributing to band broadening as shown by the van Deemter equation: (a) An open-tubular column is more efficient than a packed column. A GC analysis using a carrier gas flow rate of 20 mL/min gave a broad peak but didn't improve when the carrier gas flow rate was increased to 60 mL/min. (b) (c) A HPLC analysis using a column packed with 3 um particles gave sharper peaks compared to a column packed with 10 m particles. Note that the particles are coated with the stationary phase. (d) An open-tubular column coated with 0.25 m stationary phase gave sharper peaks than an open-tubular column coated with 1 m stationary phase.

Explanation / Answer

Chromatographic analysis

(a) An open tubular column is more efficient than a packed column. This is due to the fact that in open tubular column finer particles of the stationary phase are uniformly placed inside the surface of column, making it more efficient for separation with higher theoretical plates. On the other hand in packed columns the staionary phase is densely packed with much less theoretical plates and hence low separation efficiency.

(b) A GC analysis gave broad peak at 20 ml/min of carrier gas flow rate which did not improve by increasing it to 60 ml/min. This could b due to the fact that the temperature of the column might be too hight to allow for any separation on the column, which makes the elution time much shorter and no effect of carrier gas flow rate was seen here.

(c) HPLC analysis with stationary phase particles of 3 uM gave sharper peaks compared to column with 10 uM particles size packing. This is again because of greater number of theoretical plates presence in smaller particle column packing (3 uM) over 10 uM column packing. As the number of theoretical plates is higher in 3 uM the separation becomes more efficient and we get sharper peaks in HPLC analysis.

(d) Packing the column with finer particles of stationary phase allows for more number of theoretical plates on the open-tubular column coated with 0.25 uM size particles. The number of theoretical plates is directly proportional to the separation efficiency and hence we see sharper peaks as compared to column packed with larger particles of 1 uM size stationary phase.

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