(i) (4 marks) Calculate the resolution of both columns X and Y. (ii) (4 marks) C

Question

(i) (4 marks) Calculate the resolution of both columns X and Y. (ii) (4 marks) Calculate the number of theoretical plates in both columns X and Y. (iii) (2 marks) Calculate the H for both columns X and Y. (iv) (2 marks) State which column is most efficient and explain your choice. (v) (2 marks) With reference to the flow rate and column length explain how this efficiency has been achieved and why the resolution of the two columns is the same.
(2) (6 marks) Substances A and B give chromatographic peaks separated by 15 seconds and the retention time of A is 9.0 mins. Half height peak widths of A and B are approximately equal. How many theoretical plates are required to give a minimum acceptable resolution of 1.50?

1. (14 marks) Substances A and B were analysed chromatographically on two columns, X and Y, with the same stationary phase type and the same eluent. The outcome of the chromatography is shown in Table 1. Table 1. Chromatography of substances A and B on columns X and Y Column Column length (mm) 100 0.2 5.1 5.6 30 0.6 0.59 0.64 0.01050 0.01140 Flow rate (mL min) Retention time of A (min) Retention time of B (min) Half Height peak width of A (min) Half Height peak width of B (min) 0.1045 0.1145 (i) (4 marks) Calculate the resolution of both columns X and Y (ii) (4 marks) Calculate the number of theoretical plates in both columns X and Y (ii) (2 marks) Calculate the H for both columns X and Y (iv) (2 marks) State which column is most efficient and explain your choice. (v) (2 marks) With reference to the flow rate and column length explain how this efficiency has been achieved and why the resolution of the two columns is the same (2) (6 marks) Substances A and B give chromatographic peaks separated by 15 seconds and the retention time of A is 9.0 mins. Half height peak widths of A and B are approximately equal. Hovw many theoretical plates are required to give a minimum acceptable resolution of 1.50?

Explanation / Answer

(i) R= 2*(rtb-rta)/ (wb+ wa)

where, rta=retention time of A and ; rtb=ret. time of B

wb and wa are peak widths of B and A resspectively.

for x

RX= 2*(5.6-5.1)(0.1145+0.1045)

= 4.56

In the same way , RY= 2*(0.64-0.59)/(0.01140+0.01050)

= 4.56

(ii) N= 5.55*(rt/w)2

for the column X

N(for A)= 5.55*(5.1/0.1045)2= 13,218

N(for B)= 5.55*(5.6/0.1145)2= 13,275

for the column Y

N(for A)= 5.55*(0.59/0.01050)2 = 17,523

N(for B)= 5.55*(0.64/0.01140)2= 17,492

(iii) let us conisder substance A for a simple comparision

H=L/N ; L= length of column, N= no. of theortical plates

HX = 100/13,218

=0.00756

HY= 30/17,523

=0.00171

(iv) resolution is same for both the columns. now, the number of theoritical plates is more in the column Y and less in the column X. Therotical plates refer to the hypothetical phases of substance(liquid and gas) are in equlibirum and form plates(hypothetical).more these plates are , more distinguishable. so, a column which has higher number of theortical plates is generally considered better. Also, in column Y peak width is less that shows the peaks are narrower and sharp which is good. Hence, Column Y is the most efficient in this case

(v) for best efficiency the flow rates must be elevated. Now, efficiency is directly proportional to the column length. hence, a longer column length means more efficient column. In case of column X the flow rate is less , but the column length is more while in the case of Y if the flow rate is higher but here the column length is shorter. although in Y the number of the theoritical plates is more and peak width is less which shows this has better efficiency.

Resolution is same for both the columns becasue the ratios of the retention time of A and B in column X and Y is 10. The ratio of peak width for both A and B in both the columns is also 10. Hence, the ratios are equal which gives them the same resolution

(vi) R= 2*(rtb-rta)/ (wb+ wa)

1.50= 2*15/2w

w= 10sec= 0.167 min

N= 5.55*(rt/w)2

= 5.55*(9/0.167)2

= 16,118 plates are required to give 1.50 resolution

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