You wish to determine whether isocratic or gradient elution should be used for a

Question

You wish to determine whether isocratic or gradient elution should be used for a separation. For the initial run, a wide gradient from 5 vol% to 95 vol% B was used. For a 0.40 × 17 cm column containing 3-?m particles with a flow rate of 1.1 mL/min, what is the gradient time, tG, for this initial run? Assume S, the slope of the linear-solvent-strength model, is equal to 4.

You wish to determine whether isocratic or gradient elution should be used for a separation. For the initial run, a wide gradient from 5 vol% to 95 vol% B was used. For a 0.40 x 17 cm column containing 3- particles with a flow rate of 1.1 mL/min, what is the gradient time, to, for this initial run? Assume S, the slope of the linear-solvent-strength model, is equal to 4 Number mi n Based on your initial results, you decided that using a gradient elution would be appropriate for your separation. You optimized the gradient separation, going from 19 vol% to 32 vol% B in 18.7 min at a flow Tate of 1. 1 mLimin Calculate the average retention factor, K, for this optimized separation, rate of 1.1 m/min. Calculate the average retention factor, k*, for this optimized separation. Number k*

Explanation / Answer

a)

k* is the average retention factor for which a value of 5 is typically optimal.

S = 4

= (95-5)/100 = 0.9

Vm = pi* 0.42 * 17 / 4 = 2.14 mL

F = 1.1 mL/min

tG = 1.15 k* S Vm / F = 1.15 * 5 * 4 * 0.9 * 2.14 / 1.1

= 40.2 min

tG = tf - ti 18.7 * (32 - 19) / (95 - 5) = 2.7 min

As a good rule of thumb, Isocratic analysis is possible when tG < 0.25 tG

or tG < 0.25 * 40.2 = 10 min.

Therefore, isocratic analysis is possible.

b)

= (32-19)/100 = 0.13

tG = 18.7 min

tG = 1.15 k* S Vm / F

18.7 min = 1.15 k* (4) * 0.13 * 2.14 / 1.1

k* = 16.1

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