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Chapter 25: Q37P (page 711)

A reversed-phase separation of a reaction mixture calls for isocratic elution with 48%methanol 52%water. If you want to change the procedure to use acetonitrile/water, what is a good starting percentage of acetonitrile to try?

Short Answer

Expert verified

Considering a vertical line (intersects each solvent line at the same elegant strength) at 48% methanol intersects the acetonitrile at 38%. Use acetonitrile/water instead of 48% methanol water 52%, the good starting percentage of acetonitrile must be 38%.

Step by step solution

01

Definition of isocratic elution

When the mobile phase has a constant concentration, isocratic elution is a term used in chromatography where the concentration of the mobile phase is constant throughout the chromatographic process.

02

Percentage of acetonitrile

Considering a vertical line (intersects each solvent line at the same elegant strength) at 48% methanol intersects the acetonitrile at 38%. Use acetonitrile/water instead of 48% methanol water 52%, the good starting percentage of acetonitrile must be 38%.

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Most popular questions from this chapter

(a)explain how to measure k and resolution.

(b)state three method for measuring tmin reversed-phase chromatography.

(c)state three method for measuring tmin hydrophilic interaction liquid chromatography.

(d)Estimate tmfor 15脳0.46column containing 5-渭尘particles operating at a flow rate of 1.5mL/minEstimate tmif the particle size were 3.5-渭尘 instead.

Retention factors for three solutes separated on aC8non-polar stationary phase are listed in the table. Eluent was a 70 : 30 (vol/vol) mixture of 50 mM citrate buffer (adjusted to pH withNH3) plus methanol. Draw the dominant species of each compound at each pH in the table and explain the behavior of the retention factors.

use figure 25-17to suggest which type of liquid chromatography you could use to separate compounds in each of the following categories.

(a)Molecular mass <2000,soluble in octane

(b) Molecular mass <2000 ,soluble in methanol-water mixtures

(c) Molecular mass <2000 ,weak acid

(d)Molecular mass<2000 ,soluble highly polar

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(f)Molecular mass<2000,soluble in water in nonionic various

(g) Molecular mass<2000,soluble in water in water, variety of changes

(h) Molecular mass<2000,soluble in tetrahydrofuran

The 鈥渞ule of three鈥� states that the retention factor for a given solute decreases approximately threefold when the organic phase increases by10%.In figure 25-12,tm=2.7min. Find k for peak 5at 40%B .predict the retention time for peak at B and compare the observed and predicted times.

In monolithic columns60 the stationary phase is a single porous piece of silica or polymer filling the entire column and synthesized within the column from liquid precursors. Monolithic columns offer similar plate height to HPLC particles, but with less resistance to flow. Therefore, faster flow or longer columns can be used. The figure shows separation of isotopic molecules on a long monolithic column. Packed columns have too much resistance to flow to be made so long.

Separation of isotopic molecules on a 440-cm-long monolithic C18-silica column eluted withCH3CN/H2O(30: 70 vol/vol) at 308C. [Data from K. Miyamoto, T. Hara, H. Kobayashi, H. Morisaka, D. Tokuda, K. Horie, K. Koduki, S. Makino, O. Nu帽ez, C. Yang, T. Kawabe, T. Ikegami, H. Takubo, Y. Ishihama, and N. Tanaka, 鈥淗igh-Efficiency Liquid Chromatographic Separation Utilizing Long Monolithic Silica Capillary Columns,鈥� Anal. Chem. 2008, 80, 8741.]

(a) Unretained thiourea is eluted in 41.7 min. Find the linear velocity ux (mm/s).

(b) Find the retention factor k forC6D6

(c) Find the plate number N and plate height forC6D6

(d) Assuming that the peak widths forC6H5Dand C6D6are the same as that of C6D6, find the resolution of C6H5Dand C6D6.

(f) If we just increased the column length to increase N, what value of N and what column length would be required for a resolution of 1.000?

(g) Without increasing the length of the column, and without changing the stationary phase, how might you improve the resolution?

(h) When the solvent was changed fromCH3CN/H2O(30:70 vol/vol) toCH3CN/CH3OH/H2O(10:5:85), the relative retention for C6H5D andC6D6increased to 1.0088 and the retention factor for C6H6 changed to 17.0. If the plate number were unchanged, what would be the resolution?
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