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Chapter 25: Q1P (page 707)

(a) Why does mobile phase strength increase as solvent

becomes less polar in reversed-phase chromatography, whereas

mobile phase strength increases as solvent becomes more polar in

normal-phase chromatography?

(b) What kind of gradient is used in supercritical fluid chromatography?

Short Answer

Expert verified

(a) if a solvent is less polar, it will separate easily on the stationary phase in the column making the mobilephase strength increase.

(b)In supercritical fluid chromatography, the gradient of increasing pressure is used, which increases the eluent strength.

Step by step solution

01

Step1:Explanation ofSeparation on normal-phase chromatographyPart(a)

Separation on normal-phase chromatography implies that the stationary phase is polar and the mobile phase nonpolar. The separation depends onthe interactions of a polar analyte with the polar stationary phase.

02

Explanation ofSeparation on mobile phase chromatography

In the reverse-phase chromatography separation, the stationary phase is non-polar and the mobile phase is polar. The separation is based on thehydrophobic property of the analyte. Thus if a solvent is less polar, it will separate easily on the stationary phase in the column making the mobilephase strength increase.

03

Step 3:Definition of Supercritical fluid chromatographyPart(b)

Supercritical fluid chromatography (SFC) is a hybrid technique of gas and liquid chromatography.

04

Step 4:Importance of Supercritical fluid chromatography

The importance of SFC is that it allows the separation and determination of groups of compounds that cannot normally be determined by either gas or liquid chromatography. It is a method in which the mobile phase is a gas at temperature and pressure above critical values. It uses a gradient of increasing pressure to give increasing eluent strength.

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Most popular questions from this chapter

To which kinds of analytes do these liquid chromatography detectors respond?

(a) ultraviolet

(b) refractive index

(c) evaporative light scattering

(d) charged aerosol

(e) electrochemical

(f) fluorescence

(g) nitrogen chemiluminescence

(h) conductivity

(a) Nonpolar aromatic compounds were separated by HPLC on an octadecyl(C18)bonded phase. The eluent was 65 vol% methanol in water. How would the retention times be affected if 90% methanol were used instead?

(b) Octanoic acid and 1-aminooctane were passed through the same column described in (a), using an eluent of 20% methanol/80% buffer (pH 3.0). State which compound is expected to be eluted first and why.

role="math" localid="1656416023291" CH3CH2CH2CH2CH2CH2CH2CO2HOctanoicacidCH3CH2CH2CH2CH2CH2CH2CH2NH21鈥�Aminooctane

(c) Polar solutes were separated by hydrophilic interaction chromatography (HILIC) with a strongly polar bonded phase. How would retention times be affected if eluent were changed from 80 vol% to 90 vol% acetonitrile in water?

(d) Polar solutes were separated by normal-phase chromatographyon bare silica using methyl t-butyl ether and 2-propanol solvent. How would retention times be affected if eluent were changed from 40 vol% to 60 vol% 2-propanol? (Hint: See Table 25-4.)

Microporous silica particles with a density of2.2g/mLand a diameter10渭尘of have a measured surface area of calculated the surface area of the300m2spherical silica as if it were simply solid particles what does this calculation tell you about the shape or porosity of the particles.

A known mixture of compounds A and B gave the following HPLC results:

A solution was prepared by mixing 12.49mgof Bplus 10.00mLof unknown containing just and diluting to 25.00mL. Peak areas of 5.97and 6.38were observed for AandB, respectively. Find the concentration of A(mg/mL)in the unknown.

(a) You wish to use a wide gradient from 5 vol % to 95 vol %B for the first separation of a mixture of small molecules to decide whether to use gradient or isocratic elution. What should be the gradient time, tG,for a 15脳0.46-cmcolumn containing 3-渭尘particles with a flow of 1.0mL/min

You optimized the gradient separation going from 20vol%to 34 vol% Bin 11.5min at 1.0mLFindk*for this optimized

separation. To scale up to a 15脳1.0cmcolumn, what should be the gradient time and the volume flow rate? If the sample load on

the small column was 1mgwhat sample load can be applied to the large column? Verify that k*is unchanged.
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