A mixture of nonpolar compounds is separated by HPLC using an octyl (–(CH2)7CH3)

Question

A mixture of nonpolar compounds is separated by HPLC using an octyl (–(CH2)7CH3) stationary phase covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 55 vol% acetone and 45 vol% water to 85 vol% acetone and 15 vol% water?...

A mixture of nonpolar compounds is separated by HPLC using an octyl (-(CH2)7CH3) stationary phase Map covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 55 vol% acetone and 45 vol% water to 85 vol% acetone and 15 vol% water? O Remain the same Decrease Increase O Not enough information A mixture of polar compounds is separated by HPLC using an amino (-(CH2 3NH2) stationary phase covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 35 vol% acetonitrile and 65 vol% water to 70 vol% acetonitrile and 30 vol% water? Increase Decrease O Not enough information O Remain the same A mixture of polar compounds is separated by HPLC using bare silica as the stationary phase. How would the retention times ofthe compounds change if the solvent composition was changed from 40 vol% acetonitrile and 60 vol% tetrahydrofuran to 80 vol% acetonitrile and 20 vol% tetrahydrofuran? Scroll down to view all O Remain the same 4 parts of this question. O Not enough information Decrease Increase

Explanation / Answer

Analyte is a substance, whose chemical constituents are being identified and measured.

stationary phase that is immobilized on the support particles, or on the inner wall of the column tubing.

The mobile phase is the phase that moves in a definite direction. It may be a liquid , a gas (GC), or a supercritical fluid. The mobile phase carries the sample being separated/analyzed and the solvent that moves the sample through the column.

In the case of HPLC the mobile phase consists of a non-polar solvent(s) such as hexane in normal phase or a polar solvent such as methanol in reverse phase chromatography and the sample being separated.

Retention time is the amount of time a compound spends on the column after it has been injected.

If the polarity of the analyte compound and the stationary phase are similar, then there is likely to be a greater interaction between the two according to the rule of like attracts like. Consequently, the retention time will be longer for polar compounds on polar stationary phases and shorter on non-polar stationary phases and vice versa.

Polarity of water > ACN > methanol > THF.

1.

In this, analyte is a mixture of non-polar compound

stationary phase is a non-polar compound.

acetone is non-polar

Mobile phase 1 has 55% acetone which increases to mobile phase 2 with 85% acetone.

Here interaction is increased hence retention time will be decreased.

2.

In this, analyte is a mixture of polar compound

stationary phase is a polar compound.

water is more polar than acetonitrile

Mobile phase 1 has 65% water which decreases to mobile phase 2 with 30% water

Here interaction is decreased hence retention time will be increased.

3.

In this, analyte is a mixture of polar compound

stationary phase is a polar compound.

acetonitrile is more polar than tetrahydrofuron

Mobile phase 1 has 40% acetonitrile which increases to mobile phase 2 with 80% acetonitrile.

Here interaction is increased hence retention time will be decreased.

4.

In this, analyte is a mixture of non-polar compound

stationary phase is a non-polar compound.

methanol is less polar than water.

Mobile phase 1 has 90% methanol which decreases to mobile phase 2 with 35% methanol.

Here interaction is decreased hence retention time will be increased.

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