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Chapter 26: Q6P (page 746)

Propose a scheme for separating trimethylamine, dimethylamine, methylamine, and ammonia from one another by ion-exchange chromatography.

Short Answer

Expert verified

Principle of ion-exchange chromatography:Molecules are separated because of difference in their charges and are eluted with a solution which has a varying ionic strength. When that kind of solution passes through the column the molecules are highly separated according to their different charges.

Step by step solution

01

Order of separation of molecules

Molecules will be separated in the following order:

Ammonia < trimethylamine < methylamine < dimethylamine due topKa their values.

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Most popular questions from this chapter

In ion-exclusion chromatography, ions are separated from nonelectrolytes by an ion-exchange column. Nonelectrolytes penetrate the stationary phase, whereas ions of the same charge as the resin are repelled by the fixed charges. Because co-ions have access to less of the column volume, electrolytes are eluted before nonelectrolytes. The chromatogram shows the separation of trichloroacetic acid (TCA,pKa=-0.5), dichloroacetic acid (DCA,), and monochloroacetic acid (MCA,pKa=2.86) by passage through a cation-exchange resin eluted with 0.01 M HCl. Explain why the three acids are separated and why they emerge in the order shown.

Figure 26-24 shows the effect on resolution of increasing voltage from 28 to120kV.

(a) What is the expected ratio of migration times (t120kV/t28kV)in the two experiments? Measure the migration times for peak 1 and find the observed ratio.

(b) What is the expected ratio of plates (N120kV/N28kV)in the two experiments?

(c) What is the expected ratio of bandwidths(σ120kV/σ28kV)?

(d) What is the physical reason why increasing voltage decreases bandwidth and increases resolution?

(a) The interstitial volume, V0, in Figure 26-15 is the volume at which the curves rise vertically at the left. FindV0for the 25-nm-pore-size column. To the nearest order of magnitude, what is the smallest molecular mass of molecules excluded from this column? (b) What is the molecular mass of molecules eluted at 9.7 mL from the 12.5-nm column? (c)Vmis the volume at which the curves drop vertically at the right. Find the largest molecular mass that can freely enter the 45-nm pores.

Find the milliequivalents ofH+released if100mLof26.3mMNi2+were loaded on a cation exchange column in theH+form.

Norepinephrine (NE) in human urine can be assayed by ion-pair chromatography by using an octadecylsilane stationary phase and sodium octyl sulfate as the mobile-phase additive. Electrochemical detection (oxidation at 0.65 V versus Ag|AgCl) is used, with 2,3-dihydroxybenzylamine (DHBA) as internal standard.

  1. Explain the physical mechanism by which an ion-pair separation works.
  2. A urine sample containing an unknown amount of NE and a fixed, added concentration of DHBA gave a detector peak height ratio NE/DHAB = 0.298. Then small standard additions of NE were made, with the following results:

  3. Added concentration of NE ( ng/mL )

    Peak height ratio NE/DHBA

    12

    0.414

    24

    0.554

    36

    0.664

    48

    0.792

  • Use the graphical treatment in Section 5-3, find the original concentration of NE in the specimen.
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