Question: (a) 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

(a) 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.

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

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

NH3 NH HO Norepinephrine cation (NE) 2,3-Dihydroxybenzylamine cation (DHBA) CH,CH2CH CH CH2CH CH2CH2OSO3 Na Sodium octyl sulfate Added concentration of NE (ng/mL) 12 24 36 48 Peak height ratio NE/DHBA 0.414 0.554 0.664 0.792

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