Chromatography Forum

Hello everybody...glad to be here for the 1st time. Hope we can talk a lot.

Well, i'm doing a project here that involves metoprolol and HCTZ in association. The intention is do a dissolution profile of the product, using HPLC as quantification method.

I read some papers...all of them use a 25cm C18 collumn as stationary phase. For me, it isn't work. Both peaks elute in the beggining, and the resolution is terrible. In my opinion, that's because metoprolol is very soluble and polar, and it has no interaction with C18. Besides, HCTZ should elute in 5-6 min.... What I am doing wrong?

Method:
C18 10um, RPe (Merck)
MP: ACN: MetOH:H2O (50:15:35)
Flow: 1,0 mL/min
Injection: 20uL
DAD detector

Well, it seems a pretty basic question, but i would be very happy if I get an answer there...Thank you very much!

What I am doing wrong

Not much. I did a quick Google search and found that the references to metaprolol typically had the compound eluting at t0 (which goes to show that you really can't assume that published = competent). If you want to stick with reversed-phase, something like an "embedded polar group" ("EPG", "amide", "AQ", . . .) or maybe a phenyl column might be a better choice.

If it were my problem, I'd look at HILIC rather than RP.

At 65% organic, no wonder you see no retention for these compounds. Both metoprolol and HCT can be retained and analysed on C18, but at a higher aqueous ratio in the eluent. And in any case you should use a decent buffer, not plain water, since you're dealing with ionizable compounds.
Have a look at the application databases of column manufacturers. Just from a glance, Phenomenex (just an example) has applications for both Metoprolol and HCT on C18 columns. It might be necessary to use a gradient in order to analyze both compounds in one run.
Just as a side note, I'm not affiliated with Phenomenex . I'm sure Waters, Supelco, .... can provide applications, too.

Mixed-mode is a solution to your and other people problems