Are you running a gradient from mpa to mpb? You should specify what we see in the attached file. Anyway you should not use amine containing buffers as they will be adsorbed and thus compete with your protein. 

 I am afraid I do not know what a WCX method is. Is weak cation exchange? and what is the problem you face with high absorption at 280 nm?.I imagine that you are worried because you want to detect protein at 280 nm (?). If I may guess, I wonder why you need so many compounds in your solutions. One or more than one of the components is absorbing at 280 nm. AMP for instance. What concentration of AMP? Do you need AMP, at high concentration? Why? What absorbance at 280 nm are you having, may be is not too high and you may run the WCX assays and detect protein.

Your absorbance at 280 nm is a lot less than 0.1 and therefore within the error range of the spectrophotometer.

Absorbance near 214 nm should be part of what I would call the 'salt peak' and should increase with increasing ionic strength contributed by your buffer and NaCl components.  Big salt peaks can be large and broad enough to interfere with measurements at 260 or 280 nm - so your salt peak may actually be the cause of much of your background reading at 280 nm!  Try taking a full UV spectrum of something between 40 and 50 minutes to see what it looks like.