Hi again, DIEGC40,
I think I understand, you cannot separate successfully metformin and vildagliptin via the use of a C8 column without the Metformin peak eluting at the void time. This makes sense as the metformin is quite polar, so an ion-pairing agent is needed or perhaps a different stationary phase altogether.
The HILIC (HILIC is an HPLC separation mode) phase should work well, or perhaps a SCX phase, if the idea is to perform the separation without using an ion-pairing agent. Please note, MS detection is not mandatory for use of a HILIC separation, UV detection will work okay, I'd think, though these substances don't have much in the way of UV chromophores. It seems likely that low UV (< 220 nm) would be amenable to detect both metformin and vildagliptin.
Like Tom noted above, there are some metformin and vildagliptin methods just when you search using Google...these call for UV detection between 215 - 260 nm...checking out the UV spectra of metformin and vildagliptin would be a good start, I think. None of the methods I found easily had ion-pairing agents...and none had metformin eluting anywhere outside of the column void.
Are you prohibited from using ion-pairing, or to use a phase other than C8?