-
- Posts: 6
- Joined: Wed Nov 16, 2016 4:35 pm
I believe that I have a ghost peak in an assay / related substances method from contamination in my aqueous mobile phase. Normally I'd just redevelop the method, but we're committed to this. What is truly odd is that this ghost peak appears regardless of the mobile phase vendor, grade, instrumentation used, meticulousness of glassware washing, eluent additive vendor, etc.
The reasoning behind my supposition that the ghost peak is from mobile phase A is that:
1.) I can increase the size of the ghost peak in a proportional manner with the length of equilibration time at initial conditions (at 5%B)
2.) If (using a binary pump) a C18 column (Small, wide bore and >5um particle size so as not to increase pressure unnecessarily) is placed after pump A and before the mixing T the ghost peak disappears.
3.) If Mobile Phase A is filtered through a divinyl benzene or polystyrene solid phase extraction cartridge the ghost peak area decreases about 90%
Method Fundamentals:
1.) Gradient Method: MPA is 0.1%FA in Water; MPB is 0.1% FA in ACN
2.) Stationary Phase: Phenyl-Hexyl on positively charged hybrid silica particles, sub 2 micron particle size
3.) Ghost peak elutes at about 43% ACN (co-eluting with a specified impurity)
ID by LC/MS?
1.) Ghost Peak analyte does not seem to ionize, work ongoing to concentrate on solid phase extraction cartridge and then analyze.
ID by Spectra?
1.) Looks like a phthalate, but if it was, it should easily ionize in LC/MS
One last piece of data:
In the mobile phase A filtered through the divinyl benzene or polystyrene solid phase extraction cartridge, on day one the ghost peak is not detectable, but by 48 hours the peak has grown to a detectable level.