If I understand, you have reacted phthalolyl chloride with dimethyamine - which would be expecte to give the diamide. You have three peaks in the chromatogram - one the diamide, the second phthalic anahydride, and the third is something you have tried to remove. You want to analyze this compound - and I am not sure if you refer to the compund you can not remove or the phthaloyl chloride. And you want a separate assay for phthaloyl anhydride?
You tried to remove this other compund/peak by changes in chromatographic condition or by changes in sample handling?
Tell us what the objective of the analysis is and what analytical range is of interest.
You describe that you analyze this compund "as such with MDC" - Is this that you disolve the phthaloyl chloride in MDC (I assume dichloromethane?) and inject it in the GC without derivitization? And how do the two peaks relate to the peaks you described in the first posting? How does t his relate to the 30% solution in MDC from the supplier? And is the 100% siloxane phase your GC column or something else?
Give the details - how much sample are you using, what solvents, what quantity of reagent, what precautions are taken to be sure everything is anhydrous?