Conclusions
Most known irinotecan metabolites were identified by MS/MS and confirmed by MS3 using the LTQ XL with MALDI source directly from drug-treated tumor samples. The irinotecan metabolite SN-38-G seemed more uniformly distributed in FaDu tumors treated with both MSC and irinotecan than in those treated with irinotecan alone, where the analyte appears clumped in regions (MS/MS and MS3 data). Rastering a commercial laser with beam of ~100 ìm at a spacing of 50 ìm does improve the resolution of the 2-dimensional image5. Given the complexity of tissue samples, MS3 is truly needed in MALDI tissue imaging to filter interfering and isobaric species that are part of the baseline chemical noise and isolated during MS/MS.

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