1. We argue that low volatility additives less basic than water (pK_a < H_3O~+ < -1.7) should increase charge in positive mode ESI, while additives with cation pK_a > > -1.7 would decrease it. 2. Thus, in the absence of denaturation effects, we expect formamide and dimethyl formamide to reduce charge for analytes in aqueous and water/CH_3CN solutions. 3. pK_a measurements for the DMSO cation vary between pK_a -2.7 to +1, depending on the solvent. For aqueous solutions and low % CH_3 CN/aqueous solutions and absent denaturation effects, we expect DMSO to decrease charging in positive ESI. 4. Consistent with these predictions, our observations have shown charge decreases with DMSO, formamide, and dimethyl formamide additives.
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