All Analgesics, 55% pH=7 0.025M phosphate buffer C18, Black Trace 245 nm, Green Trace 195nm RP-Amide, Black Trace 245 nm, Green Trace 195nm C18-PFP, Black Trace 245 nm, Green Trace 200nm RP-Amide, 55% pH=5 0.025M phosphate buffer Mix A (order: aspirin and caffeine, salicyclamide, phenacetin), Black Trace 245 nm, Green Trace 220nm Acetaminophen, Black Trace 245 nm, Green Trace 195nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, 195nm RP-Amide, 55% pH=7 0.025M phosphate buffer Mix A, Black Trace 245 nm, Green Trace 220nm: aspirin, caffeine, salicyclamide, and phenacetin, respectively Acetaminophen, Black Trace 245 nm, Green Trace 195nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, Black Trace 245 nm, Green Trace 195nm RP-Amide, 55% pH=9 0.025M phosphate buffer Mix A (order aspirin, caffeine and salicyclamide, phenacetin) Black Trace 245 nm, Green Trace 230nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, Black Trace 245 nm, Green Trace 200nm C18-PFP, 55% pH=5 0.025M phosphate buffer Mix A (aspirin, caffeine, salicyclamide, phenacetin) Black Trace 245 nm, Green Trace 213nm Acetaminophen, Black Trace 245 nm, Green Trace 195nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, Black Trace 245 nm, Green Trace 195nm C18-PFP, 55% pH=7 0.025M phosphate buffer Mix A (order aspirin, caffeine, salicyclamide, phenacetin), Black Trace 245 nm, Green Trace 225nm Acetaminophen, Black Trace 245 nm, Green Trace 195nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, Black Trace 245 nm, Green Trace 200nm C18-PFP, 55% pH=9 0.025M phosphate buffer Mix A (order aspirin, caffeine and salicyclamide, and phenacetin) Black Trace 245 nm, Green Trace 210nm Ibuprofen, Black Trace 245 nm, Green Trace 195nm Diphenhydramine, Black Trace 245 nm, Green Trace 195nm All Analgesics, Black Trace 245 nm, Green Trace 200nm Unknown Tablets on C18-PFP, 55% pH=7 0.025M phosphate buffer Unknowns were crushed, dissolved in 250 mL water, and filter with 0.45 micropore filters, followed by a 1:10 dilution in methanol. Tablet 1, Black Trace 245 nm, Green Trace 195nm Tablet 2, Black Trace 245 nm, Green Trace 200nm Tablet 3, Black Trace 245 nm, Green Trace 200nm Tablet 4, Black Trace 245 nm, Green Trace 200nm Tablet 5, Black Trace 245 nm, Green Trace 195nm Supplemental Information All stock solutions were made by dissolving weight listed of analgesic in 50.0 mL methanol Stock Compounds (Amount in Stock) pKa Melting Point Structure Aspirin (acetylsalicyclic acid) 90mg 3.49 134-6˚C NaN Caffeine 30 mg 10.4 238˚C NaN Salicyclamide 90 mg 8.37 140-4˚C NaN Phenacetin 12 mg 2.2 133-8˚C NaN Acetaminophen 5 mg 9.38-9.9 169˚C NaN Ibuprofen 50 mg 4.91-5.2 76˚C NaN Diphenhydramine HCl 50 mg 9-10.10 166˚C NaN Column Chemistries C18 particle surface RP-Amide particle surface (polar embedded group) C18-PFP particle surface (polar endcapped)