## Materials and Methods
### HPLC Reagents, Equipment, and Procedure  
The standard of histamine dihydrochloride (\(\geq\)99%), sodium 1-decanesulfonate (\(\geq\)99.0%), potassium phosphate bibasic (\(\geq\)98.0%), and potassium phosphate monobasic (\(\geq\)98.0%) was supplied by Sigma-Aldrich (Steinhem, Germany). Trichoroacetic acid (TCA) of ACS grade (99%) was purchased from Carlo Erba Reagents (Rodano, Italy). Water and acetonitrile of HPLC grade were purchased from Baker (Deventer, The Netherlands). The derivatization reagents _o-phthalaldehyde_, N,N-dimethyl-2-mercaptoethanol/D (Thiofluor(tm)) and potassium borate buffer (_o-phthalaldehyde_ diluent, OD104) were from Pickering Laboratories (Mountain View, CA, USA). A 1000 mg L\({}^{-1}\) stock solution of HIM was prepared in water and stored at +4 \({}^{\circ}\)C for up to 4 months. A 10 mg L\({}^{-1}\) solution was freshly prepared in 5% TCA. Working standard solutions (0.04, 0.5, 1, 4, 8 mg L\({}^{-1}\)) were obtained by dilution with 5% TCA, just before analysis. Next, 1.70 g of potassium phosphate monobasic, 2.04 g of potassium phosphate bibasic, and 0.49 g of sodium 1-decanesulfonate were dissolved in water for the preparation of the phosphate buffer solution, containing an ion-pair reagent. The pH value of this solution was adjusted to 5.40 \(\pm\) 0.05 with concentrated hydrochloric acid and the volume was made up in a 1 L flask with water. The post-column derivatization solution was freshly prepared by dissolving 0.2 g of Thiofluor(tm) in about 1 mL of OD104 and was added to a solution consisting of 0.2 g of _o-phthalaldehyde_ in about 1 mL of methanol; the mixture was adjusted to a final volume of 200 mL by OD104. This solution was stored in a brown glass bottle and, when not in use, was kept at 4 \({}^{\circ}\)C for up to 2 days. Chromotographic separations were performed on an HPLC system, Agilent Technologies SL 1200 Series (Waldbronn, Germany) consisting of a binary pump provided with a micro vacuum degasser, a thermostatable autosampler, a column compartment, and a fluorescence detector. Online post-column chemical derivatization was performed by using a commercially available system supplied by LCTech GmbH (Dorfen, Germany) and consisting of a double-piston pump (model K-120) and a thermostatable post-column reactor (model CRX 400) equipped with a 0.5 mL knitted reaction coil. Separations were performed using Phenomenex columns: Luna C8 column (250 mm \(\times\) 4.6 mm i.d., particle size 5 \(\upmu\)m) at a flow rate of 1.0 mL min\({}^{-1}\) by isocratic elution. The mobile phase consisted of 0.01 M phosphate buffer solution, containing 0.002 M sodium 1-decanesulfonate as ion-pair reagent, at pH 5.4 (A) and acetonitrile (B) in a ratio of 80:20 (_v/v_). The injection volume was 2 \(\upmu\)L and the column temperature was set at 40 \({}^{\circ}\)C. The output of the column was connected by a T-joint with the derivatization line, pumping at a flow rate of 0.4 mL min\({}^{-1}\), and carried to the reaction coil, set at the temperature of 40 \({}^{\circ}\)C, for the derivatization process. Fluorescence detection was performed at the excitation and emission wavelengths of 343 and 445 nm, respectively. The system was interfaced, via network chromatographic software (Agilent ChemStation), to a personal computer to control instruments, data acquisition and processing. A 5 g portion of pre-treated sample was added to 30 mL of 5% TCA and the mixture was homogenized, in a single-step, with the aid of IKA ULTRA-TURRAX T18 basic (Werke GMBH & Co. KG, Germany). After centrifugation at 2112 g for 10 min at 10 \({}^{\circ}\)C and separation of the supernatant, few milliliters of 5% TCA was added to the sample extract, to obtain a final volume of 30 mL. An aliquot of 1 mL of this solution was filtered with an Anotop 10 LC filter (0.2 \(\upmu\)m, 10 mm, Whatman) obtaining an extract stable for 3 days at +4 \({}^{\circ}\)C. An amount of 100 \(\upmu\)L of this extract was diluted with 900 \(\upmu\)L of water and then injected in HPLC.