20 mM Phosphate Buffer pH 3.5 (25 mM EDTA-2Na)*, before adding to microdialysate. 
Mix ratio; [Antioxidant Solutiont] : [Microdialysis Sample] = 1: 1 to 4 (volume to volume)

*Dissolve NaH2PO4 -2H2O(Wako 192-02815) 312 mg in 100 mL water. Then add 46 μl of 1/10 diluted phosphoric acid.  This gives 20 mM Phosphate Buffer pH3.5. Add 935 mg EDTA-2Na to 100 mL of 20 mM phosphate buffer.

Solvent for Standard Solution
Catecholamine (Norepinephrine, Dopamine) : 
Put in cold storage with 0.1 M HCl containing 1 mg/mL EDTA-2Na at the catecholamine concentration of 1 ng/µL (stable for 3 months). Make fresh dilution series with 20 mM phosphate buffer (pH3.5) containing 25mM EDTA-2Na on every analysis. Catecholamines are oxidized easily at less than pg/µL. 

Indoleamine (5-HT, 5-HIAA): 
Put in cold storage with 0.1 M CH3COOH containing 1mg/mL EDTA-2Na at the concentration of 1 ng/µL (stable for 3 months). Dilute with 20 mM phosphate buffer (pH3.5) containing 25mM EDTA-2Na on every analysis. It is oxidized easily at less than pg/µL.

1 mg/ml EDTA will be saturated in cold storage.  Please use supernatant. 4/15/2010

There is a useful publication for antioxidant solution of dopamine and serotonin. 

0.1M Acetic Acid, 1.0 mM Oxalic Acid and 3.0 mM L-Cystein at pH3.2.  Our users confirmed this antioxidant does not harm the separation on the PP-ODS. 

(A. Kankaanpa¨a et al., J. Chromatogr. B, 753 (2001) 413–419, Oxalic acid stabilizes dopamine, serotonin, and their metabolites in

automated liquid chromatography with electrochemical detection.) 7/8/2011