1. Influence of pH on Retention Non-Polar pH 3.0 pH 7.0 - RCOO - RCOOH C2 Adjust pH to Suppress Ionization RCOOH pKa = 5.0

2. pH 3.0 pH 7.0 - RCOO - RCOOH SAX Adjust pH to Ensure Ionization Influence of pH on Retention Ion Exchange

3. Anion Exchange Condition Apply Sample Interference Elution Elution 1. Methanol 2. Water or Buffer 2 pH Units 2 pH Units Above Above Isolate pKa Water or Buffer Water or Buffer 2 pH Units 2 pH Units Below pKa Below pKa (500mM H 2 SO 4 ) Homovanillic Acid Analyte Analyte Elution Elution Rumsby, et. al. SAX

4. Analyte: Weak Anion, pKa 5.0 R - COOH R - COO - R - COO - Sorbent: NH 2, pKa 9.8 R- NH 3 + R- NH 3 + R- NH 2 Analyte: Strong Anion R - SO3- R - SO3- R - SO3- Sorbent: NH 2, pKa 9.8 R- NH 3 + R- NH 3 + R- NH 2 pH 3pH 7pH 11 Weak Anion Exchange (NH 2 )

5. Analyte: Weak Anion, pKa 5.0 R - COOH R - COO - R - COO - Sorbent: SAX R- NR3+ R- NR3+ R- NR3+ Analyte: Strong Anion R - SO3- R - SO3- R - SO3- Sorbent: SAX R- NR3+ R- NR3+ R- NR3+ pH 3pH 7pH 11 Strong Anion Exchange (SAX)