ACN acetonitrile water in HPLC

ACN (acetonitrile)–water in HPLC

Acetonitrile (ACN) mixed with water is a very common HPLC mobile-phase combination used in reversed‑phase and HILIC methods because ACN is highly miscible with water, has low UV absorbance at short wavelengths, and gives lower viscosity (hence lower backpressure) than methanol–water mixtures. The binary ACN–water composition is used both in isocratic runs and in gradient elution to adjust elution strength: increasing ACN generally decreases retention of nonpolar analytes on reversed‑phase columns and shortens run times.gneebio+3

Practical considerations include using HPLC‑grade ACN and high‑purity water (often with volatile additives when needed) to avoid impurities that affect baseline noise or produce ghost peaks, and degassing and filtering the mobile phase to prevent bubbles and particulates. ACN–water mixtures can show nonideal microscale structure (ACN clustering in water), which can influence retention behaviour for some solutes and make method development slightly different than with other solvents.merckmillipore+2

Safety and handling: ACN is flammable and toxic if inhaled or ingested; handle in a fume hood, wear appropriate PPE, and store away from heat sources and incompatible materials. Operational tips: use gradient profiles and small changes in %ACN when optimizing separations, monitor system pressure when switching solvents, and replace or properly condition columns if switching frequently between methanol and ACN mixtures to avoid changes in performance