Overview

Passivating and purging are two distinct procedures used to maintain the performance and longevity of HPLC systems.

Although often mentioned together, they serve entirely different purposes—one protects stainless steel surfaces from corrosion, while the other flushes solvents and contaminants from the flow path.

What Is Passivation?

Definition

• Passivation is a chemical treatment applied to stainless steel components to prevent corrosion and rust formation.

How It Works

• Stainless steel (especially 316 SS used in HPLC) is treated with nitric acid or citric acid to remove free iron from the surface.
• This process forms a protective oxide layer that reduces reactivity and significantly enhances corrosion resistance.

When Used in HPLC Systems

• In liquid chromatography, passivation means applying this protective layer to exposed stainless‑steel surfaces inside the system—tubing, valves, injectors, pump heads, and fittings.

Important Chemical Considerations

• Most mobile phases do not cause significant corrosion of 316 SS.
• However, strong non‑oxidizing acids, such as:
  • Hydrochloric acid (HCl) – rapidly removes the protective layer
  • Dilute sulfuric acid – removes it more slowly

Preferred Acid Choices

• If an acidic mobile phase is needed, recommended acids include:

  • Ortho‑phosphoric acid
  • Acetic acid or other organic acids
  • These do not aggressively strip SS surfaces.

Critical Safety Note

• If passivating an HPLC system, remove all HPLC columns and guard columns before exposing the system to strong acids:

  • Silica particles are not compatible with concentrated acids.

What Is Purging?

Definition

• Purging is a cleaning and flushing process used to replace old solvents with new mobile phase and clear the flow path of contaminants.

Purpose

• Remove remaining solvent from previous analyses
• Prevent unwanted contamination between runs
• Clear system tubing, pumps, mixers, injectors, and sample loops

How Purging Is Performed

• The HPLC column must be removed to avoid contamination from incompatible solvent carryover.
• Fresh mobile phase is introduced and allowed to replace all residual liquids throughout the system.

Where Purging Should Occur

• All solvent lines
• Sample loop
• Injector
• Autosampler needle

Passivation vs. Purging: Key Differences

Passivation

• Chemical metal‑surface treatment
• Protects stainless steel from corrosion
• Uses nitric or citric acid
• Must NOT be done with columns attached

Purging

• Cleaning/solvent‑flushing operation
• Removes old solvent and contaminants
• Performed routinely during method changes
• Column must also be removed to avoid cross‑contamination