Date: 21-APRIL-2012   Last Updated: 19-FEBRUARY-2026

Overview

Unions and adapters are common connectors used in HPLC systems to join tubing, fittings, and various instrument components. Although they may appear similar, each serves a distinct purpose in the fluid path.

Understanding when to use a union versus an adapter ensures proper compatibility, reduces leaks, and maintains chromatographic performance.

What Is a Union?

Purpose

• A union is used to connect two components that share the same thread size and thread count.

Key Characteristics

• Ideal when both sides of the connection use identical port geometry
• Typically lower cost than adapters
• Can join different types or sizes of tubing as long as the port threads match
• When connecting different tubing diameters, this is often called a reducing union

When to Use

• Straight, consistent connection between identical threaded components
• Most routine, low‑complexity HPLC tubing joins

What Is an Adapter?

Purpose

• An adapter is used to connect two different thread sizes or port configurations.

Key Characteristics

• “Adapts” one thread type to another (e.g., 1/4‑28 to 10‑32)
• Can include connections like Luer fittings or other specialty interfaces
• Used when components originate from different instrument designs or systems

When to Use

• Transitioning between mismatched thread geometries
• Connecting components from different manufacturers or systems
• When a union cannot be used because threads do not match

Practical Differences

Cost

• Unions are significantly less expensive than adapters.

Functionality

• Unions — Same threads on both sides
• Adapters — Different threads on each side

System Impact

• Unions generally create the simplest, lowest‑dead‑volume connection
• Adapters add versatility at the expense of cost and potentially increased internal volume