Introduction

Adapters and unions are common fluid-path components used throughout HPLC, UHPLC, and LC-MS systems. Although they often appear similar and serve related purposes, they are designed for different connection challenges within chromatographic systems. Understanding the distinction between adapters and unions helps ensure proper hardware selection, minimize installation issues, and maintain reliable system performance.  An additional area of confusion is the use of the term "bore" when describing unions and adapters. The presence or absence of a sufficent bore size can significantly affect fluid-path volume, dispersion, and overall chromatographic performance.

What Is an HPLC Union?

An HPLC union is used to connect two tubing assemblies or fluid-path components that share the same thread type and port configuration. The primary purpose of a union is to extend, repair, or join sections of tubing while maintaining a continuous flow path.

Characteristics of HPLC Unions

• Same thread type on both sides
• Same port configuration on both sides
• Connects tubing sections together
• Available in various internal diameters
• Commonly used throughout HPLC and UHPLC systems

Examples

• 10-32 to 10-32 unions
• Female-to-female unions
• Butt-to-butt unions
• Low dead-volume unions

What Is an HPLC Adapter?

An adapter is designed to connect two different thread types, port designs, or connection standards that would otherwise be incompatible. As the name suggests, an adapter "adapts" one connection style to another.

Characteristics of HPLC Adapters

• Different connection configuration on each side
• Converts one thread style to another
• Converts one port style to another
• Allows compatibility between different manufacturers or connection standards
• Supports integration of specialized laboratory hardware

Examples

• 10-32 to M6 adapters
• Luer-to-HPLC adapters
• Male-to-female thread conversion adapters
• Instrument-specific conversion fittings

Understanding Bore vs. Butt-to-Butt Designs

One of the most important but often overlooked specifications when selecting an adapter or union is whether the fitting contains a bore.

What Is a Bore?

A bore is a precisely machined passageway through the center of the fitting. When a fitting specification includes a bore diameter, such as 0.010 inch, 0.020 inch, or 0.030 inch, the fitting contains an internal channel between the two connection points.

In these designs:

• The tubing ends do not directly touch each other.
• The fluid travels through the bore located in the center of the fitting body.
• A small internal chamber or pathway exists between the two tubing ends.
• The fitting contains an internal wall thickness surrounding the bore.

Because the fluid must pass through this internal passage, bore size becomes an important contributor to total system volume and dispersion.

What Is a Butt-to-Butt Connection?

When no bore size is specified, the fitting is typically designed as a butt-to-butt union.  In a butt-to-butt design:

• The tubing ends meet directly inside the fitting.
• There is no intermediate bore or chamber between the tubing ends.
• The tubing is aligned end-to-end with minimal interruption to the flow path.
• Internal volume is minimized.

The term "butt-to-butt" refers to the fact that the cut faces of the tubing are positioned directly against one another, creating the shortest possible transition between the two fluid paths.

Why Does This Matter?

A fitting with a bore introduces some amount of internal volume. If the bore diameter is significantly larger than the tubing internal diameter, a small expansion chamber is created.  This can contribute to:

• Band broadening
• Peak dispersion
• Loss of chromatographic efficiency
• Reduced resolution in high-efficiency separations

By contrast, a properly installed butt-to-butt union minimizes dead volume because there is essentially no intermediate chamber between the tubing ends.  For UHPLC and modern high-efficiency HPLC methods, butt-to-butt designs are often preferred whenever compatible tubing dimensions and connection styles permit.

Through Hole Size and Tubing Compatibility

Both unions and adapters may be designed for various tubing sizes and internal diameters. Neither device is defined by tubing size.  Both can be used with:

• Different tubing internal diameters
• Different tubing materials
• Different chromatographic applications

The defining difference is not the tubing size, but the connection configuration of the ports.

What Is a Reducing Union?

A reducing union is a specialized type of union used to connect tubing with different internal diameters while maintaining the same connection style on both sides.

Examples

• 0.010" ID tubing connected to 0.020" ID tubing
• Same thread type on both ports
• Different tubing dimensions

Because both ports retain the same connection style, the fitting remains classified as a union rather than an adapter.

When Should You Use a Union?

A union is typically appropriate when:

• Connecting similar tubing systems
• Extending tubing runs
• Repairing damaged tubing assemblies
• Replacing removed components
• Maintaining consistent thread configurations

Typical Applications

• Column-to-detector connections
• Inline filter replacement
• Tubing extensions
• System troubleshooting

When Should You Use an Adapter?

An adapter is typically appropriate when:

• Connecting equipment from different manufacturers
• Converting thread standards
• Connecting HPLC components to Luer fittings
• Integrating specialized devices
• Bridging incompatible connection systems

Typical Applications

• Instrument upgrades
• Method transfer between systems
• Connection of sampling devices
• Integration of external hardware

Effects on Chromatographic Performance

Whether using an adapter or a union, proper fluid-path design remains important.

Key Considerations

• Internal volume
• Bore size
• Dead volume
• Tubing alignment
• Pressure rating
• Connection quality

Poorly matched or improperly installed fittings may contribute to:

• Peak broadening
• Reduced efficiency
• Leaks
• Increased dispersion
• Lower theoretical plate counts

Selecting appropriately sized bores and low dead-volume designs whenever possible can help preserve chromatographic performance. In critical UHPLC applications, butt-to-butt unions often provide the lowest possible internal volume and can help minimize extracolumn band broadening.

Conclusion

The primary difference between an adapter and a union is the connection configuration. Unions are designed to connect components that share the same thread and port style, while adapters are designed to connect components with different connection types.

Equally important is understanding the difference between bore-type fittings and butt-to-butt fittings. A fitting with a listed bore contains an internal flow passage between the two connections, while a fitting with no bore specification is generally a butt-to-butt design in which the tubing ends meet directly. Selecting the proper configuration can help reduce dead volume, maintain chromatographic efficiency, and optimize overall HPLC system performance.