Introduction

HPLC unions and adapters are commonly used to connect tubing, columns, detectors, valves, and other fluid path components. Although these fittings may appear similar externally, their internal design can have a significant impact on chromatographic performance.

One of the most important design differences involves the through hole, sometimes referred to as the bore, located within the union or adapter body. Understanding the distinction between bore-style and butt-to-butt connections can help chromatographers select the most appropriate fitting for a given application.

What Is a Bore in an HPLC Union or Adapter?

A bore is the internal passage that extends through the center of a union or adapter body.  In this design, the tubing ends do not directly contact each other. Instead, solvent passes through a small internal channel that connects the two tubing segments.

The bore diameter is often specified in product descriptions because it contributes to the total internal volume of the connection.

Characteristics of Bore-Style Designs

• Internal flow path passes through a central bore.
• Bore diameter is specified in product specifications.
• Creates a small amount of internal volume.
• Commonly used in many standard HPLC connections.
• May introduce a small amount of dead volume.

What Is a Butt-to-Butt Connection?

In a butt-to-butt design, there is no internal bore between the two tubing ends.  Instead, both tubing ends are aligned directly against each other within the fitting, creating a continuous flow path with minimal interruption.  Because there is no central bore, these fittings typically do not list a bore diameter specification.

Characteristics of Butt-to-Butt Designs

• No internal bore in the fitting body.
• Tubing ends align directly with one another.
• Minimal fluid path disruption.
• Extremely low dead volume.
• Often preferred for high-efficiency chromatographic applications.

Dead Volume and Chromatographic Performance

Dead volume refers to any volume within the fluid path where sample dispersion can occur outside the intended chromatographic separation.

Even small amounts of unnecessary dead volume can contribute to:

• Peak broadening
• Reduced peak height
• Lower column efficiency
• Reduced theoretical plates
• Loss of chromatographic resolution

For high-efficiency HPLC, UHPLC, and LC-MS applications, minimizing dead volume throughout the system is often critical to achieving optimal performance.

Comparing Bore and Butt-to-Butt Designs

Bore-Style Unions and Adapters

Advantages

• Robust and versatile
• Widely available
• Suitable for many routine applications

Considerations

• Introduce a small internal volume
• May contribute to minor band broadening in highly optimized systems

Butt-to-Butt Unions and Adapters

Advantages

• Minimal dead volume
• Improved fluid path continuity
• Helps preserve peak shape
• Beneficial in high-efficiency separations

Considerations

• Proper tubing alignment is important
• Installation precision can influence performance

Applications Where Dead Volume Matters Most

Connection design becomes increasingly important when working with:

• UHPLC systems
• LC-MS methods
• Narrow-bore columns
• Short columns
• Fast gradient methods
• Small particle columns
• High-efficiency separations

In these applications, even small increases in extra-column volume may have measurable effects on chromatographic performance.

Selecting the Right Union or Adapter

When evaluating unions and adapters, consider:

• Required pressure rating
• Tubing dimensions
• Internal volume requirements
• Column dimensions
• Detector type
• Desired chromatographic efficiency

For routine applications, either design may be suitable. For methods requiring maximum efficiency and minimum dispersion, low-dead-volume or butt-to-butt designs are often preferred.

Conclusion

HPLC unions and adapters are available with two primary through-hole configurations: bore-style designs and butt-to-butt designs. Bore-style fittings contain an internal flow channel that introduces a small amount of dead volume, while butt-to-butt connections align tubing ends directly and minimize internal volume. Understanding these differences can help chromatographers optimize fluid path design, reduce band broadening, and maintain the performance of modern HPLC, UHPLC, and LC-MS systems.