Two rules that solve 95% of cases

1. Match IDs: Select the coupler internal diameter (ID) to exactly match your system tubing ID. This minimizes extra‑column volume and preserves peak shape.
2. Use color codes: Coupler body colors track the tubing color codes, making it easier to pick the proper ID at a glance. 

Why ID matching matters (the science behind it)

Any mismatch between the tubing ID and the coupler’s through‑bore creates unswept (dead) volume at the junction. Dead volume acts like a micro‑mixing chamber, causing peak broadening, tailing, and resolution loss—especially with narrow‑bore columns or fast gradients. Choosing a coupler that matches the tubing ID and properly bottoms the tube in the port is the simplest, most effective way to avoid these artifacts. 

Step‑by‑step selection workflow

1. Identify your tubing ID (and OD)
   
   Confirm the ID specified for your current capillary run (e.g., 0.13–0.25 mm for micro/analytical paths) and the OD (commonly 1/16″). Many vendors use color‑coded PEEK tubing to denote ID ranges—your coupler colors should mirror these conventions. 

2. Match the coupler through‑bore to the tubing ID
   
   Pick a coupler whose through‑bore equals the tubing ID to maintain a continuous, uniform flow path. Avoid oversizing “just to be safe”—that’s how dead volume sneaks in. 

3. Confirm the port and thread standard
   
   Most column and valve connections in the high‑pressure path use 10‑32 coned ports for 1/16″ OD tubing; low‑pressure lines often use ¼‑28 flat‑bottom. Verify the geometry at both ends so the coupler (union) seats correctly. 

4. Choose body material for pressure and chemistry

   • PEEK unions/couplers: chemically inert, easy fingertight assembly; typical HPLC pressure capability; ideal for bioinert paths. 
   • Stainless‑steel unions: higher pressure tolerance and robustness; preferred for UHPLC or where pressure margins are tight. 

5. Minimize connection error during installation
   Ensure the tube bottoms out in the coupler seat (no gap) and that ferrules/nuts are correct for the receiving port. “Too short” or “too long” capillary insertion causes leaks or hidden dead volume. Finger‑tight, torque‑controlled or spring‑loaded fittings help keep connections zero‑dead‑volume and repeatable. 

Practical scenarios & recommendations

• Analytical HPLC at standard flows (e.g., 0.2–1.5 mL/min):
  If your method uses 0.17–0.25 mm ID tubing, select a same‑ID coupler (PEEK or SS depending on pressure). The color on the coupler should match the tubing’s color code to prevent mix‑ups during maintenance. 

• Microbore/fast gradients (sensitive to dispersion):
  Favor unions/couplers with through‑bores equal to the smallest adjacent tubing and verified zero‑dead‑volume design to protect efficiency and resolution. 

• Bioinert or metal‑sensitive workflows:
  Use PEEK or biocompatible couplers/unions to avoid metal contact and potential adsorption while keeping pressure ratings within method limits. 

• UHPLC (very high pressure):
  Prefer stainless‑steel (or rated biocompatible metal) bodies and verified high‑pressure fittings; follow vendor torque/swaging instructions to prevent leaks and micro‑volumes. 

Common pitfalls (and how to avoid them)

• Mismatched thread/seat geometry → check 10‑32 vs. ¼‑28 and coned vs. flat‑bottom before assembly. 
• Re‑using pre‑swaged SS ferrules in a different port can cause leaks and dead volume; in general, keep each swaged set with its original port. 
• Oversized coupler bore relative to tubing ID → creates a mixing pocket; always match IDs. 
• Color‑code confusion → use couplers that follow the same color scheme as the tubing to reduce selection errors at the bench. 

At‑a‑glance checklist

• What’s my tubing ID/OD? → Pick same‑ID coupler. 
• What’s the port geometry & thread on both sides? → Select the proper union style (10‑32 coned, ¼‑28 flat‑bottom, etc.). 
• Pressure & chemistry constraints? → Choose PEEK (bioinert, easy) or SS (UHPLC‑rated). 
• Color matching with tubing? → Prevents errors during swaps. 
• Install with bottomed‑out tubing and ZDV practice.