Overview

This document explains how Controlled‑Flow™ CE capillaries function, including the chemistry of their inner coating, how sulfonic acid groups generate a robust and pH‑independent electroosmotic flow (EOF), and why these capillaries deliver greater stability than bare fused silica. Unlike standard CE capillaries—where EOF depends heavily on buffer pH and surface silanol ionization—the Controlled‑Flow capillary uses a neutral polymer coating with a controlled number of sulfonic acid groups. These groups are fully ionized at all pH levels, creating a consistent, strong EOF.

The capillaries are manufactured in High, Medium, and Low flow versions based on differing sulfonic acid density, which can be advantageous for applications such as cIEF.

How the Controlled‑Flow CE Capillary Works

The Controlled‑Flow™ CE capillary begins as a standard bare fused silica capillary, but the inner surface is coated with a covalently bonded neutral polymer. This polymer incorporates a carefully controlled number of sulfonic acid groups.

These sulfonic acid groups are fully ionized at all pH levels, which makes the electroosmotic flow (EOF) independent of buffer pH. Because the coating provides a dense and stable negative charge, these capillaries generate an EOF that is:

• Stronger than that of bare fused silica
• More robust and consistent from run to run
• Unaffected by local pH variations, buffer depletion, or other common causes of EOF drift

The different flow rates—High, Medium, and Low Flow Controlled‑Flow capillaries—are produced by varying the amount of sulfonic acid groups in the coating. Charge density is precisely controlled during manufacturing, and each flow type exhibits a characteristic EOF strength.

These features make Controlled‑Flow capillaries especially valuable in applications such as capillary isoelectric focusing (cIEF), where consistent EOF and predictable mobility are important.