316L EP Capillary Coil Tube for HPLC Mobile Phase Delivery Systems
Product Category: Stainless Steel Coil Tube — Instrumentation & Process Analysis
Application: High Performance Liquid Chromatography (HPLC) Mobile Phase Tubing
Product Overview
This precision-engineered 316L Electropolished (EP) Capillary Coil Tube is specifically designed for use as mobile phase delivery and transfer tubing in High Performance Liquid Chromatography (HPLC) systems. In HPLC analytical workflows, the mobile phase typically comprises a combination of organic solvents (e.g., acetonitrile, methanol, tetrahydrofuran) and aqueous buffer salts (e.g., phosphate, acetate, formate buffers). These chemically aggressive media pose significant challenges to tubing integrity, including corrosion, metal ion leaching, and surface contamination that can compromise analytical accuracy.
Conventional stainless steel tubing often suffers from trace metal ion release (Fe, Cr, Ni, Mo) into the mobile phase, leading to column contamination, detector interference, baseline noise elevation, and reduced column lifetime. This product addresses these critical issues through a combination of ultra-pure 316L base material, electropolishing surface treatment, and rigorous leachability validation, ensuring minimal metal ion contribution to the chromatographic system.
Material & Properties
Base Material: ASTM A269 / AMS 5510 316L Stainless Steel
- Designation: UNS S31603 (316L)
- Carbon Content: ≤ 0.03% (low carbon grade to prevent sensitization and intergranular corrosion)
- Molybdenum Addition: 2.0–2.5% (enhanced resistance to pitting and crevice corrosion in chloride-containing environments)
- Chromium Content: 16.5–18.5% (passive oxide film stability)
- Nickel Content: 10.0–14.0% (austenitic stabilization)
Surface Treatment: Electropolishing (EP)
- Process: Electrochemical dissolution of the surface layer, resulting in a smooth, pore-free, and chemically inert surface finish
- Achieved Surface Roughness (Ra): ≤ 0.25 µm (compared to typical 0.6–1.6 µm for mechanically polished tubing)
- Benefits:
- Removal of surface impurities, decarburized layer, and grinding debris
- Enrichment of Cr and Mo in the near-surface region (enhanced passive film)
- Elimination of micro-voids and pores that can harbor contaminants
- Reduced surface area exposure to mobile phase, minimizing metal dissolution
Key Material Properties
- Bio-inertness: Verified compliant with USP <661> and <665> requirements for pharmaceutical-grade tubing
- Low Metal Leaching: Meets or exceeds ASTM F899 standard for metal ion release limits
- Chemical Resistance: Excellent resistance to organic solvents (acetonitrile, methanol, THF, DCM), aqueous buffers (pH 2–12), and dilute mineral acids
- Corrosion Potential (Pit Initiation): ≥ 200 mV (in 3% NaCl solution, per ASTM G150)
- Non-magnetic: Austenitic structure (≤ 0.5 µm/m magnetic permeability)
Specifications
| Parameter | Specification | Standard / Test Method |
|---|---|---|
| Material Grade | 316L (UNS S31603) | ASTM A269 / AMS 5510 |
| Surface Finish | Electropolished (EP), Ra ≤ 0.25 µm | ASTM A463 |
| Outer Diameter (OD) Range | 0.127 mm (1/32") to 3.175 mm (1/8") | — |
| Wall Thickness Range | 0.025 mm to 0.508 mm | — |
| Coil Winding Diameter | 10 mm to 200 mm (customizable) | — |
| Coil Length | Up to 50 meters per coil (standard) | — |
| Pressure Rating (at 20°C) | Up to 34.5 MPa (5000 psi) | ASTM A269 |
| Temperature Range | −196°C to +450°C (continuous) | — |
| Fe Leaching Rate | ≤ 1 µg/L (in mobile phase, 24-hour immersion) | ASTM F899 |
| Cr Leaching Rate | ≤ 0.5 µg/L (in mobile phase, 24-hour immersion) | ASTM F899 |
| Ni Leaching Rate | ≤ 0.5 µg/L (in mobile phase, 24-hour immersion) | ASTM F899 |
| Mo Leaching Rate | ≤ 0.3 µg/L (in mobile phase, 24-hour immersion) | ASTM F899 |
| Surface Roughness (Ra) | ≤ 0.25 µm (ID & OD) | ASTM A463 |
| Passivation | Post-EP passivation per ASTM A967 | ASTM A967 |
| Biocompatibility | USP <661> / <665> compliant | USP General Chapters |
| Winding Tolerance | ±0.5 mm | — |
| End Condition | Flush-cut, burr-free (±0.01 mm chamfer) | — |
Manufacturing Process
Step 1: Raw Material Selection & Certification
Virgin 316L stainless steel bar stock or seamless tube is sourced from certified mill suppliers with full traceability documentation. Each heat lot is accompanied by a Mill Test Certificate (MTC) verifying chemical composition, mechanical properties, and purity. The carbon content is verified to be ≤ 0.03% to ensure resistance to sensitization during subsequent heat treatment cycles.
Step 2: Seamless Tube Forming
The selected grade is processed through hot-rolling, cold-drawing, and cold-drawing to final dimensions. For capillary-grade tubing (OD < 1 mm), multi-pass cold drawing through precision tungsten carbide dies ensures dimensional accuracy within ±0.005 mm tolerance on both ID and OD.
Step 3: Solution Annealing & Pickling
Tubing is solution-annealed at 1050–1100°C followed by controlled water quenching to achieve a fully austenitic, stress-free microstructure. Subsequent acid pickling (nitric/hydrofluoric acid blend) removes any oxide scale formed during annealing, restoring a clean, passive surface.
Step 4: Electropolishing (EP)
The core differentiating process involves controlled electrochemical dissolution using a phosphoric acid/sulfuric acid electrolyte at temperatures of 50–60°C. The process parameters (voltage, current density, time, temperature) are precisely controlled to achieve:
- Uniform surface smoothing to Ra ≤ 0.25 µm
- Removal of 3–10 µm of surface layer containing impurities
- Enrichment of the passive Cr₂O₃/MoO₃ oxide layer
- Pore elimination and surface pore density reduction to < 50 pores/cm²
Step 5: Post-EP Passivation & Cleaning
Following electropolishing, tubing undergoes citric acid or nitric acid passivation per ASTM A967 to ensure a stable, protective chromium oxide passive film. Tubing is then cleaned using deionized water (≥ 18.2 MΩ·cm) and packaged in cleanroom conditions (ISO Class 7 or better) to prevent post-processing contamination.
Step 6: Precision Coiling & Winding
Tubing is wound into coils using CNC-controlled coiling machines that maintain constant winding tension and precise diameter control (±0.5 mm). Coiling is performed on mandrels with surface protection to prevent mechanical damage or scratches. Winding patterns (single-layer, multi-layer, figure-eight) are selected based on application requirements for pressure drop minimization and flow uniformity.
Step 7: Quality Inspection & Testing
Every production batch undergoes comprehensive testing:
- Dimensional Inspection: OD, ID, wall thickness, coil diameter via optical and laser micrometry
- Surface Roughness Verification: Confocal microscopy or profilometry confirming Ra ≤ 0.25 µm
- Chemical Composition Analysis: Spark OES or ICP-OES verification
- Leachability Testing: Immersion in simulated HPLC mobile phase (50% acetonitrile / 50% 10 mM phosphate buffer, pH 7.0) for 24 hours at 25°C; ICP-MS analysis of Fe, Cr, Ni, Mo, Ti, Cu concentrations
- Pressure Testing: Hydrostatic pressure test at 1.5× rated pressure for 30 minutes
- Visual Inspection: 10× magnification for surface defects, scratches, or discoloration
Applications
Primary Application: HPLC Mobile Phase Delivery
This coil tube serves as the mobile phase transport line connecting solvent reservoirs to the HPLC pump, and as post-column transfer tubing between the detector and waste or fraction collector. In process analytical technology (PAT) applications, it is used for in-line monitoring loops and real-time quality control sampling lines.
Specific Use Cases:
- Pharmaceutical Process Analysis: In-line monitoring of critical quality attributes (CQAs) during bioprocess manufacturing, where metal ion contamination is unacceptable for product purity specifications
- Environmental Monitoring HPLC: Analysis of trace organic contaminants in water and soil samples, where ultra-low metal background is essential for detection limit compliance
- Food & Beverage Analysis: Quantification of additives, preservatives, and contaminants at ppm/ppb levels
- Clinical Laboratory Diagnostics: Therapeutic drug monitoring (TDM) and biomarker quantification requiring contamination-free sample handling
- Forensic & Toxicology: Analysis of trace drug residues where any metal contamination could interfere with mass spectrometry detection
- Process Analytical Technology (PAT): Real-time in-situ monitoring of reaction progress and endpoint determination in continuous manufacturing
Mobile Phase Compatibility Matrix:
| Mobile Phase Component | Concentration Range | Compatibility |
|---|---|---|
| Acetonitrile (ACN) | 0–100% | Excellent |
| Methanol (MeOH) | 0–100% | Excellent |
| Tetrahydrofuran (THF) | 0–100% | Excellent |
| Dichloromethane (DCM) | 0–100% | Excellent |
| Phosphate Buffer | 1–100 mM, pH 2–8 | Excellent |
| Acetate Buffer | 1–100 mM, pH 3–6 | Excellent |
| Formate Buffer | 1–100 mM, pH 2–5 | Excellent |
| Triethylamine (TEA) | 0.05–0.5% v/v | Good (with periodic flushing) |
| NaCl (high concentration) | > 500 mM | Moderate (risk of pitting) |
Quality Certifications & Compliance
| Certification / Standard | Scope | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Full manufacturing process |
| ISO 13485:2016 | Medical Device Quality Management | Biocompatible applications |
| USP <661> | Leachables Testing | Pharmaceutical-grade tubing |
| USP <665> | Extractables Testing | Pharmaceutical-grade tubing |
| ASTM F899 | Implantable Metallic Materials — Chemical Compositional Requirements | Metal leaching limits |
| ASTM A269 | Seamless Austenitic Stainless Steel Pipe and Tube | Material specification |
| ASTM A463 | Electropolished Stainless Steel Tubing | Surface finish specification |
| ASTM A967 | Passivation of Stainless Steel Parts | Post-EP passivation |