316L Electropolished Stainless Steel Coiltube for GC Carrier Gas Lines
Product Overview
The 316L Electropolished Stainless Steel Coiltube (Catalog No. 164) is a precision-engineered coiled capillary tubing specifically designed for Gas Chromatography (Gas Chromatography, GC) carrier gas delivery lines. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product serves as a critical component in the process analysis segment of analytical instrumentation, ensuring the transport of high-purity carrier gases—including Helium (He), Hydrogen (H₂), and Nitrogen (N₂)—from gas cylinders or regulators to the GC inlet with exceptional flow stability and zero contamination.
In gas chromatographic systems, the carrier gas line is the conduit through which the mobile phase is introduced at a precisely controlled flow rate. Any deviation in flow, adsorption of analytes on the tubing surface, or micro-leaks at joints can compromise chromatographic resolution, peak shape, and quantitative accuracy. This coiltube addresses these challenges through a combination of ultra-pure austenitic stainless steel, electropolished internal surfaces, and rigorous leak-tight manufacturing protocols.
Material & Properties
Base Material: 316L Austenitic Stainless Steel
The tubing is fabricated from SUS 316L (UNS S31603), a low-carbon variant of the 316 grade austenitic stainless steel. The "L" designation denotes a maximum carbon content of 0.03% (wt%), which significantly reduces the risk of intergranular chromium carbide precipitation during welding or thermal processing. This is essential for maintaining corrosion resistance in high-purity gas service.
| Property | Specification | Relevance to GC Application |
|---|---|---|
| Chemical Composition (Cr) | 17.0 – 19.0 % | Provides passive oxide layer for corrosion resistance |
| Chemical Composition (Ni) | 10.0 – 14.0 % | Enhances stability in reducing atmospheres (H₂) |
| Chemical Composition (Mo) | 2.0 – 3.0 % | Improves pitting resistance against chlorides |
| Chemical Composition (C) | ≤ 0.03 % | Prevents carbide precipitation; minimizes surface outgassing |
| Tensile Strength | ≥ 515 MPa (75 ksi) | Ensures mechanical integrity under pressure cycling |
| Yield Strength | ≥ 205 MPa (30 ksi) | Maintains dimensional stability under flow pressure |
| Thermal Conductivity | ~16.3 W/(m·K) at 25°C | Facilitates uniform temperature distribution along the coil |
| Surface Roughness (Rₐ, Inner) | ≤ 0.05 μm (post-EP) | Minimizes analyte adsorption and flow turbulence |
| Maximum Operating Temperature | −196°C to +593°C | Compatible with cryogenic and heated GC applications |
Electropolished (EP) Inner Surface
The inner wall of the capillary undergoes Electropolishing (EP), an electrochemical finishing process that removes the outermost layer of the metal surface, dissolving microscopic peaks and exposing a smoother, more chemically active substrate. The resulting surface achieves a roughness of Rₐ ≤ 0.05 μm, compared to typical mechanical polishing values of Rₐ ≈ 0.2–0.4 μm. This ultra-smooth surface:
- Reduces adsorption of polar and semi-volatile analytes, which is critical for maintaining chromatographic peak shape and resolution
- Minimizes dead volume effects and flow resistance, contributing to stable carrier gas flow rates
- Eliminates surface crevices where contaminants or particulate matter could accumulate
- Enhances the passive chromium oxide layer, improving long-term corrosion resistance in the presence of trace moisture or halides
Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Material Grade | 316L (UNS S31603 / SUS 316L) | Low-carbon austenitic stainless steel |
| Tube Type | Cold-drawn capillary coiltube | Seamless; precision cold-drawn for dimensional accuracy |
| Outer Diameter (OD) | ≤ 3.0 mm | Typical range: 1.0 mm, 1.5 mm, 1.6 mm, 2.0 mm, 3.0 mm |
| Inner Diameter (ID) | 0.3 – 2.0 mm (customizable) | Selected based on required flow rate and pressure drop |
| Wall Thickness | 0.1 – 0.5 mm | Optimized for pressure rating and flexibility |
| Surface Finish (Inner) | Electropolished (EP), Rₐ ≤ 0.05 μm | Ultra-smooth for minimal analyte adsorption |
| Surface Finish (Outer) | Commercial passivated / polished | Corrosion-resistant external surface |
| Coil Configuration | Pre-coiled / Coiltube (盘管) | Compact layout; reduces installation space |
| Coil Diameter | Customizable (typically 50–200 mm) | Adaptable to instrument panel layouts |
| Tube Length | Custom (typically 0.5 m – 10 m per coil) | Ordered to specification |
| End Preparation | Flush-cut / deburred / threaded / compression fitting | Compatible with Swagelok, VCR, or custom fittings |
| Maximum Working Pressure | Up to 30 MPa (4350 psi), depending on OD/ID | Exceeds typical GC carrier gas pressure requirements |
| Leak Tightness | ≤ 1 × 10⁻⁹ mbar·L/s (Helium leak detection) | Verified via high-sensitivity helium mass spectrometry |
| Compatible Gases | He, H₂, N₂, Ar, O₂, Air (high-purity) | Non-reactive with all common GC carrier gases |
| Purity Grade | Compatible with 5.0–5.5 (99.999%–99.9999%) carrier gas | No outgassing or contamination of ultra-high-purity gas |
| Flow Stability | ±0.5% flow rate variation under steady-state conditions | Contributed to by uniform bore geometry and smooth EP surface |
Manufacturing Process
The production of this GC-grade coiltube follows a rigorous multi-stage manufacturing protocol designed to ensure dimensional precision, surface integrity, and leak-tight performance:
1. Raw Material Selection and Inspection
Virgin 316L seamless capillary stock is sourced from certified mill suppliers with documented chemical analysis and mechanical property test reports. Incoming material is inspected for surface defects, dimensional conformance, and chemical composition via Spectroscopy (Optical Emission Spectrometry, OES) and X-Ray Fluorescence (XRF) analysis. Only material meeting ASTM A269 / ASME SA-269 standards is accepted.
2. Precision Cold Drawing (重拉)
The seamless capillary undergoes precision cold drawing through hardened tungsten carbide dies to achieve the final OD and ID dimensions with tolerances of ±0.01 mm. Cold drawing:
- Refines the grain structure and eliminates casting inclusions
- Produces a uniform bore geometry critical for laminar flow and flow stability
- Induces beneficial compressive residual stresses on the surface, enhancing fatigue and corrosion resistance
- Eliminates the need for post-drawing welding, avoiding potential contamination sources
3. Electropolishing (EP) of Inner Surface
The cold-drawn capillary is subjected to electropolishing using a phosphoric acid-based electrolyte solution under controlled current density (typically 15–25 A/dm²) and temperature (50–60°C). This process:
- Anodically dissolves the surface layer (~10–20 μm removed)
- Produces a surface with Rₐ ≤ 0.05 μm and a chromium-enriched passive layer
- Removes surface contaminants, oils, and machining residues
- Is performed in a controlled environment to prevent recontamination
4. Passivation and Cleaning
Post-EP, the tubing undergoes acid passivation (nitric acid or citric acid solution) to restore and stabilize the chromium oxide passive film. The tubing is then cleaned using deionized water (DI water, ≤ 1 μS/cm) and dried with filtered dry air or nitrogen to prevent water spots or particulate contamination.
5. Coiling and Forming
The finished capillary is coiled using precision forming equipment that maintains a consistent coil diameter without introducing kinks, ovality, or plastic deformation. The coiling process is designed to:
- Preserve the internal surface finish and bore geometry
- Produce uniform pitch and coil diameter for compact installation
- Avoid work-hardening or cold-work damage that could compromise material properties
6. End Preparation and Fitting Installation
Tube ends are flush-cut, deburred, and prepared for connection to instrument fittings. Options include:
- Compression fittings (e.g., Swagelok-style) for field assembly
- Threaded ends with NPT or BSPP threads for permanent connections
- VCR (Variable Compression Radius) fittings for ultra-high vacuum or high-purity gas applications
- Custom end preparation per customer specification
7. Helium Leak Detection (氦检漏)
Every coil assembly undergoes helium mass spectrometric leak detection in accordance with ISO 14644 and ASTM E2492 standards. The process involves:
- Purging the coil with nitrogen to establish a known baseline
- Applying helium as a tracer gas at all joints and connections
- Detecting helium ingress via a high-sensitivity mass spectrometer
- Verifying leak rates ≤ 1 × 10⁻⁹ mbar·L/s (equivalent to ≤ 1 × 10⁻⁹ atm·cc/s)
This leak rate corresponds to a helium permeation of less than 1 ppb per year under standard GC operating conditions, ensuring long-term carrier gas purity and flow stability.
8. Final Inspection and Certification
Each batch undergoes final dimensional inspection (OD, ID, wall thickness via ultrasonic thickness gauging), visual surface inspection under magnification, and chemical cleanliness verification. A Mill Test Report (MTR) and Certificate of Conformance (CoC) accompany every shipment.
Applications
Primary Application: Gas Chromatography (GC) Carrier Gas Lines
This coiltube is purpose-engineered for use as the carrier gas delivery line in gas chromatographic systems. In GC analysis, the carrier gas (mobile phase) transports vaporized analytes through the separation column. The coiltube connects the gas cylinder/regulator to the GC inlet valve, and may also be used for:
- Carrier gas supply lines from bulk cylinder to instrument panel
- Makeup gas lines for split/splitless inlet systems
- Carrier gas lines for Headspace GC (HS-GC) and Gas Chromatography–Mass Spectrometry (GC-MS) systems
- Carrier gas lines for Gas Chromatography–Flame Ionization Detector (GC-FID), Thermal Conductivity Detector (GC-TCD), and Electron Capture Detector (GC-ECD) configurations
- Carrier gas lines for Gas Chromatography–Infrared Spectrometry (GC-IR) and GC-MS-MS systems
Compatible Carrier Gases
| Carrier Gas | Typical Purity | Application Notes |
|---|---|---|
| Helium (He) | 5.0 (99.999%) to 5.5 (99.9999%) | Most common GC carrier; inert; used with FID, TCD, ECD |
| Hydrogen (H₂) | 5.0 (99.999%) to 5 |