Knowledge Sharing – Fittings Used in Liquid Chromatography

Hello friends! We meet again! In the last article about connectors, I talked to you about the content of connectors (you can review the content of the previous issue if you haven’t read it) “A Brief Talk on Connectors Used in Liquid Chromatography (1)”. In addition to joints, the other major category of connectors is pipelines. In this issue, let’s talk about pipelines. Hope this content can help you too.

What factors should be considered when choosing a pipeline?

The purpose of the pipeline
Expected line pressure
Specifications and Materials of Acceptance Ports
chemical environment used
Whether disassembly is frequent

Classification of pipelines

Liquid phase piping is usually classified according to the size and material of the piping. The pipe specification is the outer diameter and inner diameter of the pipe, as well as the length. Materials are mainly divided into metal materials, non-metal materials, and composite pipelines using two or more different materials.

Specifications of the pipeline

The specification of piping is mainly composed of internal diameter (ID) and external diameter (OD), and piping in liquid phase systems is often described in imperial units. For example: OD 1/16″, ID 0.01″, where the 1/16″ and 0.01″ units are both inches, that is, the outer diameter is 1/16 inches (1.5875mm), and the inner diameter is 0.01 inches (0.254mm).

Let’s look at another example, OD 3.2mm, ID 2mm, which means the outer diameter is 3.2mm, the inner diameter is 2mm, and the metric unit is used here.

Pipe material

The liquid phase pipeline can be roughly divided into metal materials, non-metal materials and composite materials in terms of materials. Different materials have different chemical and physical properties, and the material can be selected according to the actual use.

The most commonly used metal material for liquid phase pipelines is 316 stainless steel, which has good pressure resistance, high temperature resistance, appearance gloss, and relatively good corrosion resistance. Other metal materials include titanium alloy, Hastelloy, etc., which are superior to 316 stainless steel in corrosion resistance. One of the outstanding advantages of metal materials over non-metal materials in liquid pipeline applications is better pressure resistance.

The types of non-metallic materials used in the liquid phase pipeline will be more diverse. Common are PEEK (polyetheretherketone), fused silica, PTFE (polytetrafluoroethylene), PFA (perfluoroalkylate), FEP (perfluoroethylene propylene copolymer), and ETFE (ethylene-tetrafluoroethylene copolymer) )Wait. Due to the diversity of materials, pipes of non-metallic materials will bring different chemical and physical properties, and suitable materials can be selected for some different environments.

The composite material used in the liquid phase pipeline is relatively rare. The characteristic of this material is that it can complement each other and produce synergistic effect, so that the comprehensive performance of the composite material is better than that of the original material. For example, PEEK is used as the outer layer material, fused silica is used as the inner lining material, and the PEEK-coated fused silica pipeline using polyimide in the middle layer has very excellent performance. It has high mechanical strength, good sealing adaptability, and inner diameter. Tight tolerances and good chemical compatibility.

How to choose the right pipeline

The selection of liquid pipeline needs to comprehensively consider the use of the pipeline, the use environment and pressure. For example, when the operating pressure of the pipeline is 100Mpa, the flow rate is 0.3mL/min, the mobile phase used is methanol, acetonitrile and water and other conventional reversed-phase reagents, and the pipeline is not frequently disassembled and assembled, 316 stainless steel can be selected as the pipeline material. The outer diameter of the pipe can be 1/16 inch, and the inner diameter should be less than 0.25mm (0.17mm or 0.12mm, etc.).

The pressure and material characteristics generated by the pipeline itself need to be considered when selecting the pipeline.

Common material pipeline characteristics

Material	Features
PEEK	PEEK tubing is both elastic and biocompatible, does not chemically react with the most commonly used solvents, and has a very smooth inner surface that is easily cut to the desired length. ✓ An excellent alternative to stainless steel tubing in high pressure applications. ✓ Available in a variety of colors and sizes to help identify inner diameters.
Fused silica	Due to the very tight internal diameter tolerances of fused silica, this tubing is used for microanalysis such as microliter and nanoscale HPLC and capillary electrophoresis. ✓ High quality polyimide coated fused silica.
316 stainless steel	Seamless pre-cut 316 stainless steel tubing meets the rigorous demands of today’s analyses. Sufficient treatment ensures that the pipeline is ready to use, and the two ends of the pipeline are smooth and free of burrs. ✓ Variety of inner, outer diameter and length options. ✓ Precut ensures a flat connection without burrs.
High-purity PFA	This polymer tube is made from top-quality PFA, the least polluting polymer on the market. ✓ Provides chemical stability, mechanical strength and purity suitable for medical, diagnostic, pharmaceutical, biotechnology and semiconductor applications. ✓ An excellent alternative to PTFE, which suffers from air permeability and surface texture issues.
FEP	FEP tubing is an excellent alternative to traditional PTFE tubing and is easy to use because it is non-chemically reactive with most solvents, easy to cut, and is translucent for easy monitoring of passing fluids. ✓ Ideal for common low voltage applications. ✓ Variety of sizes and colors for easy identification. ✓ Tight manufacturing tolerances ensure product consistency.
ETFE	ETFE is chemically inert and more suitable for high pressure applications (when using aqueous mobile phases) than PTFE, FEP and PFA. Additionally, since ETFE is stiffer than PTFE, FEP, and PFA, this tubing has better resistance to ID collapse. ✓ Good solvent resistance. ✓ More durable and less breathable than PTFE, FEP and PFA. ✓ Operating temperature up to 80°C.