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Fittings & Ferrules
Fittings & ferrules are used to ensure safe and tight connections between different parts of the HPLC system (column, detector, injector, etc.). They are made of stainless steel or plastic (usually PEEK). An optimal connection is crucial to avoid leaks and prevent additional dead volumes.
Fittings are connectors used to securely connect ports and capillaries. Stainless steel fittings offer high strength and chemical resistance, while PEEK fittings are valued for their flexibility and biocompatibility.
Ferrules are cylindrical components with a conical tip that firmly enclose the capillary when the fitting is tightened. Depending on the design, material and pressure requirements, ferrules can be found pointing towards or away from the port. Ferrules can be independent components or integrated into the fitting.
Technical Data
Outer shape of the fitting
Fittings consist of two main parts: The nut, which builds up the contact pressure when screwed tightly, and a ferrule that transfers the pressure to the piece of tubing and encloses it tightly. The nut is essential for the shape of the fitting, as the head and thread are part of the nut.
The nut and ferrule can also be connected in one component, which is the case with some plastic fittings. With stainless steel fittings, however, these are always two individual parts.
Head
Nuts are available with a variety of different heads, which are either tightened using tools or simply by hand.
Heads that have to be used with tools exist in the following versions:
- Hex head
- Wrench Flat
Heads that can be used by hand are
- Knurl
- Winged Head
Thread
- Thread sizes can be specified in inches or in metric units
- Always two numbers: 1. diameter and 2. turns
- In inches: 1. diameter in inches and 2. number of turns per inch (e.g. ¼'' - 28)
Alternatively: 1. span in turns and 2. turns per inch (e.g. 10 - 32) - Metric: 1. diameter in mm and 2. distance between turns in mm (e.g. M6 x 1)
- Attention! Metric and inch are inverse to each other when specifying the turns!
High and low pressure fittings
Depending on their robustness, connections can be roughly divided into high-pressure connections and low-pressure connections. In HPLC applications, for example, high-pressure fittings are required to connect your column, while low-pressure connections are generally used for pumps.
With high-pressure connections, the port and ferrule have a conical shape that tapers towards the port. Permanent stainless steel connectors or polymer-based fingertight fittings are used here.
In the case of a low-pressure connection, the port is straight with a flat base. The nose of the ferrule points away from the port towards the nut. Flange fittings or flangeless fittings are available here.
Steinless steel connectors
- Nut is tightened inside the port, ferule is pressed onto the hose
- Tool is required
- Only possible with stainless steel fittings
- Ferrule permanently connected to capillary
- Very high tightness and pressure resistance
- Fitting cannot be readjusted or used in other ports
Fingertight fittings
- Fitting is screwed on by hand
- Made of plastic (often PEEK)
- Biocompatible
- Flexible and can be adapted as often as required, port can be changed
- Fitting can be replaced independently of the tube
- Nut and ferrule may be connected to form one component
- Not quite as robust as stainless steel
Flange connections
- Hose is expanded and pressed flat against the bottom by the fitting
- Easy to use
- Cost effective
- Similar flexibility to Fingertight fittings
- Can be difficult to repair
Flangeless fittings
- Flange replaced by additional fitting with external ferrule
- No expansion of the hose necessary
- Similar flexibility to Fingertight fittings
- Easier to repair than a flange connection
Compatibility
Before purchasing a fitting, you must ensure that it is compatible with the other components in your system. The following points should be considered:
- The diameter and thread of the nut must be selected to match your port
- The outer diameter of your capillaries must match the thru-hole of the fitting
- The material combination between fitting, capillary and port must be compatible (see table)
- Note the manufacturer 's special features (e.g. Waters uses its own connections that are not compatible with other fittings!).
Fitting material | Capillary material | Port material | Compatible |
Plastic | Plastic | Plastic | + |
Plastic | Stainless steel | Plastic | + |
Plastic | Stainless steel | Stainless steel | + |
Plastic | Plastic | Stainless steel | + |
Stainless steel | Stainless steel | Stainless steel | + |
Stainless steel | Plastic | Stainless steel | - |
Stainless steel | Plastic | Plastic | - |
Stainless steel | Stainless steel | Stainless steel | - |
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