Fittings are needed whenever you want to attach, adapt or adjust piping or tubing in HPLC systems. Fittings have two primary functions: They prevent liquid or gas from leaking out of the flow path and they hold tubing in place against opposing pressure. Chromatography supplies and accessories like fittings, tubes etc. have a big impact on the performance – so take your time to identify the correct type of fitting you need for your application.

What are fittings?

A fitting is a system that consists of a nut and a ferrule. In one-piece fitting systems, the nut and ferrule come as a single piece. The nut provides the driving force that causes the ferrule to seal. One important feature of nuts is the threaded portion, which allows nuts to mate with the receiving ports. Most nuts have external threads (threads are on the outside of the nut). 

Thread call-out 

The so called „thread call-out“ describes the thread on a fitting and shows two numbers. The first number refers to the diameter of the thread and the second number tells how many threads per inch you find on the nut. In high pressure applications, 10-32 threaded nuts are very common: denser threads mean more threads to “share the load“.  

Different types of fittings

Various kinds of fittings are used in laboratory systems: flanged and flangeless, metallic and non-metallic, high-pressure and low-pressure, flat-bottomed and coned, internal and external or wrench-tightened and finger-tightened.

One main characteristic of nuts is their head-geometry. To tighten hex head and wrench flat nuts you need to use a wrench. Fingertight fittings feature a knurled head or wings – you can easily tighten those fittings using no other tool than your fingers. 

Flanged fittings are often used in low-pressure applications. Most low-pressure receiving ports do not have a coned or “tapered” internal surface and require fittings that make a seal on the flat-bottom feature. Therefore, the flow path tubing is flared out at the end. However, in a flangeless connection, typically a separate fitting and an external compression ferrule are used. 

Which fitting to use in your system

To choose the best fitting for your application, first consider what the fitting is supposed to do and how the fitting will be used. Do you need it to hold high or low pressure? Do you plan to connect a piece of tubing to a receiving port and rarely take it out or do you want to use the tubing in multiple locations? Here, the material of the fitting needs to be considered. 

A number of parameters come into play when you decide which ferrule to use. Factors such as threads of the receiving port, the size and type of tubing used, the material from which the port is made and the amount of pressure expected need to be considered. Also, the head geometry (e. g. knurled, hexagonal, square or winged) and the threaded portion (which allows nuts to mate with their receiving ports) are relevant. Another aspect is the overall length of the nut: long nuts are ideal for use in angled ports to increase the gap between adjacent fittings.

Plastic or metallic fittings

Plastic fittings: You can use a plastic fitting with nearly any type of tubing. Just make sure the fitting will hold more than the pressure you expect with your application and your tubing is suited for the style of connection you want to make (e. g. external compression, internal expansion, etc.). Polymer fittings offer superior chemical compatibility and can hold to most required pressures. A wide variety of polymer fittings is available. Polymer fittings are reusable. If you want to use the tubing in multiple locations, or if you expect to make frequent connections with the fitting, a polymer fitting suits your needs perfectly.

Commonly used polymers are PEEK, PP, PCTFE or Delrin.

Metallic fittings: For UHPLC or other super high pressure applications, most analysts prefer metallic nuts and ferrules or other specialized fittings designed for high pressures. Metallic fittings should not be used on plastic tubing. When a metallic fitting crimps down on plastic tubing, it firmly squeezes the tubing wall and can cause severe damage. If you plan to connect a piece of tubing to a receiving port and rarely take it out, a stainless steel fitting might be a good option for you.