IDEX Health & Science Blog

As UHPLC has gained a solid foothold in the market, some of the problems that have plagued chromatographers – problems like band broadening, split peaks, carryover, etc. – have begun to occur with greater frequency. While numerous things may be contributing to these problems, one common thing is often overlooked: possible problems with the system’s tubing connections.

Problem #1 – Tubing Slippage

Because system pressures in UHPLC can exceed 15,000 psi (1,034 bar), the physical demands on the tubing connections can be very significant. Due to the higher pressures of UHPLC coupled with the pressure cycling that can occur in certain areas of the system (e.g., at the injection valve), one of the biggest problems UHPLC users can face is tubing slippage.

(Diagram 1 – Tubing Slippage)

In the areas of the system where the highest amount of pressure is experienced, the conically-shaped fittings work to hold the tubing against the bottom of the receiving ports in each connection. However, at the same time, the fluid in the system’s flow path fights against the holding power of the fittings and works to drive the tubing away from bottom of the receiving port. Of course, any movement of the tubing away from the bottom of the receiving port can result in dead volume, which can lead to the plethora of chromatographic problems highlighted above.

Unfortunately, this can be very difficult to detect because the movement of the tubing often occurs slowly. It’s possible for there to be no sign of problems with the flow path connections (e.g., no evidence of leaking, no loss of pressure, etc.), while the chromatographic results continue to deteriorate due to the increasingly larger dead volume chamber being formed in the receiving port.

Problem #2 – Tubing ID Compression

Because system pressures are so high – and because traditional polymer fittings can lead to tubing slippage as described above – chromatographers are typically using all-metal fittings in UHPLC applications to ensure the most reliable connections.
However, some of the most popular all-metal fittings require a high torque load to ensure they swage properly on the tubing wall. When the need for a high tightening torque is coupled with the axiom that says “you can never tighten too much,” the stage is set to significantly compress the ID of the flow path tubing, as seen below:

(Diagram 2 – ID Compression)

Compression similar to that shown above can result in several problems:

• Higher system back pressures
• A “throttling effect” on the fluid, resulting in increased turbulence, excessive sample – mobile phase mixing, etc.
• Increased probability of tubing clogs due to suspended particulate matter

Similar to the problem with tubing slippage, tubing ID compression is also difficult to detect. There are no leaks present and no loss of pressure, making this problem tough to troubleshoot and often resulting in unnecessary system down time as well as repair expenditures.

Problem #3 – Receiving Port Damage

The high torque required to successfully swage an all-metal fitting system on the flow path tubing doesn’t just result in tubing ID compression. Depending upon the amount of tightening torque applied, it’s possible for material to be ripped from the receiving port and transferred to the surface of the ferrule.

(Diagram 3 – Galling)

When galling occurs, the receiving port is damaged – possibly beyond use. In severe cases, the material that is ripped from the receiving port serves as a type of “glue” that forces the ferrule to stick in the receiving port. This makes it difficult to remove the fitting from the port and often results in a very expensive repair.

Even if galling doesn’t occur, it’s very typical for all-metal fittings to slightly damage the conical surface inside the receiving port. As seen below, it’s possible for a ferrule to indent the conical surface by as much as 150 µm.

(Diagram 4 – Profilometer Scan)

The more often all-metal fitting systems are used in standard receiving ports, the more likely permanent damage of some sort will take place.

While the challenges linked to UHPLC connections can be significant, it’s important to remember that most can be linked to the tubing connections. Therefore, before investing a small fortune in the replacement of expensive system components, take a little time to inspect the tubing connections and replace or update them if needed. This often will be much less expensive and result in less system down time.

To learn more about how our new VHP-300 fittings can help overcome these common challenges and help improve your overall chromatography, please click here.

About the author: John Batts is a Technical Specialist and chromatography expert at IDEX Health & Science. He is also the author of the “All About Fittings Guide,” which you can download here.