Over tightening your fittings has consequences.

If you use the correct fittings and seals in the gas chromatography system, and follow some simple steps, there is absolutely no need to over tighten any fitting to achieve a leak free seal.

For almost all fittings that use seals in a GC system, whether they are ferrules, septa or o-rings, the tightening process should be the same:

  1. Make sure the place where you are applying the fitting is cooled down and the gas flow is turned off
  2. Select the correct size fitting and seal for the application
  3. Correctly fit the fitting and seal together according to the manufacturer’s guidelines
  4. Finger-tighten the fitting on to the receiving part
  5. Using the appropriate tool (wrench or fingers) tighten the fitting and extra ¼ turn.
  6. Establish a gas flow and pressurize the section of the GC system.
  7. Use a Restek electronic leak detector (28500) to check for leaks around the fitting
  8. If you do detect a leak further tighten the fitting in 1/8 turn increments until the leak ceases.

Note: If you continue to notice a leak and you have done one complete revolution of the fitting STOP.  There is something wrong.  Rather than damaging the fitting, or the receiving part, remove the fitting, get a new one and a new seal and start again.

Common fittings and seal, and the consequences of over tightening

Septa

This is the easiest seal to get leak free.  You should not need a wrench; minimal finger tightening is usually sufficient.  If you over tighten you compress the septum.  This puts the septum under stress, and so when the septum is penetrated by a needle it is more easily ripped apart leading to coring and leaks.  Coring can result in particles of septum in the liner which can adversely affect chromatography results.

You should leak check with and electronic leak detector around the septum fitting every day.  It is one of the hardest working seals, and the easiest and cheapest to change.

Reducing Nut

This is the fitting at the bottom of the inlet where the column nut attaches.  Over tightening this nut can damage the threads on both the nut and the inlet fitting.  We have also seen the inlet bent by over tightening to such an extent that liners could not fit down inside.

For GCs like Agilent, Thermo Trace 1300/1310 and Perkin Elmer 590/690 the simple way to avoid over tightening, or the use of excessive force, is to use a dual vespel ring inlet seal.  The dual vespel ring allows for a soft but effective seal, and enables you to follow the tightening procedure above.

Dual Vespel Inlet Seals come in several different surface finishes.

It should be noted that when moderately high inlet temperatures are employed the dual vespel ring can shrink slightly, which can cause a leak, but that is easily remedied by a slight additional tightening after a couple of heat cycles.  With good, frequent leak monitoring with a Restek electronic leak detector this would not cause any issues.

Column Nuts

Over tightening column nuts has many consequences.  First you could crush the column, whether that is a fused silica or MXT column.  This in turn can lead to leaks and/or flow restrictions, and in some cases the column may break off in the fitting.  Second, the ferrule may crack which can lead to leaks.  Lastly the ferrule may deform so much that it may extrude into the fittings where it can block the flow or lead to leaks, and getting stuck.

Standard connections

These are like Swagelok or Parker fitting that are brass of stainless steel, and sometimes use a graphite ferrule.  Over tightening these fittings can easily lead to stripping the threads, especially in brass fittings.  We have seen where over tightening has led to the crushing of copper tubing, and this restricting flow, and in extreme cases cracking the tubing.  If you use graphite ferrules in these fittings, as before the graphite can be extruded into the fitting which can cause leaks.

Never mix material.  If you have a steel fitting, don’t use a brass nut, and vice versa.  This can lead to easier thread stripping.  Also as a rule of thumb use brass fitting with copper piping and stainless steel fittings with stainless steel tubing.

Screw top vials

Don’t over tighten screw top vials.  This can pinch the septum resulting in a poor fit and potential for loss of sample.  We have seen it where an over tightened cap will pinch the septum so much that when a syringe tries to penetrate the septum it pushes it right into the vial.  It is almost impossible to leak check this seal.  So you have two choices: a gentle touch, or switch to crimp top vials.

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One Response to “Over tightening your fittings has consequences.”

  1. Charles Simons says:

    Great information, Mark.

    The weldment is another area that I have seen over tightened. The threads on the injection port and/or weldment can become damaged. I have seen instruments, where the glass liner ends up cracked or chipped because the weldment is over tightened.

    The GC/MS interface nut is another area that comes to mind. If one cranks down on that nut, the soft brass will deform and create a leak. Worst case scenario is snapping of the tip of the interface. That breaks the vacuum on the MS and can cause a big headache.

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