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HPLC Fittings
Fittings—typically comprised of a nut and a ferrule—have the important
function of providing the physical connection of tubing throughout an HPLC
system. However, while simple in function, fittings exhibit some complexity
regarding their description and how they are used.
To properly describe the type of fitting necessary for a particular connection,
several things must be taken into account. General descriptive terms include
the geometry of the receiving port (coned or flat-bottom); the size tubing
for which the fitting is designed; and a mechanical designation of the threads
on the nut portion of the fitting (e.g. 10-32, 1/4-28, etc.). Beyond these
general categories, fittings may be classified by the overall dimensions
(length, maximum diameter, etc.) and the type of material from which they
are manufactured: stainless steel or polymer-based.
| Stainless Steel Fittings |
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Although restrictive in use and non-universal in application,
stainless steel fittings remain the most popular fitting used on standard
HPLC systems today, due to their solvent inertness and high-pressure
holding abilities. These fittings usually come in a conical configuration,
requiring special care to be taken in choosing the proper fittings
for any given mating port.
In general, male stainless steel nuts vary in overall length and thread
length, and these two dimensions are often manufacturer-specific [Figure
4].
Also, stainless steel ferrules are available in a variety of shapes
[Figure
5].
Therefore, to ensure proper operation and long life from the chosen
fitting, it is best to use fittings specifically designed for use
with a particular manufacturer's port.
Besides the array of fittings available for use with different manufacturers'
equipment, stainless steel fittings exhibit one attribute which separates
them from polymer-based fittings; in order to use them properly, the
ferrule portion of the fitting must be swaged-or permanently attached-to
the tubing it is connecting. To do this properly, Upchurch Scientific
recommends the following procedure:
Place the nut and ferrule, in that order, on the tubing [Figure
6]. Place the loosely assembled fitting into a mating port,
and tighten the nut finger tight. While ensuring the tubing is bottomed
out inside the mating port, tighten the nut with a wrench an additional
3/4 of a turn. The ferrule should now be permanently attached to the
tubing.
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| PLEASE NOTE: Because the ferrule is permanently
attached to the tubing and because of standard machining tolerances,
Upchurch Scientific highly recommends that any pre-swaged stainless
steel fitting only be used in the mating port into which it
was initially swaged. Failure to do this may result in dead
volume or solvent leakage. |
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Additionally, for proper tightening of
a pre-swaged stainless steel fitting into its mating port, Upchurch
Scientific recommends wrench tightening only an additional 1/4 to
1/2 of a turn past finger tight, followed by subsequent monitoring
of the connection for any leaks. Should leaking occur, simply continue
to tighten the fitting a little at a time until the leak stops. Also,
should it become necessary to tighten the fitting more than one complete
revolution past finger tight, Upchurch Scientific recommends that
the fitting be replaced, as excessive tightening is typically indicative
of a damaged product.
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| Polymer-Based Fittings |
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Unlike their stainless steel counterparts, polymer-based
fittings are typically non-restrictive and fairly universal in function.
And while stainless steel fittings remain the most popular fitting
used with HPLC systems, polymer-based fittings are continuously gaining
in popularity not only due to their near-universal application but
also based on their ease of use.
Because of the nature of polymer-based fittings, the same degree of
care does not have to be taken when choosing the proper fitting to
mate with a specific manufacturer's receiving port. Primarily, the
only two characteristics of the fitting which must be known are the
geometry (coned or flat-bottom) of the receiving port and the thread
dimensions.
Also, and again unlike stainless steel fittings, polymer-based fittings
do not permanently attach to a piece of tubing and usually do not
require the use of any tool (besides your fingers!) to properly tighten
and use. Additionally, these fittings often come in a variety of polymers,
including PEEK™, Teflon®, Tefzel®,
Delrin®, PPS, polypropylene, and others,
for maximum cost and solvent resistance flexibility. |
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