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Technical Resources
Adapters andUnions
With all the different tubing sizes and threaded port confgurations,
scientists frequently use adapters to make connections. However,
adapters are not always the only choice, or even the best choice, when
making connections between dissimilar components.
Adapters have two different thread confgurations, such as 1/4-28 fat-
bottom to 10-32 coned, or 1/4-28 male fat-bottom to luer. Unions have
the same thread on both sides, such as 10-32 coned to 10-32 coned.
Please refer to Figure 9 for examples of adapters and unions.
Unions are typically less expensive than adapters while performing
equally as well. Thus, it is often advantageous to use a union wherever
possible. To determine whether a union or an adapter is appropriate
for a particular connection, frst determine if the connection is designed
for low pressure or high pressure. This is not always obvious, but you
can make some assumptions.
(For example, when connecting 1/16” OD PEEK
™
tubing to 1/8” OD
FEP or PFA tubing, you likely have a low pressure connection since
the connection pressure is limited by the amount of pressure the
fuoropolymer tubing can withstand.)
Once you know the pressure classification for your connection,
determine what connectors are available for that classifcation. For
the lowpressure example given, there are a number of unions available
with 1/4-28 internal fat-bottom geometry on both sides (see pages
40 – 41). Other options with matchingM6 and 5/16-24 internal threads
are also available (pages 40 and 51, respectively).
After you identify the connector needed, the focus turns to fnding
fttings that work with your tubing sizes to mate with each side of the
selected union. In our example, the connection is between 1/16” OD
and 1/8” OD tubing, and there are several 1/4-28 fat-bottom fttings
for both 1/16” and 1/8” OD tubing, such as those on pages 22 – 28.
Of course, a number of cases remain where only an adapter will do.
For recommendations onmaking typical threaded connections, please
see the “Connections Reference” on page 35.
Figure 10 — Internal Volume Defned
Fluid In
Fluid Out
Plugged
Dead
Volume
Swept
Volume
Connectors
Void, Dead and Swept Volume
When making connections, in most instances, what is of primary
importance is how much internal volume exists within a connection
and how that internal volume will impact chromatographic results.
Three terms describe the internal volume of a product: void volume,
dead volume and swept volume. Void volume is simply another way of
describing the total internal space within a connection into which fuid
can fow. Dead volume is that portion of the void volume that is out of
the intended fow path, while swept volume is that portion of the void
volume which is in the intended fow path (see Figure 10). Therefore,
Void Volume = Dead Volume + Swept Volume.
Dead volume, particularly in capillary connections, can cause
undesirable chromatographic effects, including:
Analysis delays
Broadened peaks
Poor resolution
Sample carry-over
Split peaks
Gas collection
Because of the negative impact of dead volume in a connection, all
dead volume should be removed from the connection if possible.
To keep most of the void volume truly swept volume, match the
tubing ID as closely as possible with the diameter of the holes in your
equipment. This ensures the fuid runs completely through the entire
passageway. Matching internal diameters also helps reduce turbulence
as the fuid passes through the connection.
Fittings Primer
Figure 9 — Adapters and Unions
Adapters
Unions
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