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Unions
Most plumbing in HPLC is done with 1/16" OD (outside diameter) tubing.
A union is used when it becomes necessary to join pieces of this tubing
together.
There are two basic types of unions commonly used in HPLC. Terminology in
HPLC is difficult due to the lack of standardization. Although, we will
call the two types External Unions and Internal Unions.
| External Unions |
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External unions have threads on the outside
of the union body. Therefore, they use fittings with internal
or female for the connection of tubing. The thru-holes of external
unions are typically .040" to .050" in diameter. This
is a relatively large ID (inside diameter) for HPLC; most tubing
is .010" or .020" ID. Why are there no external unions
available with smaller thru-holes? It is a practical manufacturing
consideration, it is simply too difficult to accurately drill
a long hole with a small diameter. A typical external union
is shown in [Figure
10].
External unions are used in non-critical areas for fluid transfer;
for example, they can be used between the pump and the sample injection
valve where thru-hole diameter is not a factor.
External unions should not be used after the sample injection valve
or before the detector. Once the sample has been introduced into the
system, smaller diameters are required to prevent dead-volumes (small
mixing chambers) that can seriously affect your results.
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| Internal Unions |
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Internal unions have threads on the inside of the union
body, and they use male, external fittings for the connection of tubing.
Internal unions have smaller IDs than external unions and are used
in the more critical areas of the HPLC system. It is in these areas,
after the injection valve, that swept-volume becomes a critical situation.
Many manufacturers call their unions "zero dead-volume unions"
when in reality they are small dead-volume unions. Let's look at what
a zero dead-volume union is and see how the various unions differ. |
| True Zero Dead-Volume Union |
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A true zero dead-volume union, or ZDV,
would connect two pieces of tubing without any dead-volume,
or mixing chamber, between the tubing. The tubing would butt
perfectly together as shown in [Figure
11]. A connection
of this type is difficult, if not impossible, to achieve. There
really is no way of knowing whether the tubing is butting together
or not, or if the tubing is butting in the center of the union.
[Figure
12] demonstrates this difficulty.
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| Webbed Union |
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The problems posed by a true zero dead-volume
union [Figure
13] & [Figure
14] are solved by the
kind of union most manufacturers term a ZDV. Although the two pieces
of tubing do not butt directly together, these unions come closest
to providing a reliable zero dead-volume connection.
The union body is designed to have a thin web of material in the center
[Figure 15]. A small thru-hole (matching the ID of the intended
tubing) is drilled through the center of the web allowing fluid to
flow from the tubing on one side of the web into the tubing on the
other side.
Even though this is not a true zero dead-volume union, it is a practical
solution and a generally accepted alternative to a true zero dead-volume
union. |
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