Adapters&Unions
With all thedifferent tubing sizes and threadedport configurations,
scientists frequently use adapters tomake connections. However,
adaptersarenotalways theonlychoice,oreven thebestchoice,when
makingconnectionsbetweendissimilar components.
Adaptershave twodifferent threadconfigurations, suchas1/4-28flat-
bottom to10-32coned,or1/4-28maleflat-bottom to luer.Unionshave
the same threadonboth sides, suchas 10-32 coned to10-32 coned.
Please refer toFigure9 forexamplesof adaptersandunions.
Unions are typically less expensive than adapterswhileperforming
equallyaswell.Thus, it isoftenadvantageous touseaunionwherever
possible. Todeterminewhether aunionor anadapter isappropriate
foraparticularconnection,firstdetermine iftheconnection isdesigned
for lowpressureor highpressure. This isnot alwaysobvious, but you
canmake someassumptions.
(For example, when connecting 1/16”ODPEEK tubing to 1/8”OD
FEPor PFA tubing, you likely have a lowpressure connection since
the connection pressure is limited by the amount of pressure the
fluoropolymer tubingcanwithstand.)
Once you know the pressure classification for your connection,
determinewhat connectors are available for that classification. For
the lowpressureexamplegiven, thereareanumberofunionsavailable
with 1/4-28 internal flat-bottomgeometry onboth sides (seepages
40–41).OtheroptionswithmatchingM6and5/16-24 internal threads
arealsoavailable (pages 40and50, respectively).
After you identify the connector needed, the focus turns to finding
fittings thatworkwith your tubing sizes tomatewitheach sideof the
selectedunion. Inour example, theconnection isbetween1/16”OD
and1/8”OD tubing, and thereare several 1/4-28flat-bottomfittings
for both1/16”and1/8”OD tubing, suchas thoseonpages 22–28.
Of course, anumber of cases remainwhereonly an adapter will do.
Forrecommendationsonmakingtypical threadedconnections,please
see the“ConnectionsReference”onpage35.
Figure 10— Internal VolumeDefined
Fluid In
FluidOut
Plugged
Dead
Volume
Swept
Volume
Connectors
Void,Dead, andSweptVolume
Whenmaking connections, inmost instances, what is of primary
importance is howmuch internal volumeexistswithin a connection
andhow that internal volumewill impact chromatographic results.
Three termsdescribe the internal volumeof aproduct: void volume,
deadvolume,andsweptvolume.Voidvolume issimplyanotherwayof
describing the total internal spacewithinaconnection intowhichfluid
canflow.Deadvolume is thatportionof thevoidvolume that isoutof
the intendedflowpath,whileswept volume is thatportionof thevoid
volumewhich is in the intendedflowpath (seeFigure10). Therefore,
VoidVolume=DeadVolume+Swept Volume.
Dead volume, particularly in capillary connections, can cause
undesirablechromatographiceffects, including:
XX
Analysis delays
XX
Broadenedpeaks
XX
Poor resolution
XX
Sample carry-over
XX
Split peaks
XX
Gas collection
Becauseof thenegative impact of dead volume in a connection, all
dead volume shouldbe removed from theconnection if possible.
To keepmost of the void volume truly swept volume, match the
tubing IDascloselyaspossiblewith thediameterof theholes inyour
equipment. Thisensures thefluid runs completely through theentire
passageway.Matching internaldiametersalsohelpsreduceturbulence
as thefluidpasses through theconnection.
Figure 9—Adapters andUnions
ADAPTERS
UNIONS
See the full rangeof
Micro/Nano connections!
186
TECHNICALRESOURCES
Fittings Primer
