130
Valves
HighPressure SwitchingValves
High pressure manual switching valves simplify procedures and
improve the speed, resolution, and sensitivity of HPLC analysis. The
switching valves are available in 316 stainless steel and PEEK
™
, with a
choice of 1.6 mm (1/16”) or 3.2 mm (1/8”) ports. See Table on page
132 for valve specifcations.
Column Selection
The six-position switching valves are used for column selection. These
valves substitute one column for another without the need tomanually
disconnect the plumbing. This makes it easy to designate a separate
column for each analysis, which helps eliminate equilibration delays,
reduce interferences and prolong column life. Turning the valve handle
selects the column desired for a particular analysis. Columns switched
to off-line are automatically sealed at both ends.
Column Switching
The two-position switching valves can be used to reroute mobile
phase during the chromatographic run without changing separation
techniques. They can also be used to perform sequential separations
with different columns and/or mobile phases.
Although the model 7000 is the most commonly used and versatile
switching valve, other models have specifc uses such as for three-way
or four-way switching patterns.
Many models have fow passages available in both standard bore and
large bore (designated with an “L” suffx). L models use 1/16” fttings
and tubing but have larger fowpassage diameters than non-Lmodels.
As such, L models can accommodate higher fow rates. Large bore
tubing can be used when the pressure drop must be limited. Large
bore valves have a lower pressure drop than standard bore valves when
both valve sizes accommodate the same fow rate.
Effects of Valves andTubingonResolution
The effect of tubing on analytical and microscale analyses can be
signifcant. Since dispersion caused by tubing is proportional to the
fourth power of diameter, large bore tubing should be avoided when
performing analytical scale or microscale analyses. Tubing ID size
≤ 0.25 mm (0.010”) is recommended.
Consider a system with injector and column switching valves
and analytical columns with small-bore connecting tubing. The
chromatograms below, made using a typical analytical chromatograph,
show these effects. Scheme A is the control (injector column
detector) with no valve in the system. In Schemes B and C, two model
7060 Six-Position Switching Valves were placed side by side (injector
valve #1 column valve #2 detector).
The injector and detector were connected to these valves by the same
tubing used in the control. The extra tubing pieces required to connect
the valves to the column were a 10 cm length for valve #1-to-column,
and a 35 cm length for column-to-valve #2. The diameters of these
tubes are indicated in the experimental details, below.
Switching Valves
These chromatograms show the loss of resolution caused by the addition of
two model 7060 column selection valves when using connection tubes of two
different inside diameters. Conditions for all cases: 4.6 mm x 12.5 cm column,
5 µm C-18 packing, 50% acetonitrile in water, 2.0 mL/min, 21 °C, 5.0 µL
sample partial flled into a model 7125 injector, 10 cm x 0.18 mm (0.007”)
bore injector outlet tube (to column or valve), 10 cm x 0.18 mm bore
detector inlet tube (from column or valve), low dispersion 1.0 cm path UV
detector cell, 0.2 sec detector time constant. See text above for details.
A Column Only
C Valve w/ 0.020”
Tubing
B Valve w/ 0.007”
Tubing
Effects of Valves and Tubing
on Resolution
7725
8125
k’ = 0.6
2930
5054
72%
k’ = 1.5
4653
6904
48%
k’ = 7.9
7875
8305
5.0%
UV detector: 1 µL volume, 4 mm path. Sample volume: 2 µL, partial-flling method.
Column: 2 mm ID x 100 mm long, 4 µm C-18. True plates of column = 11,570.
Comparison of Observed Column Plates of
Rheodyne
®
Analytical and MicroScale Injectors
Column Selection Using a 2-Position, 6-Port Switching Valve
Detector
Detector
Column 2
Position A
Column 1
Injector
Position B
Injector
Column 2
Column 1
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