138
Valves
Application Note
Application Note
Application Notes
Fluidic Movement in Tubes
Q:
“Why can I load only up to half of the volume of the loop
in partial-flling method?”
A:
Sample occupies 2 µL of loop for every 1 µL loaded from the syringe.
For example, 10 µL of sample spreads out over the entire length of a
20 µL loop. Any additional sample loaded will overfow the end of the
loop and exit out to waste. Reproducibility is poor because the volume
of sample in the loop is different from the known volume originally
loaded by your syringe.
Fluid spreads in a parabolic shape through a tube instead of moving in one
plug because the velocity is different at the center of the tube than at the
walls. The velocity at the center of the tube is twice the average velocity,
and near the wall the velocity is almost zero, creating a parabolic shape. This
fuidic movement is called laminar fow. See Figure 1.
In dual mode injectors (see “Sample Loop Loading” Application Note on page 131)
the sample from the syringe needle loads directly into the sample loop. The sample
volume is known since there is
no sample waste. The laminar
fow phenomenon accounts for
the shape of the plot as shown in
Figure 2. Note that the plot has
three regions:
a) Partial-Filling Region. When
the volume dispensed is less than
half the loop volume, the curve is
linear. Sample has not reached the
end of the loop. Within this region,
performance depends on the
syringe and operator.
b) Nonlinear Region. When the
volume dispensed is between
half the loop volume and about
two loop volumes, the curve is
nonlinear. Sample is lost from the
loop, so reproducibility is poor. If
you dispense a volume equal to
the loop size, you are in this region
of poor performance.
c) Complete-Filling Region. When the volume of sample dispensed is several loop
volumes, the loop contains only pure sample, undiluted by residual mobile phase.
Within this region, reproducibility is highest.
In the single mode injectors the sample must pass through a connecting passage
before it reaches the sample loop. Since some of the sample dispensed from the
syringe remains in the connecting passageway, an unknown amount enters the
sample loop. Therefore, single mode injectors achieve high reproducibility only by
using the complete-flling method.
Sample
Flow
Tube Wall
Mobile Phase
Figure 1 Schematic of sample fow through mobile phase between tubing walls
Sample Dispensed (loop volumes)
1 2 3 4
40 80
constant
Figure 2 Sample mass (observed peak area)
vs. volume of sample dispensed from the
syringe, in units of loop volumes, injected
onto the column from a Rheodyne
®
dual
mode injector such as model 7725
How to Find and Fix Common Sample Injector Leaks
Leaks cause valuable sample loss. Nobody wants that. The key to the valve
holding pressure is the integrity of the sealing surfaces. If there is a scratch
on the sealing surface, or the needle seal in the rotor seal is damaged, a
leak may appear. It is also important to realize what appears to be a leak
can instead be a result of siphoning. The following are the three most
common situations in which fuid leaks occur.
1. If fuid leaks out of the needle
port only while loading the loop
(i.e., while pushing down on the
plunger of the syringe), the problem
is most likely that the needle seal or
the needle port ftting in the loop fller
port is not gripping the syringe needle
tightly enough. Tighten the needle
seal grip by pushing with the eraser
end of the pencil on the needle port
(See Figure 1). The tightening reduces
the hole diameter of the needle seal
and port ftting.
2. If fuid leaks continuously from the needle port or vent lines and/or from
the stator-to-stator ring interface, replace the rotor seal and/or stator face
assembly. Scratches on the rotor seal or cracks in the stator face assembly
allow mobile phase to escape and cause cross port leakage. Genuine
Rheodyne replacement rotor seals are listed on page 134.
3. If fuid leaks from the needle port and/or vent lines but eventually stops,
the cause is most likely siphoning and not a leak. Siphoning occurs if the
vent lines are lower or higher than the needle port. Adjust the vent line(s) so
that the outlet is at the same horizontal level as the needle port to prevent
siphoning. (See Figure 2).
For other leakage or injection troubleshooting, refer to the Rheodyne
Troubleshooting Guide for HPLC Injection Problems. You may download the
Guide from the Rheodyne web site: www.idex-hs.com under Support.
Figure 1 To reform the needle seal,
push the eraser end of a pencil
against the needle port
Figure 2 Needle port level compared to the level of vent line outlet:
(A) siphoning occurs when the vent line outlet is above the needle port level
(B) siphoning does not occur if the vent line outlet is the same horizontal level
as the needle port
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