The invention relates to a pumping arrangement, for example for liquid
chromatography, wherein liquids from two or more reservoirs are delivered to an
hydraulic resistance, such as a chromatographic separation column. Such a pumping
arrangement can be used, for example, for gradient operation in preparative liquid
BACKGROUND OF THE INVENTION
High performance liquid chromatography (HPLC) is a well-known and
widespread chromatographic technique to separate and analyze liquid samples containing
different compounds of interest. The methodology of HPLC can be divided into two
areas: analytical and preparative scale liquid chromatography.
The difference is reflected in the instrumentation setup, which typically
consists of a liquid pump, an injection module, one or more chromatographic separation
columns, at least one detection module, and in preparative liquid chromatography,
always one or several fraction collection devices.
Analytical scale HPLC pumps are typically working in the flow rate
range of 1 - 5 ml /min. They use active pump valves. In most cases the inlet or
outlet valve has a spring support so that the liquid path in the pump head is actively
closed at no flow conditions. Thus, in a gradient system consisting of multi channel
pump modules, undesired mixing of different solvents will be prevented.
HPLC pumps working at flow rates above 10 ml/min up to several hundred
ml/min do not have active pump valves.
In order to achieve precise gradients at high flow rates, various
conditions with regard to liquid handling have to be fulfilled:
Three very important aspects, which are also addressed in the present invention,
are as follows:
- a) Filtration of liquids at high flow rates.
- b) Handling of liquid flow when using high volume liquid containers at different
storage levels in a laboratory.
- c) Management of the liquid priming process.
With regard to the mentioned aspects a) to c), the prior art and the
problems encountered are as follows:
SUMMARY OF THE INVENTION
- a) Filtration of liquids at high flow rates is important in order to prevent
access of fades or other small impurities that could enter into the pump head. They
can prevent proper valve sealing, causing flow irregularities. Furthermore they
may increase the system pressure over time, due to plugging of instrumental parts
or the column inlet itself.
Most often inlet filters made out of sintered metal are used. Over time they are
prone to become rusty or they may plug very soon because of their limited free surface
area and thus their overall reduced capacity. In practice they are prone to error
and have to be replaced very often. With bioanalytical applications, such as the
separation of proteins, stainless steel filtration devices are not the matter of
choice because of possible protein-metal interaction processes.
- b) High flow HPLC pumps require plungers having much greater cross section than
analytical pumps. As a matter of fact such plungers cause cavitations within the
valves very easily and will destroy the ball seat as well as the ball itself in
a very short period of time. In this case manual stop valves are used for interactive
work to prevent undesired crossflow between liquid containers at different storage
levels. For routine and automatic controlled systems electrically controlled stop
valves are used. However, they are expensive and limited in lifetime.
- c) Priming high flow HPLC instrumentation is of general importance. This can
be done by cumbersome manual sucking the air volume out of the tubing with a syringe.
This, however, is very inconvenient and time-consuming for the chromatographer.
It is thus an object of the invention to provide a pumping arrangement,
which avoids the problems and disadvantages of the prior art.
Specifically, it is an object of the invention to provide a pumping
arrangement with an improved filtering device having increased lifetime and filtering
It is a further object to prevent uncontrolled cross flow in case
that liquid reservoirs are stored at different height levels.
According to the invention, this object is achieved by a pumping arrangement
as defined in claim 1.
The pumping arrangement of the invention comprises:
- a first liquid reservoir at a first height level, connected via a first hydraulic
path to a pumping means;
- a second liquid reservoir at a second height level, connected via a second hydraulic
path to the pumping means; and
- first and second filtration devices arranged in the first and second hydraulic
path, respectively, wherein each filtration device comprises an outer member forming
a first chamber, and an inner member arranged within the outer member and forming
a second chamber, with the inner member being porous for allowing liquid flow from
the first chamber to the second chamber.
The invention has the additional advantages that it is particularly
suitable for handling non-degassed solvents and for the liquid priming process.
One of the characteristics of the present invention is that the filtration device
embodies several functions in one device, such as the filtering function and the
prevention of undesired backflow.
According to a preferred embodiment of the invention the filtration
devices are arranged at the same height level. In a further preferred embodiment,
the first (outer) chamber of the filtration device accommodates a two-phase system
comprising liquid from a liquid reservoir and vapor of this liquid.
It is another advantage of the invention that the filtration device
can be reused after blockage by impurities, since it can easily be cleaned, for
example with ultrasound cleaning. Furthermore the filtration device can easily be
sterilized before use.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following embodiments of the invention will be explained with
reference to the drawings.
- Figure 1
- illustrates an embodiment of a pumping arrangement of the invention.
- Figure 2
- shows an embodiment of the filtration device used in the pumping arrangement
according to Figure 1.
Figure 1 shows a preparative HPLC system with two high pressure pump
modules 10 and 11 to form solvent gradients. The inlet of the first pump module
10 is connected via a tube 12 to the outlet of a first filtration device 20. The
inlet of the filtration device 20 is connected via a tube 14 to a first solvent
reservoir. The height level of the first solvent reservoir is indicated at position
16. The inlet of the second pump module 11 is connected via a tube 13 to the outlet
of a second filtration device 21. The inlet of the second filtration device is connected
via a tube 15 to a second solvent reservoir. The height level of the second solvent
reservoir is indicated at position 17. The filtration devices 20, 21 will also be
referred to as "filter sink" devices 20,21, respectively.
The outlets of the pump modules 10 and 11 are connected via tubes
18 and 19, respectively, to a T-piece 23, from which a further tube 25 leads to
one or several separation devices 27, 28, 29. Arranged between the outlet of the
pump modules and the separation devices are suitable mixing and injection means
(not shown) for high precision solvent mixing before introducing samples into the
pressurized solvent stream.
In the embodiment of Figure 1, the filtration devices 20 and 21 are
identical. They are integrated into an HPLC system by means of a holder device that
keeps them at the same height level. The liquid levels in the filtration devices
are indicated at 20a and 21a, which coincide essentially with the dotted line 30.
The outlet of the T-piece 23 or of any further components, such as the separation
device 29, is at the same level or above the liquid levels 20a, 21 a, in order to
prevent uncontrolled flow at the high pressure side of the pumping arrangement.
Theoretically, the outlet at the high pressure side could also be below the level
30, if a resistance is arranged at the outlet which is so high that draining of
liquid is prevented.
In the following, the filtration device will be explained in more
detail with reference to Figure 2. The filtration device comprises an outer cylinder
1, which encloses a filtering member 2. The filtering member 2 has a cylindrical
shape with a predefined porosity and surface area. It is closed by a bottom at one
The housing of the filtering member is hermetically sealed at both
ends with two lids 3, each on either side, so that no air, any other gas or liquid
will be able to pass through. Each lid has a small tapped hole at its center. The
lids allow connecting the filtration device to further devices. In the present embodiment,
one of these devices is an HPLC pump module; the other one is a container, filled
with solvent. The housing has a predefined spatial orientation. The preferred orientation
is bottom upside of the filtering member.
At the outlet side the housing can be connected to the pump module,
using a tube. The tube holds a certain inner diameter. The diameter used has to
take minimal pressure drop into consideration. The tube itself contains a fitting
at both sides that exactly fits into the tapped hole of the chamber's lid 3. The
other end is connected to the pump module.
A second tube of the same characteristic is attached to the inlet
side of the filtering device. The other end of the tube is connected to the liquid
reservoir that contains the liquid to be filtered.
The complete filtering device is split into two different chambers:
One chamber, the outer chamber 4 contains fresh solvent sucked out
of the solvent reservoir, the second chamber, the inner chamber 5 contains pre -
filtered liquid. The porous filtering member 2 separates the two chambers. Any solvent
to be filtered for use is sucked into the apparatus-entering chamber 4, passing
the porous filtering member 2 from the outer to the inner area and thus moving into
inner chamber 5.
Prior to starting the filtration process the cylindrical device is
filled with appropriate liquid up to a predefined level. Typically the liquid level
meets the bottom of the filtration device to have full access to the total surface
area and filter capacity. All other volume above the bottom of the filtration device
remains empty (6). In practice this volume 6 will automatically become saturated
by air and/or vapor of the solvent to be filtered. Thus the outer chamber physically
contains a 2-phase system, consisting of a liquid phase to be filtered and air or
vapor phase of said solvent (6).
A typical version of the described filtration device is made of a
borosilicate glass chamber, having two outer threads made of glass. These threads
located at the end of both sides provide appropriate sealing of the glass chamber
having separate screws 7 on each side. The screws 7 are made out of polymeric material.
In each screw an insert or lid 3 is embedded, which is made of PTFE (polytetrafluoroethylene).
The insert 3 has a different thread, centered in its center point. This thread is
bottomed flat and forms the area for proper sealing with the corresponding tube
from the solvent reservoir, as well as to the tube connecting the inlet of the pump
Typically the filtering member 2 is a cylindrical sintered glass body
which is also made of borosilicate material. The pore size is in the range of 7
- 10 micrometers. The filtering member 2 and the outer glass cylinder 1 are connected
at position 8 by a special melting process. This design allows full access of filter
capacity while at the same time the already filtered liquid is kept apart from unfiltered
liquid without requiring any additional mechanical seal.
Another advantage of this embodiment consists in the properties of
the materials used: no corrosion will occur when oxidizing liquids are delivered.
The borosilicate glass as well as PTFE will withstand the complete pH range of solvents
that is typically used in HPLC. PTFE is resistant to all solvents typically used
in liquid chromatography.
In the following, it will be explained how the present invention also
provides a solution to the problem of undesired backflow, for example of mixed solvent
into pure solvent stored at a lower storage level.
Referring to Figure 1, two different situations may occur in practice:
- a) The solvent reservoirs are located at the same level compared to the pump
- b) The solvent reservoirs are located at different levels compared to the pump
modules and to each other.
In all cases the filter sink device in line with the connecting tubes
can be seen as an hydraulically communicating system, and the hydrostatic pressure,
corresponding to the maximum height difference possible between the solvent reservoirs,
has to be considered. The liquid level in the filter sink devices shown in Figure
1 fulfills the equation
P = F * s * h
- Hydrostatic pressure
- Mechanical force due to height difference
- Specific gravity
- Solvent height in filtration device
During normal operation, hydrostatic pressure is of no relevance as
the driving force for liquid flow is the adjusted pump flow, created by the motor
pump motion. However, in case the pump flow settings are at zero ml/min or if at
least one pump motor drive is off, hydrostatic pressure may become the relevant
driving force as long as no pressure balance will be achieved. The hydrostatic pressure
will force all solvents to the same liquid level (steady state). Then the flow will
stop automatically. It applies:
F * s * h (Filter sink 1) = F * s * h (Filter sink 2)
As long as the pump valves are sealing properly all solvent will remain
in steady state without any flow motion. In case of a small leakage the hydrostatic
pressure present will be the driving factor and will provide pressure balance on
the low-pressure side. This effect may cause undesired backflow of mixed solvent
into pure solvent stored at the lower storage level.
The arrangement according to the invention prevents uncontrolled cross
flow by leveling out the hydrostatically caused pressure drop with the design and
location of the filter sink (Figure 1).
Another advantage of the invention refers to handling of non-degassed
solvents: Solvents used in analytical HPLC must be degassed prior to use in order
to achieve best reproducibility of the separation results. In many high-flow preparative
HPLC applications the aspect of using degassed solvents has not played the same
important role. Thus, in preparative HPLC applications the solvents used are often
This circumstance favors the formation of tiny little air droplets
over time. Very often, they are moved forward and backward within the solvent pipe
and finally to the pump inlet. These droplets may increase their size over time
and suddenly become sucked into the pump module. Such events cause short irregularities
within the pump, resulting in unstable flow.
The filter sink prevents this effect. The filter sink's outer chamber
contains a certain volume 6 above the filter bottom portion that is not filled with
solvent. This volume can be described physically as a two-phase system: it contains
the appropriate liquid to be sucked into the pump and its vapor phase on top of
that. The two- phase system allows sucked in air droplets to be collected in the
upper vapor phase, before the liquid itself is passing the inner chamber filtration
Priming high flow HPLC instrumentation is of general importance and
is cumbersome whenever the pump to be used exceeds a certain height difference to
the level of the liquid to be pumped (Figure 1) .The most critical situation occurs,
whenever the pump inlet tube that connects the solvent reservoir to the pump is
filled with air, e.g. prior to the set up of an HPLC system with the required solvent
The present invention provides the desired convenience to start immediately
without taking care of displacing the air by solvent. Starting the pump will suck
in the solvent being stored inside the filter sink. As of pressure equalization
the empty tube that connects the solvent reservoir to the filter sink becomes filled
with solvent. Typically the vapor phase volume 6 inside the filter sink has the
same volume size as the filter sink inlet tube.
In a different embodiment, the filter sink is completely filled with
solvent prior to use. The solvent volume 6 will be displaced by the air volume inside
the inlet tube at the time the pump device is started. At the same time the inlet
tube is filled with the desired solvent sucked out of the connected solvent reservoir.
In the following, alternative embodiments and further developments
to those explained above will be described.
The filter capacity of the filter sink can be tailored to specific
application needs. This is achieved by either increasing the cross section of the
filter and / or the filter length.
The filter performance can be improved further by decreasing the pore
size and the pore size distribution. However, when decreasing the pore size the
flow resistance will increase and thus a certain amount of partial vacuum within
the apparatus will be created. To solve this drawback the pore size has to be matched
with the pump characteristics and the preferred liquid to be used.
In another technical solution the inner filtering member 2 has a cylindrical
shape with defined porosity, but an impervious bottom. In that case incoming solvent
is filtered only at the cylinder walls of the filtering member.
In a further embodiment the borosilicate glass of the outer chamber
1 is colored at its outer surface. This embodiment allows protection against UV
radiation caused by normal daylight. The colored coating diminishes the formation
of algae whenever water plays a role in the HPLC process. Typically 80 percent of
the HPLC applications are performed with a mobile phase comprising a mixture of
water and organic solvent.