Gas separation: apparatus – Apparatus for selective diffusion of gases – Membrane to degasify liquid
Reexamination Certificate
1999-07-15
2001-07-10
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
Membrane to degasify liquid
C095S046000, C096S013000
Reexamination Certificate
active
06258154
ABSTRACT:
The invention relates to an apparatus for degassing liquids. More specifically, the invention relates to an apparatus for degassing liquids in a liquid chromatograph.
BACKGROUND OF THE INVENTION
In liquid chromatography, degassers are used to reduce the amount of gas dissolved in the solvents used. The solvents used in liquid chromatography are typically stored in contact with the surrounding air so that they are usually in a gas-saturated state. These dissolved gases adversely affect the chromatographic measurements.
If a solvent mixture is used, for example alcohol/water, the gas solubility of the mixture may be less than that of the liquids it contains. This leads to the formation of gas bubbles which have detrimental effects on the stability of the system flow and the composition of the solvents. This effect plays a particularly important role in low-pressure mixing systems.
Oxygen dissolved in a solvent adversely affects detection sensitivity. In UV detection, the high solubility of oxygen in eluents is noticeable, and large fluctuations of the oxygen concentration can lead to pronounced detection noise at wavelengths below 260 nm. The high absorbance can even be used to measure the oxygen content.
In fluorescence the presence of oxygen causes the quenching effect which may lead to a suppression of the fluorescence detection. For this reason, sensitive measurements with a fluorescence detector can only be made when using degassed solvents.
In addition to the influence of dissolved gases on detection, chemical reactions of dissolved gases with the eluents also present a problem. This may adversely affect the analysis results, especially in biological analyses.
Because of the aforementioned adverse effects of dissolved gases on analysis results, it is imperative to degas the liquids being used. Several methods for degassing are known in the art, for example, heating, boiling, ultrasonic degassing, helium degassing, vacuum degassing. U.S. Pat. No. 4,469,495 describes a liquid chromatography degasser for removing oxygen or other dissolved gases from liquid, wherein the liquid being degassed is passed through a spiral-shaped tube. This tube is made of a synthetic resin material, such as polytetrafluoroethylene. U.S. Pat. No. 4,729,773 describes a device for degassing liquids in which the liquid is passed through a tube made of a fluor-resin which allows gases to pass through it while preventing the liquid from doing so. Degassers using flat membranes are known from U.S. Pat. No. 3,751,879 and from U.S. Pat. No. 3,735,562. Known degassers are often complicated to manufacture and to handle and thus costly, and may have variations in degassing efficiency when the type of solvent to be degassed changes. Another degasser using a flat membrane is known from EP 718016.
SUMMARY OF THE INVENTION
In view of the prior art, it is an object of the invention to provide an apparatus for degassing liquids which, at the same or better degassing efficiency, is substantially smaller than known degassers.
It is another object of the invention to provide a degasser which can be miniaturized and integrated into a liquid chromatograph and which only has a very small dead volume.
According to the invention, these objects are achieved by an apparatus for degassing liquids, wherein liquid is conducted through one or several cavities, and wherein gas is removed from the liquid through at least one of the limiting faces of the cavity by providing on the side of the face distal to the cavity a smaller pressure than within the cavity, said apparatus being characterized in that
the limiting face between the cavity and the region having reduced pressure compared to the cavity is formed by at least one thin membrane having a thickness of less than about 10 micrometers, and
a porous support structure for supporting the membrane is provided.
The micro degasser according to the present invention, which is preferably used in connection with a liquid chromatograph, has a very small dead volume, thus leading to a drastic reduction of the waiting period between switching on the chromatograph and the first measurement. According to a practical example, the dead volume is ten times smaller than in conventional degassers with comparable degassing efficiency. The micro degasser can be arranged between the solvent reservoir and the high pressure pump of the liquid chromatograph, or it can be directly attached to the solvent reservoir, such as a solvent bottle.
Furthermore, the manufacture of a degasser of the invention is easier and less costly than with conventional degassers.
The degasser of the invention comprises a thin membrane with a thickness of less than 10 micrometers which is supported by a porous support structure, for example a frit, or domes which are micromachined or produced by micro injection molding. Preferably, the membrane is chemically inert so that it can be used in connection with the solvents typically employed in liquid chromatography. The membrane is connected to the housing forming the degasser cavity in a gas tight manner by using a thermal process.
In order to avoid damage of the membrane at the contact points with the support structure, it is preferred to provide an additional layer between the support structure and the membrane.
According to a preferred embodiment of the invention, a thin layer is provided between the degasser housing and the membrane, said layer comprising a projecting feature, for example in the form of a bulge or nose, which, when applying underpressure at the outlet of the cavity, at least partially closes the outlet. In that way, it is avoided that a large pressure difference across the membrane builds up, which could otherwise lead to damage of the membrane. The bulge thus provides a function similar to a valve. In addition, in case of a small underpressure in the cavity, the provision of the mentioned layer ensures that a fluid connection between the inlet and the outlet of the cavity is maintained, thus preventing build-up of further underpressure in the cavity.
The mentioned thin layer also serves for relieving the membrane of the degasser at its periphery, thus contributing to a reliable operation and long life of the degasser.
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Berndt Manfred
Hempel Mario
Peters Ralf-Peter
Rummler Zeno
Schomburg Werner Karl
Hewlett--Packard Company
Spitzer Robert H.
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