Measuring and testing – Liquid level or depth gauge – Immersible electrode type
Reexamination Certificate
2002-07-22
2004-06-15
Williams, Herzon (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Immersible electrode type
C073S29000R
Reexamination Certificate
active
06748804
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a microsensor for measuring the position of liquids in capillaries which is particularly well suited for use in automated pipetting dispensers in medical laboratories and in the pharmaceutical industry.
In the analysis of clinical profiles or routine health checks, modem medicine increasingly relies on the quantitative determination of relevant substances in bodily fluids. The number of substances to be monitored is constantly growing, as is likewise the frequency of testing. Performing greater numbers of analyses while simultaneously lowering costs primarily requires a decrease in the use of reagents, which are often very costly. The tendency toward precise metering of the smallest possible quantities of liquids, with a volumetric range of 0.1 to 20 &mgr;L, is therefore a key objective.
In the metering operation, defined quantities of samples and reagents from starting containers must be distributed onto microtiter plates having many individual reaction receptacles (wells). A conventional plastic microtiter plate contains 96 wells, for example, each with a volume of 500 &mgr;L, in a 9-mm grid. Modern pipetting systems can meter between one and several hundred &mgr;L of a liquid, using stepping motor-driven injection pumps, with a piston injection precision of several percent. Typically, eight separately controllable pipettes are arranged in parallel, with the result that a microtiter plate must be filled in several passes. The throughput is therefore limited, which affects the measuring results in kinetic tests. Devices currently exist which contain 96 pipettes. However, such devices are not separately controllable; that is, with each metering operation the same quantity is dispensed to all the pipettes. In many applications, separate control of the pipettes would be preferable. In order to meter quantities of liquids from 0.1 to 20 &mgr;L with greater precision in an array of separately controllable pipettes, the metering operation must be actively monitored at each individual pipette.
In the course of a quantitative analysis, samples and reagents are successively pipetted into the appropriate wells of the microtiter plate, using an injection pump via a liquid column. The operating liquid is typically separated from the sample or reagent by an air bubble to avoid contamination. After the reactions have taken place in the wells, the concentration of one of the reaction products is photometrically determined, and the concentration of the sample component being sought is calculated therefrom.
The sample volume dispensed during a pipetting operation results from the piston feed from the injection pump. However, the sample volume is defined in the same manner both before and after the metering operation by the filling level of the sample liquid in the pipette.
Filling level sensors for monitoring liquids in reservoirs or tanks have been known for quite some time. In addition to sensors that are based on floats, there are a number of systems with no moving parts. Such systems are based, for example, on optical or electrical measurement techniques.
U.S. Pat. No. 5,138,880 describes a capacitive sensor comprising two concentric cylinders which are submerged in a dielectric medium along the measurement axis. The cylinders are divided into a number of discrete condensers. The capacitance of each individual condenser depends on whether air or the medium to be monitored is present between the electrodes. By comparison of the capacitances, the filling height of the medium in the container may be quasi-digitally determined with a precision corresponding to the number of measurement segments. The capacitive measurement principle can also be employed in the form of a planar sensor. This type of sensor must be calibrated for each liquid.
The filling level may be potentiometrically determined in conductive liquids. A rod-shaped resistor, which is vertically submerged in the liquid and together with this liquid forms the resistors of a bridge circuit, may serve as the measuring probe. The voltage drop at the resistor, measured via the liquid, is proportional to the liquid level. An example of such is disclosed in U.S. Pat. No. 5,146,785. Here, the measuring probe is additionally divided into a series of individual resistors, thus generating a stair-step, quasi-digital output signal.
A further electrical sensor principle is based on conductivity measurements. To this end, an alternating current in the kHz range is applied between two respective electrodes, and the current between the electrode pairs is measured. An example of such is disclosed in U.S. Pat. No. 5,719,556.
The electrical devices for measuring liquid levels according to the current art are not suited for measuring the position of liquids in capillaries. The use of said devices is limited to the measurement of filling levels in tanks, for example.
The object of the invention is to provide a device and a method for operating said device for electrically measuring the position of liquid levels in capillaries, particularly in metering devices, which is cost-effective to produce and which operates reliably and precisely.
The microsensor for measuring the position of liquids in capillaries according to the invention is based on the principle of conductivity measurements. However, only a change in the conductivity is essential to the measurement principle. The absolute value of the conductivity of the operating liquid plays a minor role.
Contained in the capillary is a gas bubble which is enclosed on both sides by the operating solution and which can be moved back and forth within the capillary by means of a sensor chip. A nonconductive liquid which is immiscible with the operating solution may be used instead of the gas bubble. The following description relates only to a bubble, without limiting the universality. It is essential that a significant difference in conductivity exists between the operating liquid and the contents of the bubble. It is also conceivable, therefore, that the operating liquid is nonconductive and the bubble is composed of a conductive liquid. Thus, there is at least one boundary between two different conductivities of the capillary filling in the region above the sensor element.
The sensor chip comprises a substrate preferably made of silicon, glass, or plastic. Microstructured, partially passivated metal electrodes preferably made of platinum, iridium, or gold are mounted on the sensor chip. Iridium is characterized by an especially low polarization resistance in aqueous solution. The electrodes each comprise a preferably constant number of partial electrodes which are separated by a preferably constant distance from one another and which are networked with electrical connections. The partial electrodes of preferably two electrodes are positioned pairwise opposite one another, separated preferably by a constant distance, as partial electrode pairs. The recurring basic geometry (meander) thus comprises preferably two electrode pairs, which in turn comprise partial electrode pairs. This basic geometry repeats itself periodically over the entire length of the sensor chip. The distance between the partial electrode pairs in the longitudinal direction, that is, in the direction of the bubble motion to be measured, is always the same. This also applies to adjacent partial electrode pairs which form part of adjacent meanders.
The electrical connections between the partial electrodes of the electrodes are preferably coated with a passivating layer, whereas the partial electrodes themselves represent the sensor-active regions of the sensor chip and thus are situated directly on the surface, which comes into contact with the operating liquid. The sensor is mounted laterally on the capillary, which is made of glass or plastic, for example, in such a way that the active regions of the electrodes, and thus of the partial electrodes, are located in the interior of the capillary. In contrast, the connections (bondpads) of the electrodes of individual meanders a
Lisec Thomas
Quenzer Hans Joachim
Wagner Bernd
Frank Rodney
Fraunhofer-Gesellschaft zur Foerderung der Angeandten Forschung
Williams Herzon
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