Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
2001-04-04
2003-02-18
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C204S400000, C204S434000, C204S412000, C205S775000, C205S787000
Reexamination Certificate
active
06521119
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a method for the detection of not easily volatized substances, in particular of Nitrotoluene, trinitrotoluene, dinitrotoluene or derivatives of nitrotoluene as well as chemical warfare materials such as Clark, Lewisite, Lost, Sarin, Soman, Tabun or the like in soil, liquids and gases, wherein an electrode configuration having at least one working electrode and an opposing electrode is brought into contact with the substance to be examined via an electrolyte and a voltage is applied to the working electrode whose value is increased and decreased within a predetermined measuring time at least once and having the substantially same beginning and end values, wherein the strength of the current during this at least one measurement cycle is determined in dependence on the applied voltage. The invention also concerns a device for the detection of not easily volatized substances, in particular of nitrotoluene, trinitrotoluene, dinitrotoluene or derivatives of nitrotoluenes as well as of chemical warfare materials such as Clark, Lewisite, Lost, Sarin, Soman, Tabun or the like in soil, liquids and gases having a sensor element comprising at least one working and one opposing electrode which are connected to a regulated voltage source and which come in contact with an electrolyte.
Potentiostatic (amperometric) measuring systems are usually used, e.g. to detect gaseous substances which are not easily volatized, wherein a potential is applied to a substantially porous, large electrode. Electrochemical oxidation or reduction leads to the quantitative detection of the substance present via the amount of current flow. The qualitative detection is thereby preferentially facilitated by gas chromatography or by high pressure liquid chromatography (HPLC) or other methods with which the mentioned electrochemical systems are utilized as detectors.
In the conventional electrochemical sensor systems for gas phase analysis, which can also be based on semiconductor technology, the electrodes are separated from the gas volume by means of a membrane or polymer layer, wherein the substance to be detected diffuses through a permeable membrane and is subsequently dissolved in the internal electrolytes which are in contact with the electrodes. The detection of the respective substance is thereby effected by means of electrochemical oxidation or reduction at the working electrode. As has already been mentioned, a fixed potential is applied to the working electrode.
These types of methods, e.g. gas chromatography, have the disadvantage of being time consuming and expensive. In the event that liquids or soil is investigated it is first necessary to collect samples and remove same to a predetermined analysis location.
In addition, most explosive charges are produced using trinitrotoluene (TNT) or with the addition of TNT for reasons of simplicity of production and processing. Since this type of explosive is also used for extortion or terrorist threats as well as attacks, the detection of this type of explosive, e.g. during luggage and passenger checks for air travel, is an important task in order to prevent danger to human life. Also when searching for plastic encased mines, rapid and uncomplicated detection is imperative. TNT utilized for explosive charges or for the production of mines is a solid which releases small amounts of vapor to the surrounding environment.
In order to detect this type of contamination in soil, liquids and gases, EP-A-O 665 431 has already proposed a device working on the basis of cyclic voltammetry. This device has a sensor element comprising three electrodes, a working electrode, a reference electrode as well as an opposing electrode and having a regulated voltage source. A voltage is applied to the working electrode, the cyclic reduction and increase of which accompanied by simultaneous measurement of the current flow in dependence on the applied voltage, produces so-called voltammograms, obtained in the form of voltage-current curves. The voltage values on the respective working electrodes during the occurrence of cathode or anode current peaks for reduction and oxidation of the redox pairs represent a quantitative measurement for the contamination present. The quotient between the current strength of the anode current maximum and the measured current in the so-called double layer region leads to the determination of the TNT content. The above mentioned device can determine the TNT content or that of other substances per se, however the sensitivity is insufficient in the event of very small vapor pressures. In particular, a determination of substances in the gas phase is not possible with this device, since the determination of gas phase substances requires use of a membrane by means of which a flow-through cell is separated from the electrolyte solution.
It is therefore the underlying purpose of the invention to further improve a method as well as a device of the above mentioned kind in such a fashion that electrochemically reactive substances can be detected even in very small concentrations and/or vapor pressures and in a qualitatively as well as a quantitatively reliable manner.
SUMMARY OF THE INVENTION
This purpose is achieved in accordance with the invention by means of a method of the above mentioned kind in that a plurality of measurement cycles are carried out and the differences of current strengths of sequential measurement cycles are determined and, in the event that a cathode peak maximum occurs, the associated voltage value is determined, held constant and the current strength is determined. A device in accordance with the invention is distinguished in that the electrolyte is applied in the form of a thin layer onto the end surface of a sensor element coming in contact with the substance under investigation.
It has turned out that the cyclic voltammogram, for a plurality of cycles of the measurement cycle, stabilizes itself in such a fashion that reduction leads to a characteristic maximum (cathode current peak). Determination of the difference of the measured current values allows the occurrence of such a characteristic maximum to be determined in an optimal and rapid fashion. In accordance with the invention, when a cathode current peak occurs, the associated voltage value is determined, held constant and the current strength is then determined in a potentiostatic manner over time up to saturation. In this manner, the sensitivity is increased and the time for detection reduced. The qualitative detection is done by means of the peak potential determination and the quantitative detection by means of the potentiostatic measurement of the current height. In this manner, the response time is substantially improved. For example, response times of less than 0.5 seconds can be achieved.
The device in accordance with the invention provides that the requisite electrolyte be applied in the form of a thin layer on the end of the sensor element and not, as was conventional, by submerging the sensor element into an electrolyte. The thickness of the electrolyte layer influences the detection time. The method as well as the device in accordance with the invention thereby facilitate reliable detection of the most differing of not easily volatized substances even in small concentrations. The optimal achievable potential value with which a cathode current peak (negative sign) occurs is approached in a stepwise fashion using the method in accordance with the invention.
In order to prevent the measuring time from increasing unnecessarily, one measurement cycle spans, at most, the potential region between hydrogen and oxygen development.
The extracted or determined values with respect to current height, differences and potential can be output optically or acoustically. For example, an acoustical signal can change in dependence on the determined concentration of the respective substance. This is particularly advantageous when approaching a mine due to the associated increased concentration of TNT.
If the measuring medium, under all condi
Becker Frank
Krausa Michael
Krebs Stefan
Schorb Klaus
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
Tung T.
Vincent Paul
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