Chemistry: analytical and immunological testing – Oxygen containing – Inorganic carbon compounds
Reexamination Certificate
2003-01-21
2004-04-20
Snay, Jeffrey R. (Department: 1743)
Chemistry: analytical and immunological testing
Oxygen containing
Inorganic carbon compounds
C436S146000, C422S082020, C422S078000, C422S090000
Reexamination Certificate
active
06723565
ABSTRACT:
The present invention relates to improved methods and apparatus for determination of the total concentration of organic carbon compounds in aqueous process streams. The invention is especially adapted for use in measuring carbon in deionized water or deionized water with dissolved carbon dioxide, which is often used in research and development laboratories, and in manufacture and processing of electronic components, fine chemicals and pharmaceuticals.
Particularly, the method of the present invention in a preferred embodiment includes oxidation of organic components of a sample stream of water and measurement of electrical conductance and/or electrical resistance of such stream thereafter (preferably also the temperature of such stream). The preferred embodiment requires no pump or flow control components other than an on/off valve (e.g., a solenoid valve), making it inexpensive and reliable. Further, the cell containing the conductivity electrodes and temperature sensor is removable from other components of the sensor for maintenance or calibration when required.
BACKGROUND OF THE INVENTION
Measurement of Total Organic Carbon (TOC) is a well-established method of determining the concentration of organic contaminants in water [e.g., Van Hall, C. E.; Safranko, J. and Stenger, V. A., “Rapid Combustion Method for the Determination of Organic Substances in Aqueous Solutions,” Anal. Chem., Vol. 35(3), pp. 315-319; 1963; also, Poirier, S. J. and Wood, J. H., “A New Approach to the Measurement of Organic Carbon,”
Am. Laboratory
, pp. 1-7; December 1978.]. In all TOC measurement techniques, carbon in the organic contaminants is oxidized to carbon dioxide, which is measured by a variety of means.
Water may already contain carbon dioxide and other inorganic sources of carbon, so it is necessary to either eliminate Inorganic Carbon (IC) from the sample, or measure IC, prior to TOC measurement. In those techniques that measure IC, carbon dioxide concentration, after oxidation of the organics, is the sum of carbon dioxide from organic and inorganic sources—Total Carbon (TC). TOC concentration is calculated from the difference between TC and IC. (As used herein, “carbon dioxide” is intended to include free carbon dioxide, carbonic acid and bicarbonate.)
A variety of prior art approaches for measuring TOC content of water have been proposed. The so-called “differential conductivity” technique involves measurement of conductivity and temperature before and after the oxidation of the organic components of the water sample. Oxidation is initiated by action of ultraviolet (UV) light on the sample water.
The differential conductivity technique was implemented using either a continuously flowing sample stream [e.g., Bender, D. and Bevilacqua, A. C., “Portable Continuous TOC Monitoring in a Semiconductor Water System,”
Ultrapure Water
, pp. 58-67; October 1999; see also U.S. Pat. No. 4,749,657] or a sample stream that operates in a stop-flow mode [e.g., Gallegos, P.; Stillian, J.; and Rasmussen, J., “Conductometrically Based TOC Detection Instrumentation's Accuracy in Semiconductor High-Purity Water,” Proceedings of Watertech '99 Executive Forum, pp. P1-P21; Portland, Oreg.; Oct. 5-6,1999; see also U.S. Pat. Nos. 4,868,127; 5,275,957; and 5,677,190].
Problems with the Continuous-Flow Approach
The continuous-flow approach is usually used where fast detection of rapidly changing TOC concentrations is desired. At least two conductivity cells are usually used. Water flows continuously through a first conductivity cell, then the UV reactor, and finally the second conductivity cell. The difference in temperature-compensated conductivity measurements between the two cells is used to calculate the response of the analyzer.
The flow rate of water is necessarily rapid to achieve this rapid response. To be economically competitive, the UV reactors are made too small at those flow rates for the UV exposure to oxidize organics all the way to carbon dioxide. Instead, organic acids are formed, and these acids increase conductivity of water more than carbon dioxide does at the same concentration. This produces a large positive error in measurement of TOC. To overcome this problem, those analyzers must be calibrated with solutions that contain organic compounds thought to be similar to those in the unknown water sample. Since composition of all process waters actually changes with time, these analyzers do not accurately report TOC values.
A second problem with continuous-flow techniques is that they require the flow rate be held constant, even when water pressure changes. Otherwise, already quite inaccurate measurements become unusable. This necessitates addition of relatively expensive and complex flow- and pressure-control devices to those analyzers. The analyzers also require the operator to monitor flow rate of the sample, and to make periodic manual adjustments to the flow.
Problems with the Stop-Flow Approach
In the stop-flow approach, measurements are not affected by variations in water pressure, so flow- and pressure-control devices are not required. This is because conductivity electrodes are placed inside the UV reactor, allowing conductivity measurements to be made while water is stopped in the UV reactor. Only a solenoid valve or similar device is required to stop sample flow.
Conductivity and temperature of the water are measured, then flow is stopped, the UV lamp is turned on and oxidizes organics. When oxidation is thought to be complete, conductivity and temperature of the solution are measured again. TOC concentration is calculated from the difference in temperature-compensated conductivity measurements made before and after oxidation. Then the solenoid valve is opened to allow the UV reactor to be flushed out with a fresh water sample, in preparation for the next measurement.
A problem with the stop-flow approach stems from the fact that the electrodes are in the UV reactor. The UV lamp illuminates the electrodes during the oxidation period, and they generally are coated with a photocatalytic material, such as titanium dioxide. When illuminated, this material catalyzes oxidation of organics. The problem is that some organics are converted to organic acids and later are further oxidized to carbon dioxide. Such organic acids initially make the water more conductive than it will be later when the acids are converted to carbon dioxide. This means conductivity peaks at a very high level, and then decreases as acids are converted to carbon dioxide. Conductivity asymptotically approaches a steady-state value as all of the acids are converted to carbon dioxide. If conductivity is measured too soon, a positive error results in TOC measurement.
To avoid this mistake, some stop-flow TOC analyzers make repetitive conductivity and temperature measurements. Complex algorithms must be used to monitor temperature-compensated conductivity to detect conductivity peaks. When a peak is detected, the analyzer must track the subsequent decrease in conductivity to estimate its steady-state value. This requirement increases the time for the measurement, and makes it impossible to predict how long each measurement will require. Additionally, the algorithm increases the complexity of the analyzer and cost of its electronics.
A second problem is that, because the conductivity electrodes are placed inside the UV reactor, they cannot be removed easily for maintenance or calibration unless the UV reactor is constructed with multiple parts, including seals around the conductivity electrodes. This complexity makes the UV reactor larger, increases cost, and decreases reliability.
A third problem is that stop-flow TOC analyzers are subject to errors in the conductivity measurement used to calculate the IC concentration. Hydrogen peroxide is known to be formed by electrodes in contact with water containing dissolved oxygen, even in the absence of UV light [e.g., Clechet, P.; Martelet, C.; Martin, J. R. and Olier R., “Photoelectrochemical Behaviour of TiO2 and Formation of Hydrogen Per
Davenport Ronald J.
Godec Richard D.
Andover-IP-Law
Sievers Instruments, Inc.
Silverstein David
Snay Jeffrey R.
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