This invention relates to the field of x-ray fluorescence analysis systems with pre-concentration devices, particularly for the in-situ measurement of ultra-trace levels of ionic contaminants in aqueous solutions.
Virtually all elements in the periodic table form compounds that are soluble in water. Dissolved impurities exist in aqueous streams as positive and negative charged pairs of anions and cations. Many of these impurities are toxic for human consumption or harmful to industrial processes. Therefore, the detection and quantitative measurement of the concentration of these impurities is of interest.
The present invention was developed to assist industries which have a continuous need to in-situ monitor effluent discharges to sanitary systems, and industries within the manufacturing arena where ultra pure fluids are required, such as for the production of microchips, or where fairly precise concentrations of trace elements within fluids are desired. This invention is also useful to ensure the quality and purity of intake or supply waters more generally to meet health and environmental standards. Current methodologies for the detection of trace materials, e.g., trace metals and other ionic components within process fluid streams, typically require samples to be prepared for off line analysis by specialized laboratories, which can be both costly and time consuming. Valuable production time is lost while waiting for results of required tests, or worse, when unacceptable concentrations are allowed to pass through the system for lack of continuous, on-line monitoring capability.
The usage of x-ray fluorescence (XRF) as an analytical method is well known to those skilled in the art. The XRF technique uses x-rays from a source directed towards the sample which are absorbed by the atoms in the sample. If the primary x-ray energy is sufficient, electrons in the atom are ejected from the higher binding energy inner shells creating vacancies and leaving the atom in an excited state. As the vacancy is filled by electrons from the lower binding energy outer shells, a photon with energy equal to the difference in the binding energy of the outer and inner shell electrons is ejected from the atom. Each element in the periodic chart has a unique set of energy levels, thereby allowing for identification of its presence through analysis of the energy of the emitted photon. The rate at which such photons are detected can be related to their concentration. In general, the higher the atomic number of the element, the more energetic are its characteristic fluorescence photons. By examining the intensity of the energy spectrum of the detected photons, the technique is capable of non-destructively determining the relative elemental composition of a sample. Because the emitted photons are attenuated by intervening materials between the sample and the detector, the technique is generally incapable of detecting low-atomic number (Z) elements below neon (Z=10) with energies less than 1 keV.
XRF spectroscopy has been utilized for several decades in the analysis of fossil fuels, food products such as cooking oils, soft drinks, wastewater, drinking water and medical fluids. Typical XRF processing requires the gathering of sample material so that it can be inserted into the instrument.
The sensitivity of XRF techniques depends upon the achievable signal to noise ratio. This is determined by the number of photons emitted from the element of interest at its characteristic x-ray energy that can be detected compared to photons of nearby energies scattered into the detector via other processes. Because the background is randomly distributed, the signal to noise ratio can be improved by extending the exposure time. A carefully constructed standard XRF unit is able to achieve sensitivity of about 1 to 10 parts-per-million (ppm) in mass ratio with about 30 minutes of exposure. However, to improve that sensitivity by a factor of two statistically requires a factor of four times as long a time, i.e., sensitivity improvement varies only with the square root of time. Therefore, to detect trace-level impurities at sub-ppm, the exposure times required would be inordinately long. Long exposure time usually introduces other non-statistical limitations, such as, but not limited to, gain drift in the detector, source output stability, etc. Thus, for all practical purposes, real-time analysis by direct XRF technique is limited to the ppm level since any greater sensitivity requires exposure times in excess of a few hours.
For quantitative work at better (greater sensitivity) than the 1–10 ppm level, processing of the material by means such as leaching, filtration, surface treatments, etc., by a highly trained individual is required to control “matrix” effects. This can take from several hours to days. This lengthy time between collection and reporting of analytical results prohibits XRF from being considered a ‘real time’ analytical method at ppm or better sensitivity levels.
The detection sensitivity can be improved by pre-concentrating the sample. Various filtration techniques have been tried for the detection of impurities associated with particulate matters, and ion exchange and chelating membranes have been utilized for the pre-concentration of dissolved compounds. These techniques typically require complex reversal processes such as removal or cleansing of a filter or drastically changing the pH of an ion exchange bed, for example. The capability to perform these pre-concentration steps and the subsequent analyses in-situ generally requires intervention of highly trained personnel and is not easily automated for on-site, real-time measurements.
Technologies used for extracting trace materials such as trace metals and ionic components in flow streams are often based upon or incorporated into water purification technologies. For example, U.S. Pat. Nos. 5,954,937 and 5,425,858 both by Farmer disclose an electrochemical cell for the removal of dissolved impurity ions from a liquid medium for purification purposes. The invention makes use of a highly porous carbon aerogel with very high surface density to form the electrodes of a capacitor. Upon the application of a bias voltage, the dissolved ions are attracted to the respective electrodes where they are captured in a “double layer” structure. The process can be reversed electrically to regenerate the cell. The concept is similar to earlier patents by Andelman (U.S. Pat. Nos. 5,192,432, 5,200,068, 5,360,540, 5,415,768) that make use of a carbon fiber as the porous material wherein the capacitor is wound in a spiral configuration for the fluid to flow through the electrodes, whereas Farmer uses a stack of capacitors requiring the fluid to flow between the electrodes. Farmer also recognizes that upon regeneration, the ions captured in the double layer can be discharged to a secondary chamber with significantly higher concentration to facilitate detection (see U.S. Pat. No. 5,954,937, column 27, line 43 through column 28, line 7). But, Farmer provides no disclosure on specific arrangements for in-situ or remote measurements, nor on how to configure and operate a cell for such measurements, nor on how quantitative information regarding the concentration of the impurities in the flow stream can be determined. Clearly Farmer has not considered all of the technical issues that need to be resolved in order to obtain quantitative, in-situ or remote measurements of trace level impurity concentration in a flow stream.
For detection of trace level impurities in flow streams, additional prior art is found in chemical based concentration systems. U.S. Pat. No. 5,834,633 by Davison claims the use of a permeable membrane capable of binding the impurity as a sampling device to collect and concentrate metal ions to facilitate detection using laboratory equipment such as is used in proton-induced X-ray emission (PIXE) techniques.
The prior art described above would be improved upon by an apparatus and method which provides a pre-concentration device capable of providing fully automated in-situ or remote analysis in real-time to provide quantitative measurements of trace materials in a fluid matrix, across a broad range of elements and concentration levels, by using well established XRF techniques but with significantly greater sensitivity than is made possible using standard XRF techniques and measurements.
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