WO2007081616A2

WO2007081616A2 - Method and apparatus for measuring the concentration of chlorine dioxide and other gas and liquid solutions

Info

Publication number: WO2007081616A2
Application number: PCT/US2006/061394
Authority: WO
Inventors: Thomas Mcwhorter; Aaron Rosenblatt; Andrew E. Hospador; Madhu Anand; John Peter Hobbs
Original assignee: Cdg Technology Inc.
Priority date: 2005-11-30
Filing date: 2006-11-30
Publication date: 2007-07-19
Also published as: WO2007081616A3

Abstract

The spectrophotometer apparatus of the present invention provides a system for measuring the concentration of at least one component in a fluid, for example, the concentration of chlorine dioxide in a liquid or a gas. The spectrophotometer generally includes at least one light emitting diode (LED), a photoreceptor, and a sample cell, which can accommodate either a sample fluid or gas.

Description

TITLE

METHOD AND APPARATUS FOR MEASURING THE CONCENTRATION OF CHLORINE DIOXIDE AND OTHER GAS AND LIQUID SOLUTIONS

BACKGROUND

[0001] Chlorine dioxide has become increasingly important in recent years as a gas phase disinfectant. It has been used, for example, to sterilize medical instruments and other medical articles; decontaminate buildings containing hazardous substances such as Anthrax spores or molds; and to decontaminate biosafety cabinets and other laboratory enclosures.

[0002] Chlorine dioxide is typically produced at partial pressures up to 0.1 Bar (10% at atmospheric pressure), which is currently the safety limit set by the Occupational Safety and Health Administration. Other expert sources state that production of chlorine dioxide at partial pressures up to about 0.17 Bar or even higher are safe. Higher concentrations of chlorine dioxide can decompose spontaneously and exothermically. At partial pressures above the range of about 0.3 to 0.4 Bar, chlorine dioxide can explode with increasing violence as partial pressure increases. Therefore, chlorine dioxide is usually produced under vacuum or in dilute form so that the partial pressure is strictly limited to less than the range of about 0.1 to 0.2 Bar. This equates to a concentration of about 10% to 20% chlorine dioxide by volume at atmospheric pressure.

[0003] Historically, chlorine dioxide gas has suffered in use as a decontaminant or sterilant for several reasons. Techniques for generating the gas were expensive and/or hard to perform and the chlorine dioxide they produced often contained contaminants such as chlorine, salt, water droplets, or acid vapors. These issues have been addressed by introduction of the Gas:Solid process described in U.S. Patent No. 5,290,524. In addition, techniques for measuring the concentration of the gas have been difficult, expensive, inaccurate, and/or delicate, and often require highly skilled personnel to implement.

[0004] A variety of techniques have been utilized for measuring the concentrations of chlorine dioxide, all of which have had their own unique disadvantages. [0005] For example, aqueous chlorine dioxide has been measured by titration using potassium iodide or lisamine green B. A known amount of sample gas is drawn through an impinger and the chlorine dioxide from the sample is dissolved in water, allowing the concentration in the water and ultimately, the concentration in the gas sample to be calculated. Titration methods, however, are labor to intensive. They require a skilled operator, and even then are subject to error. Further, titration is a batch method and cannot be adapted to real time control systems based on analytical feedback.

[0006] Alternatively, electrochemical cells have been used to measure the concentration of chlorine dioxide in the gas phase. The range of measurement of these cells is limited to low concentrations (typically 0.1 - 5 ppm). Electrochemical cells have been used most commonly to measure the concentration of chlorine dioxide in the workplace and in stack gas and scrubber exhaust. The problem with the use of electrochemical cells is that if they are exposed to chlorine dioxide concentrations typical of disinfection, the cells are rapidly damaged, or the electrochemical fluid can be rapidly depleted, causing unreliable measurements.

[0007] Ion mobility spectrometers have also been used to measure concentrations of chlorine dioxide, but pose their own problems. Time to of to flight mass spectrometers use the differences in transit time through a drift region to separate ions of different masses. They operate in a pulsed mode so ions must be produced or extracted in pulses. An electric field accelerates all ions into a field to free drift region with a kinetic energy of qV, where q is the ion charge and V is the applied voltage. Since the ion kinetic energy is 0.5mv², lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner. Ion mobility spectrometers are far too expensive for many applications, and are not rugged enough for many field applications and are difficult to use.

[0008] Ultraviolet (UV) spectrophotometers have been the most common way of analyzing the concentration of gaseous chlorine dioxide in the range (typically 500 ppm to 8%) used in disinfection and decontamination. Existing UV spectrophotometers suffer from a number of problems in measuring the concentration of chlorine dioxide. Many of these problems are exacerbated by the humid gas that is essential for most bio decontamination processes. [0009] As shown in FIG. 1, chlorine dioxide absorbs ultraviolet light. Air, nitrogen and other gases typically used to dilute chlorine dioxide do not absorb in this part of the spectrum. The amount of light in a given wavelength absorbed over a certain path length is a function of the concentration of the chlorine dioxide.

[0010] The UV wavelengths that are absorbed by chlorine dioxide are the same wavelengths that cause the chemical to undergo photolysis. Gas in a sample cell exposed to UV light will decompose to form chlorine and oxygen, and thereby change the composition of the sample being measured. Technically, this problem be minimized by utilizing a light of sufficiently low intensity or purging the sample so that the gas has a short retention time in the cell. However, both of these approaches create problems of their own. In present spectrophotometers, low intensity light sources, after collimation, filtration, focusing, and distances required leave little signal for measurement by a photoreceptor. Further, short retention times require small sample cells and/or high sample flow, either or both of which are undesirable in many applications.

[0011] Referring again to FIG. 1, the absorbance spectrum of chlorine dioxide gas is characterized by sharp peaks and valleys (fringes) superimposed on an overall curve that is approximately bell shaped. These fringes do not appear in the absorbance spectra of most gases, or in the spectrum for aqueous chlorine dioxide solution, where the spectrum has a shape similar to the underlying "bell curve" of the gas phase spectrum without the spikes (fringes).

[0012] The presence of these fringes causes difficulties in measuring chlorine dioxide gas concentration. If a monochromatic light source is used - such as that from a laser, or from a prism/slit aperture - the chlorine dioxide concentration measurement is extremely sensitive to small changes in wavelength of the light. For example, in FIG. 1, a shift in wavelength from 388 nm to 392 nm causes approximately a 50% change in the absorbance measure for the specific gas sample used to produce FIG. 1. This sensitivity to a slight change in wavelength means that it is impractical to use monochromatic light for spectrophotometric measurement of chlorine dioxide gas at wavelengths above the range of about 315 to 325 nm. As seen in FIG. 1, below that range, fringes do not appear, but the slope of the underlying absorbance curve is so steep that similar errors occur, if wavelength is not extremely precise and constant. Most probably, this phenomena is responsible for much of the notorious inaccuracy of present Spectrophotometers for measurement of chlorine dioxide concentrations.

[0013] In current spectrophotometers, light sources, photoreceptors and electronic components must be isolated from a gas sample by windows. These windows are subject to fouling by dust. Moisture in a sample sometimes condenses to form fog or condensation on the windows of a sample cell, which absorbs or disperses light and causes erroneously high readings. Some suppliers attempt to address this problem by using a reference beam. Typically, the reference beam is at a wavelength not absorbed by chlorine dioxide. With clean windows and only air in the sample chamber, the intensity of both the UV beam and the reference beam can be measured at the photoreceptor. Those measurements are defined as the "zero" point, or the intensity of each beam with no chlorine dioxide in the sample cell. Assuming that the reference beam and the UV beam are absorbed equally by the dirt on windows or equally dispersed by fog or condensation, then the "zero" of the UV beam can be adjusted in proportion to the change in intensity of the reference beam. By this means the electronic logic of the device can compensate for fouling in the sample cell and windows.

[0014] Many times, however, the reference beam and the UV beam are not absorbed equally by dirt on the windows of a sample cell or equally disbursed by fog or condensation. Some spectrophotometers attempt to avoid this problem by using optics that pass the reference beam and the UV beam through the same window area. However, this requires expensive and delicate precision optics, making such spectrophotometers too expensive for many applications.

[0015] Chlorine dioxide and other gases absorb light to differing degrees at different wavelengths (see FIG. 1), and photoreceptors convert light to a signal to different degrees as a function of wavelength (see FIG. 3). Therefore, it is critical that the light used in a spectrophotometer be of a narrow band of wavelength compared to the absorbance spectrum of chlorine dioxide (though broad with respect to the spectral width of the fringes). Existing spectrophotometers typically use incandescent light sources. These sources emit light in a broad spectrum. The spectrophotometers use diffraction gratings, prism/slit apertures or filters to select a narrow band of wavelength for use in measurement. These devices are expensive, and fragile. In addition, the power requirement for such light sources is much higher than the power actually required for measurement for systems where battery operation is desired.

[0016] Since the unused part of the light spectrum is dissipated as heat, heat removal and temperature are issues in many systems that use incandescent light, hi many systems, the heat from the light source is so intense that the light source must be located some distance from the sample and from the electronics of the system. This necessitates the use of mirrors and larger enclosures to provide such distance. It also necessitates the use of collimation to assure that enough light reaches the relatively remote photoreceptor to enable measurement. In addition, the spectral distribution of the incandescent light changes as bulb temperatures change and as bulbs age.

[0017] Some spectrophotometers use light that is collimated (made to travel in a single direction) as it passes through a sample. Collimation can be accomplished by passing light through lenses or it can be approximated by passing light through a series of apertures. These devices are expensive and must be perfectly aligned in order to function properly. The alignment necessity adds cost to the system and maintaining alignment makes the system fragile. It would be much less expensive to use a light that is non to directional. However, with a non directional light, some light travels directly from the light source to the photoreceptor, while some light is reflected from the walls of a sample chamber and then travels to the photoreceptor. The reflectance of the chamber walls changes with humidity. This phenomenon causes the apparent absorbance of the gas to change as a function of time and relative humidity since the reflectance of the walls changes as the surfaces equilibrate at the new humidity.

[0018] Some spectrophotometers utilizing incandescent light produce a pulsed signal, by interrupting the incandescent beam with a spinning disk with apertures in it. That approach requires collimated light, which poses the problems addressed above, and/or optical sealing so that reflected light does not interfere with the signal. Further, the rotating disk involves moving parts, which require maintenance and are subject to breakage.

SUMMARY

[0019] The spectrophotometer apparatus of the present invention provides a system for measuring the concentration of at least one component in a fluid, for example, the concentration of chlorine dioxide in a liquid or a gas. The spectrophotometer generally includes at least one light emitting diode (LED), a photoreceptor, and a sample cell, which can accommodate either a sample fluid or gas. The spectrophotometer is easy to use, provides rugged reproducible measurements and is particularly well suited for use in measuring the concentration of chlorine dioxide in gases or in solutions.

[0020] The sample cell can include two substantially parallel windows made of a material transparent to light emitted by an LED. For example, the windows can be made of quartz. The side walls of the sample cell can be machined or roughened to prevent reflection onto the photoreceptor of light coming from the direction of an LED. Further, the sample cell can be heated to a temperature higher than that of the sample fluid. For example, the sample cell can be heated to a temperature of 40 to 80 degrees Fahrenheit above the temperature of the sample fluid. The sample cell can be located between an LED and the photoreceptor, such that light produced by an LED passes through the sample cell in a direction substantially perpendicular to the two substantially parallel windows, and impinges on the photoreceptor.

[0021] In another embodiment of the apparatus, at least one LED can produce light in a wavelength range of about 250 to 450 nanometers.

[0022] In another embodiment of the apparatus, the apparatus can measure the concentration of chlorine dioxide in a gas. Further, the apparatus can measure the concentration of chlorine dioxide dissolved in water.

[0023] In another embodiment of the apparatus, the light emitted by an LED cannot only emit directly outward from the LED, but also at an angle of about 5 degrees or more outward from an axis extending directly outward from the center of an LED. This can result in an LED or array of LED's emitting light in the shape of a cone having an angle of greater than about 5 degrees.

[0024] In another embodiment of the apparatus, the angle of a line from the outermost edge of any LED to the nearest corner of the far side of the aperture to the line between an LED and the photoreceptor is greater than the angle of a cone of light emitted by an LED.

[0025] In another embodiment of the apparatus, at least one LED is pulsed with an on time of less than about 1 second every 10 seconds at a frequency of greater than about 10 Hz. [0026] In another embodiment of the apparatus, at least one LED is pulsed two or more times for each data update and the absorbance of the multiple pulses are averaged to update readout data.

[0027] In another embodiment of the apparatus, at least two LED's can be used and the light from the LED's should impinge on the photoreceptor at intervals such that only one wavelength of light is impinging on the photoreceptor at any given time.

[0028] In another embodiment of the apparatus, at least one LED can emit light in a wavelength range absorbed by the sample fluid and at least one LED can emit light in a wavelength range not absorbed by the component of the sample fluid the apparatus can measure the concentration of. The light from the LED in the wavelength range not absorbed by the component of the sample fluid being measured can be used as a reference beam such that any changes in absorbance of that beam are assumed to have also occurred in the signal beam and the "zero" setpoint of the signal beam is adjusted accordingly.

[0029] In yet another embodiment, the spectrophotometer apparatus includes at least two LED's and a photoreceptor. The photoreceptor can be positioned such that the space between the LED's and the photoreceptor is open to the surrounding atmosphere and light from the LED's can pass through the fluid between the LED's and the photoreceptor. The light from the LED's can impinge on the photoreceptor at intervals such that only one wavelength of light impinges on the photoreceptor at any given time.

[0030] In another embodiment, the apparatus can measure the concentration of two or more components in a sample fluid.

[0031] In another embodiment, at least one of the LED's produces light in the wavelength range of about 250 to 450 nanometers.

[0032] In another embodiment, the apparatus can measure at least the concentration of chlorine dioxide in a gas or the concentration of chlorine dioxide dissolved in water.

[0033] Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. BRIEF DESCRIPTION OF THE FIGURES

[0034] FIG. 1 illustrates the typical absorbance spectrum of chlorine dioxide gas.

[0035] FIG. 2 illustrates the light spectrum of a typical UV light emitting diode.

[0036] FIG. 3 illustrates the operating spectrum of a typical photoreceptor.

[0037] FIG. 4 illustrates the directional pattern of a typical light emitting diode.

[0038] FIG. 5 illustrates a summary of parameters as a function of wavelength.

[0039] FIG. 6 illustrates a side view of an embodiment of the spectrophotometer apparatus including sample cell.

[0040] FIG. 7 illustrates an open configuration of a spectrophotometer apparatus which does not necessitate a sample cell.

[0041] FIG. 8 illustrates a methodology for sizing an aperture included in a sample cell for minimizing or eliminating reflection from the sides of the sample cell.

DETAILED DESCRIPTION

[0042] The light in the spectrophotometer apparatus can be produced by LED's, which provides certain advantages over incandescent lights. LED's produce light in a narrow spectrum (see FIG. 2) with low power consumption. The LED's used in a preferred embodiment of the invention consume approximately 3.2 - 12.8 mW of power. The light that LED's produce is of relatively low intensity relative to spectrophotometers with incandescent lamps used in the past. LED's produce light in a narrow spectrum so that all of the power goes into the wavelength that is used, while incandescent bulbs produce a broad spectrum of light where much of the spectrum is unnecessary to the measurement. Because of the relatively modest power requirements, spectrophotometers of the invention can be powered by batteries in addition to alternating current sources.

[0043] Another benefit of LED's is that they can be turned on and off rapidly and reach full intensity and precise wavelength with essentially zero warm up or cool down time. LED's can be pulsed at frequencies up to many megaHertz. Whereas, incandescent lights require warm to up time and cannot be pulsed at rapid intervals. [0044] In a preferred embodiment, the spectrophotometer apparatus uses pulsed light. In one embodiment of the present invention UV light typically at about 380 nm, pulses 8 to 16 times for 50 milliseconds per pulse every 10 seconds. Each of the pulses is measured separately, and the 8 to 16 results are averaged to provide a reading update every 10 seconds. By averaging the results of multiple rapid pulses, the present invention can provide a much more statistically accurate result than a single pulse could provide.

[0045] The present invention also can utilize a reference beam at a wavelength that is not absorbed by chlorine dioxide (such as about 505 nm). In one example, the claimed apparatus uses, but is not limited to, a UV beam of approximately 380 nm and a reference beam of 505 nm. This light can also be pulsed at the same frequency as the UV beam such that it is on during the intervals when the UV beam is off. This enables a user to use a single broad sensitivity photoreceptor and to avoid the use of beam splitters, lenses and other devices, as found in spectrophotometers used in the past, to produce a reference beam.

[0046] The use of low-intensity, pulsed LED light creates several benefits: a. Low-intensity, pulsed light consumes little power. This is important to avoid heat buildup and to enable battery operation where needed. b. Low-intensity, pulsed light exposes the sample to little UV light and reduces the decomposition of the chlorine dioxide sample. c. Use of low-intensity pulsed light substantially avoids formation of the translucent film on lenses that is seen in other spectrophotometers. d. The use of pulsed light enables multiple light frequencies from multiple LED' s to be used in a rapid alternating sequence. This enables design of a device that can simultaneously measure multiple gases that absorb in different parts of the spectrum in a single sample with a single broad sensitivity photoreceptor and no beam-splitter, for example, the invention can measure chlorine dioxide using an LED that produces light in the UV range and also measure humidity or CO₂ using an LED that produces light in the Infrared (IR) range.

[0047] As indicated above, the invention can incorporate a reference beam that is pulsed so that groups of reference pulses alternate with groups of UV pulses. Thus, the reference beam and the UV beam can act on a single wide sensitivity photoreceptor.

[0048] FIG. 5 provides an LED Spectrum overlaid on the Chlorine Dioxide Absorbance Spectrum. As can be seen, LED's emit light in a spectral band that is narrow compared to the absorbance spectrum of chlorine dioxide, but wide relative to the spectral width of the fringes. Therefore, the light from the LED is absorbed in a range of frequencies that overlap one or more of the fringes and the effective absorbance is averaged over a range of the absorbance spectrum that incorporates a combination of peaks, troughs, ascending slopes and descending slopes. If the peak of the LED spectrum is in the range of about 350 to 385 nm, the measurement is relatively insensitive to small variations in the peak wavelength, since over this range, the underlying absorbance curve is relatively flat, and the fringes are relatively uniform compared to other parts of the spectrum.

[0049] In an embodiment, the invention contains heaters that heat the walls of the sample cell, for example, to greater than about 60° F above the temperature of the sample. Heating the walls of the sample cell to this temperature can prevent changes in reflectance as a function of humidity, as described above.

[0050] In another embodiment, the sample cell employs roughened cell walls, or cell walls with grooves machined perpendicular to the line between an LED array and the photoreceptor, hi yet another embodiment, the distance of the walls of the cell from the line between an LED array and the photoreceptor can be greater than the distance from the LED to the photoreceptor. (See FIG. 6). Both of these embodiments minimize the amount of reflected light that can reach the photoreceptor.

[0051] Because LED's emit light in a narrow spectrum of wavelength, (FIG. 2) the use of LED's eliminates the need for diffraction gratings or filters which minimizes the cost of the device and makes it more compact and rugged.

[0052] Unlike lasers, LED's and incandescent light bulbs do not produce collimated light. In an incandescent bulb, light is emitted in all directions from the filament. In an LED, light is generally emitted in a divergent cone from the end of the LED, however other shapes, if available, could work in the invention. The angle of the cone can be made greater or smaller in the manufacture of the LED. Typically, LED's are available that emit light in approximately 10 to 180 degree cones, hi a preferred embodiment of the present invention, LED's are used with approximately 10 to 30 degree cones. FIG. 4 shows the radial light distribution in a 30 degree LED. The boundaries of the cone are not typically defined as sharply as those of lasers or collimated light (FIG. 4). [0053] In spectrophotometers that use incandescent light sources, the light is typically collimated by means of lenses or mirrors. Collimation is required to minimize the effect of reflections from the wall and to preserve light intensity as the beam travels over the lengthy path required because of the heat of incandescent bulbs. Heating the sample cell in the present invention to greater than about 60° F above a sample temperature, machining grooves in the wall of the sample cell, or moving the sample cell walls far from the path of the light beams eliminates the need for collimation as described above, while using pulsed LED's as a light source eliminates excessive heating of electronics and other sensitive devices.

[0054] Much of the cost and fragility of spectrophotometers used in the past was caused by the need for perfect alignment of gratings used to collimate light. This was necessary because the light beams needed to be collimated and focused over substantial distances onto photoreceptors. In one embodiment of the present invention, LED's with a relatively wide angle cone of light are used. This eliminates the need for perfect alignment because: a. In embodiments where the walls of the sample cell are heated, light that strikes the wall of the sample cell instead of the photoreceptor experiences a constant reflectance. If some of the light is reflected through the sample cell to the photoreceptor, it experiences the same degree of absorbance as the light coming directly from the LED. hi other embodiments, the walls of the sample cell are machined or configured to minimize reflectance. b. hi one embodiment of the present invention, the apparatus is equipped with "automatic compensation." Zeroing can be initiated manually by pushbutton wherein intensity of the UV light signal with an empty sample cell is measured and set as the zero point, i.e. the intensity with no absorbance due to the sample. Alignment between the light source and the photoreceptor is not critical as long as the alignment does not change between them during the time the zero point is set and the time the measurement is taken, and as long as the light from the LED is sufficiently intense to effectively measure. Using an LED with an approximately 30° beam typically means that the tolerances for alignment are large.

[0055] Since alignment is not critical within broad angles, multiple LED's can be set to impinge on a single receptor without mirrors or lenses to focus the various beams. Multiple LED's can be placed adjacent to each other and imbedded in a block. If the LED's are approximately 3" from the sample cell, and the cone of light has an angle of approximately 20 degrees, the intense part of the light covers a circle 1.05" in diameter (3" x 2 x Tan 10), which is sufficient to avoid small alignment problems, compensate for the necessary eccentricity due to LED's mounted next to each other, and still cover a reasonably sized sample window.

[0056] hi many situations, it is desirable to measure the concentration of a gas in an open space such as a building or a room, hi one embodiment, an enclosed sample cell can be omitted and the fluid to be measured can circulate freely between the LED'(s) and photoreceptor. If the fluid to be so measured is corrosive or otherwise harmful to the apparatus, the electronic components of the system can be tightly sealed and windows can be provided that are transparent to the light being used. (FIG. 7). hi this embodiment, ambient light could cause errors in measurement, but, since chambers containing chlorine dioxide must be kept totally dark, stray light is not an issue with chlorine dioxide measurement.

[0057] hi one embodiment, as seen in FIG. 6, the sample fluid should be passed through a sample cell. The sample cell is equipped with windows that allow light to pass through the sample fluid. The light path in the sample is substantially equal to the distance between the inside surfaces of the windows, which can be determined by the thickness of the spacers. The length of the light path is important. If the light path is too short, the degree of absorbance can be too small to measure. If the light path is too long for a given concentration, the function of absorption versus, concentration will be in the non-linear range and will be much harder to analyze. In a preferred embodiment, for gas concentrations of approximately 500 to 4000 ppmv, the windows are typically approximately 3 inches apart but can be any distance that is sufficient for measurement of the sample by the LED and photocell of the spectrophotometer. In another preferred embodiment, for gas concentrations in the range of approximately 1 to 10%, the windows are approximately ¹A" to V₂" apart.

[0058] Little, if any, light will be reflected from the inside of the sample cell if the angle of a line from the outermost edge of any LED to the nearest corner of the far side of the aperture, to the line between the LED and the photoreceptor is greater than the angle of the cone of light from the LED. This requires smaller and smaller apertures as the LED array is placed closer and closer to the nearest window of the sample cell. This reinforces the benefits of using pulsed low-power LED's to minimize heating effects. [0059] As is known, Beer's Law defines a linear relationship between the absorbance and the concentration of a material that is absorbing light passing through it. Absorbance is defined as the log base 10 of the ratio of incident radiant power to the power received after passing through the material.

[0060] In one embodiment of the present invention, the apparatus measures and memorizes a baseline energy value for each of the LED wavelengths. Because LED' s are stable these memorized baseline values are valid for a substantial period after their capture; usually for hours.

[0061] Subsequent intensity measurements are ratioed to these baseline values, the logarithmic algorithms are performed and the linear absorbances subtracted to remove the effect of attenuation of the reference signal. The result is scaled to display the value of gas concentration and provide analog and/or digital output signals that can be used on a continuous or intermittent basis by control and/or data acquisition systems.

[0062] The invention can use analog and analog-digital mixed-signal (A-D converter) electronic technology under micro-processor control to perform many of the operational functions of this device. Analog electrical signals proportional to optical energy received after passing through the sample volume, and proportional to sample cell temperatures, can be converted to digital values which are stored and manipulated by the computer.

[0063] The micro-processor can perform functions including but not limited to, sequence and timing control of alternating illumination of two or more LED's, control of the conversion of analog values to digital values in memory, performing mathematical computations, responding to operator pushbuttons, controlling display parameters, controlling sample cell temperature and controlling output signals.

[0064] As indicated previously, a film can form on the quartz windows used to separate the gas sample from the electronics and light sources/receiver in the presence of UV light, water vapor, and chlorine dioxide. This film is translucent to UV light in the wavelengths used to measure chlorine dioxide, i.e., it absorbs and/or scatters part of the light creating artificially high absorbance readings. For dilute chlorine dioxide at relative humidity below 40% at room temperature, the film does not form, or forms much more slowly than at higher humidity. The film forms only in the part of the window through which the UV light passes and primarily on the emitter side of the cell. If pure dry air is passed through the cell, the film eventually disappears. Since the film does not form, or forms slowly, if at all, in the path of the reference beam, the reference beam cannot be used to adjust for the formation of the film unless the reference beam passes through the same window area as the UV beam. In an embodiment, this problem can be minimized by heating the sample cell as described previously.

[0065] Various embodiments of the spectrophotometer apparatus are better understood with the aid of FIGS. 6 and 7. FIG. 6 illustrates an embodiment in which light is emitted by at least one LED 100 which can be included in an array of LED's 102. A sample fluid passes through a sample cell 110. The sample fluid enters the sample cell 110 through the sample inlet 112 and exits the sample cell 110 through the sample outlet 114. The sample cell 110 is partially contained by windows 116 that allow light to pass through the sample fluid. The sample cell 110 is also partially contained by spacers 120. After light emitted by an LED 100 or array of LED's 102 passes through the sample fluid in the sample cell 110, the photoreceptor 130 receives the light and converts the light to a signal to different degrees as a function of the wavelength of the light. (See FIG. 3).

[0066] FIG. 7 illustrates an embodiment in which the spectrophotometer apparatus includes at least two LED's 200 which are part of an LED array 202 and a photoreceptor 230. The photoreceptor 230 can be positioned such that the space between the LED array 202 and the photoreceptor 230 is open to the surrounding atmosphere and light from the LED array 202 can pass through the fluid between the LED array 202 and the photoreceptor 230. The photoreceptor 230 receives the light and converts the light to a signal to different degrees as a function of the wavelength of the light. (See FIG. 3).

[0067] FIG. 8 illustrates an example of how to size the aperture for optional performance. Assume that an LED 100 which can be part of an LED array 102 is one having an angular light distribution is shown in FIG. 4. FIG. 8 illustrates that essentially zero light leaves the LED 100 at an angle of greater than about 30 degrees from the centerline of the LED 100.

Claims

CLAIMS The invention is claimed as follows:

1. A spectrophotometer comprising: a light emitting diode; a photoreceptor; a sample cell, partially defined by at least two substantially parallel windows made of a material transparent to the light emitted by the at least one light emitting diode and accommodating a sample fluid, wherein the sample cell is located between the at least one light emitting diode and the photoreceptor, such that light produced by the at least one light emitting diode passes through the sample cell in a direction substantially perpendicular to the at least two substantially parallel windows, and impinges on the photoreceptor.

2. The spectrophotometer of claim 1, wherein the at least two windows are made of quartz.

3. The spectrophotometer of claim 1, wherein one of the light emitting diodes produces light in the wavelength range of about 250 to 450 nanometers.

4. The spectrophotometer of claim 1, wherein the sample cell is heated.

5. The spectrophotometer of claim 1, wherein the sample cell is heated to a temperature of about 40 to about 80 degrees Fahrenheit above the temperature of the sample.

6. The spectrophotometer of claim 2, wherein the light emitting diode emits light, the light emitting directly outward along an axis extending substantially perpendicular from the center of a surface of the at least one light emitting diode, wherein the emitted light also emits at an angle of greater than about 5 degrees outward from the axis.

7. The spectrophotometer of claim 1 wherein the light emitting diode is included in an array including a plurality of light emitting diodes.

8. The spectrophotometer of claim 1, wherein the light emitting diode is pulsed with a frequency of about 10 Hz or greater.

9. The spectrophotometer of claim 1, wherein the light emitting diode is pulsed 2 or more times for each data update and the absorbance of the multiple pulses are averaged to update readout data.

10. The spectrophotometer of claim 1, wherein at least two light emitting diodes are used, the light from the at least two light emitting diodes impinging on the photoreceptor at intervals such that only one wavelength of light is impinging on the photoreceptor at any given time.

11. The spectrophotometer of claim 1, wherein the light emitting diode emits light in a wavelength range absorbed by the sample fluid and at least one light emitting diode emits light in a wavelength range not absorbed by the component of the sample fluid being measured.

12. The spectrophotometer of claim 1, wherein the sample cell further includes inside walls that join the sample window of the sample cell, the inside walls adapted to prevent reflection onto the photoreceptor of light coming from the direction of the at least one light emitting diode.

13. The spectrophotometer of claim 11, wherein the light from the light emitting diode in the wavelength range not absorbed by the component of the sample being measured is used as a reference beam such that any changes in absorbance of that beam are assumed to have also occurred in the signal beam and the "zero" setpoint of the signal beam is adjusted accordingly.

14. A spectrophotometer comprising: at least two light emitting diodes; a photoreceptor, positioned such that the space between the at least two light emitting diodes and the photoreceptor is open to the surrounding atmosphere and light from the at least two light emitting diodes passes through the space between the at least two light emitting diodes and the photoreceptor and impinges on the photoreceptor, wherein the light from the at least two light emitting diodes impinges on the photoreceptor at intervals such that only one wavelength of light impinges on the photoreceptor at any given time.

15. The spectrophotometer of claim 14, wherein the spectrophotometer measures the concentration of two or more components in a sample fluid.

16. The spectrophotometer of claim 14, wherein at least one of the two light emitting diodes produces light in the wavelength range of about 250 to about 450 nanometers.

17. The spectrophotometer of claim 14, further comprising a sample cell wherein the sample cell is adapted to contain a gas.

18. The spectrophotometer of claim 14, further comprising a sample cell, wherein the sample cell is adapted to contain a liquid.

19. The spectrophotometer of claim 14, wherein the at least one light emitting diode emits light, the light emitting directly outward along an axis extending substantially perpendicular from the center of the surface of the at least one light emitting diode, wherein the emitted light also emits at an angle of greater than about 5 degrees outward from the axis.

20. The spectrophotometer of claim 14, wherein the at least two light emitting diodes are pulsed with an on time of less than about 1 second every 10 seconds at a frequency greater than about 10 Hz.

21. The spectrophotometer of claim 14, wherein at least one of the at least two light emitting diodes is pulsed two or more times for each data update and the absorbance of the multiple pulses are averaged to update the readout data.

22. The spectrophotometer of claim 14, wherein at least one light emitting diode emits light in a wavelength range absorbed by the sample fluid and at least one light emitting diode emits light in a wavelength range not absorbed by the component of the sample fluid being measured.

23. The spectrophotometer of claim 25, wherein the light from the at least one light emitting diode in the wavelength range not absorbed by the component of the sample fluid being measured is used as a reference beam such that any changes in absorbance of that beam are assumed to have also occurred in the signal beam and the "zero" setpoint of the signal beam is adjusted accordingly.