Spectrophotometric determination of gas phase compositions

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Reexamination Certificate

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C250S339120

Reexamination Certificate

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06794649

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention concerns the field of spectroscopy and, more specifically, the use of spectrophotometry to determine gas phase concentrations.
2. Background Art
There are many methods used to detect and/or determine the concentration of an analyte in a mixture or solution. See, for example, U.S. Pat. Nos. 4,314,344, 4,427,772, 4,525,265, 4,795,707, 4,843,867, 5,139,957, 5,167,927, 5,474,908, 5,482,684, 5,516,489, 5,518,591, 5,600,142, 5,608,156, 5,788,925, 5,789,175, 5,847,392, 5,847,393, 5,872,359, 5,892,229, 5,938,917, 5,942,754, 5,972,199, 6,075,246, 6,156,267, and 6,189,368; and European Patent Application No. EP 1,016,421. (All of the foregoing documents, as well as all other documents cited or otherwise referenced herein, are incorporated herein in their entireties for all purposes.)
Some of those documents concern detecting and/or determining the concentration of a species in gas, vapor, or plasma. See, e.g., U.S. Pat. Nos. 4,314,344, 4,843,867, 5,139,957, 5,167,927, 5,482,684, 5,516,489, 5,600,142, 5,608,156, 5,788,925, 5,789,175, 5,847,392, 5,847,393, 5,872,359, 5,892,229, 6,075,246, 6,156,267, 6,189,368; and European Patent Application No. EP 1,016,421.
Some of those documents concern detecting and/or determining the concentration of hydrogen peroxide. See, e.g., U.S. Pat. Nos. 4,427,772, 4,525,265, 4,795,707, 4,843,867, 5,139,957, 5,167,927, 5,474,908, 5,516,489, 5,518,591, 5,600,142, 5,608,156, 5,788,925, 5,789,175, 5,847,392, 5,847,393, 5,872,359, 5,892,229, 5,938,917, 5,942,754, 5,972,199, 6,156,267, 6,189,368; and European Patent Application No. EP 1,016,421.
Some of those documents concern detecting and/or determining the concentration of hydrogen peroxide using spectrophotometry, e.g., using infrared or near-infrared energy. See, e.g., U.S. Pat. Nos. 5,600,142, 5,847,392, 5,847,393, 5,872,359, 5,892,229, 5,942,754; and European Patent Application No. EP 1,016,421.
It is known to determine successive values of a parameter for analytes that decompose after the decomposition has begun and to extrapolate from those successive values back to time zero (the moment just before decomposition begins) to estimate the value of the parameter at time zero. To applicants' knowledge, such a method has not been used for peracids or peroxides (e.g., hydrogen peroxide).
Hydrogen peroxide is used in connection with bleaching, sterilization, and other processes, and there is a need to be able to measure or determine its concentration accurately. In particular, for vapor phase sterilization, the concentration of hydrogen peroxide in the gas phase must be accurately known; however, development of a method for accurately determining the concentration of hydrogen peroxide in the gas phase is hampered by the fact that hydrogen peroxide decomposes in the gas phase. Hydrogen peroxide decomposition increases with increasing temperature (at room temperature, an increase of 10° C. is believed to more than double the rate of decomposition), with increasing pH (especially in the alkaline range), with increasing contamination (e.g., with transition metals), and with exposure to light (particularly ultraviolet light).
Hydrogen peroxide is typically sold in aqueous solution, for example, at concentrations of 3% w/w, 10% w/w, 30% w/w, 35% w/w, and higher (e.g., 70% w/w), and the manufacturers generally add proprietary stabilizers (e.g., chelants/sequestrants such as organic and inorganic phosphates and/or stannates and/or silicates) to the liquid solution to minimize decomposition. Unfortunately, these stabilizers do not function in the vapor phase and once an aqueous liquid solution of hydrogen peroxide is vaporized, as it typically is in hydrogen peroxide vapor phase sterilization processes, decomposition of the hydrogen peroxide immediately begins and continues unabated.
Continuous decomposition of hydrogen peroxide in the vapor phase makes it all the more difficult to determine a relationship between the concentration of the hydrogen peroxide in the vapor phase and a physical property of the hydrogen peroxide that can be measured rapidly (e.g., absorbance of spectral energy within a preselected spectral region characteristic of the hydrogen peroxide) and which relationship can therefore be used to monitor the hydrogen peroxide concentration (e.g., during a hydrogen peroxide sterilization process). This is because such a relationship must be established experimentally and doing so requires, among other things, collecting a sufficient number of replicate data points in real time, but that unfortunately is while the hydrogen peroxide itself is continuing to decompose. In other words, while the physical property indicative of the concentration of hydrogen peroxide is being measured repeatedly so that the relationship between concentration and the physical property can be established, the hydrogen peroxide concentration is decreasing and the measured value of the physical property is changing.
Despite all the attempts that have been made, the need still remains for a rapid and accurate method for determining hydrogen peroxide concentration in the vapor phase. More generally, the need still exists for a rapid and accurate method for determining the concentration of an analyte that decomposes.
SUMMARY OF THE INVENTION
A rapid and accurate method for determining hydrogen peroxide concentration in the vapor phase has now been developed. More generally, a rapid and accurate method for determining the concentration of an analyte that decomposes and/or whose spectral data are “pressure sensitive” (as defined herein) has now been developed. As explained below, applicants discovered that hydrogen peroxide not only decomposes but that its spectral data are pressure sensitive.
Thus, for hydrogen peroxide, the method of determining the concentration makes use of a monotonic functional relationship between the concentration of hydrogen peroxide in the vapor phase and the total (integrated) absorbance of spectral energy within a preselected spectral region characteristic of hydrogen peroxide, preferably the spectral region of wavenumbers 1180 cm
−1
(approximately 8475 nanometers) through 1331 cm
−1
(approximately 7513 nanometers). Thus, a first part of the invention concerns a method for using the relationship to determine (or estimate or predict) from the integrated absorbance of spectral energy for an unknown (i.e., unknown sample) what the concentration of hydrogen peroxide (or other analyte) is in that unknown. A second part of the invention concerns a method for establishing or determining the monotonic functional relationship for hydrogen peroxide (or other analyte).
In connection with the development of the second part of the invention as it applies to hydrogen peroxide, applicants made the surprising discovery that at pressures below about 230 torr (approximately 30.7 kPa), the integrated absorbance for hydrogen peroxide is about 20% lower than it would otherwise be if the pressure were above the pressure at which this phenomenon occurs or at least becomes noticeable (i.e., above about 230 torr). The significance of this is that use of data subject to this phenomenon (i.e., integrated absorbance data that are significantly lower than they would otherwise be) to establish the monotonic functional relationship will result in erroneous predicted hydrogen peroxide concentrations in some cases. Applicants do not know why this phenomenon of significantly lower integrated absorbance occurs.
As indicated herein, if it is desired to estimate, the vapor phase hydrogen peroxide concentration in an unknown, the integrated absorbance for the unknown over the spectral region of interest is determined and the previously established monotonic functional relationship between concentration and integrated absorbance is used. That relationship is typically established from absorbance data for different known hydrogen peroxide concentrations. Applicants discovered that, most surprisingly, for a constant amount of hydrogen peroxide in a chamber (and therefore

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