Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing liquid or solid sample
Reexamination Certificate
2001-08-14
2004-09-21
Warden, Jill (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing liquid or solid sample
C422S068100, C436S039000, C436S145000, C436S146000
Reexamination Certificate
active
06793889
ABSTRACT:
FIELD OF THE INVENTION
There are many applications for instruments capable of measuring the total organic carbon content of water (TOC) over a wide range of TOC values. The typical approach is to oxidize the carbon in the sample to carbon dioxide, and measure the latter. For analysis of the TOC of samples of relatively high purity, the sample may be exposed to UV energy (i.e. <254 nm), typically provided by a mercury lamp, possibly in the presence of a TiO
2
or other catalyst. The conductivity of a static sample (e.g., a sample taken from a process stream of interest and analyzed separately) can be monitored over time during the UV exposure to determine when the reaction is complete. See commonly-assigned U.S. Pat. No. 4,626,413, and others. Such instruments are capable of extremely accurate measurements of the TOC of water samples. However, the oxidation takes considerable time, so that it would be desirable to provide a faster-responding instrument. Furthermore, the relationship of the conductivity of the water sample to its carbon dioxide content is linear only at low concentrations, leading to complexity in analysis of samples of higher CO
2
content.
Current wide range TOC analytical processes use reagents and/or catalysts, such as sodium persulfate and phosphoric acid, to oxidize organic compounds in the sample to carbon dioxide, using either UV energy (i.e. <254 nm), typically provided by a mercury lamp, or heat, typically at least 100° C., to effect oxidation. Oxidation times are typically 5 to 15 minutes and reagents have to be replenished frequently. Commonly the resulting CO
2
is diffused across a membrane into a sample of ultrapure water, and the conductivity of the latter measured to determine the CO
2
content; this technique again becomes increasingly complicated at higher CO
2
concentrations.
As mentioned, the oxidation of carbon to CO
2
can be stimulated in several ways. An additional variation is in the treatment of the sample. The sample can be held static in an oxidation cell, or the carbon in a flowing stream can be oxidized as it flows through an oxidation cell. In the latter case, the conductivity is commonly measured at the entry into and exit from the cell, so as to provide a measure of the change in conductivity and thus of the amount of CO
2
formed. However, this technique can provide an accurate measure of TOC only if the reaction is completed or is completed to a known degree while the sample is in the cell; neither can be reliably assured.
In a non-catalyzed oxidation process, the sample is typically contained in a platinum crucible and heated to a high temperature, such as 600° C. to 900° C. The CO
2
generated is usually measured by means of a NDIR instrument.
As mentioned, the conductivity of a water sample is usually measured to determine the CO
2
content and thus measured its TOC. Other conventional CO
2
detection techniques employ non-dispersive infrared (NDIR) or Fourier-transform infrared (FTIR) techniques. In another prior art technique, exemplified in publications by Bondarowicz and by Roehl and Hoffman, plasma-stimulated emission is used to measure the CO
2
content. The TOC is oxidized by conventional means, and a RF-generated inductively coupled plasma (ICP), typically providing plasma temperatures of >2000° C., is used to heat the resulting carbon to a temperature sufficient to emit radiation; an emission spectrometer is then used to measure the carbon content, and this value is used to determine the TOC.
In a second known technique, exemplified in a paper by Emteryd et al., an ICP is used to simultaneously oxidize the TOC to CO
2
and to heat it to a temperature suitable for emission spectrometry.
In both cases, the plasma generated is too hot to allow direct physical attachment of a conventional CO
2
detector (e.g., one employing non-dispersive infrared (NDIR), Fourier-transform infrared (FTIR), or conductivity-based techniques), limiting the analytical technique to non-contact optical techniques, such as analysis of the spectral emission or mass spectrometry. This is because the previously described plasmas are “equilibrium” plasmas, that is, in which the energy of the electrons of the plasma is in equilibrium with the rotational and translational energy of the gas molecules. Accordingly, the plasma is physically extremely hot.
The present invention also employs plasma oxidation techniques in connection with the measurement of the TOC of a gaseous sample or a sample of water, but does so in a significantly different way than in these two prior art techniques.
More specifically, it would be desirable to employ plasma oxidation as the technique for converting TOC in the sample to CO
2
, so as to obtain an instrument capable of analyzing widely-varying TOC contents, yet allowing accurate analytical techniques such as NDIR, FTIR, or conductivity-based techniques to be used to measure the CO
2
thus produced.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide a wide-range TOC analytical instrument employing plasma oxidation to convert the carbon in a sample to CO
2
, but doing so at temperatures sufficiently low to allow preferred techniques to be employed for measuring the CO
2
thus produced, and doing so in a short time, so as to increase the efficiency of the instrument.
Other objects of the invention will be apparent from the following.
SUMMARY OF THE INVENTION
According to the invention, a dielectric barrier discharge (DBD), also known as a silent discharge (SD) or atmospheric pressure glow discharge (APGD), is provided to oxidize TOC in a gaseous or aqueous sample to CO
2
. DBDs are characterized by the presence of one or more insulating dielectric layers in the current path between metal electrodes in addition to the discharge gap. As known to those of skill in the art, the microdischarges generated in a DBD can be described as a weakly ionized, non-equilibrium plasma; in this connection, “non-equilibrium plasma” means that the mean energy of the electrons within the plasma is not in equilibrium with the vibration, rotation or translation energy of the bulk gas molecules. Therefore, the majority of the gas molecules stay at ambient temperatures while the electron energy is relatively high, so that the electrons of the plasma are effective in cleaving molecular bonds and thus driving the oxidation of TOC.
The DBD employed according to the present invention is referred to herein simply as “the plasma”. Table 1 lists some characteristic microdischarge properties in air at atmospheric pressure for a suitable plasma having a discharge gap of 1 mm (taken from literature).
TABLE 1
Duration:
10
−9
-10
−8
s
total charge:
10
−10
-10
−9
C
Filament
about 10
−4
m
electron
10
20
-10
21
m
−3
radius:
density:
Peak
0.1
A
mean electron
1-10
eV
current:
energy:
Current
10
6
-10
7
Am
−2
filament
close to gas
density:
temperature:
temperature in
the gap
As noted, the relatively low plasma temperature (that is, as compared to the plasmas discussed in the art cited above) of less than 50° C. allows confinement of the plasma in a glass cell; however, since the plasma is non-equilibrated, as noted the mean electron energy in this type of discharge is still sufficient to cleave chemical bonds. This allows the plasma to be used to drive the oxidation, while permitting measurement of the CO
2
thus generated by NDIR, FTIR, or conductivity-based techniques, thereby satisfying an important aspect of the invention.
More specifically, due to their suitable energy, the electrons in the plasma are capable of generating highly reactive species. The nature of these species mainly depends on the type of gas that is filling the discharge gap. In the case of oxygen, some of these species are excited oxygen, atomic oxygen, ozone, peroxides, etc. In particular, in the presence of water (vapor) and oxygen, large quantities of hydroxyl radicals are formed in high amounts responsive to the plasma discharge. Hydroxyl radicals are very strongly oxidizing, and are considered to be the ma
Asano Tsutomo
Naatz Ulf W.
Stillian John
Yamamori Yasuo
Angeli Michael de
Hach Company
Siefke Sam P.
Warden Jill
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