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Numéro de publicationUS3650696 A
Type de publicationOctroi
Date de publication21 mars 1972
Date de dépôt8 sept. 1970
Date de priorité8 sept. 1970
Numéro de publicationUS 3650696 A, US 3650696A, US-A-3650696, US3650696 A, US3650696A
InventeursEads Ewin A
Cessionnaire d'origineLamar State College Of Technol
Exporter la citationBiBTeX, EndNote, RefMan
Liens externes: USPTO, Cession USPTO, Espacenet
Sampling and analysis of sulfur compounds
US 3650696 A
Résumé
A method for accurately sampling ambient air containing sulfur compounds in the low parts per billion range and for separating, identifying and quantitatively monitoring each sulfur compound. The gaseous sample is collected in substantially sulfur-free methanol. The methanol solution is then passed through a gas chromatograph column packed with octylphenoxypolyethylene on oxyethanol polytetrafluoroethylene to separate the sulfur compounds. From the gas chromatograph the separated sulfur compounds go through a pyrolysis furnace where they are oxidized to sulfur dioxide, and thence into a microcoulometer for titration. By using the same procedure, sulfur compound impurities in solid materials can be identified and quantitatively measured.
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Description  (Le texte OCR peut contenir des erreurs.)

iJnited States Patent Eads [451 Mar. 211, i972 SAMPLING AND ANALYSIS OF SULFUR COMPOUNDS.

Ewin A. Ends, Beaumont, Tex.

[72] Inventor;

[73] Assignee: Lamar State College of Technology, Beaumont, Tex.

[22] Filed: Sept. 8, 1970 [21] Appl. No.: 70,478

[52] U.S. CL. ..23/230 PC, 23/232 C [5|] Int.Cl. ..G01n31/08,G0ln3l/l6 [58] Field of Search ..23/230 PC, 232, 254, 232 C, 23/230 [56] References Cited UNITED STATES PATENTS 3,529,937 9/1970 lhara et al. ..23/230 PC X OTHER PUBLICATIONS Glass et aL, Anal. Chem. 32, No. 10, September 1960, 1,265-

Primary Examiner-Morris O. Wolk Assistant Examiner-11. M. Reese Attorney-Bertram H. Mann, Frank B. Pugsley, James G. Ulmer, Delmar L. Sroufe and Larry B. Feldcamp [57] ABSTRACT A method for accurately sampling ambient air containing sulfur compounds in the low parts per billion range and for separating, identifying and quantitatively monitoring each sulfur compound. The gaseous sample is collected in substantially sulfur-free methanol. The methanol solution is then passed through a gas chromatograph column packed with octylphenoxypolyethylene on oxyethanol polytetrafluoroehtylene to separate the sulfur compounds. From the gas chromatograph the separated sulfur compounds go through a pyrolysis furnace where they are oxidized to sulfur dioxide, and thence into a microcoulometer for titration.

By using the same procedure, sulfur compound impurities in solid materials can be identified and quantitatively measured.

10 Claims, 1 Drawing Figure l SAMPLING AND ANALYSIS OF SULFUR COMPOUNDS LICENSE TO THE U.S. GOVERNMENT A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the U.S. Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

BACKGROUND OF THE INVENTION air pollution control work in the scientific characterization of odors from sulfur compounds.

2. Description of the Prior Art One of the primary sources of odor pollution are sulfur compounds, such as carbonyl sulfide, hydrogen sulfide and various organic sulfur compounds. The term organic sulfur compounds as used herein and in the art is intended to designate a group of compounds of thetype RSH, RS H, R 5, and R 8 in which R represents an aliphatic or aromatic radical. One or more of these sulfur compounds are emitted by rendering plants, kraft paper mills, oil refineries, Frasch sulfur plants and various chemical plants.

The odor thresholds for carbonyl sulfide, hydrogen sulfide and most organic sulfur compounds are generally regarded as being in the range from about 0.4 to about 4.0 parts per billion. Because of these low odor thresholds, it has been found almost impossible to scientifically characterize levels of odor emissions for these sulfur compounds. This has resulted from the inability to accurately sample,'separate, identify and quantitatively monitor mixtures of sulfur compounds in the low parts per billion range.

The primary reason that has deterred the development of analytical techniques for low concentrations of sulfur compounds is the reactive nature of sulfur, especially with respect to sampling and separating sulfur compounds. Sulfur compounds have a tendency to adhere to or react with the surface materials of the sampling and analytical equipment, and/or react with the liquid or gaseous materials in the equipment. Obviously, the accuracy of concentration measurements in the low parts per billion range is substantially affected by the ability to prevent sulfur compounds from reacting with or adhering to other materials.

The difficulties encountered in developing techniques for sampling and analyzing low concentrations of sulfur compounds have hindered the development of satisfactory odor regulations by air pollution control agencies. Most, if not all, air pollution control agencies that have even attempted to regulate the emission of odors use one form or another of odor pollution panels, which usually consist of three or more persons smelling air samples to determine if the odors are, offending. Obviously, the determinations made by these odor pollution panels are subject to the variations of the human nose and lack the desired scientific characterization typical of most air pollution control regulations.

In addition, progress in the development of methods and equipment for controlling the emission of odor-causing sulfur compounds has been delayed by the aforementioned difficulties. Identification of the sulfur compounds that are being emitted and a determination of the concentrations of these compounds are prerequisites to the development of techniques and equipment to abate the emissions of odor pollutants.

Most methods for measuring sulfur compounds that have gained any acceptance in air pollution control work are restricted to those where only one specific sulfur compound is measured. For example, hydrogen sulfide can be measured by using either the lead acetate tape method or the cadmium acetate-acetic acid absorption method has been developed for methyl mercaptan.

A more flexible instrument which separates and quantitatively monitors a two or more sulfur compounds is the recently developed Barton titrator. In this instrument the gas is impinged through a series of chemical solutions which selectively extract the various sulfur compounds in the gas sample. However, this instrument does have the limitation of not being capable of separating some organic sulfur compounds. Furthermore, the use of a Barton titrator for analyzing low concentrations of sulfur compounds is a relatively slow method in that each chemical solution which is used to extract a specific sulfur compound or group of compounds must be titrated separately with a microcoulometer. An additional problem is that the complexes formed in the chemical solution upon the extraction of the sulfur compound or compounds are relatively unstable, requiring therefor almost immediate titration to avoid any loss of accuracy.

Generally, previous attempts to separate sulfur compounds with gas chromatograph columns have proved to be unsuccessful because of the tenacious adherence of sulfur compounds to the column walls or solid supports, or irreversible reactions with the column walls, supports, stationary liquid phase or carrier gas. For example, in a paper presented at the 157th meeting of the American Chemical Society in Minneapolis, Minn., in April of 1969, Stevens and others related such difficulties with columns packed with 10 percent Triton X-30 S(octylphenoxypolyethylene oxyethanol) or Fluoropart T (polytetraflouoroethylene). However, as described in a recent article, Modem Aspects of Air Pollution Monitoring," by Stevens and OKeeffe, Analytical Chemistry, Volume 42, No. 2, Feb. 1970, an automated analytical gas chromatograph system has been developed which can quantitatively measure low parts per billion levels of sulfur dioxide, hydrogen sulfide, methyl mercaptan and ethyl mercaptan in ambient air.

Although this chromatographic system seems to obviate the disadvantages of the Barton titrator, as noted above, its accuracy, as is the Barton titrators, is dependent upon the sampling technique used. The possible inaccuracy results from the tendency of gaseous sulfur compounds to adhere to and/or react with the walls of the gas-sampling containers, regardless of the material used. As mentioned previously, even the smallest loss of material resulting from any adherence or reaction will cause substantial inaccuracy when measuring concentrations in the parts per billion range.

Obviously, the loss of accuracy can be minimized by directly injecting the gas sample into the analyzing equipment. However, this would require having the analyzing equipment at the point of sampling. This requirement of using sensitive analyzing equipment in the field has several disadvantages. Since the analyzing equipment would be surrounded by the atmosphere to be analyzed, any leakage in the equipment could substantially affect the results. Obviously, the more the equipment is transported from sampling point to sampling point, the greater the possibility of leakage and the more time required for maintenance. Furthermore, when numerous samples are required, the use of sensitive analyzing equipment in the field is much more expensive, both in terms of the number of trained personnel and the amount of equipment required, than if the samples were collected in the field by relatively untrained personnel and taken to a central laboratory for analysis. At such a central laboratory the samples could be run on more or less a continuous basis under controlled atmospheric conditions. In this manner, expensive analyzing equipment can be more efficiently utilized with fewer trained personnel.

SUMMARY OF INVENTION The object of the present invention is to provide an accurate method for sampling mixtures of sulfur compounds and separating, identifying and quantitatively monitoring each sulfur compound. This invention is particularly applicable to concentrations of sulfur compounds in the low parts per billion range.

Especially with respect to ambient air containing sulfur compounds, a further object of this invention is to provide a flexible method whereby gas samples can be taken in the field and then analyzed at a different location at some later time without any deterioration ofthe sample.

These objects can be obtained by impinging gaseous samples containing sulfur compounds into sulfur-free methanol. The methanol solution is then inserted into a gas chromatograph column where the sulfur compounds are separated. The sulfur compounds are then identified and measured quantitatively by first passing the separated compounds through a pyrolysis furnace where they are oxidized to sulfur dioxide, and then by passing the sulfur dioxide products into a titration cell where they are titrated coulometrically.

The use of methanol as the solvent for sulfur compounds obviates some of the problems of prior art methods. It has been found that sulfur compounds are completely soluble in methanol and do not adhere to or react with the sample container walls. Moreover, there is no reaction of the sulfur compounds with the methanol or with the other compounds in solution. Consequently, much flexibility in the sampling and analyzing of mixtures of sulfur compounds is obtained, as the samples can be accurately analyzed many days, or even weeks, after they are taken.

To separate carbonyl sulfide, hydrogen sulfide and various organic sulfur compounds, the gas chromatograph column must be inert to both the sulfur compounds and the methanol. A column packed with octylphenoxypolyethylcne oxyethanol on polytetrafluoroethylene surprisingly fulfills this criteria. By absorbing the sulfur compounds in methanol, the previous problem of retention of appreciable quantities of gaseous sulfur compounds in the column is obviated.

The use of this gas chromatograph column in conjunction with a microcoulometer provides a means for quickly separating, identifying and quantitatively monitoring mixtures of sulfur compounds at low parts per billionlevels in a matter of4 to 5 minutes.

This invention also provides a quick simple method for the analysis ofthiophene and other heterocyclic compounds, even though these compounds are normally found as impurities in solid rather than gaseous materials.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE presents a simplified flow diagram of the method ofthis invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The preferred embodiment of this invention consists of a method whereby low concentrations of odor-causing sulfur compounds in ambient air can be accurately sampled and analyzed. Such a method will allow air pollution control agencies to scientifically characterize low levels of these sulfur compounds and thereby regulate the emission os odors resulting from such compounds. Moreover, this method will aid in the development of techniques to abate the emission of odorcausing sulfur compounds.

The procedure in accordance with this invention consists of passing a measured amount of a gaseous mixture containing sulfur compounds through substantially sulfur-free methanol; passing the methanol solution in which the sulfur compounds have been absorbed through a gas chromatograph, essentially inert to methanol and sulfur compounds, to separate the sulfur compounds; passing the sulfur compounds as they are emitted from the gas chromatograph column through a pyrolysis furnace to oxidize each compound to sulfur dioxide; and passing the oxidized sulfur compounds as they are emitted from the furnace through a microcoulometer where they are titrated to determine the concentration of each sulfur compound in the gaseous mixture. This procedure provides an accurate method for sampling, separating, identifying and quantitatively monitoring sulfur compounds in the parts per billion range.

By using methanol as the solvent or scrubbing solution, the sulfur compounds are prevented from reacting with or adhering to the walls of the sampling containers. No reaction between the sulfur compounds or between the sulfur compounds and the methanol has been observed.

To test the solvent capacity of methanol for sulfur compounds, gas samples containing sulfur compounds were impinged through two sample scrubbers in series. No sulfur compounds were detected in the methanol in the second scrubber, thus indicating that all of the sulfur compounds had been absorbed by the methanol in the first scrubber.

Although heavier alcohols, such as ethanol, propanol or butanol, or ethyl ether, can be used instead of methanol, they are not preferred since methanol is the best solvent and has fewer carbon atoms which can be oxidized to carbon dioxide in the pyrolysis furnace. The presence of more than small amounts of carbon dioxide interferes with the titration in the microcoulometer.

The methanol used must be essentially sulfur free, as any sulfur present in the methanol will indicate a higher than actual concentration. It has been found that high purity, sulfurfree methanol can be more readily obtained than other sulfurfree alcohols or others.

Gas samples may be obtained by using conventional impinger gas samplers, such as a Mine Safety Appliance midget impinger. The use of such an impinger allows the volume of gas sampled and passed through the sulfur-free methanol scrubbing solution to be measured.

The absorption of the sulfur compounds in methanol allows greater flexibility in the sampling and analysis of gaseous sulfur compounds, especially in air pollution control work. For example, ambient air containing sulfur compounds can be sampled in the field and then taken to a laboratory for analysis at a later time. Even over an extended period of time, no deterioration of the sample occurs. In contrast, it has been found that one part per million of hydrogen sulfide inserted into a conventional three liter polyethylene gas sampling bag cannot be detected on analysis. This results from the hydrogen sulfide reacting with or adhering to the walls of the sampling bag. This does not occur when the gas sample is passed through methanol. Neither is there any interaction of the sulfur compounds. In fact, the stability of methanol solutions containing sulfur compounds is such that the solutions can be used as reference samples for calibrating the analytical equipment.

With this flexibility, a substantial cost savings can be realized by reducing the number of analytical equipment units and the number of trained operators of the equipment. For instance, if a sample deteriorates over a short period of time either by reaction with or adherence to the walls of the container, analytical equipment would need to be placed into the field at the point of sampling in order to maintain the accuracy required at parts per billion levels. On the basis of the number of samples taken and analyzed, such a use of the analytical equipment would require more equipment and more trained personnel as compared to having the samples taken in the field by relatively untrained personnel and then analyzed at a central laboratory. Moreover, the analytical equipment cannot be as efficiently utilized in the field as in a laboratory as much time is lost in moving the equipment from sampling point to sampling point. This movement of the equipment of the analytical equipment would also necessitate more calibration and maintenance of the equipment. Obviously, inaccurate measurements resulting from leakage of the surrounding atmosphere into the equipment can be minimized by using the equipment in an essentially sulfurfree laboratory atmosphere than in the atmosphere being sampled.

The analytical equipment used in accordance with the preferred embodiment of the present invention is shown diagrammatically in the FIGURE and consists of gas chromatograph column 1, pyrolysis furnace 2 and microcoulometer 3. The sample methanol solution containing sulfur compounds is inserted into chromatograph column 1 through line 4. Also added to column 1 is the helium carrier gas. The sulfur compounds as separated in column 1 then flow through line 5 into pyrolysis furnace 2, to which is also added oxygen through line 6. The separated sulfur compounds as oxidized to sulfur dioxide in furnace 2 then pass through line 7 into microcoulometer 3. The results of the analysis by microcoulometer 3 are usually visually recorded by a recording device, such as recorder 8, which is connected to the microcoulometer. The formula for calculating the concentration of each sulfur compound in the sample will be described hereinafter.

Besides having the capability of separating sulfur compounds, the gas chromatograph column and its contents must he inert to methanol and sulfur compounds. Such a column is u lll foot stainless steel one-fourth-inch column packed with oc-tylphenoxypolyethylene oxyethanol on polyte'trnfluoroethylene. The introduction of the sulfur compounds into the column in in methanol solution instead of in a gaseous form prevents the compounds from reacting with or adhering to the packing or the walls of the column. Helium is used as a carrier gas at flow rates from about 90 to about 170 milliliters per minute. The temperature at which the column is maintained is not critical but it must be high enough to prevent adherence of the sulfur compounds to the column walls or the packing but not so high as to cause the packing to break down. The gas chromatograph is typically operated at about 145C. However, as shown by Table 1 below, the relative retention times of sulfur compounds in methanol are somewhat variable at different column temperatures.

TABLE I Retention Time in Seconds The compounds having approximately the same retention times can be distinguished and identified by use ofwell-known sample dilution and temperature programming techniques.

The sulfur compounds as they are emitted from the gas chromatograph column in the relation shown in Table 1 above are then passed into a pyrolysis furnace where they are oxidized in oxygen at such a temperature as to form sulfur dioxide. The temperature must be maintained low enough to avoid the formation of sulfur trioxide but high enough to avoid the formation of sulfides. Both sulfides and sulfur trioxide have an adverse effect on the titration cell reactions. The preferred temperature range is from about 675C. to about 725C. The following reactions are typical of those occurring in the furnace:

The sulfur compounds, as separated and oxidized, are then passed into a conventional microcoulometer where the sulfur dioxide products are titrated with either iodine or bromine ions. In the preferred iodine cell, the contents consist of sodium azide as the buffer solution, potassium iodide, acetic acid and iodine. As the separated, oxidized sulfur compounds are drawn through the cell at a constant rate, the triiodide ions in the electrolyte are consumed The resultant drop in triiodide ion concentration is sensed as a drop in cell voltage and the triiode ion generating circuit is activated to restore the original cell voltage. The additional current flow through the generating circuit is proportioned to the quantity of triiodide ion-titrable gases reacting in the cell. The reaction at the reference sensor electrode is as follows:

S0 l] H O S0 3l' 2H At the generator anode, the reaction to replace the depleted triiodide ions is as follows:

I, l =l- The titration cell contents must be replenished periodically to maintain an approximate concentration of 0.07 percent acetic acid, 0.05 percent potassium iodide and 0.07 percent sodium azide. The concentration of these three cell components is especially affected by the presence of substantial amounts of amines and organic chlorides in the samples. Upon combustion, the amines form nitrogen oxides and the organic chlorides form acids, both of which react with the cell solution.

A sample of a methanol solution containing sulfur compounds can be completely analyzed by the described gas chromatograph, pyrolysis furnace and microcoulometer arrangement in an average of4 to 5 minutes. The results of the analysis are visually recorded on a strip chart by means of a record ing instrument connected to the microcoulometer. Using the relative retention times of the sulfur compounds in the gas chromatograph column, the peaks on the recorder strip chart can be identified as representing specific sulfur compounds. The concentration of each sulfur compound is determined by the following formula:

Peak height in millimeters X factor (cpncemrdnon micrograms per microliters of sample resistance microhter) in ohms (microcoulometer) The factor for each sulfur compound was determined by running samples of known concentrations. Factors have been obtained as set forth in Table ll:

TABLE ll Compound Factor Carbonyl sulfide 0.264 Hydrogen sulfide 102 Sulfur dioxide 0.246 Ethyl mercaptan l L9 Dimethyl sulfide 0.057 Methyl mercaptan [0.4 Diethyl disulfide 2.32 Diethyl sulfide 0.956 Dimethyl disulfide 0.228 Thiophene 0.651 2-bromothiophene 7.069 Z-nitrothiophene L360 Z-acetylthiophene 3.552 2-thiophenecarboxaldehyde 5.575 Z-thiophenecarboxylic acid 2.095

The accuracy of the method in accordance with this invention for sampling and analyzing sulfur compounds is shown by Table III. Known amounts of sulfur compounds inserted into sulfur-free methanol were analyzed using the method previously described.

TABLE III Compound Micrograms taken Micrograms recovered Dimethyl sulfide 0.085 0.085 Diethyl sulfide 0.043 0.044 Dimethyl disulfide 0.01 I 0.012 Diethyl disulfidc 0.027 0.026 Ethyl mercaptan 0.092 0.09]

Hydrogen sulfide 0.095 0.096 Carbonyl sulfide 0.084 0.084 Methyl mercaptan 0.084 0.084 Sulfur dioxide 0.108 0.106 Thiophene O. |9l 0.176

This invention can also be applied to the detection and analysis of sulfur compound impurities in solid materials. The same procedure as previously described for gaseous mixtures is used in sampling and analyzing the materials. Obviously, the material to be analyzed must be ofa small enough particle size to insure that all the sulfur compounds are completely dissolved in the methanol.

in the analysis of the solid materials, this invention has been found to be particularly applicable to the sampling and analysis of low concentrations of thiophene and other heterocyclic sulfur compounds in synthetic rubber products. Previously, very sophisticated and expensive equipment was required to separate, identify and quantitatively monitor these compounds. However, the method according to this invention allows the analysis of these compounds to be effected very quickly with relatively inexpensive equipment.

It would be obvious to persons skilled in the art that minor variations in the procedures of this invention may be used to sample and analyze sulfur compounds in gaseous mixtures or solid materials in addition to those specifically set forth and that changes and modifications of the invention can be made. lnsofar as such variations and modifications incorporate the true spirit of this invention, they are intended to be included within the scope of the appended claims.

lclaim:

l. The method of sampling a gaseous mixture containing low concentrations of sulfur compounds for the analysis of said sulfur compounds comprising passing a measured amount of said gaseous mixture through substantially sulfur-free methanol to absorb essentially all the said sulfur compounds in the said methanol.

2. The method according to claim 1 wherein said gaseous mixture is ambient air containing parts per billion concentrations of odor-causing sulfur compounds.

3. The method of sampling and analyzing a gaseous mixture containing low concentrations of sulfur compounds comprising the successive steps of:

a. passing a measured amount of said gaseous mixture through substantially sulfur-free methanol to absorb essentially all the said sulfur compounds in the methanol;

b. passing the methanol containing the absorbed sulfur compounds through a gas chromatograph column essentially inert to methanol and sulfur compounds in methanol to separate the sulfur compounds;

0. oxidizing the sulfur compounds as separated to form sulfur dioxide; and

d. titrating the separated, oxidized sulfur compounds in the form of sulfur dioxide in a microcoulometer to thereby determine the concentration of each sulfur compound in the gaseous mixture.

4. The method according to claim 3 wherein the gaseous mixture is ambient air containing parts per billion concentrations ofodor-causing sulfur compounds.

5. The method according to claim 3 wherein the separated, oxidized sulfur compounds in the form of sulfur dioxide are titrated with iodide ions in a microcoulometer.

6. The method according to claim 5 wherein the sulfur compounds as separated are oxidized at a temperature in the range from about 675C. to about 725C. to form sulfur dioxide.

7. The method of sampling, separating and quantitatively measuring low concentrations of sulfur compounds in a gaseous mixture comprising the successive steps of:

a. passing a measured amount of said gaseous mixture through substantially sulfur-free methanol to absorb essentially all the said sulfur compounds in the methanol;

b. passing the methanol containing the absorbed sulfur compounds through a gas chromatograph column packed with octylphenoxypolyethylene oxyethanol on polytetrafluoroethylene to separate the sulfur compounds;

c. oxidizing the sulfur compounds as separated to form sulfur dioxide; and

d. titrating the separated, oxidized sulfur compounds in the form of sulfur dioxide with iodide ions in a microcoulometer to thereby determine the concentration of each sulfur compound in the gaseous mixture.

8. The method according to claim 7 wherein the gaseous mixture is ambient air containing parts per billion concentrations of odor-causing sulfur compounds, and the sulfur compounds as separated are oxidized at a temperature in the range from about 675C. to about 725C. to form sulfur dioxide.

9. The method according to claim 8 wherein the sulfur compounds are heterocyclic sulfur compounds; the heterocyclic sulfur compounds as separated are oxidized at a temperature in the range from about 675C. to about 725C. to form sulfur dioxide; and the separated, oxidized heterocyclic sulfur compounds in the form of sulfur dioxide are titrated with iodide ions in a microcoulometer.

10. The method of sampling and analyzing a solid material containing low concentrations of sulfur compounds comprising the successive steps of:

a. dissolving a measured amount of said solid material in substantially sulfur-free methanol to absorb essentially all the said sulfur compounds in the methanol;

b. passing the methanol containing the absorbed sulfur compounds through a gas chromatograph column essentially inert to methanol and sulfur compounds in methanol to separate the sulfur compounds;

0. oxidizing said sulfur compounds as separated to form sulfur dioxide; and

d. titrating the separated, oxidized sulfur compounds in the form of sulfur dioxide in a microcoulometer to thereby determine the concentration of each sulfur compound in said solid material.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREQTION Patent NO. 3, Dated I Inventor(s) Ewin A. Eads It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, line 7, the word "on" should come after oxyethanol In the Abstract, lines 7 and 8, "polytetrafluoroehtylene" should be polytetrafluoroethylene Col. 2, line 19, "therefor" should be therefore Col. 2, line 30, "X-30 S" should be X-305 Col 2, line 30, "or" should be on Col. 2 line 31, "(polytetraflouroethylene)" should be (polytetrafluoroethylene) Col. 3, line 57, "as should be of Col. 6, line 6, "2H should be 2H Col. 6, line 31, (concentration micrograms per microliter) should be concentration (micrograms per microliter) Signed and sealed this 18th day or July 1972.

(SEAL) Attest:

EDWARD MFLETCHER,JR. RCBERT GOTTSCHALK Attesting OfYioer Commissioner of Patents FORM P0405) (10459) USCOMM-DC some-ps9 ".5. GOViRNMENT PRYN'HNG OFF'CEZ I959 0-366-335

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Classifications
Classification aux États-Unis436/96, 436/123
Classification internationaleG01N33/00, G01N31/16
Classification coopérativeG01N33/0044, G01N31/16, G01N33/0047
Classification européenneG01N33/00D2D4E, G01N33/00D2D4G, G01N31/16