CA1201907A - Gaseous contaminant dosimeter with blank absorbant medium - Google Patents

Abstract

TITLE
Gaseous Contaminant Dosimeter With Blank Absorbant Medium ABSTRACT

A personal dosimeter for collecting a quantity of a gaseous contaminant in proportion to its average ambient concentration over the time of collection is provided. The dosimeter consists of a closed recepta-cle containing a gas collecting absorbant medium, an added blister to contain a blank absorbant medium, a channel-containing diffusion device forming a part of the boundary of the receptacle and a porous, hydrophobic film in contact with the interior side of the barrier.

Description

Gaseous Contaminant Dosimeter With Blank Absorbant Medium BACKGROUND OF TE~E IN~NTION
Fie~ld of the Invent _n This invention i.s rel~ted to a personal do-simeter ~or registering ga~aeous contaminants in the atmosphere. More particularly, it is related to a dosimeter wherein an extra blister .is added to contain a blank absorbant medium.
Description Of The P_ior Art In response to the increasing concern about the health of workers who are exposed to harmful pollut-ants in the air, it has become necessary to monitor the concentration of the air-borne contaminants. One de-velopment ~or this purpose involved a rather large air pump which would force air to be sampled through a filter, trapping particulate contaminants. This obvi-ously is unavailing for the mon.itoring of gaseous contaminants and, even for parti~les, is not accurate to dekermine concentration of the particles in the sampled atmosphere.
Personal sampling devices which are worn by individual workers and which passively collect the con-taminants have also been used. For example, a devicewhich utilized the molecular diffusion of the gas to be monitored to collect the sample has been described in the ~merican Industrial Hygiene Association Journal, Volume 34, pages 78 81 (1973). This device and others like it, ca]led impinging tubes~ are often cumbersome to use since their design and delicate constructions necessitate that they always be oriented properly to accurately sample the atmosphere and to prevent dislo-cation of the sampling mechanlsm within the tubeO

The need arose for the development of personal - dosimeters that simply but accurately collected gaseous contaminants in proportion to their average atmospheric concentration. Examples of dosimeters designed for colorimetric analysis are disclosed in Kring U.S.
4,208,371 issued June 17, 1980; Xring et al. ~.S.
4,235,097 issued November 25, 1980; and Kring U.S.
4,269,804 issued May 26, 1981. The color intensity of the exposed collecting medium in these dosimeters is proportional to the dose-level of ~as sampled. The color stability of suitable collecting Tnediums, how-ever is effected by storage variables.
A need became apparent for the availability of a standard to compare with the collecting a~sorbant medium.
SUMMARY OF T~E INVENTION
According to the present invention, there is provided a personal dosimeter for collecting a gaseous contaminant in proportion to its average ambient con-centrati.on during the collection time where the do-simeter consists essentially of a closed receptacle;
a collecting medium for the gaseous contami-nant within the receptacle;
a blister separately sealed and containing a blank absorbant medium;
a diffusion device, forming part of the boundary of the recPptacle, the device containing a plurality of through-and~through channels adapted for the gaseous contaminant to diffuse the.rethrough from the atmosphere to the interior of the receptacle, said channels each having a length-to-diameter ratio of at least 3 and sald channels providing the only communica-tton between the atmosphere and the interior of the receptacle; and a porous, hydrophobic, inert film covering the interlor openinqs of said channels.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a ma~nified perspective vie~ of a gas dl~fusion device usable in -the present invention.
Fig. 2 is top view of a gaseous contaminant dosimeter utilizing the di~fusion device of Fig. 1 and having a sealed blister of ~lank absorbant medium.
Fig. 3 i5 a partlal perspecti~e view of the ~ dosimeter of Fig. 2.
Fig. 4 is a perspective view of a gaseous contaminant dosimeter utilizing a membrane as the gas diffusion device and havi.ng a sealed blister of blank absorbant medium.
DET~ILED DESCRIPTION OF THE INVENTION
The dosimeters of this invention collect a gaseous contaminant in proportion to its average concentration in the atmosphere during the collection pPriod and provide for the expedient determination of this concentration. This is achieved by passively sampling the gaseous contaminant in ambient air in proportion to i~s concentration therein by allowing the contaminant to diffuse into an interior portion o~
the dosimeter where i-t is maintained, by a collecting medium situated therein~ until it is analyzed.
The blank absor~ing medium contained in the added blister i5 the same absorbing material as present in the diffuser containing absorbing blister. Both mediums are pre.sent in measured amounts. The blank does not increase in absorbance after the dosimeter is exposed to a gaseous contaminant.
When the collecting medium is analyzed, the blank absorbant medium ~rom the added, separately sealed blister is used as a standard which has been exposed to the same environmental conditions, e.g., shelf-life, as the collecting absorbant medium.

The collecting medium holds the gaseous con-taminant ox its ~ons in a form that is more readily analyzable than is the gaseous form. After collection, the collecting medlum and the blank medium are removed from the dosimeter and treated with appropriate re-agents to prodwce color, the intensity of which is dependent upon the amount of gaseous contaminant col-lected and analysis of the blank medium. The time-aYerage ambient concentration can then be determined, as later explained, with a previously-calibrated color-lmeter or spectrophotometerO Alternatively, the con-taminant can he separated from the collecting medium and its quantity determined, for example, hy gas chromatography wherein the results of the gas chroma-tography analysis have been previously callbratedagainst known time-average ambient concentrations of the contaminant. The preerred method of determination is colorimetric.
Generally, the collecting medium is a material that absorbs, adsorbs, reacts or otherwise combines with the gaseous contaminant being measured. Regard-less of the manner in which the medium interacts, as above, with the contaminant, the quantity or strength of the collecting medium in the dosimeter should be sufficlent to interact completely with the total quantity of gaseous contaminant which is anticipated to be collected. The collecting medium will often be specific to the particular gaseous contaminant being monitoredO Examples, meant to be representative but not limiting, include aqweous solutlons of oxidiæing agents or triethanol amine to absorb nitrogen dioxide, solutions of potassium or sodium tetrachloromercurate to absord sulur dioxide, solutions of sulfuric or othex acids to absorb ammonia, and distilled water or a s~lution of sodium bisul~ite to absorb ~ormaldehyde.
Charcoal or powdered carbon of high surface area, powders of metals, or metal salts can be used to absorb many other organic contaminants.
Methods for colorimetric anal~sis, ~or example, fQr sulfur dioxide, nitrogen dioxide, ammonia, or formaldehyde, in air, are discribed in National ~nstitute for Occupational Saety and Health method numbers 160 (publication 121, 1975), 108 (publication 136, 1974), 205 (publication 121, 1975) and 125 (publi-cation 136, 1974), respectively. The techniques therein described are readily adaptab]e with respect to absorbing solutions and color-form.ing reagents for use in connection with collection by the dosimeter or the present invention~
One embodiment of the present invention is shown in Figures 2 and 3 and is described and can be formed as follows. A base sheet 6 of impermeable poly-meric material is provided to form one side of thereceptacle portion of the dosimeter. The base sheet is provided with at least two depressions ~ and S whlch can be linearly spaced parallel to the gas diffusion 1 device and the opposite edge of base sheet. The sheet is preferably transparent and thermoplastic and can be made of polymers of olefin, halogenated polymers, poly-esters, or ionomer resins. Preferred materials are the ionomer resins shown in U.S Patent No. 3,264,272 issued August 2, 1966 to R. W. Rees. They are the ionic copol~mers o~ alpha-olefins and alpha, beta~ethylenically unsaturated carboxylic acids of 3-8 carbon atoms having 10-90~ of the carboxylic acid groups neutralized with metal ions.

~ J

The size of sheet 6 is not critical but is preferabl~ a size easily adaptable for use in a per-sonal dosimeter which is to be worn or readily carried.
The depressions 4 and 5 can easily be formed by applying pressure to sheet 6 with an appropriate die, heated or otherwise.
After the collectlng medium and blank have been placed on sheet 6, a second top-sheet 7 correspond-int to sheet 6 in composition and substantially in size is placed over sheet 6. Heat and pressure are then applied surrounding the two depressions 4 and 5 to pro-vide permanentr fluid-tiyht bonding at the corresponding areas 3 of sheets 6 and 7~ Adhesives or other forms of bonding can also be used provi.ded the bonds are per-manent and fluid-tight and the adhesive is inert to the collecting medium.
An elonyate gas diffusion device l having a plurality o~ through-and-through channels 2 is positioned parallel and proximate to the fourth, unbonded edge of base sheet 6 and parallel and flush with the fourth, un-bonded edge of top sheet 7. The open channels 2 of device l are thus orien~ed horizontally with respect ~o the plane of sheet 6 and perpendicularly with respect to the fourth edges of sheets 6 and 7. On the interior side 8 of device l, covering the channel openings of this interior side, is a porous, hydrophobic film, de-scribed in more detail below. Diffusion device l, thus placed be~ween sheets 6 and 7, is bonded to the sheets by the application of heat and pressure or by use of adhesives which should be impermeable and chemically inert to the collecting medium.
The bond between diffusion device 1 and each of sheets 6 and 7 should be liquid-tight and ~ir-tight, thus completely enclosing the interior 4 of the receptacle R~3~

formed by sheets 6 and 7. The relative positions of di~us~on device 1 and ~heets 6 and 7 are such that the channels 2 pro~ide the only communication between the atmosphere and the interior 4 of the receptacle.
It is also possi~le to form the dosimeter of Figures 2 and 3 saving the placement o~ the collect-ing medium and the blank when they are a liquid r for last. In such a case, the dosimeter is otherwise fo~ned as described above. The collectlny medium can be placed by piercing top sheet 7 at an appropriate spot with a hypodermic needle and injecting a measured amount of the collecting medium into the interior. The hole made by the hypodermic needle can then be thermally sealed.
Difusion device 1 allows the gaseous contami-nant to diffuse through each of channels 2 according to Fick's Law, which is expressed in relevant form as M =D C-t A/L
~here M = quantity of gaseous contaminant transferred (mg) D = diffusion coefficient of the gaseous contaminant through air (cm2/min) C = concentration of the contaminant in the atmosphere (mg/cm3~
t = time of exposure (minutes) = cross sectional area oE the channel (cm2) L = distance in direction of diffusion, herein channel length (cm).
Values of D for various gaseous contaminants are readily available from the literature. The purely difusional nature of the transfer of the gaseous con-taminant through the channels, at a rate in linear proportion to its atmospheric concentration, provides the integrating character of the dosimeter.

Gas dif$usion de~ice 1 is pre~e~bl~ ~ade from materials that are non-hygroscopic and both chemi-cally and physically inert to the gaseous contaminant and to the collecting medium. Examples are polyethylene, polypropylene, polymers or copolymers of tetrafluoro-ethylene and hexafluoropropylene, and stainless steel.
The above-named polymers are preferred since they can be e~sily injection-molded.
As can be seen Xrom Fick's Law, the number and diameter of the channels a~fect the ~uantity of ~aseous contaminant collected since they affect the total cross-sectional area available ~or transfer.
The quantity of contaminant collected is also inversely proportional to the length of the channels. Althouyh these parameters are not necessarily critical to the integrating operation of the diffusion device, it has been found that when each of the channels has a length-to-diameter ratio of at least 3, preferably at least 4.5, the dosimeter attains the desired insensi-tivity to relative atmospheric motion caused by windor movement Gf the wearer. It has further been found that the use of about 5-500 channels, preferably 10-100 channels, each having a diameter of about 50-1000 microns and a length of about 1.0-25.0 mm, preferably 3.0-8.0 mm, provides a device that is sufficiently sensitive to low ambient contaminant concentrations but is still of a conveniently small size.
A porous, hydrophobic film of 15 1000 micron thickness is placed over the channel openings on the interior slde 8 of diffusion device 1, the side com-municating with the interior 4 of the dosimeter. The film can be made, for example, of polymers or copoly-mexs of tetrafluoroethylene and hexafluoropropylene.

The function of the ilm is to prevent the ~bsorhing solution, if that ~orm oE collecting medium is used, from ~lowing into the channels of diffusion device 1 and to ~urther reduce sensitivity to atmospheric motion. ~ccordingly, the porosity o~ the film and the size of its pores should be se]ected so th~t these ~unctions are performed without inter~errin~ with the passage of the gaseous contaminant from the interior ends of the channels to the absorbing solutlon, That ts, the diffusion of gaseous contaminant through this film should be signlficantly ~reater than the diffusion throuyh the channels so that the overall rate of dif-fusion is essentially controlled only by the channels.
~t has been ~ound that a film that is 50-80% porous ~ith a pore size of 0.1-3.0 microns is sufficient for this purpose when channels as previously described are used.
The dosimeter of Figure 2 is an example of pre~erred embodiments of the present invention but the invention is not limited thereto. Another such embodi-ment of the present invention is shown in Figure 4.
In use, a dosimeter of this invention is ex-posed to the air containing the gaseous contaminant for a period of time for ~hich the average contaminant concentration is sought. When the collecting medium is an absorbing solution, for example, a measured amount o~ the solution is then withdrawn from the dosimeter by, for example, a hypodermic syringe.

When the analysis is to be m~de ph~tometri call~, the withdrawn absorbin~ solution is mixed with appropriate color ~ormin~ reagents which chang~ the color of the absorbing solution. The intensity of ¦ 5 color so formed is dependent upon the amount of gaseous ' contaminant collected. Alt:hough it is o~ten desirable to ha~e a self-contained dosimeter, as shown in U.S.
Patent 4,208,371, in which the reagents are contained in the dosimeter and no wit:hdrawal of material is necessary, this is sometimes impractical. An example of this is where the reagents are highly acidicl as in the color production for formaldehyde where the reagents are chro~otropic acid and sulfuric acid. In such cases, it is difflcult to package the reagents in a stable and safe form, and the simple dosimeter of the present invention is well suited for these appli-cations.
The dosimeter of this invention can be cali-brated to give a direct relationship between colori-metric or spectrophotometric readings and averageambient concentration of the gaseous contaminant. This can be accomplished by following a calibration pro-cedure similar to that described in U.S. Patent 4,208,371. In such a procedure, several dosimeters are exposed over a given period o~ time to various known concentrations of a contaminant for which cali-bration is sought. The dosimeters contain the same kinds and amounts of collecting rnedium. Spectrophoto-metric readings, for example, are determined for at least two dosimeters at each of several known concen-trations, and a straight line is plotted, using a least-squares analysis, through the data points thus obtained.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A personal dosimeter for collecting a gaseous contaminant in the atmosphere during a given collection time in proportion to its average ambient concentration during the collection time, the dosimeter consisting essentially of a closed receptacle;
a liquid collecting absorbant medium for the gaseous contaminant within the receptacle;
a sealed blister positioned in the receptacle separate from the liquid collecting absorbant medium and containing a liquid blank absorbant medium;
a diffusion device, forming a part of the boundary of said receptacle, the device containing a plurality of through-and-through channels, each channel having an opening to the atmosphere and an interior opening adapted for the gaseous contaminant to diffuse therethrough from the atmosphere and into contact with the liquid collecting absorbant medium, said channels each having a length-to-diameter ratio of at least 3 and said channels providing the only communication between the atmosphere and the liquid collecting absorbant medium; and a porous, hydrophobic, inert film covering the interior openings of said channels thereby prevent-ing the flow of liquid absorbant collecting medium into said channels.

2. The dosimeter of Claim 1 in which said receptacle is a pliable pouch of polymeric material.

3. the dosimeter of Claim 1 in which there are 5-500 channels, each having a diameter of 50-1000 microns and a length of 1.0-25.0 mm.

4. The dosimeter of Claim 3 in which the channel length is 3.0-8.0 mm and there are 10-100 channels.

5. The dosimeter of Claim 4 in which the film has a thickness of 15-1000 microns, a porosity of 50-80%, and a pore size of 0.1-3.0 microns.

6. The dosimeter of Claim 5 in which the absorbing solution is for formaldehyde collection.

7. The dosimeter of Claim 2 in which there are 5-500 channels, each having a diameter of 50-1000 microns and a length of 1.0-25.0 mm.

8. The dosimeter of Claim 7 in which the channel length is 3.0-8.0 mm and there are 10-100 channels.

9. The dosimeter of Claim 8 in which the film has a thickness of 15-1000 microns, a porosity of 50-80%, and a pore size of 0.1-3.0 microns.

10. The dosimeter of Claim 9 in which the absorbing solution is for formaldehyde collection.